Earthquake Report: New Ireland

This region of Earth is one of the most seismically active in the past decade plus. This morning, as I was preparing for work, I got an email notifying me of an earthquake with a magnitude M = 7.5 located near New Ireland, Papua New Guinea.

There are every type of plate boundary fault in this region. There are subduction zones, such as that forms the New Britain and San Cristobal trenches. There are transform faults, such as that responsible for the M 7.5 temblor. There are also spreading ridges, such as the one that forms the Manus Basin to the northwest of today’s quake.

I interpret this M 7.5 earthquake to be a left-lateral strike slip earthquake based on (1) the USGS mechanism (moment tensor), (2) our knowledge of the faulting in the region, and (3) historic analogue earthquake examples. There was an earthquake on a subparallel strike-slip fault on 8 March 2018 (here is the earthquake report for that event). Also in that report, I discuss an earthquake from November 2000 that had a magnitude M = 8.0.

After my work on the 28 September 2018 Donggala-Palu earthquake, landslides, and tsunami, I am open minded about the possibility of strike-slip earthquakes as having tsunamigenic potential. There are actually many examples of strike-slip earthquakes causing tsunami, including the 1999 Izmit, 2012 Wharton Basin, and the 2000 New Ireland earthquake too! (see Geist and Parsons, 2005 for more about the small 2000 tsunami.) There was initially a tsunami notification from tsunami.gov about the possibility of a tsunami. Here is a great website where I usually visit when I am looking for tsunami records on tide gage data. This is the closest gage to the quake, but it is not located optimally to record a small tsunami as might have been generated today (I checked).

The Weitin fault is a very active fault, with a slip rate of about 130 mm/yr (Tregoning et al, 1999, 2005). For a comparison, the San Andreas fault has a slip rate of about 25-35 mm/year. Here is a great treatise on the SAF.

There are also examples of earthquake triggering in this region. For example, the 2000.11.16 M 8.0 strike-slip earthquake triggered the 2000.11.16 M 7.8 thrust fault earthquake. It is not unreasonable to consider it possible that there may be triggered earthquakes from this M 7.5 earthquake. Of course, we won’t know until it happens because nobody has the capability to predict earthquakes (regardless of what the charlatans may claim).

The USGS has a variety of products associated with their earthquake pages. I use many of these products in these earthquake reports, so I especially appreciate them. One of the recently added products is a landslide and a liquefaction probability model output. Based on our knowledge of how earthquake release energy, and our knowledge of how earth materials respond to this energy release, people have developed models that allow us to estimate the possibility any given region may experience landslides or liquefaction. I spent some time discussing this in the 28 Sept. 2018 Donggala-Palu earthquake report here.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 3.0 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the North Pole becomes the South Pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly east-west trends of these red and blue stripes in the Woodlark Basin. These lines are parallel to the ocean spreading ridges from where they were formed. The stripes disappear at the subduction zone because the oceanic crust with these anomalies is diving deep beneath the upper plate, so the magnetic anomalies from the overlying plate mask the evidence for the lower plate.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the lower left corner is a figure from Oregon State University (Geology). This shows a cartoon view of the tectonic plates in the region. Note the subduction zone where the Solomon Sea late dives beneath the South Bismarck and Pacific plates. Of particular interest today is the transform (strike-slip) plate boundary between the North and South Bismarck plates.
  • In the upper left corner are two more detailed tectonic maps from Holm et al. (2019). The upper panel shows the plate boundary faults (active subduction zones are symbolized with dark triangles, fossil subd. zones are shown as open triangles). I plate a blue star int eh location of today’s earthquake (as for all inset figures). The lower panel shows the source of volcanic rocks as they have been derived from different subducted oceanic crust and overlying mantle. The geochemistry of these volcanic rocks helps us learn about the tectonic history of this complicated region.
  • The figure in the lower right corner (Holm et al., 2019) shows the current configuration of the different plate boundary faults. Note the left lateral strike-slip relative motion on the (labeled here) Bismarck Sea fault. When this fault crosses New Ireland, it splays into a series of different faults. The most active fault is the Weitin fault.
  • The figure in the upper right corner has lots of information, including cross sections showing the subduction zones (Holm et al., 2016). The oceanic crust created by spreading centers is highlighted for the Woodlark Basin, as well as the Manus Basin northwest of today’s M 7.5 earthquake. The cross section A-B shows these spreading centers.
  • Here is the map with a month’s seismicity plotted. This map includes magnetic anomaly data.

  • Here is the map with a century’s seismicity plotted for magnitudes M ≥ 7.5. Because of the complexity of this figure, the magnetic anomaly data are not included.

M 7.5 Landslide and Liquefaction Models

There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:

FOS = Resisting Force / Driving Force

When FOS > 1, the slope is stable and when FOS < 1, the slope fails and we get a landslide. The illustration below shows these relations. Note how the slope angle α can take part in this ratio (the steeper the slope, the greater impact of the mass of the slope can contribute to driving forces). The real world is more complicated than the simplified illustration below.


Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.

Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.

An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.

Below is a diagram showing how an increase in pore pressure can push against the sediment particles so that they are not touching any more. This allows the particles to move around and this is why our feet sink in the sand in the analogy above. This is also what changes the strength of earth materials such that a landslide can be triggered.


Below is a diagram based upon a publication designed to educate the public about landslides and the processes that trigger them (USGS, 2004). Additional background information about landslide types can be found in Highland et al. (2008). There was a variety of landslide types that can be observed surrounding the earthquake region. So, this illustration can help people when they observing the landscape response to the earthquake whether they are using aerial imagery, photos in newspaper or website articles, or videos on social media. Will you be able to locate a landslide scarp or the toe of a landslide? This figure shows a rotational landslide, one where the land rotates along a curvilinear failure surface.


Here is an excellent educational video from IRIS and a variety of organizations. The video helps us learn about how earthquake intensity gets smaller with distance from an earthquake. The concept of liquefaction is reviewed and we learn how different types of bedrock and underlying earth materials can affect the severity of ground shaking in a given location. The intensity map above is based on a model that relates intensity with distance to the earthquake, but does not incorporate changes in material properties as the video below mentions is an important factor that can increase intensity in places.

If we look at the map at the top of this report, we might imagine that because the areas close to the fault shake more strongly, there may be more landslides in those areas. This is probably true at first order, but the variation in material properties and water content also control where landslides might occur.

There are landslide slope stability and liquefaction susceptibility models based on empirical data from past earthquakes. The USGS has recently incorporated these types of analyses into their earthquake event pages. More about these USGS models can be found on this page.

I prepared some maps that compare the USGS landslide and liquefaction probability maps.

  • Here is the landslide probability map (Jessee et al., 2018). Below the poster I include the text from the USGS website that describes how this model is prepared. The topography and bathymetry come from the National Science Foundation funded GeoMapApp.


Nowicki Jessee and others (2018) is the preferred model for earthquake-triggered landslide hazard. Our primary landslide model is the empirical model of Nowicki Jessee and others (2018). The model was developed by relating 23 inventories of landslides triggered by past earthquakes with different combinations of predictor variables using logistic regression. The output resolution is ~250 m. The model inputs are described below. More details about the model can be found in the original publication. We modify the published model by excluding areas with slopes <5° and changing the coefficient for the lithology layer "unconsolidated sediments" from -3.22 to -1.36, the coefficient for "mixed sedimentary rocks" to better reflect that this unit is expected to be weak (more negative coefficient indicates stronger rock).To exclude areas of insignificantly small probabilities in the computation of aggregate statistics for this model, we use a probability threshold of 0.002.

  • Here is the liquefaction probability (susceptibility) map (Zhu et al., 2017). Note that the regions of low slopes in the valleys and coastal plains are the areas with a high chance of experiencing liquefaction.


Zhu and others (2017) is the preferred model for liquefaction hazard. The model was developed by relating 27 inventories of liquefaction triggered by past earthquakes to globally-available geospatial proxies (summarized below) using logistic regression. We have implemented the global version of the model and have added additional modifications proposed by Baise and Rashidian (2017), including a peak ground acceleration (PGA) threshold of 0.1 g and linear interpolation of the input layers. We also exclude areas with slopes >5°. We linearly interpolate the original input layers of ~1 km resolution to 500 m resolution. The model inputs are described below. More details about the model can be found in the original publication.

Other Report Pages

Some Relevant Discussion and Figures

    • Here is the generalized tectonic map of the region from Holm et al., 2015. I include the figure caption below as a blockquote.

    • Tectonic setting and mineral deposits of eastern Papua New Guinea and Solomon Islands. The modern arc setting related to formation of the mineral deposits comprises, from west to east, the West Bismarck arc, the New Britain arc, the Tabar-Lihir-Tanga-Feni Chain and the Solomon arc, associated with north-dipping subduction/underthrusting at the Ramu-Markham fault zone, New Britain trench and San Cristobal trench respectively. Arrows denote plate motion direction of the Australian and Pacific plates. Filled triangles denote active subduction. Outlined triangles denote slow or extinct subduction. NBP: North Bismarck plate; SBP: South Bismarck plate; AT: Adelbert Terrane; FT: Finisterre Terrane; RMF: Ramu-Markham fault zone; NBT: New Britain trench.

    • In earlier earthquake reports, I discussed seismicity from 2000-2015 here. The seismicity on the west of this region appears aligned with north-south shortening along the New Britain trench, while seismicity on the east of this region appears aligned with more east-west shortening. Here is a map that I put together where I show these two tectonic domains with the seismicity from this time period (today’s earthquakes are not plotted on this map, but one may see where they might plot).

    • Here is the slab interpretation for the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

    • 3-D model of the Solomon slab comprising the subducted Solomon Sea plate, and associated crust of the Woodlark Basin and Australian plate subducted at the New Britain and San Cristobal trenches. Depth is in kilometres; the top surface of the slab is contoured at 20 km intervals from the Earth’s surface (black) to termination of slabrelated seismicity at approximately 550 km depth (light brown). Red line indicates the locations of the Ramu-Markham Fault (RMF)–New Britain trench (NBT)–San Cristobal trench (SCT); other major structures are removed for clarity; NB, New Britain; NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; TLTF, Tabar–Lihir–Tanga–Feni arc. See text for details.

    • Here are the forward models for the slab in the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

    • Forward tectonic reconstruction of progressive arc collision and accretion of New Britain to the Papua New Guinea margin. (a) Schematic forward reconstruction of New Britain relative to Papua New Guinea assuming continued northward motion of the Australian plate and clockwise rotation of the South Bismarck plate. (b) Cross-sections illustrate a conceptual interpretation of collision between New Britain and Papua New Guinea.

    • Here is a map showing some detailed mapping of the Weitin fault (Lindley, 2006).

    • Weitin Fault, Southern New Ireland, showing trace of fault, topography and evidence used by Hohnen (1978) to tentatively suggest sinistral fault movement (after Hohnen, 1978).

    • This figure shows details of the regional tectonics (Holm et al., 2016). I include the figure caption below as a blockquote.

    • a) Present day tectonic features of the Papua New Guinea and Solomon Islands region as shown in plate reconstructions. Sea floor magnetic anomalies are shown for the Caroline plate (Gaina and Müller, 2007), Solomon Sea plate (Gaina and Müller, 2007) and Coral Sea (Weissel and Watts, 1979). Outline of the reconstructed Solomon Sea slab (SSP) and Vanuatu slab (VS)models are as indicated. b) Cross-sections related to the present day tectonic setting. Section locations are as indicated. Bismarck Sea fault (BSF); Feni Deep (FD); Louisiade Plateau
      (LP); Manus Basin (MB); New Britain trench (NBT); North Bismarck microplate (NBP); North Solomon trench (NST); Ontong Java Plateau (OJP); Ramu-Markham fault (RMF); San Cristobal trench (SCT); Solomon Sea plate (SSP); South Bismarck microplate (SBP); Trobriand trough (TT); projected Vanuatu slab (VS); West Bismarck fault (WBF); West Torres Plateau (WTP); Woodlark Basin (WB).

    • Here is a larger scale map showing lineaments (thin black lines) which represent structures formed at the spreading ridges (Lindley, 2006). These spreading ridges are perpendicular to the Weitin and sister transform faults (like the Sapom fault).

    • Map showing onshore structures of the Gazelle Peninsula and New Ireland and those interpreted from SeaMARC II sidescan backscatter data in the Eastern Bismarck Sea. BSSL, Bismarck Sea Seismic Lineation (BSSL). SeaMARC II backscatter data from which lineations have been picked are from Taylor et al. (1991 a-c). Modified after Madsen and Lindley (1994).

    • The interpretive poster above shows the 2007 M 8.1 tsunamigenic subduction zone earthquake. I presented information about this earthquake in a report from 22 Jan. 2017 here. Below are some of the interpretive posters from that report that show excellent examples of subduction zone earthquakes along the San Cristobal trench.
    • Here is my interpretive poster from the 12/17 M 7.9 Bougainville Earthquake, possibly (probably) related to today’s M 7.9 earthquake. This is my Earthquake Report for the 12/17 earthquake.

    Here is a visualization of the seismicity as presented by Dr. Steve Hicks.

    • Here are the maps from Holm et al. (2019) that show the sources of volcanic rocks in the region.

    • Tectonic setting of Papua New Guinea and Solomon Islands. A) Regional plate boundaries and tectonic elements. Light grey shading illustrates bathymetry <2000m below sea level indicative of continental or arc crust, and oceanic plateaus. The New Guinea Orogen comprises rocks of the New Guinea Mobile Belt and the Papuan Fold and Thrust Belt; Adelbert Terrane (AT); Aure-Moresby trough (AMT); Bougainville Island (B); Bismarck Sea fault (BSF); Bundi fault zone (BFZ); Choiseul Island (C); Feni Deep (FD); Finisterre Terrane (FT); Guadalcanal Island (G); Gazelle Peninsula (GP); Kia-Kaipito-Korigole fault zone (KKKF); Lagaip fault zone (LFZ); Malaita Island (M); Manus Island (MI); New Britain (NB); New Georgia Islands (NG); New Guinea Mobile Belt (NGMB); New Ireland (NI); Papuan Fold and Thrust Belt (PFTB); Ramu-Markham fault (RMF); Santa Isabel Island (SI); Sepik arc (SA); Weitin Fault (WF); West Bismarck fault (WBF); Willaumez-Manus Rise (WMR). Arrows indicate rate and direction of plate motion of the Australian and Pacific plates (MORVEL, DeMets et al., 2010); B) Pliocene-Quaternary volcanic centres and magmatic arcs related to this study. Figure modified from Holm et al. (2016). Subduction zone symbols with filled pattern denote active subduction; empty symbols denote extinct subduction zone or negligible convergence.

    • This is a series of plate reconstructions from Holm et al. (2019), the final panel is in the interpretive poster above.

    • Selected tectonic reconstructions and mineral deposit formation for key areas and times within the eastern Papua New Guinea and Solomon Islands region. A) Formation of the Panguna and Fauro Island Deposits above the interpreted subducted margin of the Solomon Sea plate-Woodlark Basin, and Mase deposit above the subducting Woodlark spreading center; B) Formation of the New Georgia deposits above the subducting Woodlark spreading center, and Guadalcanal deposits above the subducting margin of the Woodlark Basin; C) Formation of the Solwara deposits related to transtension along the Bismarck Sea fault above the subducting Solomon Sea plate, and deposits of the Tabar- Lihir-Tanga-Feni island arc chain related to upper plate extension (normal faulting indicated by hatched linework between New Ireland and Bougainville), while the Ladolam deposit forms above a tear in the subducting slab. Interpreted Solomon Sea slab (light blue shaded area for present-day) is from Holm and Richards (2013); the reconstructed surface extent or indicative trend of slab structure is indicated by the dashed red lines. Green regions denote the present-day landmass using modern coastlines; grey regions are indicative of crustal extent using the 2000m bathymetric contour. The reconstruction is presented here relative to the global moving hotspot reference frame, please see the reconstruction files in the supplementary material for specific reference frames.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

  • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

    Social Media

    References:

  • Baldwin, S.L., Monteleone, B.D., Webb, L.E., Fitzgerald, P.G., Grove, M., and Hill, E.J., 2004. Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea in Nature, v. 431, p/ 263-267, doi:10.1038/nature02846.
  • Baldwin, S.L., Fitzgerald, P.G., and Webb, L.E., 2012. Tectonics of the New Guinea Region, Annu. Rev. Earth Planet. Sci., v. 40, pp. 495-520.
  • Cloos, M., Sapiie, B., Quarles van Ufford, A., Weiland, R.J., Warren, P.Q., and McMahon, T.P., 2005, Collisional delamination in New Guinea: The geotectonics of subducting slab breakoff: Geological Society of America Special Paper 400, 51 p., doi: 10.1130/2005.2400.
  • Hamilton, W.B., 1979. Tectonics of the Indonesian Region, USGS Professional Paper 1078.
  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Geist, E. L., and T. Parsons (2005), Triggering of tsunamigenic aftershocks from large strike-slip earthquakes: Analysis of the November 2000 New Ireland earthquake sequence, Geochem. Geophys. Geosyst., 6, Q10005, https://doi.org/10.1029/2005GC000935.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Highland, L.M., and Bobrowsky, P., 2008. The landslide handbook—A guide to understanding landslides, Reston, Virginia, U.S. Geological Survey Circular 1325, 129 p.
  • Holm, R. and Richards, S.W., 2013. A re-evaluation of arc-continent collision and along-arc variation in the Bismarck Sea region, Papua New Guinea in Australian Journal of Earth Sciences, v. 60, p. 605-619.
  • Holm, R.J., Richards, S.W., Rosenbaum, G., and Spandler, C., 2015. Disparate Tectonic Settings for Mineralisation in an Active Arc, Eastern Papua New Guinea and the Solomon Islands in proceedings from PACRIM 2015 Congress, Hong Kong ,18-21 March, 2015, pp. 7.
  • Holm, R.J., Rosenbaum, G., Richards, S.W., 2016. Post 8 Ma reconstruction of Papua New Guinea and Solomon Islands: Microplate tectonics in a convergent plate boundary setting in Eartth Science Reviews, v. 156, p. 66-81.
  • Holm, R.J., Tapster, S., Jelsma, H.A., Rosenbaum, G., and Mark, D.F., 2019. Tectonic evolution and copper-gold metallogenesis of the Papua New Guinea and Solomon Islands region in Ore Geology Reviews, v. 104, p. 208-226, https://doi.org/10.1016/j.oregeorev.2018.11.007
  • Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
  • Johnson, R.W., 1976, Late Cainozoic volcanism and plate tectonics at the southern margin of the Bismarck Sea, Papua New Guinea, in Johnson, R.W., ed., 1976, Volcanism in Australia: Amsterdam, Elsevier, p. 101-116
  • Keefer, D.K., 1984. Landslides Caused by Earthquakes in GSA Bulletin, v. 95, p. 406-421
  • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
  • Lindley, I.D., 2006. Extensional and vertical tectonics in the New Guinea islands: implications for island arc evolution in Annals of Geophysics, suppl to v. 49, no. 1, p. 403-426
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • Tregoning, P., Jackong, R.J., McQueen, H., Lambeck, K., Stevens, C., Little, R.P., Curley, R., and Rosa, R., 1999. Motion of the South Bismarck Plate, Papua New Guinea in GRL, v. 26, no. 23, p. 3517-3520
  • Tregoning, P., McQueen, H., Lambeck, K., Jackson, R. Little, T., Saunders, S., and Rosa, R., 2000. Present-day crustal motion in Papua New Guinea, Earth Planets and Space, v. 52, pp. 727-730.
  • Tregoning, P., Sambridge, M., McQueen, H., Toulin, S., and Nicholson, T., 2005. Motion of the South Bismarck Plate, Papua New Guinea in GJI, v. 160, p. 1103-111, https://doi.org/10.111/j.1365-246X.2005.02567.x
  • USGS, 2004. Landslide Types and Processes, U.S. Geological Survey Fact Sheet 2004-3072
  • Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385, doi: 0.1785/0120160198

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Posted in earthquake, Extension, geology, landslides, pacific, plate tectonics, strike-slip, subduction, Transform, tsunami

Earthquake Report: Panamá

There was just now an earthquake beneath Panamá. The major plate boundary in the region is a subduction zone (convergent plate boudary) where the Cocos and Nazca plates dive northwards beneath the Caribbean plate forming the Middle America trench (MAT).

This magnitude M = 6.1 earthquake appears to be associated with the transform plate boundary (strike-slip fault) that is formed between the Cocos and Nazca plates. I initially interpreted the earthquake mechanism (e.g. moment tensor) shows this to be a strike-slip earthquake along the Panamá fracture zone (PFZ). However, the earthquake is not currently deep enough according to the USGS Slab 2.0 data (that shows the depth of the megathrust subduction zone, or the top of the downgoing oceanic crust/slab). This is still possible, but it is also possible that this is in the upper plate, the Caribbean plate.

If this is in the upper plate (seems more probable), then there are several reasons for the temblor. Perhaps the PFZ is causing differential stress in the overriding plate (causing strike-slip faults to form subparallel to the PFZ and sister faults). Perhaps there is oblique relative plate motion, that is causing strain and slip partitioning in upper plate crustal faults. Perhaps there is some other complicated faulting in the upper plate that exists for some other reason (e.g. pre-existing structures inherited from the tectonic history). OR, it may be due to a combination of any of these possibilities. The fact that this earthquake (and a Christmas temblor in 2003) are aligned with the PFZ suggests that these quakes may be related to the PFZ. As Mr. Spock (Star Trek) would say, “fascinating.”

I have some early reports for quakes along this fz, though the quality of my reports have improved over time. See the May 2014 and January 2015 reports.

The Panama fracture zone (PFZ) has a few sister fracture zones, subparallel dextral (right-lateral) strike-slip faults that have been studied by looking at seismicity and structures of the seafloor. There was a series of large earthquakes in the region south of the MAT in 1934 (Camacho, 1991) ewith the largest magnitude quake at M = 7.5. Earthquakes in the magnitude 6 range are quite common for this system, with temblors M ≥ 6 over once a year.

After tweeting this report, Dr. Kristen Morell (assistant professor at U.C. Santa Barbara) pointed out to me that they did lots of work on the tectonics in the region for their Ph.D. research. I have added some figures from her work below. Morell shows that there are upper plate crustal faults that are associated with the PFZ. Dr. Morell uses a variety of methods to come to this conclusion, including geomorphology (always a great tool), fault mapping (and cross sections of thrust faults and folds), relative plate motions and reconstructions, exhumation analysis, etc. These articles are fundamental to our understanding in this region and we are lucky to have them.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 3.0 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault. Note how the slab contours end at the longitude of the PFZ.

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the North Pole becomes the South Pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly east-west trends of these red and blue stripes. These lines are parallel to the ocean spreading ridges from where they were formed, at the Galapagos Spreading Ridge.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper right corner is a map from Hoernle et al. (2002) that shows the general plate tectonic configuration in this region. We can see the subduction zones, the spreading ridge, and the transform faults (e.g. the PFZ).
  • In the lower left corner there is a figure that shows how the spreading ridges (extensional plate boundaries) have been offset by the fracture zones (transform plate boundaries). Note how some of the spreading ridges are inactive (Meschede and Barckhausen, 2000)
  • In the upper left corner is a large scale map showing the detailes of the magnetic anomalies as they relate to the faults in the region ((Marcaillou et al., 2006).
  • In the lower right corner is a map that shows the details of the different earthquakes during the 1934 sequence (Camacho, 1991).
  • Here is the map with a month’s seismicity plotted.

  • Here is the map with a century’s seismicity plotted.

  • Here is the map with the Global Earthquake Model (GEM) strain map as an overlay (Kreemer et al., 2014). Strain is a quantification of the amount of deformation of the Earth over time. Technically, it is the change in shape (length, volume) per unit time. Note how the strain is localized along the subduction zone, as well as along the PFZ.

Other Report Pages

Some Relevant Discussion and Figures

  • Here is the tectonic overview figure from Hoernle et al. (2002).

  • Gala´pagos Islands and hotspot tracks (Cocos, Coiba, Malpelo, and Carnegie Ridges), igneous complexes in Central America with Gala´pagos geochemical affinities, and western portion of Caribbean plate. Is. is Island, S.C. is spreading
    center.

  • This is another good overview map that shows the spreading center that creates the Nazca and Cocos plates (Coates et al., 2004).

  • Map of southern Central America (dark shading) and the Panama microplate (pale shading). Darien is picked out in pale shading. Dashed lines with teeth mark zones of convergence; zippered line is Panama-Colombia suture. Very heavy dashed line marks location of Neogene volcanic arc; black circles mark Paleogene-Eocene volcanic arc. NPDB – North Panama deformed belt; SPDB – South Panama deformed belt; PFZ – Panama fracture zone. Principal Neogene sedimentary basins located by striped ovals.

  • Here is a map showing an early interpretation of the magnetic anomalies of the Panama Basin (Lonsdale and Klitgord, 1978). The following 2 figures show an example of how shipboard measurements of magnetic intensity are interpreted for the seafloor shown in this figure. The gray bands are parts of the oceanic crust that share a similar magnetic polarity and a similar range in ages. They also present their interpretations of the crustal structures in the region. Recall that this map was prepared long before we had the excellent detailed bathymetry data that we may view in Google Earth (prepared as an inversion of gravity data collected from satellites and the Space Shuttle). Many of the subsequent interpretations of the tectonics in the region is based on this initial analysis.

  • Crustal structure between Malpelo and Panama, showing 1965 to 1975 epicenters (defining present plate boundaries), magnetic anomalies, tracks of profiles shown in Figures 6 and 8, and locations of sampling sites.

  • Here is a map showing the magnetic intensity observations as recorded from research cruises (Lonsdale and Klitgord, 1978). The wiggles are shown along the ship tracklines. The next figure shows one example of these magnetic profiles and how they interpret these data into the magnetic anomaly map of the seafloor.

  • Magnetic anomalies between Malpelo and Carnegie Ridges, numbered according to time scales listed in caption of Figure 2. Note anomaly 5 to 5B sequence along long 81°W, mirrored around extinct Malpelo rift spreading center at lat 1°40’N. Magnetic anomalies on western (Costa Rica rift) segment are from Hey and others (1977). Tracks are labeled for Conrad 11.11 (CON 11) Yaloc 69 (Y 69), Iguana 3, Cocotow 2 (CCTW 2), Cocotow 3 (CCTW 3), F. Drake 3 (FD 3), and some lines of Oceanographer 70 (OC 70). Unlabeled profiles on Costa Rica rift are from Oceanographer 69; those on Malpelo rift are from Oceanographer 70.

  • Here is the profile from Malpelo Ridge to Carnegie Ridge (Lonsdale and Klitgord, 1978). They present the observations compared to their modeled results at the top and a bathymetric profile along with sediment thickness on the bottom.

  • Profile of Cocotow 3 traverse between Malpelo and Carnegie Ridges (see Fig. 4 for location). Synthetic magnetic profile was generated using reversal time scale discussed in Figure 2 caption, spreading rates indicated at top of this figure, magnetization of 4 A/m, and magnetized layer that is 500 m thick and has an upper surface that coincides with basement relief. Note that anomaly match is poor
    within 50 to 100 km of Carnegie and Malpelo Ridges. Fit could have been improved by postulating a somewhat faster spreading rate for this region, but even so, we could not achieve as good a match as for central part of profile.

  • This is the result of their analyses (Lonsdale and Klitgord, 1978). These authors prepared a tectonic reconstruction given their knowledge of crustal structures and magnetic anomaly data.

  • Tectonic reconstructions tracing inferred history of eastern Panama Basin. (A) Middle Oligocene: Farallon plate interacting with Caribbean and South American plates, just before splitting into Nazca and Cocos plates. (B) Middle Miocene: Malpelo and Carnegie Ridges are being formed by hot spot centered on Nazca plate near axis of Nazca-Cocos spreading and are being continuously separated by spreading at boundary. (C) Late Miocene: slowdown of spreading on Malpelo rift, rejuvenation of fracture zone at long 83°W, and cessation of subduction at Panama Trench. (D) Early Pliocene: continued northward migration of Cocos Ridge, stagnation of Malpelo Ridge, and uplift of Coiba Ridge near Nazca-Cocos-Caribbean triple junction. (E) Pleistocene: Cocos and Carnegie Ridges have just arrived at Middle America and Ecuador Trenches, and triple junction has jumped west from Coiba to Panama fracture zone.

  • Here is the map from Meschede and Barchkausem (2000) that shows the complexity of the spreading ridges and the fracture zones in the region.

  • Overview of the eastern Panama Basin (modified from Meschede et al., 1998). Numbers indicate the ages of oceanic crust. The distribution of extinct spreading systems is from Meschede et al. (1998). CNS = Cocos-Nazca spreading system. RSB = rough/smooth boundary.

  • Here is the figure from Camacho (1991) showing their analysis of the faults and earthquake mechanisms from teh 1934 series of earthquakes (and others too). I include their focal mechanism in the posters above.

  • Map depicting lhe southwestern Panama continental manin and lhe Panama-Coiba Fracture Zone wilh some of its characteristic focal mechanisms.

  • Here is the detailed map from Marcaillou et al. (2006) showing the ages of the seafloor for the Panama Basin (the oceanic crust near and east of the PFZ).

  • Interpretation of the pattern of crustal isochrons (Hardy 1991; Lonsdale 2005) and plate boundaries in the Panama Basin (modified from Lonsdale 2005). Earthquakes (black dots) and fault plane solution are from the Harvard University archive of centroid-moment tensor solutions. Plain lines are active spreading axis and transform faults: Costa Rica Rift (CRR) and Panama Fracture Zone (PFZ). Dashed and dotted lines are fossil spreading axis and transform faults: Buenaventura Rift (BR), Malpelo Rift (MR), Coiba Fracture Zone (CFZ) and Yaquina Graben. Possible spreading activity along Sandra Rift (SR) is still in discussion.

  • UPDATE: Here are some key figures from Dr. Morell’s research.
  • The interaction of the Cocos Ridge with the MAT appears to be a key part of the tectonics associated with the PFZ. Here is the plate tectonic map from Morell et al. (2012).
  • My original maps included left-lateral strike-slip arrows on the subduction zone east of the PFZ because I had interpreted this as likely given the obliquity of relative plate motion there, but I was unsure if this was reasonable (I had not seen any maps showing this). Given the analysis by Dr. Morell, I have replaced these arrows in the interpretive posters. Sometimes it is good to go with one’s interpretations of their observations, regardless if they find these interpretations elsewhere. Good lesson.

  • (a) Digital elevation model of the plate tectonic setting surrounding the Cordillera de Talamanca (CT), southern Costa Rica and Cordillera Central (CC), western Panama. Tectonic plates shown are the Cocos plate (CO), Nazca plate (NZ), Caribbean plate (CA), Panama microplate (PM), with plate velocities relative to a fixed CA plate [Bird, 2003; DeMets et al., 1990; DeMets, 2001; Jin and Zhu, 2004; Kellogg and Vega, 1995]. MAT, Middle America Trench; EPR, East Pacific Rise; CNS-2, Cocos-Nazca-Spreading; PTJ, Panama Triple Junction; PFZ, Panama Fracture Zone; BFZ, Balboa Fracture Zone; CFZ, Coiba Fracture Zone; NP, Nicoya Peninsula; AG, Aguacate Range; OP, Osa Peninsula; BP, Burica Peninsula; FCTB, Fila Costeña Thrust Belt; NPDB, North Panama Deformed Belt; TV, Tisingal Volcano; IV, Irazú volcano; BV, Barú volcano; YV, La Yeguada volcano; EV, El Valle volcano. Bathymetric data supplied by ETOPO1 combined from Amante and Eakins [2009], Smith and Sandwell [1997], and Ranero et al. [2003]. Topography supplied by CGIR-CSI based on NASA’s SRTM4 data set. White triangles indicate active volcanoes. Yellow dashed lines indicate on-land projection of Cocos Ridge boundaries. (b) Inset showing location of Figure 1a based on ETOPO1 data. SA refers to the South American plate. (c) Velocity triangle for Panama Triple Junction. CR represents the axis of the Cocos Ridge. Red lines denote the PM-CO and PM-NZ vectors, respectively. Numbers shown are in mm/yr.

  • This figure shows how Dr. Morell interprets how the PFZ projects beneath the upper plate (Morell et al. (2013). Note how the faults in the Fila Costeña Thrust Belt terminate where this tear is projected.

  • Map of southern Central America, showing plate tectonic setting surrounding the Panama triple junction (PTJ) and Barú Volcano based on a stable Caribbean plate (DeMets et al., 1990; Kellogg and Vega, 1995; DeMets, 2001; Bird, 2003). MAT—Middle America Trench; CO—Cocos plate; NZ—Nazca plate; CA— Caribbean plate; PM—Panama micro plate; PTJ—Panama triple junction; PFZ—Panama fracture zone; BFZ—Balboa fracture zone; CFZ—Coiba fracture zone; TV—Tisingal Volcano; OP—Osa Peninsula; BP—Burica Peninsula; VG— Valle General. Elevations are based on National Aeronautics and Space Administration’s (NASA) SRTM v4 imagery. Bathymetry is combined from ETOPO1 and Ranero et al. (2003). Thrusts and shortening estimates outlined for Fila Costeña thrust belt are combined from Sitchler et al. (2007), Fisher et al. (2004), and Morell et al. (2008). Fault traces on Burica Peninsula are from Morell et al. (2011a) and back arc is from Brandes et al. (2007). Contour interval for bathymetry is 250 m.

  • Here is a map that shows how geomorphology can be used to interpret the tectonics of a region (Morell et al., 2013). The color of the stream channel networks represents the steepness of those channels. We can interpret steeper channels to represent regions of higher (or more recent) tectonic uplift rates.
  • Note how the steeper channels are to the west of the projection of the PFZ tear, coincident with the eastern termination of the Fila Costeña Thrust Belt.

  • Map of normalized steepness (ksn) values calculated over a 0.5 km window for drainage basins >107 m2 and excluding valley bottoms for Cordillera de Talamanca and western Cordillera Central. Numbers in northeastern flank of Talamanca correspond to knickpoint numbers shown in Table 2. The locations of longitudinal profiles in Figure 7 are marked as A, B and C, respectively. Faults shown in the Fila Costeña are based on Fisher et al. [2004], Morell et al. [2008], and Sitchler et al. [2007]. Faults drawn in the Limón back arc are approximated from topographic lineaments. Location shown in Figure 1. Base map sourced from DEM draped over slope map derived from SRTM data set. Inset in upper right is simplified geologic map for Cordillera de Talamanca region based on Denyer and Alvarado [2007].

  • Here is a larger scale map showing the geological mapping or late Cretaceous, Tertiary, and Quaternary rock units, and the detailed fault mapping (Morell et al., 2009). The inset shows a shaded relief map showing some of the north-south faults in the upper plate, which are suggested to reflect the crustal response of the PFZ in the upper plate.

  • Simplified geologic map of the southeast Fila Costeña Thrust Belt in the inner forearc of Costa Rica and western Panama (see Fig. 1 for location). Combined data from Sitchler et al. (in press) and this study, revised after Kolarsky and Mann (1995) and Mora (1979). Although the thrust belt continues to the northwest, we focus on the southeast termination. Black boxes indicate location of Figs. 3 and 5. OPFZ = On-land projection of the Panama Fracture Zone. Geology is draped on 90-m DEM supplied by NASA’s SRTM-3 dataset. Stratigraphic column modified after Sitchler et al. (in press), Phillips (1983) and Fisher et al. (2004). Inset A shows shaded DEM of area in white dotted box denoting scarps visible for right-lateral faults A and B based on SRTM-3 dataset.

  • This is an even more large scale map showing strike and dip measurement of the thrust faults mapped in the above figure, as well as more details of the north-south strike-slip raults. Note how they offset some of the thrust faults with a right-lateral _dextral_sense of motion. This is the same sense of motion as the PFZ. This is probably not a coincidence!

  • Bedrock geologic map of the southeastern termination of the Fila Costeña Thrust Belt showing strike and dip measurements within thrust sheets that dip to the northeast. The southeastern termination of the thrust belt roughly coincides with the on-land projection of the Panama Fracture Zone (OPFZ, red dashed line), which is migrating to the southeast with the Panama Triple Junction. F1, F2, F3, F4, and F5 refer to thrust faults 1, 2, 3, 4 and 5, respectively. Cross-sections show locations of balanced cross-sections in Fig. 4. All fault traces and contacts are approximated. Inset index map shows figure location in red box relative to the on-land projection of the Panama Fracture Zone.

  • The following two figures show the Tertiary to recent tectonic reconstructions from Morell (2015). These reconstructions are based on relative plate motion rates as constrained by magnetic anomalies mapped in the oceanic crust, as well as GPS and plate circuit relative plate motions from other researchers (e.g. MOREVEL from DeMets et al., 2010 and others). Frist we see the Morell (2015) magnetic anomaly map, then the plate history maps.

  • Digital elevation model [Smith and Sandwell, 1997; Amante and Eakins, 2009] showing Panama Basin and seafloor magnetic anomaly data surrounding the southern Central America subduction zone [Lonsdale and Klitgord, 1978; Wilson and Hey, 1995; Barckhausen et al., 2001; Lonsdale, 2005] based on chron time scale of Cande and Kent [1995]. The 22000 m contour is shown for prominent bathymetric features in the region, including Malpelo Ridge (MaR) and Coiba Ridge (CoR). BFZ, Balboa Fracture Zone; CFZ, Coiba Fracture Zone; CNS, Cocos-Nazca spreading center; COL, Colombia; CR, Costa Rica; EC, Ecuador; GHS, Galapagos hot spot; MR, Malpelo Rift; MAT, Middle America Trench; MoR, Morgan Rift; NI, Nicaragua; PA, Panama; PFZ, Panama Fracture Zone; SR, Sandra Rift; YG, Yaquina Graben. Inset: Present day plate boundaries of Cocos (CO), Nazca (NZ), Caribbean (CA), and South American (SA) plates. East Pacific Rise (EPR) and Cocos-Nazca Spreading Center (CNS) are also shown.


    Plate reconstruction models for the Cocos (CO) and Nazca (NZ) plates relative to the Caribbean plate from 4 Ma to recent. BFZ, Balboa Fracture Zone; CaR, Carnegie Ridge; CFZ, Coiba Fracture Zone; CNS, crust derived from the Cocos-Nazca spreading center; CocR, Cocos Ridge; CoR, Coiba Ridge; CR, Costa Rica; EPR, crust derived from the East Pacific Rise; GH, Galapagos hot spot; MaR, Malpelo Ridge; MoR, Morgan Rift; MR, Malpelo Rift; PA, Panama; PFZ, Panama Fracture Zone; PTJ, Panama Triple Junction; RSB, rough-smooth boundary; SMD, seamount domain; SR, Sandra Ridge; YG, Yaquina Graben.


    Plate reconstruction models for the Cocos and Nazca plates relative to the Caribbean plate from 6 to 10 Ma. CaR, Carnegie Ridge; CFZ, Coiba Fracture Zone; CNS, crust derived from the Cocos-Nazca spreading center; CocR, Cocos Ridge; CR, Costa Rica; EPR, crust derived from the East Pacific Rise; GH, Galapagos hot spot; MaR, Malpelo Ridge; MR, Malpelo Rift; PA, Panama; PFZ, Panama Fracture Zone; PTJ, Panama Triple Junction; SMD, seamount domain, SR, Sandra Ridge; YG, Yaquina Graben.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

  • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

    References:

  • Camacho, E., 1991. The Puerto Armuelles Earthquake (southwestern Panama) of July 18, 1934 in Rev. Geol. Amer. Central, v. 13, p. 1-13.
  • Coates, A.G., Collins, L.S., Aubry, M.-P., and Berggren, W.A., 2004. The Geology of the Darien, Panama, and the late Miocene-Pliocene collision of the Panama arc with northwestern South America in GSA Bulletin, v. 116, no. 11/12, p. 1327-1344, doi: 10.1130/B25275.1;
  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Hoernle, K., van den Bogaard, P., Werner,R., Lissinna, B., Hauff, F., Alvarado, G., Garbe-Schönberg, D., 2002. Missing history (16–71 Ma) of the Gala´pagos hotspot: Implications
    for the tectonic and biological evolution of the Americas in Geology, v. 30, no. 9, p. 795-798
  • Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
  • Lonsdale, P. and Klitgord, K.D., 1978. Structure and tectonic history of the eastern Panama Basin in GSA Bulletin, v. 89, p. 981-999
  • Morell, K. D., Fisher, D.M., Gardner, T.W., 2008. Inner forearc response to subduction of the Panama Fracture Zone, southern Central America in EPSL, v. 268, p. 82-85, http://doi.org/10.1016/j.epsl.2007.09.039
  • Morell, K. D., Kirby, E., Fisher, D. M., and van Soest, M. 2012. Geomorphic and exhumational response of the Central American Volcanic Arc to Cocos Ridge subduction in J. Geophys. Res., v. 117, B04409, https://doi.org/10.1029/2011JB008969.
  • Morell, K. D., Gardner, T.W., Fisher, D.M., Idleman, B.D., and Zellner, H.M., 2013. Active thrusting, landscape evolution, and late Pleistocene sector collapse of Barú Volcano above the Cocos-Nazca slab tear, southern Central America in GSA Bulletin, v. 125, no. 7/8, p. 1301-1318 https://doi.org/10.1130/B30771.1
  • Morell, K. D., 2015. Late Miocene to recent plate tectonic history of the southern Central America convergent margin in Geochem. Geophys. Geosyst., v. 16, p. 3362–3382, https://doi.org/10.1002/2015GC005971.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Marcaillou, B., Charvis, P., and Collot, J.-V., 2006. Structure of the Malpelo Ridge (Colombia) from seismic and gravity modelling in Mar Geophys Res., DOI 10.1007/s11001-006-9009-y
  • Meschede, M., and Barckhausen, U., 2000. Plate tectonic evolution of the Cocos-Nazca spreading center in Silver, E.A., Kimura, G., and Shipley, T.H. (Eds.), Proc. ODP, Sci. Results, 170: College Station, TX (Ocean Drilling Program), p. 1–10
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743

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Posted in earthquake, education, geology, plate tectonics, strike-slip, Transform

Earthquake Report: Papua New Guinea

Earlier today, there was an intermediate depth beneath eastern Papua New Guinea (PNG). With a magnitude M = 7.2, this is one of the largest earthquake so far in 2019. Here is the USGS website for this earthquake.

Today’s earthquake was quite deep, about 130 km. There are several ways that people have interpreted the tectonics here (which is more common than not).

PNG and New Britain are a region of convergence, where the Australia plate to the south is moving northwards to the Pacific plate (and lots of smaller plates are moving around too).

To the east is a subduction zone (convergent plate boundary) where the Solomon Sea plate dives north beneath the South Bismarck plate. I have prepared many earthquake reports for earthquakes in this region, most of them thrust (compressional) earthquakes related to subduction.

To the north of PNG is a transform plate boundary (strike-slip) that begins at the eastern boundary of the New Britain trench and extends along the north side of PNG, eventually turning into the Sorong fault, then the Palu Koro system in Sulawesi. On 28 September 2018 was an interesting earthquake and tsunami, along with some mega landslides. Here is my report for that series of events.

In the center of PNG, running east-west, is a collision zone formed by the north-south compression I mentioned above. There is a series of compressional folds and faults called the Papua Fold Belt. There have been several large quakes recently in this fold belt. Here is a report for one of those thrust earthquakes, much shallower than today’s eq.

The convergent plate boundary faults in this region have been long lived and have an interesting history. Some of the subduction zones that show up on the maps we will look at are fossil subduction zones (they are no longer active). However, just because they are not active does not mean that there are no earthquakes there. Often, earthquakes can happen along pre-existing zones of weakness. Today’s earthquake may be such a quake. It is difficult to really know.

There have been about 4 earthquakes in the area of today’s quake, with magnitudes M > 7.0. Today’s earthquake is extensional, but intermediate depth earthquakes can be of all types. The 2 quakes that have USGS mechanisms were strike-slip, but one was oblique (it was extensional and strike-slip).

Today, there was also a thrust earthquake, associated with the San Cristobal Trench (the subduction zone to the east of the New Britain trench). I did not label this subduction zone in the map below, but here is an earthquake sequence where I describe this fault zone in greater detail.

Today’s M 7.2 temblor is a cool mystery!

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 3.0 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.
  • Note the interesting orientation of the slab contours near today’s quake. Along the New Britain trench, they get deeper to the north (red contours near the trench and blue contours to the north). These contours wrap around on the west, so in the region of today’s quake, they get deeper to the south. There may be a subducting slab dipping to the south here, perhaps associated with the Trobriand Trench. This is one interpretation, which suggests today’s temblor was in an oceanic slab dipping to the south. Holm et al. (2015) favor a different interpretation, where the fossil subduction zone (forming the Pockington Trough) left behind a delaminated slab. So, today’s quake would be in the oceanic plate from the Australia plate that used to be dipping to the north. I don’t know if we can tell which is the correct hypothesis.

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the North Pole becomes the South Pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly east-west trends of these red and blue stripes in the Woodlark Basin and Solomon Sea plate. These lines are parallel to the ocean spreading ridges from where they were formed. The stripes disappear at the subduction zone because the oceanic crust with these anomalies is diving deep beneath the overlying plate, so the magnetic anomalies mask the evidence for the downgoing plate.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper right corner is a great figure showing the generalized plate tectonic boundaries in this region of the equatorial Pacific Ocean (Holm et al., 2016). I place a blue star in the general location of the M 6.5 earthquake (also plotted in other inset figures). This map shows the major plate boundary faults. Active subduction zones have shaded triangle fault symbols, while inactive subduction zones have un-shaded triangle fault line symbols. I place a blue star in the general location of today’s temblor (as in other inset figures).
  • In the lower left corner is a map showing the fault systems in the region (Cloos et al., 2005). The legend allows us to distinguish between active and inactive fault systems.
  • In the lower right corner is a time history of this plate boundary from Holm et al. (2015). This is generalized with south on the left and north on the right. The plate on the left is the Australia plate and the downgoing plate was previously a subduction zone that formed the Pockington Trough.
  • In the upper left corner is a map that shows the probability (likelihood) for liquefaction (Zhu et al;., 2017). This is a recent addition to the USGS earthquake pages. Read more about this modeling here. These areas that may experience liquefaction are areas that are low lying topography with underlying sediment (as opposed to bedrock) that have the correct properties (e.g. water saturated) to liquefy. I discuss liquefaction in the Donggala-Palu earthquake report here.
  • Here is the map with a month’s seismicity plotted.

  • Here is the map with a century’s seismicity plotted.

  • Here is the map with a month’s seismicity plotted, but i have plotted the active faults in the CCOP database (white lines). These fault lines nicely highlight the Papua fold belt.

Other Report Pages

Some Relevant Discussion and Figures

  • Here is the Holm et al. (2016) figure, showing the major plate boundary faults (symbols tell us which plate boundaries are no longer active: dark symbols = active, hollow symbols = inactive).

  • Topography, bathymetry and regional tectonic setting of New Guinea and Solomon Islands. Arrows indicate rate and direction of plate motion of the Australian and Pacific plates (MORVEL, DeMets et al., 2010); Mamberamo thrust belt, Indonesia (MTB); North Fiji Basin (NFB)

  • This is the Cloos et al. (2005) map from the poster.

  • Tectonic map of New Guinea, adapted from Hamilton (1979), Cooper and Taylor (1987), Dow et al. (1988), and Sapiie et al. (1999). AFTB—Aure fold and thrust belt, FTB—fold-and-thrust belt, IOB—Irian Ophiolite Belt, TFB—thrust-and-fold belt, POB—Papuan Ophiolite Belt, BTFZ—Bewani-Torricelli fault zone, MDZ—Mamberamo deformation zone, YFZ—Yapen fault zone, SFZ—Sorong fault zone, WO—Weyland overthrust. Continental basement exposures are concentrated along the southern fl ank of the Central Range: BD—Baupo Dome, MA—Mapenduma anticline, DM—Digul monocline, IDI—Idenberg Inlier, MUA—Mueller anticline, KA—Kubor anticline, LFTB—Legguru fold-and-thrust belt, RMFZ—Ramu-Markham fault zone, TAFZ—Tarera-Aiduna fault zone. The Tasman line separates continental crust that is Paleozoic and younger to the east from Precambrian to the west.

  • This is the Cloos et al. (2005) cross section, showing a different interpretation of the delaminated slab.

  • Lithospheric-scale cross section at 2 Ma. Plate motion is now focused along the Yapen fault zone in the center of the recently extinct arc. This probably occurred because this zone of weakness had a trend that could accommodate the imposed movements as the corner of the Caroline microplate ruptured, forming the Bismarck plate, and the corner of the Australian plate ruptured, forming the Solomon microplate. The collisional delamination-generated magmatic event ends in the highlands as the lower crustal magma chamber solidifies. Upwelled asthenosphere cools and transforms into lithospheric mantle. This drives a slow regional subsidence of the highlands that will continue for tens of millions of years or until other plate-tectonic movements are initiated. Deep erosion is still concentrated on the fl anks of the mountain belt. RMB—Ruffaer Metamorphic Belt, AUS—Australian plate, PAC—Pacific plate.

  • Here is the tectonic map figure from Sappie and Cloos (2004). Their work was focused on western PNG, so their interpretations are more detailed there (and perhaps less relevant for us for these eastern PNG earthquakes).

  • Seismotectonic interpretation of New Guinea. Tectonic features: PTFB—Papuan thrust-and-fold belt; RMFZ—Ramu-Markham fault zone; BTFZ—Bewani-Torricelli fault zone; MTFB—Mamberamo thrust-and-fold belt; SFZ—Sorong fault zone; YFZ—Yapen fault zone; RFZ—Ransiki fault zone; TAFZ—Tarera-Aiduna fault zone; WT—Waipona Trough. After Sapiie et al. (1999).

  • This is the two panel figure from Holm and Richards (2013) that shows how the New Britain trench megathrust splays into three thrust faults as this fault system heads onto PNG. They plot active thrust faults as black triangles (with the triangles on the hanging wall side of the fault) and inactive thrust faults as open triangles. So, either the NG trench subduction zone extends further east than is presented in earlier work or the Bundi Fault Zone is the fault associated with this deep seismicity.

  • Topography, bathymetry and major tectonic elements of the study area. (a) Major tectonic boundaries of Papua New Guinea and the western Solomon Islands; CP, Caroline plate; MB, Manus Basin; NBP, North Bismarck plate; NBT, New Britain trench; NGT, New Guinea trench; NST, North Solomon trench; PFTB, Papuan Fold and Thrust Belt; PT, Pocklington trough; RMF, Ramu-Markham Fault; SBP, South Bismarck plate; SCT, San Cristobal trench; SS, Solomon Sea plate; TT, Trobriand trough; WB,Woodlark Basin; WMT,West Melanesian trench. Study area is indicated by rectangle labelled Figure 1b; the other inset rectangle highlights location for subsequent figures. Present day GPS motions of plates are indicated relative to the Australian plate (from Tregoning et al. 1998, 1999; Tregoning 2002; Wallace et al. 2004). (b) Detailed topography, bathymetry and structural elements significant to the South Bismarck region (terms not in common use are referenced); AFB, Aure Fold Belt (Davies 2012); AT, Adelbert Terrane (e.g. Wallace et al. 2004); BFZ, Bundi Fault Zone (Abbott 1995); BSSL, Bismarck Sea Seismic Lineation; CG, Cape Gloucester; FT, Finisterre Terrane; GF, Gogol Fault (Abbott 1995); GP, Gazelle Peninsula; HP, Huon Peninsula; MB, Manus Basin; NB, New Britain; NI, New Ireland; OSF, Owen Stanley Fault; RMF, Ramu-Markham Fault; SS, Solomon Sea; WMR, Willaumez-Manus Rise (Johnson et al. 1979); WT, Wonga Thrust (Abbott et al. 1994); minor strike-slip faults are shown adjacent to Huon Peninsula (Abers & McCaffrey 1994) and in east New Britain, the Gazelle Peninsula (e.g. Madsen & Lindley 1994). Circles indicate centres of Quaternary volcanism of the Bismarck arc. Filled triangles indicate active thrusting or subduction, empty triangles indicate extinct or negligible thrusting or subduction.

  • Here is the slab interpretation for the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

  • 3-D model of the Solomon slab comprising the subducted Solomon Sea plate, and associated crust of the Woodlark Basin and Australian plate subducted at the New Britain and San Cristobal trenches. Depth is in kilometres; the top surface of the slab is contoured at 20 km intervals from the Earth’s surface (black) to termination of slabrelated seismicity at approximately 550 km depth (light brown). Red line indicates the locations of the Ramu-Markham Fault (RMF)–New Britain trench (NBT)–San Cristobal trench (SCT); other major structures are removed for clarity; NB, New Britain; NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; TLTF, Tabar–Lihir–Tanga–Feni arc. See text for details.

  • Here are the forward models for the slab in the New Britain region from Holm and Richards, 2013. I include the figure caption below as a blockquote.

  • Forward tectonic reconstruction of progressive arc collision and accretion of New Britain to the Papua New Guinea margin. (a) Schematic forward reconstruction of New Britain relative to Papua New Guinea assuming continued northward motion of the Australian plate and clockwise rotation of the South Bismarck plate. (b) Cross-sections illustrate a conceptual interpretation of collision between New Britain and Papua New Guinea.

  • Earlier, in other earthquake reports, I have discussed seismicity from 2000-2015 here. The seismicity on the west of this region appears aligned with north-south shortening along the New Britain trench, while seismicity on the east of this region appears aligned with more east-west shortening. Here is a map that I put together where I show these two tectonic domains with the seismicity from this time period (today’s earthquakes are not plotted on this map, but one may see where they might plot).

  • This map shows plate velocities and euler poles for different blocks. I include the figure caption below as a blockquote.

  • Tectonic maps of the New Guinea region. (a) Seismicity, volcanoes, and plate motion vectors. Plate motion vectors relative to the Australian plate are surface velocity models based on GPS data, fault slip rates, and earthquake focal mechanisms (UNAVCO, http://jules.unavco.org/Voyager/Earth). Earthquake data are sourced from the International Seismological Center EHB Bulletin (http://www.isc.ac.uk); data represent events from January 1994 through January 2009 with constrained focal depths. Background image is generated from http://www.geomapapp.org. Abbreviations: AB, Arafura Basin; AT, Aure Trough; AyT, Ayu Trough; BA, Banda arc; BSSL, Bismarck Sea seismic lineation; BH, Bird’s Head; BT, Banda Trench; BTFZ, Bewani-Torricelli fault zone; DD, Dayman Dome; DEI, D’Entrecasteaux Islands; FP, Fly Platform; GOP, Gulf of Papua; HP, Huon peninsula; LA, Louisiade Archipelago; LFZ, Lowlands fault zone; MaT, Manus Trench; ML, Mt. Lamington; MT, Mt. Trafalgar; MuT, Mussau Trough; MV, Mt. Victory; MTB, Mamberamo thrust belt; MVF, Managalase Plateau volcanic field; NBT, New Britain Trench; NBA, New Britain arc; NF, Nubara fault; NGT, New Guinea Trench; OJP, Ontong Java Plateau; OSF, Owen Stanley fault zone; PFTB, Papuan fold-and-thrust belt; PP, Papuan peninsula; PRi, Pocklington Rise; PT, Pocklington Trough; RMF, Ramu-Markham fault; SST, South Solomons Trench; SA, Solomon arc; SFZ, Sorong fault zone; ST, Seram Trench; TFZ, Tarera-Aiduna fault zone; TJ, AUS-WDKPAC triple junction; TL, Tasman line; TT, Trobriand Trough;WD, Weber Deep;WB, Woodlark Basin;WFTB, Western (Irian) fold-and-thrust belt; WR,Woodlark Rift; WRi, Woodlark Rise; WTB, Weyland thrust; YFZ, Yapen fault zone.White box indicates the location shown in Figure 3. (b) Map of plates, microplates, and tectonic blocks and elements of the New Guinea region. Tectonic elements modified after Hill & Hall (2003). Abbreviations: ADB, Adelbert block; AOB, April ultramafics; AUS, Australian plate; BHB, Bird’s Head block; CM, Cyclops Mountains; CWB, Cendrawasih block; CAR, Caroline microplate; EMD, Ertsberg Mining District; FA, Finisterre arc; IOB, Irian ophiolite belt; KBB, Kubor & Bena blocks (including Bena Bena terrane); LFTB, Lengguru fold-and-thrust belt; MA, Mapenduma anticline; MB, Mamberamo Basin block; MO, Marum ophiolite belt; MHS, Manus hotspot; NBS, North Bismarck plate; NGH, New Guinea highlands block; NNG, Northern New Guinea block; OKT, Ok Tedi mining district; PAC, Pacific plate; PIC, Porgera intrusive complex; PSP, Philippine Sea plate; PUB, Papuan Ultramafic Belt ophiolite; SB, Sepik Basin block; SDB, Sunda block; SBS, South Bismarck plate; SIB, Solomon Islands block; WP, Wandamen peninsula; WDK, Woodlark microplate; YQ, Yeleme quarries.

  • This figure incorporates cross sections and map views of various parts of the regional tectonics (Baldwin et al., 2012). These deep earthquakes are nearest the cross section D (though are much deeper than these shallow cross sections). I include the figure caption below as a blockquote.

  • Oblique block diagram of New Guinea from the northeast with schematic cross sections showing the present-day plate tectonic setting. Digital elevation model was generated from http://www.geomapapp.org. Oceanic crust in tectonic cross sections is shown by thick black-and-white hatched lines, with arrows indicating active subduction; thick gray-and-white hatched lines indicate uncertain former subduction. Continental crust, transitional continental crust, and arc-related crust are shown without pattern. Representative geologic cross sections across parts of slices C and D are marked with transparent red ovals and within slices B and E are shown by dotted lines. (i ) Cross section of the Papuan peninsula and D’Entrecasteaux Islands modified from Little et al. (2011), showing the obducted ophiolite belt due to collision of the Australian (AUS) plate with an arc in the Paleogene, with later Pliocene extension and exhumation to form the D’Entrecasteaux Islands. (ii ) Cross section of the Papuan peninsula after Davies & Jaques (1984) shows the Papuan ophiolite thrust over metamorphic rocks of AUS margin affinity. (iii ) Across the Papuan mainland, the cross section after Crowhurst et al. (1996) shows the obducted Marum ophiolite and complex folding and thrusting due to collision of the Melanesian arc (the Adelbert, Finisterre, and Huon blocks) in the Late Miocene to recent. (iv) Across the Bird’s Head, the cross section after Bailly et al. (2009) illustrates deformation in the Lengguru fold-and-thrust belt as a result of Late Miocene–Early Pliocene northeast-southwest shortening, followed by Late Pliocene–Quaternary extension. Abbreviations as in Figure 2, in addition to NI, New Ireland; SI, Solomon Islands; SS, Solomon Sea; (U)HP, (ultra)high-pressure.

  • In 1977, D.B. Dow published a Geological Synthesis of Papua New Guinea. There are some excellent low angle oblique views of tectonic geomorphologic features, including the Papua fold belt. Below are two examples, one with an arc volcano formed in the midst of the fold belt. These images are based on RADAR imagery.

  • Radar image of Mount Murray stratovolcano (lat. 6°45’S, long. 144°00’E)—of late Pliocene or Quaternary age—surmounting the prominent strike ridges of folded Miocene Darai Limestone. Deep erosion of the crater has exposed the intrusive core of the volcano. (Scale about 1:250 000.)


    Side-looking radar image of the eastern end of the Papuan Fold Belt between Mount Murray and Mount Karimui. The prominent ridges are steeply dipping Darai Limestone which has been repeated by folding and thrust-faulting. The karst surface developed on the limestone is evident despite the very heavy jungle cover. This image was obtained with the radar looking from the south, so the image is oriented with north to the bottom of the page to prevent the viewer seeing inverted topography. (Scale about 1:250 000.)

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

  • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

    Social Media

    References:

  • Abers, G. and McCaffrey, R., 1988. Active Deformation in the New Guinea Fold-and-Thrust Belt: Seismological Evidence for Strike-Slip Faulting and Basement-Involved Thrusting in JGR, v. 93, no. B11, p. 13,332-13,354
  • Baldwin, S.L., Monteleone, B.D., Webb, L.E., Fitzgerald, P.G., Grove, M., and Hill, E.J., 2004. Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea in Nature, v. 431, p/ 263-267, doi:10.1038/nature02846.
  • Baldwin, S.L., Fitzgerald, P.G., and Webb, L.E., 2012. Tectonics of the New Guinea Region, Annu. Rev. Earth Planet. Sci., v. 40, pp. 495-520.
  • Cloos, M., Sapiie, B., Quarles van Ufford, A., Weiland, R.J., Warren, P.Q., and McMahon, T.P., 2005. Collisional delamination in New Guinea: The geotectonics of subducting slab breakoff: Geological Society of America Special Paper 400, 51 p., doi: 10.1130/2005.2400.
  • Dow, D.B., 1977. A Geological Synthesis of Papua New Guinea, Bureau of Mineral Resources, Geology, and Geophysics, Bulltein 201, Australian Government Publishing Sevice, Canberra, 1977, 58 pp.
  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Hamilton, W.B., 1979. Tectonics of the Indonesian Region, USGS Professional Paper 1078.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Holm, R. and Richards, S.W., 2013. A re-evaluation of arc-continent collision and along-arc variation in the Bismarck Sea region, Papua New Guinea in Australian Journal of Earth Sciences, v. 60, p. 605-619.
  • Holm, R.J., Richards, S.W., Rosenbaum, G., and Spandler, C., 2015. Disparate Tectonic Settings for Mineralisation in an Active Arc, Eastern Papua New Guinea and the Solomon Islands in proceedings from PACRIM 2015 Congress, Hong Kong ,18-21 March, 2015, pp. 7.
  • Holm, R.J., Rosenbaum, G., Richards, S.W., 2016. Post 8 Ma reconstruction of Papua New Guinea and Solomon Islands: Microplate tectonics in a convergent plate boundary setting in Eartth Science Reviews, v. 156, p. 66-81.
  • Johnson, R.W., 1976, Late Cainozoic volcanism and plate tectonics at the southern margin of the Bismarck Sea, Papua New Guinea, in Johnson, R.W., ed., 1976, Volcanism in Australia: Amsterdam, Elsevier, p. 101-116
  • Koulali, A., tregoning, P., McClusky, S., Stanaway, R., Wallace, L., and Lister, G., 2015. New Insights into the present-day kinematics of the central and western Papua New Guinea from GPS in GJI, v. 202, p. 993-1004, doi: 10.1093/gji/ggv200
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • Sapiie, B., and Cloos, M., 2004. Strike-slip faulting in the core of the Central Range of west New Guinea: Ertsberg Mining District, Indonesia in GSA Bulletin, v. 116; no. 3/4; p. 277–293
  • Tregoning, P., McQueen, H., Lambeck, K., Jackson, R. Little, T., Saunders, S., and Rosa, R., 2000. Present-day crustal motion in Papua New Guinea, Earth Planets and Space, v. 52, pp. 727-730.
  • Wells, D., l., and Coppersmith, K.J., 1994. New Empirical Relationships among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement in BSSA, vol. 84, no. 4, pp. 974-1002

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Earthquake Report: Sulawesi, Indonesia

Today I awoke to the USGS earthquake notification service email about an earthquake offshore of Sulawesi, Indonesia. There was an earthquake with a magnitude M 6.8 to the southeast of the Donggala/Palu earthquake from 28 September 2018. Here is the comprehensive earthquake report for the Donggala/Palu earthquake, landslides, and tsunami.

Just like the September quake, today’s event was a strike-slip earthquake, where the crust moves side-by-side (like the San Andreas fault).

This region of the world is complicated and special. There are subduction zone and transform plate boundaries. I use several maps below to present how these plate boundaries control the types of earthquakes. First I plot the earthquakes from the past year, then for the past century. Of course, let’s remember that seismometers are not that old, so the first half of the 20th century, there were not many seismometers. So, the earthquake record before the 1950s is generally composed of earthquakes with larger magnitude.

There are many many faults in this region, overlapping each other, offsetting each other. And, there have been earthquakes along many of these systems over the past year and past century that represent these different systems and how they interact.

The M 6.8 temblor is strange because it is oriented in a way that is different from the mapped faults in the region. The mainshock/aftershock sequence suggests a northeast-southwest oriented fault (making this a right-lateral strike slip earthquake). The mapped faults with this orientation are instead left-lateral faults.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 4.5 and M ≥ 7.5 in different versions.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    Magnetic Anomalies

  • In the M 7.5 map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the North Pole becomes the South Pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly east-west trends of these red and blue stripes. These lines are parallel to the ocean spreading ridges from where they were formed.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper right corner is an overview map from Zahirovic et al. (2014) that shows the major plate boundary faults and tectonic plates. I placed a blue star in the general location of yesterday’s M 6.8 temblor.
  • The map in the upper left corner shows one interpretation of these faults as presented by Bellier et al. (2006). The M 6.8 quake happened somewhere in the intersection of the Batui thrust (an extension of the Molucca Collision) and the Sorong fault. There are half a dozen different interpretations for the tectonics here, this is but one.
  • The map in the lower left corner is a map from Cipta et a. (2006) that shows the relative seismic hazard for Sulawesi. Compare this map with the Bellier map above. Note how the seismic hazard is directly related to the known earthquake faults.
  • In the lower right corner is a low-angle oblique view of the plates and their boundaries in this part of the world (Hall, 2011). I present this figure alone below to highlight the details of how these faults interact near the M 6.8 quake.
  • Here is the map with a year’s seismicity plotted for quakes M ≥ 4.5.
    • Earthquakes from the past year represent well many of the plate boundaries here. Notably is the Donggala/Palu sequence, with all the aftershocks, that align with the trend of the Palu-Koro fault as it connects to the south tot he Sorong fault system.
    • There are several quakes along the Java trench (the Sunda subduction zone), showing thrust quakes (e.g. 2.18, 8.28, and 10.1 in 2018 and 1.21 and 1.22 in 2019). The Lombok sequence of 2018 is also evidence of this north-south convergence.
    • As the Australia plate dives deep beneath the Sunda plate, the slab (the oceanic crust) pulls downwards, causing extension. The 2018.07.28 M 6.0 and 2019.04.06 normal fault earthquakes (orange arrows) are great examples of this. Intermediate depth earthquakes are not completely understood, but we learn more every year. For example, sometimes there are compressional quakes (thrust/reverse) that happen at these depths, e.g. the 2018.08.17 M 6.5 quake.
    • One of the more active regions is the Molucca Strait, where there are subduction/convergent zones that oppose each other. The 2019.01.06 M 6.6 shaker is a good example of what can happen here (and does rather frequently).
    • Further north is the subduciton zone that forms the Philippine trench. There was a M 7.0 earthquake on 2018.12.29 that shows evidence of the subduction zone megathrust.


  • Here is the map with a century’s seismicity plotted for quakes M ≥ 7.5.
    • The USGS earthquake catalog includes additional examples of larger quakes over the past century that represent the range of plate boundary types in the region. The global earthquake catalog is better after 1950 due to the increase in seismic monitoring during the cold war (monitoring for nuclear weapons testing).
    • Evidence for subduction along the Sunda subduction zone include subduction zone earthquakes (1977.08.19 M 8.3, 1994.06.02 M 7.8). Also, evidence for down-dip slab-pull extension as evidenced by the 1996.06.17 M 7.9 temblor. similar to the 2018 Lombok sequence, there are also examples of the backthrust to the subduction zone (e.g. 1992.12.12 M 7.8 and 2004.11.11 M 7.5).
    • The Molucca Strait thrust earthquakes are evidence for this bivergent convergence (e.g. 1986.08.14 M 7.5 and 2007.01.21 M 7.5).
    • There is also a subduction zone on the north side of Sulawesi, with several good example earthquakes (e.g. 1990.04.18 M 7.5, 1991.06.20 M 7.5, and 1996.01.01 M 7.9, which was tsunamigenic).
    • There was a strike-slip earthquake on 1998.11.29 that is related to the Sorong fault system, a magnitude M 7.7 shaker.


Other Report Pages

Some Relevant Discussion and Figures

  • This is the small scale tectonic map of the region (Zahirovic et al., 2014). This gives us the overview we need so we can understand the wide variety of plate boundary faults and how they interact with each other.

  • Regional tectonic setting with plate boundaries (MORs/transforms = black, subduction zones = teethed red) from Bird (2003) and ophiolite belts representing sutures modified from Hutchison (1975) and Baldwin et al. (2012). West Sulawesi basalts are from Polvé et al. (1997), fracture zones are from Matthews et al. (2011) and basin outlines are from Hearn et al. (2003). ANI – Andaman and Nicobar Islands, BD– Billiton Depression, Ba – Bangka Island, BI – Belitung (Billiton) Island, BiS – Bismarck Sea, BP – Benham Plateau, CaR – Caroline Ridge, CS – Celebes Sea, DG– Dangerous Grounds, EauR – Eauripik Ridge, FIN – Finisterre Terrane, GoT – Gulf of Thailand, GR– Gagua Ridge, HAL– Halmahera, HBa – Huatung Basin, KB–Ketungau Basin, KP – Khorat Platform, KT – Kiilsgaard Trough, LS – Luconia Shoals, MacB – Macclesfield Bank, ManTr – Manus Trench, MaTr – Mariana Trench, MB– Melawi Basin, MDB– Minami Daito Basin, MG– Mangkalihat, MIN – Mindoro, MN– Mawgyi Nappe, MoS – Molucca Sea, MS– Makassar Straits, MTr – Mussau Trench, NGTr – New Guinea Trench, NI – Natuna Islands, ODR– Oki Daito Ridge, OJP –Ontong Java Plateau, OSF – Owen Stanley Fault, PAL – Palawan, PhF – Philippine Fault, PT – Paternoster Platform, PTr – Palau Trench, PVB – Parece Vela Basin, RB – Reed Bank, RMF– Ramu-Markham Fault, RRF – Red River fault, SEM– Semitau, ShB – Shikoku Basin, Sol. Sea – Solomon Sea, SPK – Sepik, SPT – abah–Palawan Trough, STr – Sorol Trough, Sul – Sulawesi, SuS – Sulu Sea, TPAA– Torricelli–Prince Alexander Arc, WB–West Burma, WCT–W Caroline Trough, YTr –Yap Trough.

  • Here is the tectonic map from Bellier et al., 2006. I include their caption below in blockquote. Note how the Molluca Collision faults trend towards the Batui thrust. However, when we look more closely at the faulting on a local scale, things get much more complicated.


  • Regional geodynamic sketch that presents the present day deformation model of Sulawesi area (after Beaudouin et al., 2003) and four main deformation systems around the Central Sulawesi block, highlighting the tectonic complexity of Sulawesi. Approximate location of the Central Sulawesi block rotation pole (P) [compatible with both GPS measurements (Walpersdorf et al., 1998a) and earthquake moment tensor analyses (Beaudouin et al., 2003)], as well as the major active structures are reported. Central Sulawesi Fault System (CSFS) is formed by the Palu–Koro and Matano faults. Arrows correspond to the compression and/or extension directions deduced from both inversion and moment tensor analyses of the focal mechanisms; arrow size being proportional to the deformation rate (e.g., Beaudouin et al., 2003).We also represent the focal mechanism provided by the Harvard CMT database [CMT data base, 2005] for the recent large earthquake (Mw=6.2; 2005/1/23; lat.=0.928S; long.=120.108E). The box indicates the approximate location of the Fig. 6 that corresponds to the geological map of the Palu basin region. The bottom inset shows the SE Asia and Sulawesi geodynamic frame where arrows represent the approximate Indo-Australian and Philippines plate motions relative to Eurasia.

  • Here is the larger scale map showing the fault configuration in this region (Bellier et al., 2006). I include this so we can see how the Sorong fault system extends and relates to the Palu-Koro system.


  • Sketch map of the Cenozoic Central Sulawesi fault system. ML represents the Matano Lake, and Leboni RFZ, the Leboni releasing fault zone that connects the Palu–Koro and Matano Faults. Triangles indicate faults with reverse component (triangles on the upthrown block). On this map are reported the fault kinematic measurement sites (geographic coordinates in Table 3).

  • Here is the low-angle oblique view of this region. Note the left-lateral strike-slip fault bisecting Sulawesi. Note the Sorong fault system that trends towards this system. The Sorong fault ends in a convergent plate boundary in eastern Sulawesi (the Batui thrust). There is a small north-south fault linking these two systems on the western part of hte N Banda Sea. The M 6.8 earquake happened in this area. We will look at more detailed maps of this area.

  • 3D cartoon of plate boundaries in the Molucca Sea region modified from Hall et al. (1995). Although seismicity identifies a number of plates there are no continuous boundaries, and the Cotobato, North Sulawesi and Philippine Trenches are all intraplate features. The apparent distinction between different crust types, such as Australian continental crust and oceanic crust of the Philippine and Molucca Sea, is partly a boundary inactive since the Early Miocene (east Sulawesi) and partly a younger but now probably inactive boundary of the Sorong Fault. The upper crust of this entire region is deforming in a much more continuous way than suggested by this cartoon.

  • Here is another interpretation showing how these faults map interact in the region (Simandjuntak and Barber, 1996). Yesterday’s M 6.8 quake happened southwest of Banggai.

  • Talaud orogeny in the North Moluccas. Line of section illustrated in Fig. 9 is indicated.

  • This is larger scale, showing details for the Sulawesi region (Simandjuntak and Barber, 1996).

  • Sulawesi orogeny. Line of section illustrated in Fig. 9 is indicated.

  • Below are a couple maps from Watkinson et al. (2011) that show detailed mapping in this area.
  • First here is a fault tectonic map based on new (2011) interpretations. These interpretations are based on detailed seismic reflection data, as well as high resolution multibeam mapping (detailed information about the surface of the seafloor).

  • Map of the same area as Figure 1, and drawn largely after the same sources, but modified in the light of the present study. Revised faults are shown in red. Principal differences include the absence of a through-going Sula Thrust, the Sorong Fault as a plate boundary which does not reach the surface, and connection of the Poh Head fault to the region of dextral transpression in the west of the study area. Sources of deformation in the region are indicated by regions of colour.

  • Here is a regional map showing multibeam bathymetry along with fault line interpretations. This is “figure 3; note the extent for “figure 8,” which is the figure i present next.

  • (a) Shaded relief map of the multibeam data. See inset map for location. Illumination from the NW. (b) Interpreted structural map, showing fault kinematics, basin areas, and fields of debris derived from the collapsing slope in the south. Locations of subsequent figures shown.

  • Here is the detailed map of the seafloor geomorphology (Watkinson et al., 2011). This map is northeast of the island of Banggai, but it informs us about the northeast oriented faults, along with the northwest oriented faults. Note that the northwest oriented faults are right lateral (opposite sense of motion compared to the Sorong fault system, which makes interpreting the M 6.8 more complicated). Also, north how the northwest striking (oriented) faults are left-lateral strike-slip systems. This is also opposite the sense of motion for the M 6.8 earthquake (and also for the 1999.08.12 M 6.2 quake (see the year’s seismicity interpretive poster above).
  • So, we have a mystery. What fault system is responsible for the 2019.04.12 M 6.8 and 1999.08.12 M 6.2 quakes. So exciting!

  • Multibeam image showing details of the region of dextral transpression in the west of the study area. See Figure 3b and inset map for location. Antiformal hinge lines marked by black dashed lines, thrusts marked by white dashed lines. Strike-slip faults marked by double half arrows. Maximum horizontal stress orientations for various structures shown in top right.

  • Next, lets look at the evidence for subduction along the Sunda subduction zone. Below is a map showing historic seismicity (Jones et al., 2014). Cross sections B-B’ and C-C’ are shown. The seismicity for the cross sections below are sourced from within each respective rectangle.

  • Here are the seismcity cross sections.

  • Below are the maps and cross sections from Darman et al., 2012.

  • Tectonic map of the Lesser Sunda Islands, showing the main tectonic units, main faults, bathymetry and location of seismic sections discussed in this paper.

  • Here is the seismicity cross section in the interpretive poster above.

  • This plot shows the earthquake localizations on a South-North cross section for the lat -14°/-4° long 114°/124° quadrant corresponding to the Lesser Sunda Islands region. The localizations are extracted from the USGS database and corresponds to magnitude greater than 4.5 in the 1973-2004 time period (shallow earthquakes with undetermined depth have been omitted.

  • Here is their interpretations of seismic data used to interpret the tectonics of the subduction zone and Flores thrust.

  • Six 15 km deep seismic sections acquired by BGR from west to east traversing oceanic crust, deep sea trench, accretionary prism, outer arc high and fore-arc basin, derived from Kirchoff prestack depth migration (PreSDM) with a frequency range of 4-60 Hz. Profile BGR06-313 shows exemplarily a velocity-depth model according to refraction/wide-angle
    seismic tomography on coincident profile P31 (modified after Lüschen et al, 2011).

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

  • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

    Social Media

    References:

  • Audley-Charles, M.G., 1986. Rates of Neogene and Quaternary tectonic movements in the Southern Banda Arc based on micropalaeontology in: Journal of fhe Geological Society, London, Vol. 143, 1986, pp. 161-175.
  • Audley-Charles, M.G., 2011. Tectonic post-collision processes in Timor, Hall, R., Cottam, M. A. &Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 241–266.
  • Baldwin, S.L., Fitzgerald, P.G., and Webb, L.E., 2012. Tectonics of the New Guinea Region in Annu. Rev. Earth Planet. Sci., v. 41, p. 485-520.
  • Bellier, O., Se´brier, M., Seward, D., Beaudouin, T., Villeneuve, M., and Putranto, E., 2006. Fission track and fault kinematics analyses for new insight into the Late Cenozoic tectonic regime changes in West-Central Sulawesi (Indonesia) in Tectonophysics, v. 413, p. 201-220.
  • Benz, H.M., Herman, Matthew, Tarr, A.C., Hayes, G.P., Furlong, K.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011. Seismicity of the Earth 1900–2010 New Guinea and vicinity: U.S. Geological Survey Open-File Report 2010–1083-H, scale 1:8,000,000.
  • Cipta, A., Robiana, R., Griffin, J.D., Horspool, N., Hidayati, S., and Cummins, P., 2016. A probabilistic seismic hazard assessment for Sulawesi, Indonesia in Cummins, P. R. &Meilano, I. (eds) Geohazards in Indonesia: Earth Science for Disaster Risk Reduction, Geological Society, London, Special Publications, v. 441, http://doi.org/10.1144/SP441.6
  • Darman, H., 2012. Seismic Expression of Tectonic Features in the Lesser Sunda Islands, Indonesia in Berita Sedimentologi, Indonesian Journal of Sedimentary Geology, no. 25, po. 16-25.
  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Gómez, J.M., Madariaga, R., Walpersdorf, A., and Chalard, E., 2000. The 1996 Earthquakes in Sulawesi, Indonesia in BSSA, v. 90, no. 3, p. 739-751
  • Hall, R., 2011. Australia-SE Asia collision: plate tectonics and crustal flow in Geological Society, London, Special Publications 2011; v. 355; p. 75-109 doi: 10.1144/SP355.5
  • Hangesh, J. and Whitney, B., 2014. Quaternary Reactivation of Australia’s Western Passive Margin: Inception of a New Plate Boundary? in: 5th International INQUA Meeting on Paleoseismology, Active Tectonics and Archeoseismology (PATA), 21-27 September 2014, Busan, Korea, 4 pp.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
  • Jones, E.S., Hayes, G.P., Bernardino, Melissa, Dannemann, F.K., Furlong, K.P., Benz, H.M., and Villaseñor, Antonio, 2014. Seismicity of the Earth 1900–2012 Java and vicinity: U.S. Geological Survey Open-File Report 2010–1083-N, 1 sheet, scale 1:5,000,000, https://dx.doi.org/10.3133/ofr20101083N.
  • Koulali, A., S. Susilo, S. McClusky, I. Meilano, P. Cummins, P. Tregoning, G. Lister, J. Efendi, and M. A. Syafi’i, 2016. Crustal strain partitioning and the associated earthquake hazard in the eastern Sunda-Banda Arc in Geophys. Res. Lett., 43, 1943–1949, doi:10.1002/2016GL067941
  • Lin, J., and R. S. Stein (2004), Stress triggering in thrust and subduction earthquakes and stress interaction between the southern San Andreas and nearby thrust and strike-slip faults, J. Geophys. Res., 109, B02303, doi:10.1029/2003JB002607.
  • Lüschen, E., Müller, C., Kopp, H., Engels, M., Lutz, R., Planert, L., Shulgin, A., Djajadihardja, Y. S., 2011. Structure, evolution and tectonic activity of the eastern Sunda forearc,Indonesia from marine seismic investigations, Tectonophysics, 508, p. 6-21
  • McCaffrey, R., and Nabelek, J.L., 1984. The geometry of back arc thrusting along the Eastern Sunda Arc, Indonesia: Constraints from earthquake and gravity data in JGR, Atm., vol., 925, no. B1, p. 441-4620, DOI: 10.1029/JB089iB07p06171
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
  • Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
  • Okal, E. A., & Reymond, D., 2003. The mechanism of great Banda Sea earthquake of 1 February 1938: applying the method of preliminary determination of focal mechanism to a historical event in EPSL, v. 216, p. 1-15.
  • Silver, E.A., Breen, N.A., and Prastyo, H., 1986. Multibeam Study of the Flores Backarc Thrust Belt, Indonesia, in JGR., vol. 91, no. B3, p. 3489-3500
  • Simandjuntak, T.O. and Barber, A.J., 1996. Contrasting tectonic styles in the Neogene orogenic belts of Indonesia in Hall, R. & Blundell, D. (eds), 1996, Tectonic Evolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 185-201.
  • Socquet, A., Simons, W., Vigny, C., McCaffrey, R., Subarya, C., Sarsito, D., Ambrosius, B., and Spakman, W., 2006. Microblock rotations and fault coupling in SE Asia triple junction (Sulawesi, Indonesia) from GPS and earthquake slip vector data, J. Geophys. Res., 111, B08409, doi:10.1029/2005JB003963.
  • Walpersdorf, A., Rangin, C., and Vigny, C., 1998. GPS compared to long-term geologic motion of the north arm of Sulawesi in EPSL, v. 159, p. 47-55.
  • Watkinson, I.M. Hall, R., Ferdian, F., 2011. Tectonic re-interpretation of the Banggai-Sula–Molucca Sea margin, Indonesia in Hall, R., Cottam, M. A. &Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 203–224. http://doi.org/10.1144/SP355.10
  • Watkinson, I.M. and Hall, R., 2017. Fault systems of the eastern Indonesian triple junction: evaluation of Quaternary activity and implications for seismic hazards in Cummins, P. R. & Meilano, I. (eds) Geohazards in Indonesia: Earth Science for Disaster Risk Reduction, Geological Society, London, Special Publications, v. 441, https://doi.org/10.1144/SP441.8
  • Zahirovic, S., Seton, M., and Müller, R.D., 2014. The Cretaceous and Cenozoic tectonic evolution of Southeast Asia in Solid Earth, v. 5, p. 227-273, doi:10.5194/se-5-227-2014

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Posted in earthquake, education, geology, Indonesia, plate tectonics, strike-slip

18 April 1906 San Francisco Earthquake

Today is the anniversary of the 18 April 1906 San Francisco Earthquake. There are few direct observations (e.g. from seismometers or other instruments) from this earthquake, so our knowledge of how strong the ground shook during the earthquake are limited to indirect measurements.

Below I present a poster that shows a computer simulation that provides an estimate of the intensity of the ground shaking that may happen if the San Andreas fault slipped in a similar way that it did in 1906.

The USGS prepares these ShakeMap scenario maps so that we can have an estimate of the ground shaking from hypothetical earthquakes. I present a poster below that uses data from one of these scenarios. This is a scenario that is similar to what we think happened in 1906, but it is only a model.

There is lots about the 1906 Earthquake that I did not include, but this leaves me room for improvement for the years into the future, when we see this anniversary come again.

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1900-2018 with magnitudes M ≥ 5.5.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the North Pole becomes the South Pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly north-south trends of these red and blue stripes. These lines are parallel to the ocean spreading ridges from where they were formed.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • On the right is a map from Wallace (1990) that shows the main faults that are part of the Pacific – North America plate boundary. The San Andreas fault is the locus of a majority of this relative plate motion.
  • In the upper right, to the left of the Wallace map, is a map of the entire state of California. This map shows the shaking potential for different regions based on an estimate of earthquake probability. Pink areas are more likely to experience stronger ground shaking, more frequently, than areas colored green.
  • In the lower right, to the left of the Wallace map, is a photo showing a fence that was offset during the 1906 earthquake. The relative distance between these fences is about 2.6 meters (Lawson, 1908; Aargard and Bowza, 2008).
  • In the upper left corner is a map showing an estimate of the ground motions produced by the 1906 San Francisco earthquake, based on Song et al. (2008) source model (Aargard et al., 2008).
  • In the lower left corner is a figure that shows the historic earthquakes for hte San Francisco Bay region (Aagaard et al., 2016). Note that they find there to be a 72% chance of an earthquake with manitude 6.7 or greater between 2014 and 2043.
  • Here is the map with a month’s seismicity plotted.

  • Here is the photo of the offset fence (Aargard and Bowza, 2008).

  • Fence half a mile northwest of Woodville (east of Point Reyes), offset by approximately 2.6 m of right-lateral strike-slip motion on the San Andreas fault in the 1906 San Francisco earthquake (U.S. Geological Survey Photographic Library, Gilbert, G. K. 2845).

  • Here is the USGS ShakeMap (Aargard et al., 2008)

  • ShakeMap for the 1906 San Francisco earthquake based on the Boatwright and Bundock (2005) intensities (processed 18 October 2005). Open circles identify the intensity sites used to construct the ShakeMap.

  • In the map above, we can see that the ground shaking was quite high in Humboldt County, CA. Below is a photo from Dengler et al. (2008) that shows headscarps to some lateral slides that failed as a result of the 1906 earthquake. This is the tupe of failure that extended across a much larger landscape for the 28 September 2018 Dongalla / Palu earthquake and tsunami.

  • Spread failures on the banks of the Eel River near Port Kenyon in 1906. Photo E. Garrett, courtesy of Peter Palmquist.

  • Here is a map that shows the estimate for the location of the epicenter for the mainshock of the 1906 earthquake. See Lomax (2008) for more on this.

  • I place a map shows the configuration of faults in central (San Francisco) and northern (Point Delgada – Punta Gorda) CA (Wallace, 1990). Here is the caption for this map, that is on the lower left corner of my map. Below the citation is this map presented on its own.

  • Geologic sketch map of the northern Coast Ranges, central California, showing faults with Quaternary activity and basin deposits in northern section of the San Andreas fault system. Fault patterns are generalized, and only major faults are shown. Several Quaternary basins are fault bounded and aligned parallel to strike-slip faults, a relation most apparent along the Hayward-Rodgers Creek-Maacama fault trend.

  • Here is the figure showing the evolution of the SAF since its inception about 29 Ma. I include the USGS figure caption below as a blockquote.

  • EVOLUTION OF THE SAN ANDREAS FAULT.

    This series of block diagrams shows how the subduction zone along the west coast of North America transformed into the San Andreas Fault from 30 million years ago to the present. Starting at 30 million years ago, the westward- moving North American Plate began to override the spreading ridge between the Farallon Plate and the Pacific Plate. This action divided the Farallon Plate into two smaller plates, the northern Juan de Fuca Plate (JdFP) and the southern Cocos Plate (CP). By 20 million years ago, two triple junctions began to migrate north and south along the western margin of the West Coast. (Triple junctions are intersections between three tectonic plates; shown as red triangles in the diagrams.) The change in plate configuration as the North American Plate began to encounter the Pacific Plate resulted in the formation of the San Andreas Fault. The northern Mendicino Triple Junction (M) migrated through the San Francisco Bay region roughly 12 to 5 million years ago and is presently located off the coast of northern California, roughly midway between San Francisco (SF) and Seattle (S). The Mendicino Triple Junction represents the intersection of the North American, Pacific, and Juan de Fuca Plates. The southern Rivera Triple Junction (R) is presently located in the Pacific Ocean between Baja California (BC) and Manzanillo, Mexico (MZ). Evidence of the migration of the Mendicino Triple Junction northward through the San Francisco Bay region is preserved as a series of volcanic centers that grow progressively younger toward the north. Volcanic rocks in the Hollister region are roughly 12 million years old whereas the volcanic rocks in the Sonoma-Clear Lake region north of San Francisco Bay range from only few million to as little as 10,000 years old. Both of these volcanic areas and older volcanic rocks in the region are offset by the modern regional fault system. (Image modified after original illustration by Irwin, 1990 and Stoffer, 2006.)

Tectonic History of Western North America and Southern California

  • Here is an animation from Tanya Atwater that shows how the Pacific-North America plate margin evolved over the past 40 million years (Ma).

Some Relevant Discussion and Figures

  • Here is the shaking potential map for California.

  • Here is the earthquake timeline (Aagaard et al., 2016).

  • This map shows the relative contribution that each fault has for the chance of earthquakes in the region. For example, this shows that the Hayward fault is the fault with the highest chance of rupture (Aagaard et al., 2016).

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

  • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

    Social Media

Return to the Earthquake Reports page.

Posted in earthquake, education, geology, los angeles, pacific, plate tectonics, San Andreas, strike-slip, Transform

Earthquake Report: central Aleutians

A couple days ago, in my inbox, there was an email from the Pacific Tsunami Warning Center about an earthquake along the Aleutian Islands, near Rat Island, Alaska. However, this earthquake was not along the megathrust subduction zone fault there and it was rather deep (~19 km). Also, this earthquake was strike-slip (not thrust or reverse), so probably did not produce much vertical ground motion. These two factors combined (deep and strike-slip) suggest to me that there would not be a tsunami generated from this earthquake. BUT we learn new things every month.

There was a subduction zone earthquake nearby on 15 August 2018. Learn more about the subduction zone in my earthquake report for this M 6.6 earthquake here.

There was a similar earthquake in 2017 further to the west, which was also a strike-slip earthquake and it produced a small sized tsunami (Lay et al., 2017). However, the 17 July 2017 magnitude M 7.9 earthquake was much larger in magnitude. Here is my earthquake report and update for this 2017 earthquake. These reports include information about the intersection of the Aleutian and Kuril plate boundaries.

The majority of the Aleutian Islands are volcanic arc islands formed as a result of the subduction of the Pacific plate beneath the North America plate. To the west, there is another subduction zone along the Kuril and Kamchatka volcanic arcs. These subduction zones form deep sea trenches (the deepest parts of the ocean are in subduction zone trenches).

In the eastern part of the Aleutian/Alaska subduction zone (e.g. Alaska Peninsula or Prince William Sound), the plates converge in the direction of subduction (perpendicular to the fault orientation or “strike”). Further to the west, the plates converge obliquely compared to the fault orientation.

This oblique convergence results in the development of additional special faults that accommodate the plate convergence not perpendicular to the faults. These are typically strike-slip faults parallel to the subduction zone (they accommodate the proportion of relative motion parallel to the fault), called forearc sliver faults.

Along the central and western Aleutian plate boundary, this strike-slip relative motion also creates blocks in the upper North America plate that rotate relative to the forearc sliver fault. Imagine how ball bearings rotate when the two planes that they are contained within move relative to each other.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the North Pole becomes the South Pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly east-west trends of these red and blue stripes. These lines are parallel to the ocean spreading ridges from where they were formed. The stripes disappear at the subduction zone because the oceanic crust with these anomalies is diving deep beneath the North America plate \, so the magnetic anomalies from the overlying North America plate mask the evidence for the Pacific plate.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the lower right corner is a figure that shows the historic earthquake ruptures along the Aleutian Megathrust (Peter Haeussler, USGS). I placed a blue star in the general location of this M 6.5 quake (same for the other inset figures).
  • In the upper left corner is a figure that shows how oblique relative motion between plates results in a variety of faults and fault bounded blocks (Lange et al., 2008). This example is from southern Chile (near the 1960 subduction zone earthquake).
  • In the upper right corner is a plate tectonic map showing the plate boundaries (inset) and the crustal faults in the North America plate (Krutikov et al., 2008). The wide arrows show motion of Pacific plate relative to the North America plate (the direction the plate is subducting). These authors used the paleomagnetic data as evidence for rotation of fault bounded blocks.
  • In the lower left corner is a figure from Bassett and Watts (2015 B) that shows the results of their analyses using gravity data.
  • Here is the map with a month’s seismicity plotted. I outlined the blocks and labeled using Ryan and Scholl (1989) as a basemap (but very similar to Krutikov). I outlined some lineaments in the magnetic anomaly data for crust on both sides of the Amlia fracture zone and labeled these B and A (near label for Pacific plate). Note how they are offset relative to each other, demonstration of the left-lateral sense of motion here.

  • Here is the map with a century’s seismicity plotted. Check out the example strike slip earthquakes, including the 2017.06.02 M 6.8 quake (that was interpreted by Lay et al., 2017 to be right lateral). Also shown is the 2003.11.17 M 7.8 subduction earthquake. Many of the other earthquakes plotted in this map are also subduction earthquakes.

  • Here is the map with a century’s seismicity plotted, with megathrust earthquake patches from Peter Haeussler (USGS) outlined. I outlined the subduction zone slip patches shown in the Peter Haeussler (USGS) map. Consider how the structures in the different plates may interact with each other.

Other Report Pages

Some Relevant Discussion and Figures

  • Here is a map that shows historic earthquake slip regions as pink polygons (Peter Haeussler, USGS). Dr. Haeussler also plotted the magnetic anomalies (grey regions), the arc volcanoes (black diamonds), and the plate motion vectors (mm/yr, NAP vs PP).

  • Here is the figure from Sykes et al. (1980) that shows the space time relations for historic earthquakes in relation to the map.

  • Above: Rupture zones of earthquakes of magnitude M > 7.4 from 1925-1971 as delineated by their aftershocks along plate boundary in Aleutians, southern Alaska and offshore British Columbia [after Sykes, 1971]. Contours in fathoms. Various symbols denote individual aftershock sequences as follows: crosses, 1949, 1957 and 1964; squares, 1938, 1958 and 1965; open triangles, 1946; solid triangles, 1948; solid circles, 1929, 1972. Larger symbols denote more precise locations. C = Chirikof Island. Below: Space-time diagram showing lengths of rupture zones, magnitudes [Richter, 1958; Kanamori, 1977 b; Kondorskay and Shebalin, 1977; Kanamori and Abe, 1979; Perez and Jacob, 1980] and locations of mainshocks for known events of M > 7.4 from 1784 to 1980. Dashes denote uncertainties in size of rupture zones. Magnitudes pertain to surface wave scale, M unless otherwise indicated. M is ultra-long period magnitude of Kanamori 1977 b; Mt is tsunami magnitude of Abe[ 1979]. Large shocks 1929 and 1965 that involve normal faulting in trench and were not located along plate interface are omitted. Absence of shocks before 1898 along several portions of plate boundary reflects lack of an historic record of earthquakes for those areas.

  • Here is a great illustration that shows how forearc sliver faults form due to oblique convergence at a subduction zone (Lange et al., 2008). Strain is partitioned into fault normal faults (the subduction zone) and fault parallel faults (the forearc sliver faults, which are strike-slip). This figure is for southern Chile, but is applicable globally.

  • Proposed tectonic model for southern Chile. Partitioning of the oblique convergence vector between the Nazca plate and South American plate results in a dextral strike-slip fault zone in the magmatic arc and a northward moving forearc sliver. Modified after Lavenu and Cembrano (1999).

  • Here is a figure from Krutikov et al. (2008) that shows how blocks in the Aleutian Arc may accommodate the oblique subduction, along forearc sliver faults. Note that these blocks may also rotate to accommodate the oblique convergence. There are also margin parallel strike slip faults that bound these blocks. These faults are in the upper plate, but may impart localized strain to the lower plate, resulting in strike slip motion on the lower plate (my arm waving part of this). Note how the upper plate strike-slip faults have the same sense of motion as these deeper earthquakes.

  • Location map for the Aleutian Islands. The outline blocks and shaded summit basins are from Geist et al. [1988], showing a possible rotation mechanism. The heavy arrows show the mean rotations with respect to North America indicated by paleomagnetic data, the lighter arrows the motion of the Pacific plate with respect to North America. (inset) General location map modified from Chapman and Solomon [1976], Mackey et al. [1997], and Pedoja et al. [2006]. Solid lines show boundaries of plates and blocks: NA, North American Plate; B, Bering Block; PA, Pacific Plate; OKH, Okhotsk Plate; KI, Komandorsky Island Block; EUA, Eurasian Plate

  • Here is a figure from Ryan and Scholl (1989) that shows their interpretation of the fault bounded blocks within the forearc shear couple.

  • Map showing the boundaries of clockwise-rotating and westward translating blocks that comprise the Aleutian Ridge [from Geist et al. 1988]. Summit basins and transverse Pacific slope canyons are extensional structures that formed in the wake of these rotating and translating blocks. Arrows show relative plate motion between the Pacific and North American plates; convergence is increasingly oblique to the west. The central Aleutian sector lies within the Andreanof block located between Adak Canyon and Amukta Basin. A prominent summit basin has formed in the eastern part of the block (the composite Amlia and Amukta Basins). However, a summit basin is not present in the western part of the Andreanof block between Adak and Atka Islands. Asterisks show the location of active and dormant volcanoes; the star denotes the approximate location of the 1986 Andreanof earthquake.

  • This is a figure from Lay et al. (2017) that shows their estimate for fault slip for the 2017 temblor. This shows a northwest-southeast trending (striking) strike-slip fault. This is the slip model they used as input for their tsunami model.

  • (a) Bilateral slip model for the 2017 earthquake and USGS/NEIC catalog seismicity from 1900 to 16 July 2017 (blue circles, scaled proportional to magnitude, with events larger than M ~ 7 being labeled), along with all moment tensor solutions from the GCMT catalog from 1976 to 16 July 2017 (red-filled compressional quadrant focal mechanisms). (b) Foreshock seismicity on 17 July 2017 (blue circles) and aftershock seismicity in the first 2 weeks (magenta circles) along with the MW 6.3 foreshock GCMT focal mechanism (cyan focal mechanism). The large focal mechanism is the W-phase moment tensor from this study. The boxes indicate short-period radiators from the Eurasia-Greenland back projection, and stars indicate radiators from the North American back projection (Figure 3). The slip distribution is shown in detail in Figure S12. White vectors indicate the relative motion of the Pacific Plate to North America (almost identical to that relative to the Bering Plate). The large red star indicates the main shock epicenter.

  • Here is a figure from Lay et al. (2017) that shows (a) their initial condition (the amount of seafloor vertical land motion that initiated the tsunami, (b) the maximum wave height map, and (c) the comparison between their model results (in red) and the observations (in black) for water surface elevations after the earthquake.

  • Predicted tsunami from the bilateral faulting model. (a) Final seafloor deformation with the red star indicating the epicenter and the dashed line delineating projection of the faulting model on the seafloor. (b) Predicted tsunami amplitude and DART stations (circles) considered in this study. (c) Comparison of filtered sea surface recordings (black) at DART stations with predictions (red) along with corresponding amplitude spectra (right). The recorded and predicted time series were filtered to remove signals shorter than 5 min period and the full 5 h time series were used in the computation of the amplitude spectra. The strike-slip faulting and position of the stations result in weak tsunami waves, but the timing and height of long-period arrivals provide bounds on the source.

  • Here is the figure from Bassett and Watts (2015) for the Aleutians.

  • Aleutian subduction zone. Symbols as in Figure 3. (a) Residual free-air gravity anomaly and seismicity. The outer-arc high, trench-parallel fore-arc ridge and block-bounding faults are dashed in blue, black, and red, respectively. Annotations are AP = Amchitka Pass; BHR = Black-Hills Ridge; SS = Sunday Sumit Basin; PD = Pratt Depression. (b) Published asperities and slip-distributions/aftershock areas for large magnitude earthquakes. (c) Cross sections showing residual bathymetry (green), residual free-air gravity anomaly (black), and the geometry of the seismogenic zone [Hayes et al., 2012].

  • Here is the schematic figure from Bassett and Watts (2015).

  • Schematic diagram summarizing the key spatial associations interpreted between the morphology of the fore-arc and variations in the seismogenic behavior of subduction megathrusts.

  • Here are several figures from Konstantnovskaia et al. (2001) showing their tectonic reconstructions. I include their figure captions below in blockquote. The first figure is the one included in the poster above.

  • Geodynamic setting of Kamchatka in framework of the Northwest Pacific. Modified after Nokleberg et al. (1994) and Kharakhinov (1996)). Simplified cross-section line I-I’ is shown in Fig. 2. The inset shows location of Sredinny and Eastern Ranges. [More figure caption text in the publication].

  • Here are 4 panels that show the details of their reconstructions. Panels shown are for 65 Ma, 55 Ma, 37 Ma, and Present.


  • The Cenozoic evolution in the Northwest Pacific. Plate kinematics is shown in hotspot reference frame after (Engebretson et al., 1985). Keys distinguish zones of active volcanism (thick black lines), inactive volcanic belts (thick gray lines), deformed arc terranes (hatched pattern), subduction zones: active (black triangles), inactive *(empty triangles). In letters: sa = Sikhote-aline, bs = Bering shelf belts; SH = Shirshov Ridge; V = Vitus arch; KA = Kuril; RA = Ryukyu’ LA = Luzon; IBMA = Izu-Bonin-Mariana arcs; WPB = Western Philippine, BB = Bowers basins.

  • Here is a beautiful illustration for the Aleutian Trench from Alpha (1973) as posted on the David Rumsey Collection online.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

  • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

    Social Media

Return to the Earthquake Reports page.

Posted in alaska, earthquake, pacific, plate tectonics, strike-slip, subduction

Earthquake Report: Northern Alaska

At shortly before 13:30 today in northern Alaska there was a large earthquake, with a magnitude of M=7.1.

Many of us are familiar with the Good Friday earthquake, a megathrust subduction zone earthquake. This earthquake has a birthday tomorrow, from 27 March, 1964 (55 years ago).

The M=9.2 1964 temblor created a tsunami that traveled across the Pacific Ocean. More about the Good Friday earthquake and tsunami can be found here.

Alaska has a variety of major fault systems in addition to the subduction zone. There are also large strike-slip faults (move side by side) such as the Denali fault and the Kaltag fault. There are even more strike slip systems too, like the Queen Charlotte / Fairweather fault in southeastern Alaska and the Bering-Kresla shear zone in the extreme western part of the Aleutian Islands. Alaska is so cool, they even have extensional (normal) earthquakes, such as on 1 December 2018.

Recently, there was a series of strike-slip earthquakes in the Gulf of Alaska probably related to reactivation of pre-existing structures in the Pacific plate. We continue to have aftershocks in this area.

Also, there is an ongoing sequence of earthquakes (now, maybe it is a swarm?) in northeastern Alaska. The largest quake was in August last year (2018), with a magnitude of M=6.3.

Today’s earthquake happened away from one of the mapped faults in the USGS Quaternary Active Fault and Fold Database (the Kaltag fault). The earthquake mechanism shows this earthquake may have been a slightly oblique normal type of an earthquake. I placed strike-slip arrows on the 2 possible nodal planes.but this is mainly a normal earthquake.

There was also a normal earthquake in 1958, when a M=7.1 quake struck about 50 km (35 miles) to the southeast of today’s quake. However, the 1958 event was oriented perpendicular to today’s quake. Below are some observations made following the 1958 earthquake. There was evidence of liquefaction, with sand volcanoes about a meter thick extending for hundreds of meters laterally.

I need to get to bed, but will try to write more tomorrow.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 3.0 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the North Pole becomes the South Pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the trends of these red and blue stripes. These lines are parallel to the ocean spreading ridges from where they were formed.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the lower left corner is a map from the USGS that shows the major fault systems and historic earthquakes in Alaska. Note the large area in pink from the 1964 Good Friday earthquake.
  • In the upper right corner is a low angle oblique figure showing the subduction zone (see the Pacific plate subduct beneath the North America plate). Some of the strike-slip faults are shown, including the location of the 2002 Denali earthquake sequence. This is from USGS Fact Sheet fs014-03 (USGS, 2003). I placed a blue star in the general location of today’s M=5.2.
  • In the upper left corner is a map from Fletcher and Christensen (1966). In their paper, they describe a sequence of earthquakes in the 1950s. I placed a blue star in the general location of today’s M=5.2.
  • Here is the map with a month’s seismicity plotted.

  • In commemoration of the 55th anniversary of the Good Friday earthquake and tsunami, below is the poster from my report here.

Other Report Pages

Some Relevant Discussion and Figures

  • Davis (1960) includes some fantastic photo records, which some are shown below. Here is a great map showing their observations following the earthquake. Below the map is the legend and caption.


  • Map of a portion of the field epicenter. Alaska earthquake of 7 April 1958. (Compiled from vertical air photos and USGS Alaska Topographic Series 1:63,360, Melozitna and Kateel River Quadrangles, 1954.

  • Here is the map from Davis (1960) that shows their estimate of the ground shaking intensity (using the MMI scale as described above).

  • Isoseismal map of the intensities of the April 7, 1958 earthquake, (Modified Mercalli scale).

  • Here is a photo of one of the sand blows from Davis (1960).

  • Surface of one of the major sand flows covering an area greater than 1 square mile. The silty sand has a relatively uniform thickness of approximately 2½ feet.

  • There was a lake in the middle of some sand dune deposits, which were overlying alluvial (river lain) sediments. Below is a photo showing some of the landsliding in the sediments and below the photo is a cross section drawing. Note the large spatial extent of this slope failure.

  • A conical collapse nearly 20 feet deep. It occurred approximately 200 yards from the nearest sand flow.


    Cross-section A-A’ showing the arrested sand dune deposits resting on the alluvium below. Location of the cross-section is shown on the map (figure 5). [Figure 5 is the map and legend.]

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

  • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

Return to the Earthquake Reports page.

Posted in alaska, earthquake, education, Extension, geology, plate tectonics

Earthquake Report: Peru

On 01 March 2019 there was an intermediate depth earthquake near the border of Peru and Bolivia. In the past century, this is the first earthquake in this area at this depth. There are some historic quakes to the east, but they are much deeper. However, if we take a look at the 1994 M=8.2 shaker, it has a similar orientation as yesterday’s M=7.0 quake.

Another similarity with the 1994 temblor, is that they are both extensional (normal) earthquakes. The majority of intermediate depth earthquakes are extensional, but not all.

Here are the USGS web pages for these two earthquakes.

Recently (22 February 2019), there was an intermediate depth M 7.5 earthquake to the northwest of this one, in Ecuador. Here is my report for that earthquake. I spend some time discussing intermediate depth earthquakes and the landslide probability failure maps from the USGS.

On 08 April 2018 there was a M=6.8 deep earthquake tot he southeast of this M 7.0. Please visit my earthquake report page here for more background material on deep earthquakes and what may cause them.

Later, on 24 August 2018 there was another deep quake in Peru, and my earthquake report for that event describes in greater detail the subduction zone in this region.

I include interpretive posters from these reports below.

Yesterday’s M 7.0 earthquake shows extension in the north-south direction, which is oblique to the orientation of the subducting plate (the “slab”). However, it is oriented the same as the 1994 quake, so there is possibly something shared between these two quakes that may tell us something about the plate here. Read my report on the 22 Feb ’19 quaKe as I discuss various factors that may control the orientation of these intermediate depth earthquakes as they relate to the orientation of the subducting plate and structures in the downgoing plate.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1919-2019 with magnitudes M ≥ 6.5 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the North Pole becomes the South Pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly northwest-southeast trends of these red and blue stripes in the Nazca plate, as formed along the East Pacific Rise oceanic spreading center. These lines are parallel to the ocean spreading ridges from where they were formed. #Awesome

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper right corner is a section of the map from Hayes et al. (2015), which is a USGS map documenting the seismicity of the earth in this region. The cross section B-B’ is shown below. The cross section plots the earthquake depths along the profile shown on the map. The B-B’ profile crosses the subduction zone very close to where this earthquake happened. I place a blue star in the general location of today’s M 7.0 earthquake.
  • In the lower right corner is the B-B’ cross section. Note the location of the earthquake (blue star). There have not been many earthquakes in this area, though our history of earthquakes is very very short (so it is difficult to say that this earthquake is rare).
  • In the upper left corner is a plate tectonic map from Hu et al. (2016), which shows the major plate boundaries in the region. The subduction zone is indicated as a black line with triangles (the triangles show the direction that the Nazca plate is subducting below the South America plate).
  • In the lower left corner is a figure from Villegas-Lanza et al. (2016). We can use the slab contours to help us navigate today’s earthquake location relative to this map. On the left is a map showing historic subduction zone earthquakes. The center shows the spatial extent of these quakes and plots them on a time scale (the horizontal axis). On the right are earthquake mechanisms for quakes in the Peru/Ecuador area.
  • Here is the map with a month’s seismicity plotted. I include a comparison of the Intensity model and the observations.
  • The horizontal axis is distance from the earthquake and the vertical axis is shaking intensity using the MMI scale (discussed above). The observations (people can report their observations of intensity using the USGS “Did You Feel It?” form posted on their earthquake pages) are the dots and the modeled regressions are the solid lines.
  • I also note the areas in the main map that are where the shaking is modeled to be felt. There are 3 main areas. These show up in the comparison plot below, where I compare the 1994 M 8.2 quake with this M 7.0 quake.

  • Here is the map with a century’s seismicity plotted.

  • Here is a figure that shows the MMI maps for these two earthquakes. Earthquake magnitudes scale with energy (how strong they might shake) on a logorhythmic scale. A M 8.0 earthquake releases ~32 times more energy than a M 7.0 quake. So, the M 8.2 quake released much much more energy than the M 7.0.
  • The take away with this figure is that the M 8.2 earthquake, even though it is twice as deep, it was much more energetic. The ground shaking was much higher than for the M 7.0 temblor… I include red stars showing the comparison epicenter for the adjacent quake.

Other Report Pages

Some Relevant Discussion and Figures

  • This is the Hu et al. (2016) tectonic map. Note the slab contours and how they help us understand the shape of the downgoing Nazca plate.

  • Geological setting of South America with depth contours of slab 1.0 (Hayes et al., 2012)indicated by thin black lines, subducting oceanic plateaus translucent gray and continental cratons translucent white. The major flat slabs in South America are outlined with thick black lines. The locations of oceanic plateaus, cratons and flat slabs are modified from Gutscher et al.(2000), Loewy et al.(2004)and Ramos and Folguera (2009), respectively. The present-day plate motion is shown as black arrows. Tooth-shaped line represents the South American trench. Seafloor ages to the west of South America are shown with colorful lines with numbers indicating the age in Ma.

  • Here are some cross sections that show the geometry of the slab, as modeled by Hu et al. (2016). Cross section C is almost exactly where the 01 March 2019 M 7.0 and 9 June 1994 M 8.2 earthquakes are.

  • Cross sections of the best-fit model from 5◦to 30◦S at an interval of 5◦. Orange arrows mark the location of these cross sections. In each cross section, background color represents the temperature field with the yellow lines indicating the interpolated Benioff zone from slab 1.0(Hayes et al., 2012). Gray circles represent the locations of earthquakes with magnitude >4.0 from IRIS earthquake catalog for years from 1970 to 2015. Black lines above each cross section delineate the topography, with the vertical scale amplified by 20 times. Note the overall match of the slab geometry to both individual seismicity and slab 1.0 contour.

  • Here is an animation from IRIS that reviews the tectonics of the Peru-Chile subduction zone. For the animation, first is a screen shot and below that is the embedded video. This animation is from IRIS. Written and directed by Robert F. Butler, University of Portland. Animation and Graphics: Jenda Johnson, geologist. Consultant: Susan Beck, University or Arizona. Narration: Elayne Shapiro, University of Portland.

  • Here is a download link for the embedded video below (34 MB mp4)
  • The Rhea et al. (2016) document is excellent and can be downloaded here. The USGS prepared another cool poster that shows the seismicity for this region (though there does not seem to be a reference for this).

  • This is a great visualization from Dr. Laura Wagner. This shows how the downgoing Nazca plate is shaped, based upon their modeling.

  • Below are all figures from Scire et al. (2017).
  • This first one shows the location of (1) their cross sections (see below), (2) the locations of the seismometers and other equipment used in this study, and (3) historic seismicity used in their analyses.

  • Map showing seismic station locations (squares—broadband; inverted triangles—short period) for individual networks used in the study and topography of the central Andes. Slab contours (gray) are from the Slab1.0 global subduction zone model (Hayes et al., 2012). Earthquake data (circles) for deep earthquakes (depth >375 km) are from 1973 to 2012 (magnitude >4.0) and were obtained from the U.S. Geological Survey National Earthquake Information Center (NEIC) catalog (https://earthquake.usgs.gov/earthquakes/). Red triangles mark the location of Holocene volcanoes (Global Volcanism Program, 2013). Plate motion vector is from Somoza and Ghidella (2012). Cross section lines (yellow) are shown for cross sections in Figures 5 and 8.

  • Here are all the tomographic cross sections.


  • Trench-perpendicular cross sections through the tomography model. Velocity anomalies are shown in blue for fast anomalies, red for slow anomalies. Cross section locations are as shown in Figure 1. Dashed lines are the same as in Figure 6. Yellow dots are earthquake locations from the EHB catalog (Engdahl et al., 1998). Solid black line marks the top of the Nazca slab from the Slab1.0 model (Hayes et al., 2012).

  • This figure that shows an estimate of the geometery of the slab (scire et al., 2016). This surface is based on a contrast between material properties of the slab and the overlying material (mantle). Note the north arrow. These authors were interested in many things, including how the Nazca Ridge changes the geometry of ht emegathrust fault. Today’s M 7.1 happened in a place where the fault is steeply dipping. Use the latitude and longitude to findthe location of today’s earthquake relativ to this figure. 11° South and 70.8° East, with a depth of 610 km.

  • 3-D diagram of the resolved subducting Nazca slab and prominent mantle low-velocity anomalies inferred from our tomographic models. The isosurfaces for this diagram are obtained by tracing the most coherent low-velocity anomalies (less than negative 3 per cent) and slab-related (greater than positive 3 per cent) coherent fast anomalies in the tomographic model. Geomorphic provinces (fine dashed lines) are the same as in Fig. 1(a). Heavy black outline marks the projection of the subducted Nazca Ridge from Hampel (2002). Anomalies A, C, D and E labelled as in previous figures. Downloaded from http://gji.oxfordjournals.org/ at Yale University on December 1, 2015

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechanisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

  • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

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Posted in earthquake, education, pacific, plate tectonics, subduction

Earthquake Report: Ecuador

Well, as I was getting up this morning, my social media feed was abuzz about the intermediate depth earthquake in Peru. While it was quite deep (<130km), it was still widely felt and probably caused lots of damage. After just starting a new job and still in the midst of moving in, I am a little late getting this out. But there are some cool learning moments here... This part of the world enjoys a variety of plate boundary fault systems and interesting plate tectonic interactions. There are multiple spreading ridges, creating oceanic plates, and these ridges and plates interact in complicated ways. The results from these complicated relations (e.g. how a hotpot near a spreading ridge affects the thickness of the crust formed at that spreading ridge) can also impact the convergent plate boundaries as these plates subduct beneath the South America continental plate. The subduction zone megathrust fault, formed where the Nazca plate dives under the South America plate, has an historic and prehistoric record of earthquakes. However, today's MW=7.5 earthquake is not a megathrust event.

The earthquake is an extensional (normal) type of an earthquake. It probably occurred along a fault in the downgoing Nazca plate.

The plate here is undergoing extension either from some internal deformation within the plate, or due to what we call “slab pull” (the plate that is diving down and deep is pulling the plate that is less deep). So, the fault may be oriented perpendicular to the direction the plate is going down. However, sometimes there are pre-existing faults (like fracture zones, etc.) that may reactivate under different conditions from when they were formed.

On most subduction zones, these faults will, thus, be parallel to the subduction zone fault.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1919-2019 with magnitudes M ≥ 6.5 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.li>

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly east-west trends of these red and blue stripes near the spreading ridge separating the Cocos and Nazca plates. These lines are parallel to the ocean spreading ridges from where they were formed. The Cocos plate is formed at two spreading ridges, so check out how the magnetic anomalies in the northern part of the Cocos plate are not parallel to the other anomalies. #Awesome

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper right corner is a section of the map from Rhea et al. (2010). This shows earthquakes and the major plate boundary faults. Note the profile A-A’ in green. The rectangle shows from where earthquakes are selected to plot in the cross section below. I placed a blue star in the general location of today’s M=7.5 quake.
  • In the lower right corner is the A-A’ cross section. Note the location of the earthquake (blue star). Why is is not “well behaved” (i.e. it does not look like it belongs in this data set)? My hypothesis is that the plate is misshapen here (as evidenced by the slab contours in the maps below). So, the cross section is not oriented perfectly for this part of the plate (in addition, the earthquake is also not within the rectangle of selected seismicity).
  • In the lower left corner is a great map detailing the plate tectonics of the Cocos and Nazca plates, where the spreading ridges are, fracture zones, and the main terrane boundaries in the South America plate (e.g. sliver or “block” boundaries, broken up parts of the South America plate). This is from Gutscher et al., 1999.
  • In hte upper left corner is a map from Rhea et al. (2010) that shows an estimate of the seismic hazard in this part of the world. This is largely based on seismicity rates. Some seismic hazard model maps also incorporate other measures of hazard like strain rates.
  • Here is the map with a month’s seismicity plotted.

  • The first thing I noticed is that the orientation of the earthquake is not parallel to the deep sea trench formed by the subduction zone as mentioned above.
  • If we look at the shape of the Nazca plate beneath South America where this earthquake happened (shown as “slab contours” in these maps), we will see that the contours are not parallel to the subduction zone fault.
  • Where today’s M=7.5 earthquake happened, the contours are closer to east-west, which means the fault is dipping downward slightly more to the north, rather than to the east. This may explain why the moment tensor and the earthquake show extension in the northeast/southwest direction.
  • In the historic earthquake poster below, check out how there are analogical earthquakes to today’s quake, while further to the south, these extensional quakes are oriented closer to being perpendicular to the trench.
  • There are a few historic quakes on the map below that are thrust events (compressional), but they are much shallower in depth (about 17 and 24 km), compared to all other quakes are deeper than 70 km.
  • Here is the map with a century’s seismicity plotted.

  • Below I present a series of maps that are intended to address the excellent ‘new’ products included in the USGS earthquake pages: landslide probability and liquefaction susceptibility (a.k.a. the Ground Failure data products).
  • First I present the landslide probability model. This is a GIS data product that relates a variety of factors to the probability (the chance of) landslides as triggered by this earthquake. There are a number of assumptions that are made in order to be able to produce this model across such a large region, though this is still of great value (like other aspects from teh USGS, e.g. the PAGER alert). Learn more about all of these Ground Failure products here.
  • There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity).
  • This model, like all landslide computer models, uses similar inputs. I review these here:
    1. Some information about ground shaking. Often, people use Peak Ground Acceleration, though in the past decade+, it has been recognized that the parameter “Arias Intensity” is a better measure of the energy imparted by the earthquake across the land and seascape. Instead of simply accounting for the peak accelerations, AI integrates the entire energy (duration) during the earthquake. That being said, PGA is a more common parameter that is available for people to use. For example, when I was modeling slope stability for the 2004 Sumatra-Andaman subduction zone earthquake, the only model that was calibrated to observational data were in units of PGA. The first order control to shaking intensity (energy observed at any particular location) is distance to the earthquake fault that slipped.
    2. Some information about the strength of the materials (e.g. angle of internal friction (the strength) and cohesion (the resistance).
    3. Information about the slope. Steeper slopes, with all other things being equal, are more likely to fail than are shallower slopes. Think about skiing. Beginners (like me) often choose shallower slopes to ski because they will go down the slope slower, while experts choose steeper slopes.
  • Areas that are red are more likely to experience landslides than areas that are colored blue. I include a coarse resolution topographic/bathymetric dataset to help us identify where the mountains are relative to the coastal plain and continental shelf (subnarine). Note the blue line is the shoreline and that North is to the left. The M=7.5 epicenter is the green dot to the east of the mountains.

  • Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.
  • Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.
  • An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.
  • Below is the liquefaction susceptibility map. I discuss liquefaction more in my earthquake report on the 28 September 20018 Sulawesi, Indonesia earthquake, landslide, and tsunami here.

  • Here is a map that shows shaking intensity using the MMI scale (mentioned and plotted in the main earthquake poster maps). I present this here in the same format as the ground failure model maps so we can compare these other maps with the ground shaking model (which is a first order control on slope failure).
  • Let’s compare the MMI map below with the liquefaction susc. map. What might we conclude may be the largest factor for the landscape being susceptible to liquefaction?
  • Check out how the liquefaction map more directly resembles this MMI map, than the landslide map. In this case, my interpretation is that for the landslide model, slope is a larger controlling factor than ground shaking (though still a major factor).
  • And to answer my question, you were correct, liquefaction appears to be more highly controlled by ground shaking intensity.

  • Something else that is cool about the liquefaction map is we can see where the river valleys are. These regions have a higher liq. susc. because they are (1) closer to the earthquake and (2) they are composed of materials that are more susceptible to liquefaction (e.g. sediment rather than bedrock).

Other Report Pages

Some Relevant Discussion and Figures

  • Here is the Gutscher et al. (1999) map. Check out the complicated interaction between the structures in the Nazca and South America plates. The upper plate (South America) appears to have a major change in structures related to the Carnegie Ridge and the fracture zones (e.g. Grijalva FZ).

  • Tectonic setting of the study area showing major faults, relative plate motions according to GPS data [7] and the NUVEL-1 global kinematic model [8], magnetic anomalies [13] and active volcanoes [50]. Here and in Fig. 4, the locations of the 1906 (Mw D 8:8, very large open circle) and from south to north, the 1953, 1901, 1942, 1958 and 1979 (M  7:8, large open circles) earthquakes are shown. GG D Gulf of Guayaquil; DGM D Dolores–Guayaquil Megashear.

  • Here is another map from Gutscher et al. (1999) that shows several seismicity cross sections and we can see how the shape of the Nazca plate is different in different places. Today’s M=7.5 earthquake happened just south of the C’ label on the map (near the label “Region 3”).

  • Shaded hill relief and seismicity of the study area. Bathymetry and topography from Smith and Sandwell’s TOPEX database [51] with active volcanoes (red triangles) [50]. Seismicity (1964–1995), 1230 events Mb > 4:0, from Engdahl et al.’s global hypocenter relocation [18] scaled by depth and magnitude, omitting upper plate seismicity (<70 km depth >200 km east of the trench) in map. Oceanic plateaus defined by 2500 m contour. Location and sampling boxes of seismological sections indicated. Depth contours to the Wadati–Benioff zone indicated as dotted lines. Seismotectonic Regions 1–4 (see text) also shown. Section A–A0, Region 1, steep ESE-dipping subduction, narrow volcanic arc. Section B–B0, Region 2, intermediate-depth seismic gap, subduction of Carnegie Ridge with inferred flat slab shown, broad volcanic arc is spread out over 150 km. Section C–C0 , Region 3, narrow volcanic arc, steep NE-dipping slab. Section D–D0 , Region 4, Peru flat slab segment, no volcanic arc. Section F–F0, Andes-parallel profile illustrating the intermediate depth seismic gap and inferred Carnegie ‘flat slab’ in Region 2. Note the tear north of the steep NE-dipping slab in Region 3.

  • Here is a great visualization from Gutscher also. This shows how the structures in the Nazca plate may be controlling the tectonics here. Note their interpretation of several tears in the plate, and how the plate on the left shows that it is dipping to the northeast (consistent with the northeast extension from today’s temblor).

  • 3-D view of the two-tear model for the Carnegie Ridge collision featuring: a steep ESE-dipping slab beneath central Colombia; a steep NE-dipping slab from 1ºS to 2ºS; the Peru flat slab segment south of 2ºS; a northern tear along the prolongation of the Malpelo fossil spreading center; a southern tear along the Grijalva FZ; a proposed Carnegie flat slab segment (C.F.S.) supported by the prolongation of Carnegie Ridge.

  • This is a fantastic map from Chlieh et al. (2014) which shows how the earth moves based on GPS rate data. We can see how the major South America fault systems (e.g. GG and DGFZ) are accumulating tectonic strain (as evidenced by the shange in plate rate / vecor lenght at GPS sites on either sides of this fault system.

  • Seismotectonic setting of the oceanic Nazca plate, South America Craton (SoAm) and two slivers: the North Andean Sliver (NAS) and the Inca Sliver (IS). The relative Nazca/SoAm plate convergence rate in Ecuador is about 55mm/yr (Kendrick et al., 2003). Black arrows indicate the diverging forearc slivers motions relative to stable SoAm are computed from the pole solutions of Nocquet et al.(2014). The NAS indicates a northeastward long-term rigid motion of about 8.5 ±1mm/yr. The ellipse indicatesthe approximate rupture of the great 1906 Mw=8.8Colombia–Ecuador megathrust earthquake. The Carnegie Ridge intersects the trench in central Ecuador and coincides with the southern limit of the great 1906 event. Plate limits (thick red lines) are from Bird(2003). DGFZ =Dolores–Guayaquil Fault Zone; GG =Gulf of Guayaquil; GR =Grijalva Ridge; AR =Alvarado Ridge; SR =Sarmiento Ridge.

  • Since we are talking about the subduction zone megathrust, we can take a look at the history of subduction zone earthquakes here. This map is from Villegas-Lanza et al. (2016). We can use the slab contours to help us navigate today’s earthquake location relative to this map. Ecuador is unlabeled on the map, but is located between Colombia and Peru.

  • (a) Seismotectonic setting of the South American subduction zone. The red ellipses indicate the approximate rupture areas of large subduction earthquakes (M≥ 7.5) between 1868 and 2015 [Silgado, 1978; Beck and Ruff, 1989; Dorbath et al., 1990; Beck et al., 1998]. The blue ellipses indicate the locations of moderate tsunami-earthquakes [Pelayo and Wiens, 1990; Ihmle et al., 1998]. The bathymetry from GEBCO30s highlights the main tectonic structures of the subducting Nazca Plate, which are from north to south: Carnegie Ridge (CR), Grijalva Ridge (GR), Alvarado Ridge (AR), Sarmiento Ridge (SR), Virú Fracture Zone (VFZ), Mendaña Fracture Zone (MFZ), Nazca Ridge (NR), Nazca Fracture Zone (NFZ), Iquique Ridge, Juan Fernandez Ridge, Challenger Fracture Zone (CFZ), and Mocha Fracture Zone (MCFZ). The white arrow indicates the convergence of the Nazca Plate relative to the stable South America (SSA) reference frame [Kendrick et al., 2003]. The slab geometry isodepth contours are reported every 50 km (solid lines) and 10 km (dashed lines), based on the Slab1.0 model [Hayes et al., 2012]. The dashed rectangle corresponds to Figures 1b and 1c. The N.A.S. and C.A.S. labels indicate the North Andean and the Central Andes Slivers [Bird, 2003], respectively. (b) Temporal and spatial distributions of large subduction earthquakes with Mw ≥ 7.5 that occurred in Peru since the sixteenth century. The rupture extent values (in km) of historical (gray) and recent (red) megathrust earthquakes along the Peruvian margin are shown as a function of time (in years). A triangle indicates if a tsunami was associated with the event. The orange bands denote the entrance of the NR and the MFZ delimiting the northern, central, and southern Peru subduction segments. The rupture lengths were taken from its corresponding published slip models [Silgado, 1978; Beck and Ruff, 1989; Dorbath et al., 1990; Pelayo and Wiens, 1990; Ihmle et al., 1998; Giovanni et al., 2002; Salichon et al., 2003; Pritchard et al., 2007; Bilek, 2010; Delouis et al., 2010; Moreno et al., 2010; Schurr et al., 2014], and for historical earthquakes, we estimated its approximated lengths using scaling law relationships [Wells and Coppersmith, 1994]. (c) A map of the rupture areas of large subduction earthquakes that occurred in the twentieth century [Silgado, 1978; Beck and Ruff, 1989; Dorbath et al., 1990; Ihmle et al., 1998; Giovanni et al., 2002; Sladen et al., 2010; Chlieh et al., 2011], with their associated gCMT focal mechanisms. In northern Peru, the 1960 (Mw = 7.6) Piura earthquake and the 1996 (Mw = 7.5) Chimbote earthquake, which are shown by cyan-colored polygons, were identified as tsunami-earthquake events [Pelayo and Wiens, 1990; Ihmle et al., 1998; Bilek, 2010].

  • Part of looking at the above map is that we can look at the same article to find this great figure, that we can compare with the Gutscher and Chlieh maps.

  • Schematic description of the principal continental slivers contributing to the deformation partitioning of the Peruvian margin: North Andean Sliver (NAS; yellow) Peruvian Sliver (PS; in red), and Eastern Cordillera–Subandean regions (in green), which are separated by the limit between Western Cordillera and Eastern Cordillera. All of the motions are in reference to SSA and are expressed in millimeters per year (mm/yr). The inset shows the kinematic triangles and obliquity partitioning vectors for Ecuador (latitude 1°N), the Guayaquil Bend (latitude 5°S), and the Arica Bend (latitude 18°S). The lines with triangle symbols indicate the local trench axis. The green and purple lines are, respectively, the along- and normal trench components of Nazca/SSA convergence vector. The blue arrows indicate the Nazca/NAS and Nazca/PS convergence vectors, and the red arrows are the NAS/SSA and PS/SSA convergence vectors.

  • Finally, lets look here to see how Chlieh et al. (2014) use their modeling of the megathrust to infer the percent that the megathrust fault is accumulating tectonic strain (fault coupling). Warmer colors = higher coupling (like the fault has brand new vecro in red areas and worn out velcro in white areas).
  • See how there is low coupling in the area of the fault that where there are fracture zones or oceanic ridges in the Nazca plate (e.g. the Nazca Ridge and Mendaña fracture zone. Something that they did not label is the Carnegie Ridge, which is the area of low coupling near distance 1800-2000, near Plura, Ecuador and the North arrow.

  • (left) Along-trench variations of moment deficit rate for (middle) minimum and (right) maximum interseismic coupling models. Even though the interseismic pattern might vary significantly between models, the locations of the peaks and valleys in the rate of moment deficit are very persistent characteristics that highlight the locations of the principal
    asperities (peaks) and creeping barriers (valleys). The dashed ellipse contours in the middle map show the approximate rupture area of large earthquakes, as described in Figure 1.

  • Here are a couple figures I just came across (h/t Pablo, see tweet below). This first one shows an excellent visualization of the Nazca plate (Yepes et al., 2016).

  • Slab bending depicted as a hypothetical contorted surface. The drawings represent the subduction and bending of Farallon and Nazca plates from three different perspectives. The margin convexity (concavity from the perspective of the continental plate) forces the slab to flex and shorten at depth which accumulates stresses in most strained areas. Present-day position of the Grijalva rifted margin at the trench coincides with a noticeable inflection point of the trench axis (in red). A horizontal grid has been added to help visualize the plates dipping angles. A transparent 100 km thick volume has been added below the contorted surface to simulate the plate, but at intermediate depths the depicted surface should be representing the plate inner section. (a) South to north perspective showing the different dipping angles of Farallon and Nazca plates. The slab depth color scale is valid for the three drawings. (b) West to east oblique perspective at approximately the same angle as Nazca plate’s dip. The contortion of the Farallon plate at depth south of the Grijalva rifted margin is clearly noticeable from this perspective. (c) East to west perspective. Intermediate depth seismicity (50–300 km) from the instrumental catalog [Beauval et al., 2013] is drawn at the reported hypocentral depth. Two areas ofmaximum strain in the Farallon plate are shown (hachured): the El Puyo seismic cluster (SC) and the 100–130 km depth stretch of high moment release seismicity related to a potential hinge in the subducting plate. Lack of seismicity in the Nazca plate is explained due to the fact that this young plate, even though it is also strained, is too hot for brittle rupture.

  • This figure shows how convergence rate (through obliquity) varies along strike (Yepes et al., 2016).

  • Convergence obliquity. Circles represent obliquity values calculated at each epicentral latitude for Mw ≥6 earthquakes. Plus signs represent ruptured fault plane strikes obtained from focal mechanisms assuming the most probable fault plane candidate. We have used the Harvard focal mechanism catalog (http://www.globalcmt.org) from 1976 to 2013. Colors indicate focal mechanism. Tsunami events are highlighted, but they also correspond to thrust events. Light gray lines are the trench azimuth (crosses) ± 10% error. Notice the good agreement between fault strike and trench azimuth for thrust interface events. Some Mw <6 earthquakes have been included to show lack of agreement for noninterface events such as the Yaquina graben normal and the Grijalva rifted margin strike-slip events.

  • This figure shows their interpretation about how the Nazca plate (Nazca vs. Farallon slab) changes the subduction zone along strike (Yepes et al., 2016).

  • Inslab seismogenic source zones. Seven inslab SSZs have been defined along with three complementary sources (see the text for explanation). SSZs are colored according to their Mo release density (MoRD). Farallon slab zones are overlapped as seen in Figure 9a. Only colors in the “visible” part of the plate in plan view are shown as original. They are identified by three labels: source zone name and depth range, western boundary (W), and eastern boundary (E). Nazca plate SSZs are not overlapped. Complementary zones below the interface SSZs are hachured. Yellow triangles are active late Holocene volcanoes. Circles and stars correspond to seismicity presented in Figure 2b. (a) A-B 150 km wide cross section in Farallon domain to show how plates overlap. (b) Normal faulting, shallow (Z ≤ 50 km) focal mechanisms related to the Yaquina graben extracted from the Harvard global centroid moment tensor catalog [Dziewonski et al., 1981] from 1976 to 2013. Focal mechanisms correspond mainly to internal tearing. Outer trench bending is ruled out as the cause for normal faulting since dilatational focal mechanisms are obviously present east of the trench, where the slab is already plunging with normal angles and is not subjected to bending forces.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

  • Here is a great tweet that discusses the different parts of a seismogram and how the internal structures of the Earth help control seismic waves as they propagate in the Earth.

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Posted in earthquake, education, plate tectonics, subduction

Earthquake Report: Guatemala and Mexico

This morning (my time) there was a moderately deep earthquake along the coast of southern Mexico and northern Guatemala. Here is my Temblor article about this M=6.6 earthquake and how it might relate to the 2017 M=8.2 quake.

Offshore of Guatemala and Mexico, the Middle America trench is formed by the subduction of the oceanic Cocos plate beneath the North America and Caribbean plates.

To the east of Guatemala and Mexico, the North America and Caribbean plates are separated by a left lateral (sinistral) strike-slip plate boundary fault (that forms the Cayman Trough beneath the Caribbean Sea).

As this plate boundary comes onshore, this fault forms multiple splays, including the Polochi-Montagua fault. As this system trends westwards across Central America, it joins another strike-slip plate boundary associated with the subduction zone (the Volcanic Arc fault).

South of about 15°N, the relative plate motion between the Caribbean and Cocos plates is oblique (they are not moving towards each other in a direction perpendicular to the subduction zone fault). At plate boundaries where plate convergence is oblique (like also found in Sumatra), the strain is partitioned onto the subduction zone (for fault normal component of the relative plate motion) and a forearc sliver fault (for the fault parallel relative motion).

The Tehuantepec fracture zone (TFZ) is a major structure in the Cocos plate. Coincidentally, the strike-slip fault systems trend towards where the TFZ intersects the trench.

There is left-lateral offset of the seafloor across the TFZ so the crust is about 10 million years older on the north side of the eastern TFZ. This age offset changes the depth of the crust across the TFZ and also may affect the megathrust fault properties on either side of the TFZ.

In addition, the TFZ may have geological properties that also affect the fault properties when this part of the plate subducts (affecting where, when, and how the fault slips).

There are so many things going on, but I will mention one more thing. Something that also appears to be happening in this part of the subduction zone is that there may be gaps in the slab beneath the megathrust. If this is true (Mann, 2007), then there may be changes in slab pull tension along strike as a result of different widths of attached downgoing slab.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1919-2019 with magnitudes M ≥ 6.5 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include a transparent version of the slab 2.0 contours plotted (Hayes, 2018), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault.li>

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.

    Age of Oceanic Lithosphere

  • In one map below, I include a transparent overlay of the age of the oceanic crust data from Agegrid V 3 (Müller et al., 2008).
  • Because oceanic crust is formed at oceanic spreading ridges, the age of oceanic crust is youngest at these spreading ridges. The youngest crust is red and older crust is yellow (see legend at the top of this poster).

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the lower left corner is a pair of figures from Manea et al. (2013). On the left is a map showing some major plate boundary faults and other fault systems relevant to this region. On the right is a low angle oblique visualization of the Cocos plate. North is to the lower right. The depth of the slab is shown in shades of blue (see legend). Note the offset of blue color across the TFZ.
  • In the upper right corner is another low angle oblique visualization of the structures (Manea et al., 2013). Note the difference in depth of the slab across the TFZ and how the forearc sliver and North America / Caribbean strike-slip faults cross the upper plate. Read more about the forearc sliver in this report about an earthquake in El Salvador.
  • In the lower right corner is a map of the region showing details of the structures in the Cocos plate (Mann, 2007). There are an abundance of faults associated with the spreading ridges and offsets of these by numerous fracture zones. Note how the Cocos plate is formed by 2 different spreading ridges.
  • Here is the map with a century’s seismicity plotted.

  • Here is the map with a century’s seismicity plotted, using the age of the crust as an overlay.

There are also some interesting relations between different historic earthquakes.

In 2017 there was a series of large magnitude earthquakes in the region of today’s M=6.6 and further to the south. These quakes are highlighted in the posters above, notable are the 6 Jun M=6.9 and 22 Jun M=6.8. The first quake was a deep extensional event, followed by a thrust event (possibly triggered by the M=6.9). In addition, there was a M=6.9 extensional earthquake in 2014 that also may have been a player.

I presented an interpretive poster showing the zone of aftershocks associated with the June sequence. Later, in Sept, there was a M=8.2 extensional tsunamigenic earthquake to the north of the June sequence. If we look at the aftershock zone for the M=8.2 quake, it looks like a sausage link adjacent to the sausage link formed by the June aftershocks. mmmm veggie sausages.

However there was no megathrust earthquake in the area of the M=8.2 sequence.

  • Here is an interpretive poster showing how the 2017 June and September sequences spatially relate.

  • Here is a report where I discuss the June 2017 sequence in greater detail.

Other Report Pages

Some Relevant Discussion and Figures

  • Here are some figures from Manea et al. (2013). First are the map and low angle oblique view of the Cocos plate.

  • A. Geodynamic and tectonic setting alongMiddle America Subduction Zone. JB: Jalisco Block; Ch. Rift—Chapala rift; Co. rift—Colima rift; EGG—El Gordo Graben; EPR: East Pacific Rise; MCVA: Modern Chiapanecan Volcanic Arc; PMFS: Polochic–Motagua Fault System; CR—Cocos Ridge. Themain Quaternary volcanic centers of the TransMexican Volcanic Belt (TMVB) and the Central American Volcanic Arc (CAVA) are shown as blue and red dots, respectively. B. 3-D view of the Pacific, Rivera and Cocos plates’ bathymetrywith geometry of the subducted slab and contours of the depth to theWadati–Benioff zone (every 20 km). Grey arrows are vectors of the present plate convergence along theMAT. The red layer beneath the subducting plate represents the sub-slab asthenosphere.

  • Here is the figure that shows how the upper and lower plate structures interplay.

  • Kinematic model (mantle reference frame) of the subducting Cocos slab along the MAT in the vicinity of Cocos–Caribbe–North America triple junction since Early Miocene. The evolution of Caribbean–North America tectonic contact is based on the model of Witt et al. (2012). The blue strips represent markers on the Cocos plate. Note how trench roll forward is associated with steep slab in Central America, whereas trench roll back is associated with flat slab in Mexico.

  • Here are 2 different figures from Mann (2007). First we see a map that shows the structures in the Cocos plate. Note the 3 profile locations labeled 1, 2, and 3. These coincide with the profiles in the lower panel.

  • Present setting of Central America showing plates, Cocos crust produced at East Pacifi c Rise (EPR), and Cocos-Nazca spreading center (CNS), triple-junction trace (heavy dotted line), volcanoes (open triangles), Middle America Trench (MAT), and rates of relative plate motion (DeMets et al., 2000; DeMets, 2001). East Pacifi c Rise half spreading rates from Wilson (1996) and Barckhausen et al. (2001). Lines 1, 2, and 3 are locations of topographic and tomographic profi les in Figure 6.

  • Here are 2 different views of the slabs in the region. These were modeled using seismic tomography (like a CT scan, but using seismic waves instead of X-Rays). The upper maps show the slabs in map-view at 3 different depths. The lower panels are cross sections 1, 2, and 3. Today’s M=6.6 earthquake happened between sections 1 & 2.

  • (A) Tomographic slices of the P-wave velocity of the mantle at depths of 100, 300, and 500 km beneath Central America. (B) Upper panels show cross sections of topography and bathymetry. Lower panels: tomographic profi les showing Cocos slab detached below northern Central America, upper Cocos slab continuous with subducted plate at Middle America Trench (MAT), and slab gap between 200 and 500 km. Shading indicates anomalies in seismic wave speed as a ±0.8% deviation from average mantle velocities. Darker shading indicates colder, subducted slab material of Cocos plate. Circles are earthquake hypocenters. Grid sizes on profi les correspond to quantity of ray-path data within that cell of model; smaller boxes indicate regions of increased data density. CT—Cayman trough; SL—sea level (modifi ed from Rogers et al., 2002).

  • These figures are from the USGS publication (Benz et al., 2011) that presents an educational poster about the historic seismicity and seismic hazard along the Middle America Trench.
  • First is a map showing earthquake depth as color (green depth > red). Seismicity cross section B-B’ is shown on the map. Today’s M=6.6 quake is nearest this section.


  • Franco et al. (2012) used GPS observations to evaluate the kinematics (how the plates move and interact relative to each other) of this region. Below is a map that shows earthquake mechanisms that reveal the strike-slip faults as they converge. The forearc sliver (the block between the megathrust and the forearc sliver fault) is shaded gray.
  • These authors also use a model to estimate how much the megathrust is locked and accumulating elastic strain. They evaluate a range of possible physical properties of the find that the megathrust north of the forearc sliver is more highly locked (seismogenically coupled).

  • Proposed model of faults kinematics and coupling along the Cocos slab interface, revised from Lyon-Caen et al. (2006). Numbers are velocities relative to CA plate in mmyr−1. Focal mechanisms are for crustal earthquakes (depth ≤30 km) since 1976, from CMT Harvard catalogue.

  • Here is a map from Benz et al. (2011) that shows the seismic hazard for this region.

  • Below is a video that explains seismic tomography from IRIS.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

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Posted in caribbean, earthquake, education, Extension, geology, mexico, pacific, plate tectonics, subduction