I had been making an update to an earthquake report on a regionally experienced M 5.6 earthquake from coastal northern California when I noticed that there was a M 7.3 earthquake in eastern Indonesia.
https://earthquake.usgs.gov/earthquakes/eventpage/us600044zz/executive
This earthquake is in a region of strike-slip faulting (if in downgoing plate for example) or subduction thrusting, so I thought it may or may not produce a tsunami. There are also intermediate depth quakes here (deeper than subduction zone megathrust events), like this earthquake (which reduces the chance of a tsunami). While we often don’t think of strike-slip earthquakes as those that could cause a tsunami, they can trigger tsunami, albeit smaller in size than those from subduction zone earthquakes or locally for landslides. But, I checked tsunami.gov just in case (result = no tsunami locally nor regionally). I also took a look at the tide gages in the region here and here (result = no observations).
South of this earthquake is a convergent plate boundary, where the Australia plate dives northwards beneath a part of the Sunda plate (Eurasia) forming the Java and Timor trenches (subduction zones). Far to the west, on 2 June 1994 there was a subduction zone megathrust earthquake along the Java Trench. Earlier, on 19 August 1977 there was an M 8.3 earthquake, but it was not a subduction zone thrust event, but an extensional earthquake in the downgoing Australia plate (Given and Kanamori, 19080). Both 1977 and 1994 events are shown on one of the maps below. The 1977 earthquake was tsunamigenic, creating a wave observed on tide gages at Damier, Hampton, and Port Hedland in Australia (Gusman et al., 2009).
To the north of the subduction zone, there is a parallel fault system that dips in the opposite direction as the subduction zone. This is referred to as a backthrust fault (it is a thrust fault and “backwards” to the main fault). The Wetar and Flores faults are both part of this backthrust system. In JUly and August of 2018 there was a series of earthquakes near the Island of Flores associated with this backthrust. Here is my final of 3 reports on those earthquakes.
The Timor trough wraps around to the north on its eastern end and eventually forms the Seram Trench, which dips to the south. The shape of these linked trenches forms a “U” shape with the open part of the U pointing to the west. Recently it has been published that the basin formed by these fault systems is the deepest forearc basin on Earth (Pownall et al., 2016). There was a subduction zone earthquake in 1938, called the Great Banda Sea Earthquake. Okal and Reymond (2003) prepared an earthquake mechanism for this M 8.5 earthquake.
To complicate matters, there is a large strike-slip system that comes into the area from the east (Papua New Guinea) and bisects the crest of the “U” shape. This strike slip system feeds into the backthrust so that the backthrust is both a thrust fault and a strike-slip fault. There are probably separate faults that accommodate these different senses of motion. There have been a series of strike-slip earthquakes in the 20th century associated with the strike-slip motion along this boundary. For example, Osada and Abe (1981) uses seismologic records (e.g. from seismometers) to prepare an earthquake mechanism for this M 8.1 earthquake. They found that it was an oblique strike-slip earthquake. The depth was pretty shallow compared to the M 7.3 earthquake I am reporting about today.
On 17 June 1987 there was another relatively shallow M 7.1 strike-slip earthquake on this strike-slip fault system.
However, there is also a deeper strike-slip fault within the Australia plate. This fault is probably what ruptured on 2 March 2005 (M 7.1) and 10 December 2012 (M 7.1). The M 7.3 earthquake from a day ago had a similar magnitude, depth, mechanism, and location as these earlier quakes. These may have all ruptured the same fault (or not).
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 ≥ 7.0 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. Some earthquakes have older focal mechanisms plotted in black and white.
- 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.
- 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 Caroline and Australia plates. 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 Sunda plate (part of Eurasia), so the magnetic anomalies from the overlying Sunda plate mask the evidence for the Australia plate.
Magnetic Anomalies
- In a map below, I include a transparent overlay of the Global Strain Rate Map (Kreemer et al., 2014).
- The mission of the Global Strain Rate Map (GSRM) project is to determine a globally self-consistent strain rate and velocity field model, consistent with geodetic and geologic field observations. The overall mission also includes:
- contributions of global, regional, and local models by individual researchers
- archive existing data sets of geologic, geodetic, and seismic information that can contribute toward a greater understanding of strain phenomena
- archive existing methods for modeling strain rates and strain transients
- The completed global strain rate map will provide a large amount of information that is vital for our understanding of continental dynamics and for the quantification of seismic hazards.
- The version used in the poster(s) below is an update to the original 2004 map (Kreemer et al., 2000, 2003; Holt et al., 2005).
Global Strain
- In the upper left corner, I include a map from Benz et al. (2011) that shows historic earthquake locations (epicenters) along with some of the plate boundary faults. Note the strike slip fault (with the opposing black arrows) that cross the location of the 1938 earthquake (labeled in yellow on that map). I placed a blue star in the location of the M 7.3 quake. There is a cross section to the right of the map that shows how earthquakes dive down with a westward trend (following the plate down the subduction zone). The cross section location is shown on the map (B-B’).
- In the upper right corner is a larger scale tectonic map from Audley (2011) showing the major thrust faults and the large forearc basin is labeled “Weber Deep.”
- Hangesh and Whitney (2016) did lots of work on the faulting in the region to the south of the M 7.3. They show block boundaries and relative plate motion arrows in white. Note how they extend strike-slip motion along the Timor trough. This may be in addition to the strike-slip along the backthrust.
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
- Here is the map with a month’s seismicity plotted. I included MMI contours from a recent M 6.3 earthquake in PNG, which led to a sequence of additional M~6 quakes to the southeast of that main shock. I won’t be writing a report for those quakes, even though it is interesting (check it out!). Sorry to have misspelled Hengesh as Hangesh.
- Here is the map with a month’s seismicity (M ≥ 0.5) plotted with the Global Strain data plotted. We can see the 2018 Flores swarm show up here.
Other Report Pages
Some Relevant Discussion and Figures
- Here is a tectonic map for this part of the world from Zahirovic et al., 2014. They show a fracture zone where the M 7.3 earthquake happened. I left out all the acronym definitions (you’re welcome), but they are listed in the paper.
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).
- This is a great visualization showing the Australia plate and how it formed the largest forearc basin on Earth (Pownall et al., 2014).
- The maps on the left show a time history of the tectonics. The low angle oblique view on the right shows the dipping crust (north is not always up, as in this figure).
- In the lower right, they show how there is strike-slip faulting along the Seram trough also (I left out the figure caption for E).
Reconstructions of eastern Indonesia, adapted from Hall (2012), depict collision of Australia with Southeast Asia and slab rollback into Banda Embayment. Yellow star indicates Seram. Oceanic crust is shown in purple (older than 120 Ma) and blue (younger than 120 Ma); submarine arcs and oceanic plateaus are shown in cyan; volcanic island arcs, ophiolites, and material accreted along plate margins are shown in green. A: Reconstruction at 15 Ma. B: Reconstruction at 7 Ma. C: Reconstruction at 2 Ma. D: Visualization of present-day slab morphology of proto–Banda Sea based on earthquake hypocenter distribution and tomographic models
- Here is a map and some cross sections showing seismic tomography (like C-T scans into the Earth using seismic waves instead of X-Rays). The map shows the location of the cross sections (Spakman et al., 2010).
The Banda arc and surrounding region. 200 m and 4,000 m bathymetric contours are indicated. The numbered black lines are Benioff zone contours in kilometres. The red triangles are Holocene volcanoes (http://www.volcano.si.edu/world/). Ar=Aru, Ar Tr=Aru trough, Ba=Banggai Islands, Bu=Buru, SBS=South Banda Sea, Se=Seram, Sm=Sumba, Su=Sula Islands, Ta=Tanimbar, Ta Tr=Tanimbar trough, Ti=Timor, W=Weber Deep.
Tomographic images of the Banda slab. Vertical sections through the tomography model along the lines shown in Fig. 1. Colours: P-wave anomalies with reference to velocity model ak135 (ref. 30). Dots: earthquake hypocentres within 12 km of the section. The dashed lines are phase changes at ~410 km and ~660 km. The sections are plotted without vertical exaggeration; the horizontal axis is in degrees. The labelled positive anomalies are the Sunda (Su) and Banda (Ba) slabs: BuDdetached slab under Buru, FlDslab under Flores, SDslab under Seram, TDslab under Timor. a, The Sunda slab enters the lower mantle whereas the Banda embayment slab is entirely in the upper mantle with the change under Sulawesi. b–e, Banda slab morphology in sections parallel to Australia plate motion shows a transition from a steep slab with a flat section (fs) (b) to a spoon shape shallowing eastward (c–e).
- Here is the map from Benz et al. (2011).
- Here is the tectonic map from Hengesh and Whitney (2016)
Illustration of major tectonic elements in triple junction geometry: tectonic features labeled per Figure 1; seismicity from ISC-GEM catalog [Storchak et al., 2013]; faults in Savu basin from Rigg and Hall [2011] and Harris et al. [2009]. Purple line is edge of Australian continental basement and fore arc [Rigg and Hall, 2011]. Abbreviations: AR = Ashmore Reef; SR = Scott Reef; RS = Rowley Shoals; TCZ = Timor Collision Zone; ST = Savu thrust; SB = Savu Basin; TT = Timor thrust; WT =Wetar thrust; WASZ = Western Australia Shear Zone. Open arrows indicate relative direction of motion; solid arrows direction of vergence.
- Here is the Audley (2011) cross section showing how the backthrust relates to the subduction zone beneath Timor. I include their figure caption in blockquote below.
Cartoon cross section of Timor today, (cf. Richardson & Blundell 1996, their BIRPS figs 3b, 4b & 7; and their fig. 6 gravity model 2 after Woodside et al. 1989; and Snyder et al. 1996 their fig. 6a). Dimensions of the filled 40 km deep present-day Timor Tectonic Collision Zone are based on BIRPS seismic, earthquake seismicity and gravity data all re-interpreted here from Richardson & Blundell (1996) and from Snyder et al. (1996). NB. The Bobonaro Melange, its broken formation and other facies are not indicated, but they are included with the Gondwana mega-sequence. Note defunct Banda Trench, now the Timor TCZ, filled with Australian continental crust and Asian nappes that occupy all space between Wetar Suture and the 2–3 km deep deformation front north of the axis of the Timor Trough. Note the much younger decollement D5 used exactly the same part of the Jurassic lithology of the Gondwana mega-sequence in the older D1 decollement that produced what appears to be much stronger deformation.
- Here is a figure showing the regional geodetic motions (Bock et al., 2003). I include their figure caption below as a blockquote.
Topographic and tectonic map of the Indonesian archipelago and surrounding region. Labeled, shaded arrows show motion (NUVEL-1A model) of the first-named tectonic plate relative to the second. Solid arrows are velocity vectors derived from GPS surveys from 1991 through 2001, in ITRF2000. For clarity, only a few of the vectors for Sumatra are included. The detailed velocity field for Sumatra is shown in Figure 5. Velocity vector ellipses indicate 2-D 95% confidence levels based on the formal (white noise only) uncertainty estimates. NGT, New Guinea Trench; NST, North Sulawesi Trench; SF, Sumatran Fault; TAF, Tarera-Aiduna Fault. Bathymetry [Smith and Sandwell, 1997] in this and all subsequent figures contoured at 2 km intervals.
- Whitney and Hengesh (2015) used GPS modeling to suggest a model of plate blocks. Below are their model results.
Plate boundary segments in the Banda Arc region from Nugroho et al (2009). Numbers inside rectangles show possible micro-plate blocks near the Sumba Triple Junction (colored) based on GPS velocities (black arrows) with in a stable Eurasian reference frame.
- Here is the conceptual model from Whitney and Hengesh (2015) that shows how left-lateral strike-slip faulting can come into the region.
Schematic map views of kinematic relations between major crustal elements in the Sumba Triple Junction region. CTZ= collisional tectonic zone. Red arrow size designates schematic plate motion relations based on geological data relative to a fixed Sunda shelf reference frame (pin).
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.
The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj
— AK Earthquake Center (@AKearthquake) August 15, 2018
- 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.
- 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.
- 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.
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.)
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.
Today, on #SeismogramSaturday: what are all those strangely-named seismic phases described in seismograms from distant earthquakes? And what do they tell us about Earth’s interior? pic.twitter.com/VJ9pXJFdCy
— Jackie Caplan-Auerbach (@geophysichick) February 23, 2019
- M 9.2 Andaman-Sumatra subduction zone 2014 Earthquake Anniversary
- M 9.2 Andaman-Sumatra subduction zone SASZ Fault Deformation
- M 9.2 Andaman-Sumatra subduction zone 2016 Earthquake Anniversary
- 2019.06.23 M 7.3 Banda Sea
- 2019.04.12 M 6.8 Sulawesi, Indonesia
- 2018.09.28 M 7.5 Sulawesi
- 2018.10.16 M 7.5 Sulawesi UPDATE #1
- 2018.08.19 M 6.9 Lombok, Indonesia
- 2018.08.05 M 6.9 Lombok, Indonesia
- 2018.07.28 M 6.4 Lombok, Indonesia
- 2017.12.15 M 6.5 Java
- 2017.08.31 M 6.3 Mentawai, Sumatra
- 2017.08.13 M 6.4 Bengkulu, Sumatra, Indonesia
- 2017.05.29 M 6.8 Sulawesi, Indonesia
- 2017.03.14 M 6.0 Sumatra
- 2017.03.01 M 5.5 Banda Sea
- 2016.10.19 M 6.6 Java
- 2016.03.02 M 7.8 Sumatra/Indian Ocean
- 2015.07.22 M 5.8 Andaman Sea
- 2015.11.08 M 6.4 Nicobar Isles
- 2012.04.11 M 8.6 Sumatra outer rise
- 2004.12.26 M 9.2 Andaman-Sumatra subduction zone
Indonesia | Sumatra
General Overview
Earthquake Reports
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- 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.
- 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.
- Given, J. W., and H. Kanamori (1980). The depth extent of the 1977 Sumbawa, Indonesia, earthquake, in EOS Trans. AGU., v. 61, p. 1044.
- Gusnman, A.R., Tanioka, Y., Matsumoto, H., and Iwasakai, S.-I., 2009. Analysis of the Tsunami Generated by the Great 1977 Sumba Earthquake that Occurred in Indonesia in BSSA, v. 99, no. 4, p. 2169-2179, https://doi.org/10.1785/0120080324
- 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.
- 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.
- Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
- Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
- Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
- 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.
- 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.
- Osada, M. and Abe, K., 1981. Mechanism and tectonic implications of the great Banda Sea earthquake of November 4, 1963 in Physics of the Earth and Plentary Interiors, v. 25, p. 129-139
- Pownall, J.M., Hall, R., Armstrong,, R.A., and Forster, M.A., 2014. Earth’s youngest known ultrahigh-temperature granulites discovered on Seram, eastern Indonesia in Geology, v. 42, no. 4, p. 379-282, https://doi.org/10.1130/G35230.1
- Spakman, W. and Hall, R., 2010. Surface deformation and slab–mantle interaction during Banda arc subduction rollback in Nature Geosceince, v. 3, p. 562-566, https://doi.org/10.1038/NGEO917
- Whitney, B.B. and Hengesh, J.V., 2015. A new model for active intraplate tectonics in western Australia in Proceedings of the Tenth Pacific Conference on Earthquake Engineering Building an Earthquake-Resilient Pacific 6-8 November 2015, Sydney, Australia, paper number 82
- 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
References:
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Well, I was on the road for 1.5 days (work party for the Community Village at the Oregon Country Fair). As I was driving home, there was a magnitude M 5.6 earthquake in coastal northern California. 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 ≥ 5.0 in one version. From the USGS:
The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj — AK Earthquake Center (@AKearthquake) August 15, 2018
Strike Slip: 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.)
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.
Today, on #SeismogramSaturday: what are all those strangely-named seismic phases described in seismograms from distant earthquakes? And what do they tell us about Earth’s interior? pic.twitter.com/VJ9pXJFdCy — Jackie Caplan-Auerbach (@geophysichick) February 23, 2019
There was just an earthquake associated with the plate boundary that forms the Kermadec Trench, a deep oceanic trench that extends north from New Zealand, towards the Fiji Islands. In this part of the world, there is a convergent plate boundary where the Pacific plate dives westward beneath the Australia plate forming the Kermadec megathrust subduction zone fault. This fault has a history of earthquakes with magnitudes commonly exceeding M 7 and some exceeding M 8. While we cannot predict earthquakes, based on the historic record, this earthquake may be all that happens right now. But our historic record is incredibly short, so people must remain vigilant at all times. 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 ≥ 6.0 and 7.0 in two 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 (including a M 6.1 earthquake that happened about an hour prior to the M 7.2. This is very close in time. The M 6.1 is too small of a magnitude to change the static coulomb stress significantly. It seems possible that there was dynamic triggering though (???). I will need to think about this a little more (check out the literature on dynamic triggering, to see what time window that may be a relevant trigger).
Map of the Southwest Pacific Ocean showing the regional tectonic setting and location of the two dredged profiles. Depth contours in kilometres. The presently active arcs comprise New Zealand–Kermadec Ridge–Tonga Ridge, linked with Vanuatu by transforms associated with the North Fiji Basin. Colville Ridge–Lau Ridge is the remnant arc. Havre Trough–Lau Basin is the active backarc basin. Kermadec–Tonga Trench marks the site of subduction of Pacific lithosphere westward beneath Australian plate lithosphere. North and South Fiji Basins are marginal basins of late Neogene and probable Oligocene age, respectively. 5.4sK–Ar date of dredged basalt sample (Adams et al., 1994).
Kermadec Trench from Woods Hole Oceanographic Inst. on Vimeo.
The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj — AK Earthquake Center (@AKearthquake) August 15, 2018
Strike Slip: 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.)
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.
Today, on #SeismogramSaturday: what are all those strangely-named seismic phases described in seismograms from distant earthquakes? And what do they tell us about Earth’s interior? pic.twitter.com/VJ9pXJFdCy — Jackie Caplan-Auerbach (@geophysichick) February 23, 2019
Mw=6.2, TONGA ISLANDS (Depth: 15 km), 2019/06/15 21:56:09 UTC – Full details here: https://t.co/5PNNCtfejY pic.twitter.com/xIF3FBF4Ba — Earthquakes (@geoscope_ipgp) June 15, 2019 Mw=7.4, KERMADEC ISLANDS, NEW ZEALAND (Depth: 37 km), 2019/06/15 22:55:02 UTC – Full details here: https://t.co/NtMjNEXgZ1 pic.twitter.com/lacrYnNpHi — Earthquakes (@geoscope_ipgp) June 15, 2019 Mw=6.4, KERMADEC ISLANDS REGION (Depth: 25 km), 2019/06/16 05:17:16 UTC – Full details here: https://t.co/zq4F7wwAvo pic.twitter.com/oLX1Ep8Bp1 — Earthquakes (@geoscope_ipgp) June 16, 2019 Preliminary Magnitude 7.3 #earthquake in KermadecIslandsRegion, 2019-06-15 22:54 UTC time by #RaspberryShake #CitizenScience #seismograph network. Join the largest citizen #seismic community – https://t.co/bkUOY6dBm2. https://t.co/Ten7tGxaTS pic.twitter.com/FzthIcYzHE — Raspberry Shake (@raspishake) June 15, 2019 Meanwhile, in other news that's much nicer to think about, a large earthquake deep below the Pacific that didn't hurt people showed up beautifully in my central London flat on the @raspishake pic.twitter.com/i2F3fWGrjP — Austin Elliott (@TTremblingEarth) June 16, 2019 Check out large amplitude long period waves from morning M7.2 kermadec's earthquake at GRZ (geonet.bottom plot) compared to more distant 2011 M7.4 but similar depth& rupture mechanism (top plot). Similarity of path, mechanism&depth points towards slow rupture mechanism. Thoughts? pic.twitter.com/6iJP4OKOdu — Caroline Holden (@Carolinenz59) June 16, 2019
This morning (my time) there was a magnitude M 6.4 earthquake offshore of Chile. While it was in the correct location to possibly cause a tsunami, the magnitude was too small. I plot the seismicity from the past year, with color representing depth and diameter representing magnitude (see legend), for earthquakes M ≥ 4.0. I include earthquake epicenters from some specific historic earthquakes with magnitudes M ≥ 4.0 in one version.
Maps of (A) tectonic framework, (B) topography, and (C) sedimentary basin configuration of South America. (A) Map of plate boundaries, Andean magmatic arc (including the northern, central, and southern volcanic zones), regions of flat slab subduction, modern stress orientations from earthquake focal mechanisms, eastern front of Andean fold-thrust belt, and key segments of the retroarc foreland basin system. Plate velocities are shown relative to stable South American plate (DeMets et al., 2010). (B) DEM topographic map showing the Andes mountains and adjacent foreland region, including the Amazon, Parana, Orinoco, and Magdalena (Mag) river systems. (C) Map of Andean retroarc basins, showing isopach thicknesses (in km) of Cretaceous-Cenozoic basin fill, forebulge axis (from Chase et al., 2009), and locations of 13 sites (8 foreland basins, 5 hinterland basins) considered in this synthesis
Left estimated extent of large historical or instrumental ruptures along the Chilean margin adapted from ME´ TOIS et al. (2012). Gray stars mark major intra-slab events. The recent Mw[8 earthquakes are indicated in red. Gray shaded areas correspond to LCZs defined in Fig. 3. Right seismicity recorded by the Centro Sismologico Nacional (CSN) during
a Histogram depicts the rate of Mw>3 earthquakes registered by the CSN catalog during the interseismic period defined for each zone (see Fig. 2) on the subduction interface, on 0.2° of latitude sliding windows. Stars are swarm-like sequences detected by HOLTKAMP et al. (2011) depending on their occurrence date. Swarms located in the Iquique LCZ and Camarones segment are from RUIZ et al. (2014). Empty squares are significant intraplate earthquakes. b Red curve variations of the average coupling coefficient on the first 60 km of depth calculated on 0.2° of latitude sliding windows for our best model including an Andean sliver motion. Dashed pink curves are alternative models with different smoothing options that fit the data with nRMS better than 2 (see supplementary figure 6): the pink shaded envelope around our best model stands for the variability of the coupling along strike. Green curves coseismic distribution for Maule (VIGNY et al. 2011), Iquique (LAY et al. 2014) and Illapel earthquakes (RUIZ et al. 2016). Gray shaded areas stand for the identified low coupling zones (LCZs). LCZs and high coupling segments are named on the left. The apparent decrease in the average coupling North of 30°S is considered as an artifact of the Andean sliver motion (see Sect. 5.2). c Best coupling distribution obtained inverting for Andean sliver motion and coupling amount simultaneously. The rupture zones for the three major earthquakes are indicated as green ellipses. White shaded areas are zones where we lack resolution.
Left coupling maps (color coded) versus coseismic slip distributions (gray shaded contours in cm) for the last three major Chilean earthquakes (epicenters are marked by white stars). From top to bottom Iquique area, white squares are pre-seismic swarm event in the month before the main shock, green star is the 2005, Tarapaca´ intraslab earthquake epicenter, blue star is the Mw 6.7 Iquique aftershock; Illapel area, green squares show the seismicity associated with the 1997 swarm following the Punitaqui intraslab earthquake (green star); Maule area, green star is the epicenter of the 1939 Chillan intraslab earthquake. Right interseismic background seismicity in the shallow part of the subduction zone (shallower than 60 km depth) for each region (red dots) together with 80 and 90 % coupling contours. White dots are events identified as mainshock after a declustering procedure following GARDNER and KNOPOFF (1974). Yellow areas extent of swarm sequences identified by HOLTKAMP et al. (2011) for South and Central Chile, and RUIZ et al. (2014) for North Chile.
Comparison between the uplift rates, interseismic coupling, major bathymetric features, and peninsulas along the Andean margin (10°S–40°S). (a) Uplift rates of marine terraces reported in the literature (we present the average rate since terrace abandonment; Table S1 in the supporting information [Jara-Muñoz et al., 2015]). Each color corresponds to a marine terrace assigned to a marine isotopic stage (MIS). Gray dots are the uplift rates of the central Andean rasa estimated from a numerical model of landscape evolution [Melnick, 2016]. (b) Major bathymetric features and peninsulas and pattern of interseismic coupling of the Andean margin from GPS data inversion (this study). Gray shaded areas correspond to the areas where the spatial resolution of inversion is low due to the poor density of GPS observations (see text and supporting information for more details). The Peru-Chile trench (thick black line), the coastline (thin black line), and the convergence direction (black arrows) are indicated. We superimposed the curve obtained by shifting the trench geometry eastward by 110 km (trench-coast distance of 110 km; blue line) with the curve reflecting the 40 km isodepth of the subducting slab (red line; Slab1.0 from Hayes and Wald [2009]), a depth which corresponds approximately with the downdip end of the locked portion of the Andean seismogenic zone (±10 km) [Ruff and Tichelaar, 1996; Khazaradze and Klotz, 2003; Chlieh et al., 2011; Ruegg et al., 2009; Moreno et al., 2011; Métois et al., 2012]. The two curves are spatially similar in the erosive part of the Chile margin (north of 34°S), whereas they diverge along the shallower slab geometry in the accretionary part of the Chile margin (south of 34°S), where the downdip end of the locked zone may be shallower (Figure 4b). Red arrows indicate the low interseismic coupling associated with peninsulas and marine terraces and evidence of aseismic afterslip (after Perfettini et al. [2010] below the Pisco-Nazca Peninsula; Pritchard and Simons [2006], Victor et al. [2011], Shirzaei et al. [2012], Bejar-Pizarro et al. [2013], and Métois et al. [2013] for the Mejillones Peninsula; Métois et al. [2012, 2014] below the Tongoy Peninsula; and Métois et al. [2012] and Lin et al. [2013] for the Arauco Peninsula). FZ: Fracture zone. Horizontal blue bands are the areas where coastline is less than 110 km (light blue) or 90 km (dark blue) from the trench (see Figure 1).
Maps showing the location of the study and the events used ((a)–(c)). In red we present interplate earthquakes, while in blue, the intermediate depth, intraplate ones. We used beach balls to plot those events with known focal and circles for those without. White triangles mark the position of the Chilean Seismological Network used to locate the events; those with names represent stations used in the waveform analysis (either accelerometers or broadbands with known instrumental response). Labels over beach balls correspond to CMT codes.
Cross-section at (a) 33.5◦S and (b) 36.5◦S showing the events used in this study. In red we present interplate earthquakes, while in blue, the intermediate depth, intraplate ones.We used beach balls (vertical projection) to plot those events with knownfocal and circles for those without. In light gray is shown the background seismicity recorded from 2000 to 2006 by the Chilean Seismological Service
The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj — AK Earthquake Center (@AKearthquake) August 15, 2018
Strike Slip: 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.)
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.
Today, on #SeismogramSaturday: what are all those strangely-named seismic phases described in seismograms from distant earthquakes? And what do they tell us about Earth’s interior? pic.twitter.com/VJ9pXJFdCy — Jackie Caplan-Auerbach (@geophysichick) February 23, 2019
Well, yesterday was the start of a sequence of earthquakes offshore of San Clemente Island, about 100 km west of San Diego, California. The primary tectonic player in southern CA is the Pacific – North America plate boundary fault, the San Andreas (SAF). The region offshore where this ongoing sequence is called the California Continental Borderlands (CCB). There exists an excellent record of how the North America – Pacific plate margin boundary has evolved through time (remember, prior to about 29 million years ago, this plate boundary in southern CA was a subduction zone). 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.
EVOLUTION OF THE SAN ANDREAS FAULT.
Map of the California Continental Borderland showing major tectonic features and moderate earthquake locations (M >5.5). The dashed box shows area of this study. The large arrows show relative plate motions for the Pacific-North America transform fault boundary (~N40° ± 2°W; RM2 and PA-1 [Plattner et al., 2007]). BP = Banning Pass, CH = Chino Hills, CP = Cajon Pass, LA = Los Angeles, PS = Palm Springs, V = Ventura, ESC = Santa Cruz Basin, ESCBZ = East Santa Cruz Basin fault zone, SCI = Santa Catalina Island, SCL = San Clemente Island, SMB = Santa Monica Basin, and SNI = San Nicolas Island. Base map from GeoMapApp/Global Multi-Resolution Topography (GMRT) [Ryan et al., 2009].
Shaded relief map of Santa Catalina Island and vicinity, where several restraining-bend pop-ups and releasing-bend basins exist along major fault zones. Epicentres for two moderate earthquakes (1981 Santa Barbara Island, M 6.0; 1986 Oceanside, M 5.8) and aftershocks bound the Santa Catalina Island restraining bend (locations by Astiz & Shearer 2000; focal mechanism from Corbett 1984). Other restraining-bend pop-ups include the Palos Verdes Hills (PVH) and Lasuen knoll along the Palos Verdes fault zone, and Signal Hill (SH) and possibly the San Joaquin Hills (SJH) along the Newport–Inglewood fault zone. Small pop-ups and pull-apart basins in the vicinity of Crespi knoll are shown in Figure 14. Total relief across the Catalina Fault is almost 2000 m, from Catalina Basin to Mt Orizaba. From 60 to 72 km of right-slip on San Clemente Fault is inferred from offset of Emery Knoll crater rim (Legg et al. 2004b).
Seismic-reflection profile USGS-116 across the Catalina basin (see Fig. 12 for profile location). Note the thin sediment cover over an irregular basement surface. A pull-apart basin exists where the San Clemente Fault steps to the NE to eventually merge with the Catalina Fault. The major faults have subvertical dips, typical of strike-slip faults. Convergence across the Catalina Fault has elevated Santa Catalina Island, and uplift occurs on both sides of the PDZ. Seismic data from USGS (J. Childs 2005, pers. comm.) FK migration at 4800 fps velocity was applied to 22-fold USGS stacked data.
Map showing bathymetry, Quaternary faults, and recent seismicity in the Outer Borderland. Fault locations are based on the high-resolution bathymetry, available high-resolution seismic reflection profiles, and published fault maps [cf. California Geological Survey (CGS), 2010]. The red symbols show magnitude-scaled (M>4) epicenters for seismicity recorded for the period of 1932 to 2013. Seismicity data and focal mechanisms are derived from the Southern California Seismograph Network catalogs, National Earthquake Information Center [2012–2013], and Legg [1980]. Focal mechanism event numbers correspond to Table S2 in the supporting information. The black rectangle shows location of Figure 10. The light blue lines show tracklines of multichannel seismic profiles—the labeled white profiles are shown in Figures 12 (124) and 13 (108 and 126).
Map showing major active tectonic elements of the northern part of the California Continental Borderland. Major active (Quaternary) faults are shown in red (SAF = San Andreas fault, ABF = Agua Blanca fault, SCF = San Clemente fault, and SCCR = Santa Cruz-Catalina Ridge, Ferrelo). Major strike-slip offsets are shown by shaded areas with estimated displacement (EK = Emery Knoll crater; Tanner Basin near DB = Dall Bank; and SDT = San Diego Trough, small pull-apart near Catalina). Other symbols show oblique fault character including transpressional restraining bends (CAT = Santa Catalina Island, CB = Cortes Bank, and TB = Tanner Bank), uplifts (SRI = Santa Rosa Island, SCz = Santa Cruz Island, SNI = San Nicolas Island, CB = Cortes Bank, TB = Tanner Bank, and SBM = San Bernardino Mountains), and transtensional pull-apart basins (SD = San Diego, ENS = Ensenada, SCB = San Clemente Basin, and SIB = San Isidro Basin). The large arrows show Pacific-North America relative plate motions with the blue dashed line (PAC-NAM) along a small circle for the RM2 [Minster and Jordan, 1978] plate motions model through San Clemente Island (SCL). Boundary between the Inner and Outer Borderland follows the East Santa Cruz Basin fault zone (dotted line; modified from Schindler [2010] and De Hoogh [2012]). Holocene volcanoes exist along the coast (SQ= San Quintín) and within the Gulf of California Rift (CP = Cerro Prieto and Obsidian Buttes, Salton Trough). Dates show year of earthquakes with mapped focal mechanisms (see Table S2 in the supporting information). SB = Santa Barbara, LA = Los Angeles, and PS = Palm Springs.
High-resolution 24-channel 4 kJ sparker seismic profiles along the Santa Cruz-Catalina Ridge (C. Goldfinger, personal communications, 2001). The profiles are arranged from north to south along the southern part of the ridge (see Figure 6a for profile locations). Profile OSU-128 located near the bifurcation of the Santa Cruz-Catalina Ridge east of Santa Barbara Island. The colored horizons are correlated to stratigraphy mapped by Sorlien et al. [2013] (see Figure 6 for profile location). The East San Clemente fault zone merges with the Catalina Ridge fault zone within the eastern valley (Figure 5a). Reverse slip is evident on several steep faults that bound blocks of sedimentary rocks that were squeezed up in this transpressional fault system.
Color-contoured slope-shaded multibeam bathymetry gridded at 10 meters. A) The Catalina Basin and the San Gabriel Canyon–Channel depositional system. Dashed line in the Catalina Basin indicates approximate extent of channels resolved on the seafloor.
Northwest channels and lobes. See Figure 1A for profile locations. Gray lines represent profile intersections. A) Chirp profile across the northwestern Catalina Basin shows the stacking of lobes that do not reach the Kimki Fault (KF). B) An obliquely oriented chirp profile shows that the lobe deposits originate from the northwest channels, end before reaching the San Clemente Fault (SCF), and do not overlap in extent with lobe b.
Chronology of major Cenozoic events in the Southern California region (after Wright [1991] and Legg and Kamerling [2012]). Intensity of tectonic deformation is represented by the curve. Local (Los Angeles Basin) biostratigraphic zonation is shown. The slanted labels for Neogene stages represent the time-transgressive nature of these boundaries.
Emergent Pleistocene marine terraces on the west side of the island between Norton and Box canyons.
Map showing the plate tectonic setting of western North America (simplified from Drummond (1981) and Simkin et al. (2006)). SAF, San Andreas Fault; MTJ, Mendocino Triple Junction; CSZ, Cascadia subduction zone. Also shown are marine terrace localities with reliably dated ~120 ka, ~80 ka, or ~49 ka corals, or amino acid ratios in mollusks that permit correlation to ~120 ka, ~80 ka, or ~49 ka terrace localities, and elevation data that allow calculations of late Quaternary uplift rates. Paleo-sea levels, relative to present, used for uplift rate calculations are þ6 m (~120 ka), 11 m (~80 ka), and 62 m (~49 ka), derived from data in Muhs et al. (2012). Abbreviations and sources of data, south to north: CP, Cabo Pulmo (Muhs et al., 2002a); LP, La Paz (Sirkin et al., 1990); BH, Bahía Magdalena (Omura et al., 1979); IC, Isla Coronados and PC, Punta Chivato (Johnson et al., 2007; see also Table 2); MU, Mulege (Ashby et al., 1987); BT, Bahía de Tortugas (Emerson et al., 1981); PB, Punta Banda (Rockwell et al., 1989; Muhs et al., 2002a); PL, Point Loma (Kern, 1977; Muhs et al., 2002a); SCI, San Clemente Island (Muhs et al., 2002a, 2014); NB, Newport Bay (Grant et al., 1999); SNI, San Nicolas Island (Muhs et al., 2012); PV, Palos Verdes Hills (Muhs et al., 2006); NCI, Northern Channel Islands (this study); V, Ventura (Lajoie et al., 1979; Kennedy et al., 1982;Wehmiller, 1982); IV, Isla Vista (Gurrola et al., 2014; see also Table 2); SB, Shell Beach (Stein et al., 1991; Hanson et al., 1994); PSL, Point San Luis (Hanson et al., 1994; Muhs et al., 1994); C, Cayucos (Stein et al., 1991; Muhs et al., 2002a); AN, A~no Nuevo (Muhs et al., 2006); PA, Point Arena (Muhs et al., 2006); PD, Point Delgada (McLaughlin et al., 1983a, 1983b; Merritts and Bull, 1989); CC, Crescent City (Kennedy et al., 1982; Polenz and Kelsey, 1999); CB, Cape Blanco (Kelsey, 1990; Muhs et al., 1990); B, Bandon (McInelly and Kelsey, 1990; Muhs et al., 1990, 2006); YB, Yaquina Bay (Kennedy et al., 1982; Kelsey et al., 1996).
The two beach balls show the stike-slip fault motions for the M6.4 (left) and M6.0 (right) earthquakes. Helena Buurman's primer on reading those symbols is here. pic.twitter.com/aWrrb8I9tj — AK Earthquake Center (@AKearthquake) August 15, 2018
Strike Slip: 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.)
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.
Today, on #SeismogramSaturday: what are all those strangely-named seismic phases described in seismograms from distant earthquakes? And what do they tell us about Earth’s interior? pic.twitter.com/VJ9pXJFdCy — Jackie Caplan-Auerbach (@geophysichick) February 23, 2019
Earthquake Report: Mendocino triple junction
https://earthquake.usgs.gov/earthquakes/eventpage/nc73201181/executive
I didn’t realize this until I was almost home (finally hit the sack around 4 am).
This earthquake follows a sequence of quakes further to the northwest, however their timing is merely a coincidence. Let me repeat this. The M 5.6 earthquake is not related to the sequence of earthquakes along the Blanco fracture zone.
Contrary to what people have posted on social media, there was but a single earthquake. This earthquake happened beneath the area of Petrolia, nearby the 1991 Honeydew Earthquake. More about the Honeydew Earthquake can be found here.
This region also had a good sized shaker in 1992, the Cape Mendocino Earthquake, which led to the development of the National Tsunami Hazard Mitigation Program. More about the Cape Mendocino Earthquake can be found on the 25th anniversary page here and in my earthquake report here.
The regional tectonics in coastal northern California are dominated by the Pacific-North America plate boundary. North of Cape Mendocino, this plate boundary is convergent and forms the Cascadia subduction zone (CSZ). To the south of Cape Mendocino, the plate boundary is the right-lateral (dextral) San Andreas fault (SAF). Where these 2 fault systems meet, there is another plate boundary system, the right-lateral strike-slip Mendocino fault (don’t write Mendocino fracture zone on your maps!). Where these 3 systems meet is called the Mendocino triple junction (MTJ).
The MTJ is a complicated region as these plate boundaries overlap in ways that we still do not fully understand. Geologic mapping in the mid- to late-20th century provides some basic understanding of the long term history. However, recent discoveries have proven that this early work needs to be revisited as there are many unanswered questions (and some of this early work has been demonstrated to be incorrect). Long live science!
Last night’s M 5.6 temblor happened where one strand of the MF trends onshore (another strand bends towards the south). But, it also is where the SAF trends onshore. At this point, I am associating this earthquake with the MF (so, a right-lateral strike-slip earthquake). The mechanism suggest that this is not a SAF related earthquake. However, it is oriented in a way that it could be in the Gorda plate (making it a left-lateral strike-slip earthquake). However, this quake is at the southern edge of the Gorda plate (sedge), so it is unlikely this is a Gorda plate event.Below is my interpretive poster for this earthquake
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
Magnetic Anomalies
Global Strain
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Below the CSZ map is an illustration modified from Plafker (1972). This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes.
Be ready for more earthquakes
What we think will happen next
About our earthquake forecasts
USGS Landslide and Liquefaction Ground Failure data products
Other Report Pages
Some Relevant Discussion and Figures
I have compiled some literature about the CSZ earthquake and tsunami. Here is a short list that might help us learn about what is contained within the core that I collected.
Geologic Fundamentals
Compressional:
Extensional:
Cascadia subduction zone
General Overview
Earthquake Reports
Gorda plate
Blanco fracture zone
Mendocino fault
Mendocino triple junction
North America plate
Explorer plate
Uncertain
Social Media
References:
Return to the Earthquake Reports page.
Earthquake Report: Kermadec Trench
https://earthquake.usgs.gov/earthquakes/eventpage/us6000417i/executive
A minor tsunami (~25 cm in size) has been recorded at Raoul Island, due west of the earthquake, the closest gage to the temblor. Tide gages in New Zealand just began recording a small tsunami the moments I started writing this report (about an hour ± after the earthquake).
This tsunami is small enough that it probably won’t cause much damage. However, tidal inlets and harbors can have currents that are higher in response to even small tsunami, if the shape of the seafloor/harbor is optimal for this. However, further away from the earthquake, the tsunami will be even smaller; so small that it may not be observable in tide gage data.
There was recently an M 6.9 earthquake in this same area and here is my earthquake report for that shaker.Below is my interpretive poster for this earthquake
Magnetic Anomalies
Global Strain
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Other Report Pages
Tide Gage Data
Some Relevant Discussion and Figures
Geologic Fundamentals
Compressional:
Extensional:
New Britain | Solomon | Bougainville | New Hebrides | Tonga | Kermadec Earthquake Reports
General Overview
Earthquake Reports
Social Media
References:
Return to the Earthquake Reports page.
Earthquake Report: Chile
https://earthquake.usgs.gov/earthquakes/eventpage/us600040ja/executive
The major plate boundary here is the megathrust subduction zone that forms the Peru-Chile trench. Here, the Nazca plate dives eastwards beneath the South America plate.
Many people are familiar with subduction zone earthquakes which are responsible for the largest size temblors possible, as well as tsunami capable of travelling across the entire Pacific Ocean. The largest earthquake recorded on modern instruments is the 22 May 1960 M 9.5 Chile earthquake. There have been 2 large transoceanic tsunami caused by subduction zone earthquakes in 2010 and 2015. At the bottom of this report is a list of other earthquakes in this region.
A few months ago, there was an earthquake with a magnitude of M 6.7. However, this earthquake was an extensional earthquake, instead of a compressional earthquake that we typically associate with subduction zones.
This M 6.7 was down-dip (east) of today’s quake. It is possible that the M 6.7 terremoto caused “static coulomb” stress changes in the surrounding region that may have led to today’s earthquake. Someone would need to conduct some numerical analyses to test this hypothesis (I don’t currently have a matlab license, so cannot run Coulomb software to do this analysis myself). I wrote about the M 6.7 earthquake in an earthquake report, as well as for a Temblor article.
There have been several sequences in this same area of the subduction zone that people have used to suggest other types of stress changes from earlier quakes that led to later quakes (e.g. a sequence in 1997, e.g. Leyton et al., 2009 and Gardi et al., 2006).
There are a number of examples at other subduction zones where extensional and compressional earthquakes in different regions can trigger earthquakes of the opposite type. In 2009 earthquakes along the Kuril subduction zone and in 2011 earthquakes east of Japan are good examples.Below is my interpretive poster for this earthquake
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
Magnetic Anomalies
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Other Report Pages
Some Relevant Discussion and Figures
interseismic period, color-coded depending on the event’s depth. Three zones have been defined to avoid including aftershocks and preshocks associated with major events: (1) in North Chile, we plot the seismicity from 2008 to january 2014, i.e., between the Tocopilla and Iquique earthquakes; (2) in Central Chile, we plot the seismicity on the entire 2000–2014 period; (3) in South-Central Chile, we selected events that occurred between 2000 and 2010, i.e., before the Maule earthquake.
Geologic Fundamentals
Compressional:
Extensional:
Chile | South America
General Overview
Earthquake Reports
Social Media
References:
A theoretical and observational comparison in Physics of the Earth and Planetary Interiors, v. 175, p. 37-46, https://doi.org/10.1016/j.pepi.2008.03.017Return to the Earthquake Reports page.
Earthquake Report: San Clemente Island
Here are the earthquakes in this sequence:
There was an earthquake offshore of Los Angeles last year. Check out my earthquake report and report update.
In places the SAF is a single thoroughgoing fault (e.g. in the southern San Joaquin Valley), in others it splays into multiple strands (in Orange County between the Santa Ana Mtns and Lake Elsinore), and in other places it bends to create regions of uplift (like in Ventura or the Santa Monica Mtns). The active faulting in the CCB is basically a series of right-lateral faults that step and bend to form uplifted islands and terraces, along with pull-apart sedimentary basins.
San Clemente Island is a region of uplifted non-marine Tertiary volcanic rocks (andesite and dacite) with ages ranging from 14.8 – 16.5 million years ago (Yeats, 1968; Merifield et al., 1971; Ward and Valenise, 1996). These rocks are overlain by Tertiary (Miocene) sediments (limestone, siltstone, shale, and diatomite; correlates to the Monterey Formation) and Plio-Pleistocene sediments (sandstones and conglomerates; correlates to the Fernando Formation found onshore; Stadum & Susuki, 1976; Ward and Valenise, 1996).
The bedrock is folded into a northwest trending anticline (rocks are folded upwards with the crest in the center of the island, forming a convex upward fold). Moore (1969) use regional compilations of seismic reflection data to show that this type of tectonic folding is ubiquitous throughout the CCB.
Ward and Velensise (1996) suggest that the San Clemente island formed via uplift during progressive slip on two, southeast striking, southwest dipping, blind thrust faults. These faults initiated movement between 3 and 5 Ma. There are a suite of Pleistocene marine terraces (2.56 Ma and younger) that provide evidence that uplift is continuing. Using fossil age determinations and correlation of marine terrace elevations with global eustatic sea level curves, the island is currently uplifting at rates between 0.2 and .5 mm/year. So, the underlying thrust faults are slipping at about 0.6-1.5 mm/yr (Ward and Velensise, 1996).
Muhs et al. (2014) used numerical ages (uranium-series analysis of corrals and amino acid geochronology of mollusks) to calculate marine terrace uplift rates in the CCB. When compared to uplift rates from different tectonic regimes, the terrace uplift rates in CCB is comparable to regions where strike-slip tectonics are dominant. These authors suggest that uplift like that found at the Big Bend (e.g. Ventura and Santa Monica Mtns) is not influencing terrace uplift rates in the CCB.
Along with this compression, there is a right-lateral (dextral) strike-slip fault on the east side of the island, the San Clemente fault, which has a slip rate of about 1 – 4 mm.yr (Ward and Valensise, 1996). The Southern California Earthquake Center suggests the slip rate is about 1.5 mm/yr for the SCF.
The ongoing sequence of earthquakes near the San Clemente Island are small in magnitude. If these were foreshocks to a larger earthquake, this would be felt across the southland, possibly cause damage on the island (where there is a U.S. Naval base), could possibly trigger submarine landslides or a small tsunami. Strike-slip earthquakes are not always considered a significant source for large tsunami, but there is abundant evidence that they do, though often much smaller than tsunami generated from thrust or subduction zone earthquakes. It is possible, if not probable, that this sequence will fizzle out.Below is my interpretive poster for this earthquake
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 include some inset figures. Some of the same figures are located in different places on the larger scale map below.
Other Report Pages
Some Relevant Discussion and Figures
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.)
Pleistocene Marine Terraces
Geologic Fundamentals
Compressional:
Extensional:
San Andreas fault
General Overview
Earthquake Reports
Northern CA
Central CA
Southern CA
Eastern CA
Southern CA
Earthquake Reports
Social Media
References:
http://digitalcommons.unl.edu/usgsstaffpub/932Return to the Earthquake Reports page.