Monthly Archives: September 2022

Earthquake Report for M 6.9 Earthquake in Taiwan

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I don’t always have the time to write a proper Earthquake Report. However, I prepare interpretive posters for these events.

Because of this, I present Earthquake Report Lite. (but it is more than just water, like the adult beverage that claims otherwise). I will try to describe the figures included in the poster, but sometimes I will simply post the poster here.

There was a magnitude M 6.9 earthquake in Taiwan on 18 September 2022.

https://earthquake.usgs.gov/earthquakes/eventpage/us7000i90q/executive

Taiwan is an interesting place, from a tectonic perspective. There is an intersection of several plate boundary fault systems here. Along the western boundary of Taiwan the Eurasia plate subducts (dives beneath) the Philippine Sea plate forming the Manila trench. This megathrust subduction zone fault system terminates somewhere in central-northern Taiwan.

Intersecting central Taiwan from the east is another subduction zone where the Philippine Sea plate subducts beneath the Eurasia plate, forming the Ryukyu trench.

There was an earthquake in Taiwan in 1999 that has been commemorated by creating a park and museum that preserves some of the evidence of the earthquake. This Chi-Chi earthquake cause lots of damage and, sadly, lots of suffering. In addition, because of the dominance of the computer chip manufacturing industry in Taiwan at the time, the price of computer chips was greatly inflated. The global economy suffered following this earthquake.

This 18 September 2022 M 6.9 earthquake occurred on a crustal fault that strikes (trends) parallel to the coast. Because of the mapped faults, I interpret this to have been a left-lateral strike slip earthquake.

There was a foreshock, a mag M 6.5 earthquake, nearby, the day before.

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1921-2021 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.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    I include some inset figures.

  • In the upper left corner is a map that shows the plates, their boundaries, and a century of seismicity.
  • In the upper right are two maps that show models of how there may have been landslides or liquefaction because of the earthquake shaking and impacts. Read more about landslides and liquefaction here. I include both the USGS epicenter and the Central Weather Bureau Seismological Center epicenter (which is probably more accurate). However, these ground failure models are based on the USGS epicenter/location.
  • To the left of those two maps is a low angle oblique view of the tectonic plates and how they are oriented relative to each other.
  • Below that figure, in the center, is a map from Chen at al. (2020) that shows the earthquake fault mapping along eastern Taiwan. I place a yellow star in the location of the M 6.9 epicenter (the location of the earthquake on the ground surface).
  • In the lower right corner is a map that shows the ground shaking from the earthquake, with color representing intensity using the Modified Mercalli Intensity (MMI) scale. The closer to the earthquake, the stronger the ground shaking. The colors on the map represent the USGS model of ground shaking. The colored circles represent reports from people who posted information on the USGS Did You Feel It? part of the website for this earthquake. There are things that affect the strength of ground shaking other than distance, which is why the reported intensities are different from the modeled intensities.
  • To the left of the intensity map is a map that shows seismicity from the Central Weather Bureau Seismological Center. The locations of earthquakes from this center are better than those from the USGS since this organization runs a local seismic network (the USGS runs a global network). The local network uses more seismometers than the global network (so can detect more events, in this region).
  • To the left of this seismicity map is a plot that shows how the shaking intensity models and reports relate to each other. The horizontal axis is distance from the earthquake and the vertical axis is shaking intensity (using the MMI scale, just like in the map to the right: these are the same datasets).
  • In the upper left-center is a figure that shows the USGS earthquake slip model. This shows how much the fault slipped in different areas (based on their modeling, not observation). The model shows that there were places that may have slipped over 1.5 meters (5 feet).
  • Here is the map with 3 month’s seismicity plotted.


    Supportive Figures

  • I could not help myself. I am so excited to have this website back up and running, like a fully operational space station, that I include below some additional figures that help us understand the tectonic setting.
  • Here is the low angle oblique view of the plate configuration in Taiwan.


  • Here is the map from Chen at al. (2020) that shows the fault mapping in this area of eastern Taiwan.


    • Geologic map of the Coastal Range on shaded relief (after Wang and Chen, 1993). The Longitudinal Valley Fault (LVF) can be subdivided into the Linding and Juisui locked Fault and the Chihshang and Lichi creeping Fault. Vertical cross-sections of VS perturbation tomography along the AeA0 and BeB0 profiles denote the Central Range, the Coastal Range, and the LVF. EU: Eurasian Plate; PH: Philippine Sea Plate.

  • Here is an oblique view of the plate configuration in this region. This is from Chang (2001).


  • Here is a great interpretation showing how the Island of Taiwan is being uplifted and exhumed. This is from Lin (2002).


  • Needless to say, this is an excellent map showing the complicated faulting of this region. This is from Theunissen et al. (2012).


  • Here is another tectonic interpretation map from here.

  • Here is a great general overview of the tectonics of the region from Shyu et al. (2005). I include their figure caption below the image as a blockquote.


    • A neotectonic snapshot of Taiwan and adjacent regions. (a) Taiwan is currently experiencing a double suturing. In the south the Luzon volcanic arc is colliding with the Hengchun forearc ridge, which is, in turn, colliding with the Eurasian continental margin. In the north both sutures are unstitching. Their disengagement is forming both the Okinawa Trough and the forearc basins of the Ryukyu arc. Thus, in the course of passing through the island, the roles of the volcanic arc and forearc ridge flip along with the flipping of the polarity of subduction. The three gray strips represent the three lithospheric pieces of Taiwan’s tandem suturing and disarticulation: the Eurasian continental margin, the continental sliver, and the Luzon arc. Black arrows indicate the suturing and disarticulation. This concept is discussed in detail by Shyu et al. [2005]. Current velocity vector of the Philippine Sea plate relative to the Eurasian plate is adapted from Yu et al. [1997, 1999]. Current velocity vector of the Ryukyu arc is adapted from Lallemand and Liu [1998]. Black dashed lines are the northern and western limits of the Wadati-Benioff zone of the two subducting systems, taken from the seismicity database of the Central Weather Bureau, Taiwan. DF, deformation front; LCS, Lishan-Chaochou suture; LVS, Longitudinal Valley suture; WF, Western Foothills; CeR, Central Range; CoR, Coastal Range; HP, Hengchun Peninsula. (b) Major tectonic elements around Taiwan. Active structures identified in this study are shown in red. Major inactive faults that form the boundaries of tectonic elements are shown in black: 1, Chiuchih fault; 2, Lishan fault; 3, Laonung fault; 4, Chukou fault. Selected GPS vectors relative to the stable Eurasian continental shelf are adapted from Yu et al. [1997]. A,Western Foothills; B, Hsueshan Range; C, Central Range and Hengchun Peninsula; D, Coastal Range; E, westernmost Ryukyu arc; F, Yaeyama forearc ridge; G, northernmost Luzon arc; H, western Taiwan coastal plains; I, Lanyang Plain; J, Pingtung Plain; K, Longitudinal Valley; L, submarine Hengchun Ridge; M, Ryukyu forearc basins.

  • This figure from Shyu et al. (2005) shows their interpretation of the different tectonic domains in Taiwan. This is a complicated region that includes collision zones in different orientations as the Okinawa Trough, Ryukyu Trench, and Manila Trench (all subduction zones) each intersect beneath and adjacent to Taiwan. I include their figure caption below the image as a blockquote.


    • Map of major active faults and folds of Taiwan (in red) showing that the two sutures are producing separate western and eastern neotectonic belts. Each collision belt matures and then decays progressively from south to north. This occurs in discrete steps, manifested as seven distinct neotectonic domains along the western belt and four along the eastern. A distinctive assemblage of active structures defines each domain. For example, two principal structures dominate the Taichung Domain. Rupture in 1999 of one of these, the Chelungpu fault, caused the disastrous Chi-Chi earthquake. The Lishan fault (dashed black line) is the suture between forearc ridge and continental margin. Thick light green and pink lines are boundaries of domains.

  • This map from Shyu et al. (2005) shows the earthquake slip regions for proposed earthquake scenarios. I include their figure caption below the image as a blockquote.


    • Proposed major sources for future large earthquakes in and around Taiwan. Thick red lines are proposed future ruptures, and the white patches are rupture planes projected to the surface. Here we have selected only a few representative scenarios from Table 1. Earthquake magnitude of each scenario is predicted value from our calculation.

  • This map from here shows the basement geology of Taiwan. Note the accretionary belts, including the forearc basin. This is a compilation from Teng et al. (2001) and Hsiao et al. (1998) as presented in Ustaszewski et al. (2012).


Return to the Earthquake Reports page.

Earthquake Report: M 7.6 Papua New Guinea

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I was travelling to southern California to attend the annual meeting for the Southern California Earthquake Center. This was the first in person meeting since 2019, my first SCEC meeting. I had landed and was waiting for the luggage to arrive when I saw the CSEM earthquake app notification that there was a large earthquake in Papua New Guinea (PNG).

I  put together a quick tweet before anything was posted on the USGS earthquake page other than the location and depth.

When I got to my hotel room later, more information was up.

However, due to some problems with Dreamhost (my website hosting company), I am migrating to a different company. For a little while, parts of this website (like the links and the images) will be non-functional. I will not be using Dreamhost again. I don’t have any more to say about this since they have not returned any of my emails in over a week, basically abandoning my website in the middle of the night without any warning.

https://earthquake.usgs.gov/earthquakes/eventpage/us6000iitd/executive

The depth increased to about 90 km and has a normal-oblique sense of motion. This means that the earthquake was the result of a combination of extension (stretching) and strike-slip.

This area is a complicated region from a tectonic perspective. There are old faults and old plate boundaries that may no longer be active and there are known active faults that juxtapose these older structures.

For example, there is a convergent (moving together) plate boundary fault system on the north side of Papua New Guinea. This ‘subduction zone’ formed a deep sea trench called the New Guinea trench, where the Caroline plate subducted south beneath Papua New Guinea. This plate boundary fault system is thought to be inactive on the west side of the island and active, but with a slow convergence rate, on the eastern side of the island.

Then, to the southeast of PNG, there is a deep sea trench formed by the subduction of the Australia plate diving to the north beneath the island. This fault is inactive offshore and turns into the Papua fold and thrust belt (PFTB) onshore. The PFTB onshore is inactive in the western part of the island and has a slow convergence rate on the eastern side of the island. On 25 February 2018 there was a M 7.5 earthquake associated with the PFTB. Here is the earthquake report for that earthquake.

Between these two subduction zones, that dip in opposite directions, are several large strike-slip fault systems, which also have varying levels of activity.

“Yesterday’s” M 7.6 occurred in one of the plates that is/was being subducted. It was probably in the Australia plate that dives beneath PNG (which is responsible for the PFTB).

I updated this page a bit on 13 March 2023 and make some corrections. I note these changes below.

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1920-2020 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.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    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 map that shows the earthquake intensity using the modified Mercalli intensity scale. Earthquake intensity is a measure of how strongly the Earth shakes during an earthquake, so gets smaller the further away one is from the earthquake epicenter. The map colors represent a model of what the intensity may be. The USGS has a system called “Did You Feel It?” (DYFI) where people enter their observations from the earthquake and the USGS calculates what the intensity was for that person. The dots with yellow labels show what people actually felt in those different locations.
  • In the lower left center, there is a plate tectonic map showing the tectonic plates and their boundaries (Cloos et al., 2005). I place a yellow star in the general location of this M 7.6 earthquake.
  • On the right, above the intensity map, is a low angle oblique view of the tectonic plate configuration from Cloos et al. (2005). This shows how the Australia plate may be oriented and I place a yellow star to show how the earthquake may be located within the plate (the “slab”).
  • In the upper left corner are two maps showing the probability of earthquake triggered landslides and possibility of earthquake induced liquefaction. I will describe these phenomena below.
  • Here is the map with 3 month’s seismicity plotted.


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.

UPDATE: 2023.03.13

  • Today (13 March 2023) there was an M 6.3 earthquake that made me look back at this M 7.6 event.
  • This area is interesting, in how there are relict plate boundaries surrounded by active ones. The maps shown above make that clear.
  • The series of figures that I want to add are from Holm et al. (2015). They have some excellent cross sections that show their interpretation of the evolution of the plate boundary between the Australia and Pacific plates (including the Bismarck plates as well).
  • Holm et al. (2015) use radiometric ages and geochemical analysis of the rocks in the region to show that there was once a subduction zone, followed by a collision zone, slab foundering, all leading to the existing configuration.
  • This first figure shows (b) the plate boundary faults and topography, (a) their study area and the magmatic arc geology and geochronology, and (c) a large scale map showing their study sites.


    • Panel A: Distribution of mapped magmatic rock units (from Australian Bureau of Mineral Resources (1972)) and major tectonic boundaries of central Papua New Guinea. Published K/Ar (white boxes) and U–Pb ages (grey boxes) for Late Cenozoic magmatic rocks are from Page and McDougall (1972), Grant and Nielsen (1975), Page (1976), Whalen et al. (1982), Rogerson and Williamson (1985), Richards and McDougall (1990), and van Dongen et al. (2010). Panel B: Major tectonic elements of Papua New Guinea; Adelbert Terrane (AT); Aure trough (AuT); Bundi fault zone (BFZ); Bismarck Sea seismic lineation (BSSL); Fly Platform (FP); Finisterre Terrane (FT); Lagaip fault zone (LFZ); Manus Basin (MB); New Britain (NB); New Britain trench (NBT); New Guinea trench (NGT); North Sepik arc (NSA); Owen Stanley fault zone (OSFZ); Papuan Fold and Thrust Belt (PFTB); Papuan Peninsula (PP); Pocklington trough (PT); Ramu–Markham fault zone (RMFZ); South Bismarck plate (SBP); Solomon Sea (SS); Trobriand trough (TT);Woodlark Basin (WB). Panel C: Regional geology of the eastern Papuan Highlands (modified from Australian Bureau of Mineral Resources (1989)).

  • This second figure shows their interpretation of the evolution for this plate boundary in cross sectional view.
  • Time spans 15 million years. The most important figure in this entire report is this one, particularly the lowest panel.
  • The lowest panel shows how there are three plates atop each other and this 7.6 is somewhere within this stack of plates. (Same for the 13 March 2023 earthquake)


    • Simplified geodynamic evolution of the Maramuni arc. Time steps represent: a) north-dipping subduction of the Pocklington Sea slab at the Pocklington trough and associated magmatism of the Maramuni arc intruding the New Guinea Mobile Belt (NGMB); b) onset of continental collision and growth of the New Guinea Orogen from ca. 12 Ma as the Australian continent enters the Pocklington trough. Collision between the Australian continent and the New Guinea Mobile Belt leads to cessation of subduction at the Pocklington trough and associated foundering and steepening of the Pocklington Sea slab in the mantle; c) continued orogenesis results from convergence between the Australian continent and New Guinea Mobile Belt, associated with under thrusting of the leading Australian continental margin at the Pocklington trough. Northward motion of the Australian plate relative to the foundering Pocklington Sea slab leads to buckling and overturning of the slab. A reduction in convergence at the Pocklington trough is accommodated by initiation of north-dipping subduction of the Solomon Sea plate beneath the Finisterre Terrane; d) lithospheric delamination of the Pocklington Sea slab some 6 m.y. after continental collision results in renewed orogenesis in the New Guinea Orogen, and HREE-depleted magmatism at Kokofimpa. Subduction of the Solomon Sea plate continues at the New Britain trench but also begins to underthrust the New Guinea Mobile Belt at the Trobriand trough (Holmet al., unpublished data); e) Maramuni arc magmatism migrates southward into the Papuan Fold and Thrust Belt. Subduction of the Solomon Sea plate at the New Britain and Trobriand subduction systems results in closure of the Solomon Sea and collision and overthrusting of the Finisterre Terrane above the New Guinea Mobile Belt. Magmatism of the West Bismarck arc begins northward of the Finisterre Terrane; and f) convergence between the Finisterre Terrane and New Guinea Mobile Belt continues at the Ramu–Markham fault. Underthrusting of the New Guinea Mobile Belt beneath the Finisterre Terrane results in crustal-derived magmatism in theWest Bismarck arc. The foundered Solomon Sea slab is laterally continuous with the present day Solomon Sea plate to the east (see Fig. 1). Present day cross-section is modified from Holm and Richards (2013).

  • Here is the tweet for the 13 March 2023 M 6.3 earthquake.


Shaking Intensity

Luckily I updated this page because I noticed that the interpretive figure below was incorrect (it was for a different earthquake).

  • Here is a figure that shows a more detailed comparison between the modeled intensity and the reported intensity. Both data use the same color scale, the Modified Mercalli Intensity Scale (MMI). More about this can be found here. The colors and contours on the map are results from the USGS modeled intensity. The DYFI data are plotted as colored dots (color = MMI, diameter = number of reports).
  • In the upper panel is the USGS Did You Feel It reports map, showing reports as colored dots using the MMI color scale. Underlain on this map are colored areas showing the USGS modeled estimate for shaking intensity (MMI scale).
  • In the lower panel is a plot showing MMI intensity (vertical axis) relative to distance from the earthquake (horizontal axis). The models are represented by the green and orange lines. The DYFI data are plotted as light blue dots. The mean and median (different types of “average”) are plotted as orange and purple dots. Note how well the reports fit the green line (the model that represents how MMI works based on quakes in California).
  • Below the lower plot is the USGS MMI Intensity scale, which lists the level of damage for each level of intensity, along with approximate measures of how strongly the ground shakes at these intensities, showing levels in acceleration (Peak Ground Acceleration, PGA) and velocity (Peak Ground Velocity, PGV).


    •

Potential for Ground Failure

Luckily I updated this page because I noticed that the interpretive figure below was incorrect (it was for a different earthquake).

  • Below are a series of maps that show the potential for landslides and liquefaction. These are all USGS data products.
    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.


  • Below is the liquefaction susceptibility and landslide probability map (Jessee et al., 2017; Zhu et al., 2017). Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides.
  • I use the same color scheme that the USGS uses on their website. Note how the areas that are more likely to have experienced earthquake induced liquefaction are in the valleys. Learn more about how the USGS prepares these model results here.


    References:

    Basic & General References

  • 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>
  • 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
  • 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
  • Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
  • Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
  • 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, https://doi.org/0.1785/0120160198

    • Specific 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.

    • ..

  • Hamilton, W.B., 1979. Tectonics of the Indonesian Region, USGS Professional Paper 1078.
  • 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
  • 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.

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Earthquake Report: M 6.9 Mid Atlantic Ridge

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There have not been that many large earthquakes this year. This is good for one main reason, there is a lower potential for human suffering.

Therefore, there are fewer Earthquake Reports for this year.

This morning (my time) there was a magnitude M 6.9 earthquake along the Romanche transform fault, a right-lateral strike-slip fault system that offsets the Mid Atlantic Ridge in the equatorial Atlantic Ocean. The fault is part of the Romanche fracture zone.

https://earthquake.usgs.gov/earthquakes/eventpage/us7000i53f/executive

The transform faults in this part of the Mid Atlantic Ridge plate boundary have a pattern of earthquakes that seem to max out in the lower 7 magnitudes. This may be (at least partly) due to the maximum length of these faults (?).

The Romanche fault is about 900 kilometers long. The Chain fault is about 250 km long. The St. Paul fault is about 350 km long.

    Earthquake magnitude is controlled by three things:

  1. the size of the earthquake slip area, for most events, this is basically the length of the fault (since the width of the fault is controlled by the thickness of the lithosphere, or the crust)
  2. the amount that the fault slipped
  3. physical properties of the lithosphere or crust on either side of the fault (how “elastic” the Earth is)

Using empirical (data) based relations between earthquake subsurface rupture length and earthquake magnitude (Wells and Coppersmith, 1994), I calculate the maximum earthquake magnitude we may get on these three faults listed above.

Here are the data that Wells and Coppersmith use to establish these relations.


(a) Regression of subsurface rupture length on magnitude (M). Regression line shown for all-slip-type relationship. Short dashed line indicates 95% confidence interval. (b) Regression lines for strike-slip relationships. See Table 2 for regression coefficients. Length of regression lines shows the range of data for each relationship.

Here are the magnitude estimates for each of these fault systems.


Looking at the interpretive poster, we can see that there have not been any temblors that approach the sizes listed in this table. The largest historic earthquake was M 7.1 (there were several).

So, we may ask ourselves one of the most common questions people ask regarding earthquakes. Was this M 6.9 a foreshock to a larger earthquake?

Obviously, we cannot yet know this. Nobody can predict the future (at least not yet).

However, based on the incredibly short historic record of earthquakes, we may answer this question: “no, probably not.” This answer is tempered by the very short seismic record. If magnitude 8 earthquakes occur, on average, every 1000 years, then our ~100 year record might be too short to “notice” one of these M 8 events.
If we continue to look at the historic record, we will see that there appear to be three instances where one of these M 6.5-7 earthquakes had a later earthquake of a similar magnitude.
When an earthquake fault slips, the crust surrounding the fault squishes and expands, deforming elastically (like in one’s underwear). These changes in shape of the crust cause earthquake fault stresses to change. These changes in stress can either increase or decrease the chance of another earthquake.
I wrote more about this type of earthquake triggering for Temblor here. Head over there to learn more about “static coulomb stress triggering.”
In the poster, I label these earthquakes as “Linked Earthquakes.” Perhaps the later of each earthquake pair (or triple) was triggered by the change in static coulomb stress.

    Here are the three sets of “Linked Earthquakes:”

  • In 1992, along the Chain fault, the 16 Feb M 6.6 appears to have triggered the 18 Feb M 6.6. More speculatively, about 6 months later, it seems that there was a triggered M 6.9 on 18 Aug. Static Coulomb triggering typically has a limit of about 2-3 times the rupture length (and this depends of the pre-existing stress on the receiver fault, the fault that may be having triggered slip). A M 6.6 may have a rupture length of 50 km, so could possibly affect faults as far as 100-150 km away. The M 6.9 is about 70 km from the easternmost M 6.6, so it seems possible that the M 6.9 was triggered by the M 6.6.
  • In 2003, along the Romanche fault, there were two M 6.6 earthquakes separated by about 6 weeks. These quakes are about 100 km apart, possibly close enough to be triggered.
  • In 2020, along the St. Paul transform fault, there was a pair of quakes about 3 weeks and 340 km apart. The first quake was M 6.5, so this pair of events seems to far apart to be related.

So, given the historic record, it sure seems likely that there may be another M6-7 earthquakes in the region of the fault sometime in the next couple of months. And, given our lack of knowledge about the long term behavior of these faults, it is also possible that there could be a larger M 8 event.
Since we cannot yet know the real answer to this question, we are reminded of the advice that educators and emergency response people provide: If one lives in Earthquake Country, get earthquake prepared. Just a little effort to get better prepared makes a major difference in the outcome.
Head over to Earthquake Alliance where there are some excellent brochures about how to be better prepared and more resilient to earthquake and tsunami hazards. Living on Shaky Ground is one of my favorites!

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1920-2020 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.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    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 I include a map that shows the age of the oceanic crust in the Atlantic Ocean. Oceanic crust (or lithosphere) is created at mid ocean ridges, where there is extension that allows upward movement of magma, leading to the formation of oceanic crust. The Mid Atlantic Ridge system is one of these types of plate boundaries.
  • Above the crust age map is an illustration showing the how the crust moving away from the ocean ridges leaves behind oceanic crust. The Earth’s magnetic polarity changes at times and the oceanic crust records these changes in magnetic polarity. These changes are the main reason why we know that the crust is formed along these ridge systems. Read more here.
  • In the upper left corner is a small scale map that shows the historic seismicity, the plate boundary fault systems, and the magnetic anomalies. Places with crust formed when the magnetic field is like today, is colored red (a.k.a. normal polarity) and crust formed when the poles were reversed relative to today is blue (i.e., reversed polarity).
  • In the upper right corner is a map that shows the earthquake intensity from this earthquake (using the modified Mercalli Intensity Scale). Intensity is a measure of how strongly the shaking is felt, not a measure of the earthquake size. So, the intensity gets smaller with distance (see how the highest intensity is nearest the earthquake epicenter).
  • In the lower left center there is a map from Heezen et al. (1964). Heezen was an oceanographer that contributed greatly to our knowledge of the oceans. In this study, one of the things that they were studying is the flow of deep water (deep water flows largely because of changes in density of the seawater, controlled by salinity and temperature). Because of this, they were mapping the shape of the seafloor to see where this deep water could flow. Ths location of this map is outlined by a dashed rectangle in the main map.
  • Here is the map with a month’s seismicity plotted.


    •

Some Relevant Discussion and Figures

  • Here is the Müller et al. (2008) figure from the interpretive poster above.


    •

  • Here a the Bonatti et al. (2001) figure showing the bathymetry of this area. I include the figure caption as a blockquote below.


    • A: Multibeam topography of Romanche region, showing north-south profiles where sampling was carried out. Black dots and red numbers indicate estimated age (in million years) of lithosphere south of Romanche Transform, assuming spreading half-rate of 17 mm/yr within present-day ridge and transform geometry. White dots indicate epicenters of teleseismically recorded 1970–1995 events (magnitude . 4). FZ is fracture zone. B: Topography and petrology at eastern intersection of Romanche Fracture Zone with Mid-Atlantic Ridge. Data were obtained during expeditions S-16, S-19, and G-96 (Bonatti et al., 1994, 1996). C: Location of A along Mid-Atlantic Ridge.

  • Dr. Stephen Hicks and their colleagues conducted a fascinating study of the 2016 M 7.1 earthquake. They hypothesize that the Romanche fault slipped in different parts of the fault at different times (during the earthquake).
  • This map shows the historic seismicity of the region.


    • Seismotectonic context. The map location is given by the red rectangle on the inset globe. Focal mechanisms are shown for events with Mw > 6 (ref. 30). Mw > 7.0 events are labelled. Stations of the PI-LAB ocean bottom seismometer network are indicated by triangles. Our relocated hypocentre and low-frequency RMT of the 2016 earthquake are shown by the red star and red beach ball, respectively. The orange beach ball is a colocated Mw 5.8 used for the Mach cone analysis. The black rectangle shows the location of the map in Fig. 2. ISC Bulletin, Bulletin of the International Seismological Centre.

  • Here is where Hicks et al. (2020) hypothesize that the slip slipped.


    • Interpretation of rupture dynamics for the 2016 Romanche earthquake. Top: perspective view of bathymetry along the Romanche FZ. Bottom: interpretive cross-section along the ruptured fault plane. Colours show a thermal profile based on half-space cooling. The green line denotes the predicted transition between velocity-strengthening and velocity-weakening frictional regimes (as expressed by the a – b friction rate parameter) from Gabbro data35. The numbers show the key stages of rupture evolution: (1) rupture initiation (star) in the oceanic mantle, (2) initiation phase has sufficient fracture energy to propagate upwards to the locked section of fault, (3) weak subshear rupture front travels east in the lower crust and/or upper mantle, (4) rupture reaches the locked, thinner crustal segment close to the weaker RTI (SE1), (5) sufficient fracture energy for a westward supershear rupture in the crust along the strongly coupled fault segment (SE2) and (6) rupture possibly terminated by a serpentinized and hydrothermally altered fault segment.

    References:

    Basic & General References

  • 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>
  • 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
  • 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
  • Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
  • Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
  • 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, https://doi.org/0.1785/0120160198

    • Specific References

  • Abercrombie, R.E. and Ekstrom, G., 2001. Earthquake slip on oceanic transform faults in Nature, v. 410, p. 74-77
  • Bonatti, E., Brunello, D., Fabretti, P., Ligi, M., Porcaro, R.A., and Sealer, M., 2001. Steady-state creation of crust-free lithosphere at cold spots in mid-ocean ridges in Geology, v. 29, no. 11, p. 979-982.
  • Hicks, S.P., Okuwaki, R., Steinberg, A., Rychert, C.A., Harmon, N. Abercrombie, R.E., Bogiatzis, P., Cataphors, D., Zahradnik, J., Kendall, J-M., Yagi, Y., Shimizu, K., and Sudhaus, H., 2020. Back-propagating supershear rupture in the 2016 Mw 7.1 Romanche transform fault earthquake in Nature Geoscience, v. 13, p. 647-653, https://doi.org/10.1038/s41561-020-0619-9
  • Heezen, B.C., Bunce, E.T., Hersey, J.B., and Tharp, M., 1964. Chain and Romanche fracture zones in Deep-Sea research, v. 11, p. 11-33
  • Müller, R.D., Sdrolias, M., Gaina, C., and Roest, W.R., 2008. Age, spreading rates and spreading symmetry of the world’s ocean crust in Geochem. Geophys. Geosyst., 9, Q04006, doi:10.1029/2007GC001743
  • Torsvik, T.H., Tousse, S., Labaila, C., and Smethurst, M.A., 2009. A new scheme for the opening of the South Atlantic Ocean and the dissection of an Aptian salt basin in Geophysical Journal International, v. 177, p. 1315-1333.

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