Category Archives: oceanography

Cascadia subduction zone: M 9.0 tsunami simulation video

NOAA Center for Tsunami Research just released an animation that shows a numerical simulation of what a tsunami may appear like when the next Cascadia subduction zone earthquake occurs. I present a summary about the CSZ tectonics on a 316th year commemoration page here. I include a yt link and embedded video and an mp4 embedded video and download link.

Here is a screenshot from the video:


Cascadia Update: new tsunami simulation!

The NOAA/NWS/Pacific Tsunami Warning Center has updated their animation of the simulation of the 1700 “Orphan Tsunami.”

Source: Nathan C. Becker, Ph.D. nathan.becker at noaa.gov


    I provide some other background information about Cascadia here:

  • Cascadia’s 315th Anniversary 2015.01.26
  • Earthquake Information about the CSZ 2015.10.08

Below are some links and embedded videos.

  • Here is the yt link for the embedded video below.
  • Here is the mp4 link for the embedded video below. (2160p 145 mb mp4)
  • Here is the mp4 link for the embedded video below. (1080p 145 mb mp4)
  • Here is the text associated with this animation:

    Just before midnight on January 27, 1700 a tsunami struck the coasts of Japan without warning since no one in Japan felt the earthquake that must have caused it. Nearly 300 years later scientists and historians in Japan and the United States solved the mystery of what caused this “orphan tsunami” through careful analysis of historical records in Japan as well as oral histories of Native Americans, sediment deposits, and ghost forests of drowned trees in the Pacific Northwest of North America, a region also known as Cascadia. They learned that this geologically active region, the Cascadia Subduction Zone, not only hosts erupting volcanoes but also produces megathrust earthquakes capable of generating devastating, ocean-crossing tsunamis. By comparing the tree rings of dead trees with those still living they could tell when the last of these great earthquakes struck the region. The trees all died in the winter of 1699-1700 when the coasts of northern California, Oregon, and Washington suddenly dropped 1-2 m (3-6 ft.), flooding them with seawater. That much motion over such a large area requires a very large earthquake to explain it—perhaps as large as 9.2 magnitude, comparable to the Great Alaska Earthquake of 1964. Such an earthquake would have ruptured the earth along the entire length of the 1000 km (600 mi) -long fault of the Cascadia Subduction Zone and severe shaking could have lasted for 5 minutes or longer. Its tsunami would cross the Pacific Ocean and reach Japan in about 9 hours, so the earthquake must have occurred around 9 o’clock at night in Cascadia on January 26, 1700 (05:00 January 27 UTC).

    The Pacific Tsunami Warning Center (PTWC) can create an animation of a historical tsunami like this one using the same too that they use for determining tsunami hazard in real time for any tsunami today: the Real-Time Forecasting of Tsunamis (RIFT) forecast model. The RIFT model takes earthquake information as input and calculates how the waves move through the world’s oceans, predicting their speed, wavelength, and amplitude. This animation shows these values through the simulated motion of the waves and as they race around the globe one can also see the distance between successive wave crests (wavelength) as well as their height (half-amplitude) indicated by their color. More importantly, the model also shows what happens when these tsunami waves strike land, the very information that PTWC needs to issue tsunami hazard guidance for impacted coastlines. From the beginning the animation shows all coastlines covered by colored points. These are initially a blue color like the undisturbed ocean to indicate normal sea level, but as the tsunami waves reach them they will change color to represent the height of the waves coming ashore, and often these values are higher than they were in the deeper waters offshore. The color scheme is based on PTWC’s warning criteria, with blue-to-green representing no hazard (less than 30 cm or ~1 ft.), yellow-to-orange indicating low hazard with a stay-off-the-beach recommendation (30 to 100 cm or ~1 to 3 ft.), light red-to-bright red indicating significant hazard requiring evacuation (1 to 3 m or ~3 to 10 ft.), and dark red indicating a severe hazard possibly requiring a second-tier evacuation (greater than 3 m or ~10 ft.).

    Toward the end of this simulated 24-hours of activity the wave animation will transition to the “energy map” of a mathematical surface representing the maximum rise in sea-level on the open ocean caused by the tsunami, a pattern that indicates that the kinetic energy of the tsunami was not distributed evenly across the oceans but instead forms a highly directional “beam” such that the tsunami was far more severe in the middle of the “beam” of energy than on its sides. This pattern also generally correlates to the coastal impacts; note how those coastlines directly in the “beam” have a much higher impact than those to either side of it.

Earthquake report: Aleutian subduction zone at the Amlia fracture zone update

Today (2015.11.09) we had another moderate sized earthquake along the Aleutian subduction zone, the largest of four earthquakes of magnitude mid 5-6. My initial earthquake report form 2015.11.02 can be found here.

    Here are the USGS web sites for the largest magnitude earthquakes plotted below.

  • 2015.11.02 M 5.9
  • 2015.11.06 M 5.5
  • 2015.11.08 M 5.4
  • 2015.11.08 M 5.7
  • 2015.11.09 M 6.2

Below is a map showing the epicenters from earthquakes during the past 30 days in the region of some earthquakes with largest magnitudes ranging from 5.4-6.2 (linked above). I plot the moment tensors from the earthquakes with the 4 largest magnitudes. These five earthquakes are the result of north-northwest compression from the subduction of the Pacific plate underneath the North America plate to the north. The majority of these earthquakes occurred in the region of the subduction zone where the Amlia fracture zone is aligned. The AMZ is a left lateral strike slip oriented fracture zone, which displaces crust of unequal age, beneath the megathrust. The difference in age results in a variety of factors that may contribute to differences in fault stress across the fracture zone (buoyancy, thermal properties, etc). For example, older crust is colder and denser, so it sinks lower into the mantle and exerts a different tectonic force upon the overriding plate.

I placed a moment tensor / focal mechanism legend in the upper right corner of the map. 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.

This is a USGS graphic that shows a cross section of the Aleutian subduction zone that is a part of the “Eastern Aleutian Volcanic Arc Digital Model.’

This shows a cross section of a subduction zone through the two main parts of the earthquake cycle. The interseismic part (inbetween earthquakes) and the coseismic part (during earthquakes). This was developed by George Plafker and published in his 1972 paper on the Good Friday Earthquake.


I also place a map created by Peter Haeussler, USGS, which shows the historic earthquakes along the Alaska and Aleutian subduction zones.


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


This region was active in September of 2015 also. Here is my earthquake report from that series of earthquakes.

This map shows some late 20th and early 21st century earthquakes and their moment tensors for this region of the Aleutian subduction zone.

Cascadia subduction zone: Tectonic Earthquakes of the Pacific Northwest

IRIS and the US Geological Survey have recently produced an educational video about tectonic earthquakes in the region of the US Pacific Northwest. The project was funded by the National Science Foundation.

The Video


YT link for the embedded video below.
mp4 link for the embedded video below.

mp4 embedded video:


YT embedded video:

I recently collected a core with a thick sandy deposit that is hypothesized to be the sedimentary deposit that was the result of tsunami deposition following the 1700 A.D. Cascadia subduction zone earthquake. Here is my post about that core.

Here is a composite of the two cores that I collected. The top of the core is on the left.

  • YT link for the embedded video below:

Earlier this year was the 315th anniversary of the 1700 AD Cascadia subduction zone earthquake and tsunami. I compiled some information about that earthquake and tsunami. I included some information about the plate tectonics of the region. Here is the post for that anniversary.

Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2006). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate).

This map (McCrory et al., 2006) shows the secular (ongoing modern) rates of motion for the Juan de Fuca and Gorda plates relative to the North America plate (Wilson, 1998; McCrory, 2000). Red triangles denote active arc volcanoes.

Here is a view of the subduction zone showing the landscape and the plate configuration within the Earth. The cross section is located near the southern Willamette Valley. This is schematic and does not completely match the real geography. Note how the downgoing plate melts and the rising magma leads to volcanism of the Cascade volcanoes (a volcanic arc).

Here is a version of the CSZ cross section alone (Plafker, 1972).

This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. We also can see how a subduction zone generates a tsunami. Atwater et al., 2005.

Here is an animation produced by the folks at Cal Tech following the 2004 Sumatra-Andaman subduction zone earthquake. I have several posts about that earthquake here and here. One may learn more about this animation, as well as download this animation here.

This figure shows the regions that participate in this interseismic and coseismic deformation at Cascadia. Atwater et al., 2005. Black dots on the map show sites where evidence for coseismic subsidence has been found in coastal marshes, lakes, and estuaries.

This shows how the CSZ is deforming vertically today (Wang et al., 2003). The panel on the right shows the vertical motion in mm/yr.

This figure, also from Wang et al. (2003), shows their estimate of how the coseismic vertical motion may happen. Contours are in meters.

Here is a graphic showing the sediment-stratigraphic evidence of earthquakes in Cascadia. Atwater et al., 2005. There are 3 panels on the left, showing times of (1) prior to earthquake, (2) several years following the earthquake, and (3) centuries after the earthquake. Before the earthquake, the ground is sufficiently above sea level that trees can grow without fear of being inundated with salt water. During the earthquake, the ground subsides (lowers) so that the area is now inundated during high tides. The salt water kills the trees and other plants. Tidal sediment (like mud) starts to be deposited above the pre-earthquake ground surface. This sediment has organisms within it that reflect the tidal environment. Eventually, the sediment builds up and the crust deforms interseismically until the ground surface is again above sea level. Now plants that can survive in this environment start growing again. There are stumps and tree snags that were rooted in the pre-earthquake soil that can be used to estimate the age of the earthquake using radiocarbon age determinations. The tree snags form “ghost forests.

Here is a photo of the ghost forest, created from coseismic subsidence during the Jan. 26, 1700 Cascadia subduction zone earthquake. Atwater et al., 2005.

Here is a photo I took in Alaska, where there was a subduction zone earthquake in 1964. These tree snags were living trees prior to the earthquake and remain to remind us of the earthquake hazards along subduction zones.

This shows how a tsunami deposit may be preserved in the sediment stratigraphy following a subduction zone earthquake, like in Cascadia. Atwater et al., 2005. If there is a source of sediment to be transported by a tsunami, it will come along for the ride and possibly be deposited upon the pre-earthquake ground surface. Following the earthquake, tidal sediment is deposited above the tsunami transported sediment. Sometimes plants that were growing prior to the earthquake get entombed within the tsunami deposit.

Here is a new animation of the tsunami that was triggered during the 1700 AD CSZ earthquake. This is just a model and has considerable uncertainty associated with it. From the US NWS Pacific Tsunami Warning Center (PTWC).

This is the timeline of prehistoric earthquakes as preserved in sediment stratigraphy in Grays Harbor and Willapa Bay, Washington. Atwater et al., 2005. This timeline is based upon numerous radiocarbon age determinations for materials that died close to the time of the prehistoric earthquakes inferred from the sediment stratigraphy at locations along the Grays Harbor, Willapa Bay, and Columbia River estuary paleoseismic sites.

Offshore, Goldfinger and others (from the 1960’s into the 21st Century, see references in Goldfinger et al., 2012) collected cores in the deep sea. These cores contain submarine landslide deposits (called turbidites). These turbidites are thought to have been deposited as a result of strong ground shaking from large magnitude earthquakes. Goldfinger et al. (2012) compile their research in the USGS professional paper. This map shows where the cores are located.

Here is an example of how these “seismoturbidites” have been correlated. The correlations are the basis for the interpretation that these submarine landslides were triggered by Cascadia subduction zone earthquakes. This correlation figure demonstrates how well these turbidites have been correlated. Goldfinger et al., 2012

This map shows the various possible prehistoric earthquake rupture regions (patches) for the past 10,000 years. Goldfinger et al., 2012. These rupture scenarios have been adopted by the USGS hazards team that determines the seismic hazards for the USA.

I have a paper that also discusses the paleoseismology and sedimentary settings in Cascadia (and Sumatra). Patton et al., 2013.

Here is my abstract:

Turbidite deposition along slope and trench settings is evaluated for the Cascadia and Sumatra–Andaman subduction zones. Source proximity, basin effects, turbidity current flow path, temporal and spatial earthquake rupture, hydrodynamics, and topography all likely play roles in the deposition of the turbidites as evidenced by the vertical structure of the final deposits. Channel systems tend to promote low-frequency components of the content of the current over longer distances, while more proximal slope basins and base-of-slope apron fan settings result in a turbidite structure that is likely influenced by local physiography and other factors. Cascadia’s margin is dominated by glacial cycle constructed pathways which promote turbidity current flows for large distances. Sumatra margin pathways do not inherit these antecedent sedimentary systems, so turbidity currents are more localized.

The Gorda plate is deforming due to north-south compression between the Pacific and Juan de Fuca plates. There have been many papers written about this. The most recent and comprehensive review is from Jason Chaytor (Chaytor et al., 2004). Here is a map of the Cascadia subduction zone, as modified from Nelson et al. (2006) and Chaytor et al. (2004). I have updated the figure to be good for projections in a dark room (green) and to have the correct sense of motion on the two transform plate boundaries at either end of the CSZ (Queen Charlotte and San Andreas faults).

Here is the Chaytor et al. (2004) map that shows their interpretation of the structural relations in the Gorda plate.

Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the Januray 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004).

The Blanco fracture zone is also an active transform plate boundary. The BFZ is a strike slip fault system that connects two spreading ridges, the Gorda Rise and the Juan de Fuca Ridge. Here is a map that shows the tectonic setting and some earthquakes related to the BFZ from April 2015. There are some animations on this web page showing seismicity with time along the BFZ, over the past 15
years.

References:

Plate Tectonics: 200 Ma

Gibbons and others (2015) have put together a suite of geologic data (e.g. ages of geologic units, fossils), plate motion data (geometry of plates and ocean ridge spreading rates), and plate tectonic data (initiation and cessation of subduction or collision, obduction of ophiolites) to create a global plate tectonic map that spans the past 200 million years (Ma). Here is the facebook post that I first saw to include this video.

Here is a view of their tectonic map at a specific time.


Gibbons et al. (2015) created several animations using their plate tectonic model. I have embedded both of these videos below. Other versions of these files are placed on the NOAA Science on a Sphere Program website.

They also compare their model with P-wave tomographic analytical results. P-Wave tomography works similar to CT-scans. CT-scans are the the result of integrating X-Ray data, from many 3-D orientations, to model the 3-D spatial variations in density. “CT” is an acronym for “computed tomography.” Both are kinds of tomography. Here is a book about seismic tomography. Here is a paper from Goes et al. (2002) that discusses their model of the thermal structure of the uppermost mantle in North America as inferred from seismic tomography.

Here is an illustration from the wiki page that that attempts to help us visualize what tomography is.


P-Wave tomography uses Seismic P-Waves to model the 3-D spatial variation of Earth’s internal structure. P-Wave tomography is similar to Computed-Tomography of X-Rays because the P-wave sources are also in different spatial locations. For CT-scans, the variation in density is inferred with the model. For P-Wave tomography, the variation in seismic velocity. Typically, when seismic waves travel faster, they are travelling through old, cold, and more dense crust/lithosphere/mantle. Likewise, when seismic waves travel slower, they are travelling through relatively young, hot, and less dense crust/lithosphere/mantle.

Regions of Earth’s interior that have faster seismic velocities are often plotted in blue. Regions that have slower velocities are often plotted in red.

Here are their plots showing the velocity perturbation (faster or slower). I include the figure caption below the image.



Plate reconstructions superimposed on age-coded depth slices from P-wave seismic tomography (Li et al., 2008) using first-order assumptions of near-vertical slab sinking, with a) 3.0 and 1.2 cm/yr constant sinking rates in the upper and lower mantle, respectively, following Zahirovic et al. (2012), and b) 5.0 and 2.0 cm/yr upper and lower mantle sinking rates, respectively, following Replumaz et al. (2004). Both end-member sinking rates indicate bands of slab material (blue, S1–S2) offset southward from the Andean-style subduction zone along southern Lhasa, consistentwith the interpretations of Tethyan subducted slabs by Hafkenscheid et al. (2006). However, although the P-wave tomography provides higher resolution than S-wave tomography, the amplitude of the velocity perturbation is significantly lower in oceanic regions (e.g., S2) and the southern hemisphere due to continental sampling biases. Orthographic projection centered on 0°N, 90°E.

Chile Historic Tsunami Comparisons

Here are three tsunami wave amplitude models from NOAA for three Great (magnitude > 8.0) earthquakes in Chile.

    Here are the USGS websites for the three earthquakes that generated these earthquakes, plus the one from 2015.09.16 not shown on the map below. The USGS magnitudes are listed.

  • 1960.05.22 M 9.6 Bio-Bio, Chile 25 km (the USGS lists the M as 9.6, while most everyone else states M = 9.5
  • 2010.02.27 M 8.8 Bio-Bio, Chile 22.9 km
  • 2014.04.01 M 8.2 Iquique, Chile 25 km
  • 2015.09.16 M 8.3 Illapel, Chile 25 km
    Here are the NOAA Center for Tsunami Research websites for the three tsunamis plotted in the map below, plus the one from 2015.09.16 not shown on the map below.

  • 1960.05.22 M 9.5 (There is no page for the 1960 earthquake, so this map is located on the 2010 page.)/li>
  • 2010.02.27 M 8.8
  • 2014.04.01 M 8.2
  • 2015.09.16 M 8.3

Here is the map. These three maps use the same color scale. There is not yet a map with this scale for the 2015 tsunami, so we cannot yet make the comparison.

Here is an animation of these three tsunami from the US NWS Pacific Tsunami Warning Center (PTWC). This is the YouTube link.

Here are all the earthquakes of magnitude greater than M 8.0 in Chile from between 1900 and today (2015.09.18).


Here is a space time diagram from Beck et al. (1998). The 2015 earthquake occurs in the region of the 1943 and 1880 earthquakes.


Here is the latest map that shows the 2015 earthquake and aftershocks in relation to some of the historic maps shown on the Beck et al. (1998 ) space time diagram. I describe this map and its contents more on this page (Hayes et al., 2012).


    These are the posts that I have put together regarding the 2015.09.16 earthquake and tsunami.

  • 2015.09.16 First Post
  • 2015.09.16 First Update
  • 2015.09.17 Tsunami Observations
  • 2015.09.17 Second Update
  • http://www.science.earthjay.com/wp-admin/media-upload.php?post_id=3022&type=image&TB_iframe=1

2015.09.16 Chile Update #2

Here I post some updates about the M 8.3 Great Earthquake in Chile and the associated tsunami observations and models.

Here is an updated map, that shows the aftershocks are extending further north and south, out of the 1943 strike length region. The 1922 earthquake to the north was much larger, with a magnitude of M 8.5.


Here are some observations from tide gages along the west coast of the US. These all come from here.

Crescent City

North Spit (Humboldt Bay)

Here are some tsunami simulations from NOAA here.

Here is the wave height map, based on numerical simulations of wave propagation throughout the Pacific Ocean. These simulations are recurrently compared with tide gage and buoy data.


This plot shows their model results as compared to tide gages in Chile.


This plot shows their model results as compared to tide gages in California.


This is an animation of their wave propagation model. Here is the link to the embedded mp4 video below.

Here is some drone footage of the tsunami damage in Chile.

RT:

emol TV:

The Guardian:

Here is a large scale map showing an update of the epicenter plots.


Earthquake along the Alaska Penninsula!

We just had an earthquake along the Alaska Peninsula. The magnitude is currently set at 6.7 on the USGS website. The Peninsula is a volcanic arc that forms as a result of the subduction of the Pacific plate beneath the North America plate. The second largest earthquake ever recorded by seismometers occurred on March 27, 1964, known as the Good Friday Earthquake. I have posted some material about this earthquake.

Here is an early map showing the epicenter of the Mw = 6.7 earthquake. This earthquake has an oblique strike-slip moment tensor fault plane solution. Based upon the block rotation in the forearc to the west, I suspect that this may be a N-S right-lateral earthquake.

    Here are a couple pages that discuss the Good Friday Earthquake and the general tectonic setting along this plate boundary.

  • 1964 Earthquake Before and After Photo Tour
  • 1964 Earthquake Seismic Wave Propagation Animation
  • June 2013 M 7.9 Rat Islands Earthquake. This page also has some cool animations from the Rat Island Earthquake and tectonic maps of the Aleutian Islands.

Here is a graphic showing what a forearc sliver might look like (from GSA).

Here is a map from Krutikov et al. 2008 (Active Tectonics and Seismic Potential of Alaska, Geophysical Monograph Series 179 Copyright 2008 by the American Geophysical Union. 10.1029/179GM07)
Note that there are blocks that are rotating to accomodate the oblique convergence. There are also margin parallel strike slip faults that bound these blocks. These faults are in the upper plate, but may impart localized strain to the lower plate, resulting in strike slip motion on the lower plate (my arm waving part of this). Note how the upper plate strike-slip faults have the same sense of motion as these deeper earthquakes.

This map shows the region with historic earthquakes extending back to 1960, of magnitude 6,0 and larger. The largest circle in the upper right part of the map is the epicenter for the Good Friday Earthquake.

Here is a map that shows the regional extent of the 1964 earthquake. Regions of coseismic uplift/subsidence are delineated by blue/red polygons.

This shows a cross section of a subduction zone through the two main parts of the earthquake cycle. The interseismic part (inbetween earthquakes) and the coseismic part (during earthquakes). This was developed by George Plafker and published in his 1972 paper on the Good Friday Earthquake.

Here is a map showing the historic earthquakes along this subduction zone. This is from Peter J. Haeussler, USGS, Alaska Science Center.

These are leveling data from the earthquake cycle along the subduction zone in southeastern Japan.

Here is a video that discusses the 1964 earthquake.

Youtube Source IRIS

WMV file for downloading.

Animation & graphics by Jenda Johnson, geologist
Directed by Robert F. Butler, University of Portland
U.S. Geological Survey consultants: Robert C. Witter, Alaska Science Center Peter J. Haeussler, Alaska Science Center
Narrated by Roger Groom, Mount Tabor Middle School
Maps from Google Earth. Video from US Army Corps of Engineers. Tsunami animation from National Oceanic & Atmospheric Administration (NOAA). Photographs from US Geological Survey.

    References:

  • Hyndman and Wang, 1995
  • Plafker, 1972

Mid Atlantic Ridge Plate Boundary

In the past week, there have been a few earthquakes along the Mid Atlantic Ridge (MAR) and some associated fracture zones. The Mid Atlantic Ridge is a divergent plate boundary. As the plates move apart, the asthenosphere is decompressed and magma rises to the surface to create new oceanic lithosphere. The youngest oceanic crust is along these oceanic spreading centers/ridges. When these spreading ridges are offset laterally, transform plate boundaries called fracture zones form. The MAR has many fracture zones.

Here is a map showing the southern swarm is related to the spreading center and that the seismicity in the north is strike-slip motion on a fault probably related to a fracture zone, possibly the Vernadsky or Bogdanov fracture zones (looks like it is on a fracture zone between these two, but I am uncertain about which fz is which). The southern earthquake magnitude M = 6.3 moment tensor for the spreading ridge earthquake is extensional, consistent with being related to a divergent plate boundary (orange arrows). This spreading ridge is between the St. Helena and Hotspur fracture zones. The northern earthquake magnitude M = 5.1 moment tensor matches what we would expect for a fracture zone in this region (green arrows). I found these fracture zones labeled on a couple maps (Bonatti et al., 2010 and online from Woods Hole, and the USGS earthquake maps).

Click on the map to be able to read the labels for the fracture zones.

    Here are the USGS web pages for the three largest magnitude earthquakes in the above map:

  • 2015.05.24 M 5.1 northern earthquake
  • 2015.05.24 M 6.3 southern mainshock
  • 2015.05.25 M 5.2 southern aftershock

M 7.0 south Atlantic Ocean

How exciting! This earthquake is on the other side of the Scotia plate from the earthquake swarm from last week.

This M7.0 earthquake appears to have slipped along a fault associated with the North Scotia Ridge (NSR). The NSR is mapped as a left-lateral transform fault (not a ridge, like it is named). There is not yet a moment tensor available, but I suspect this may be a left-lateral strike-slip earthquake.

Here is the USGS earthquake page.

Indeed, the moment tensor shows a possible left-lateral strike slip earthquake.

Here is a tectonic map of the region from here:

Here is a regional map with the epicenter in orange and historic earthquake epicenters in gray.

Here is a local map showing Modified Mercalli Intensity contours and historic earthquake epicenters.

Here is the USGS page showing the contributed moment tensors: