Triggered Earthquakes: Izu-Bonin

Well, we had a large magnitude earthquake (M = 6.2) triggered by the M 7.8 deep earthquake along the Izu-Bonin Trench. Here is my post from yesterday. Note the triggered earthquake is in a region where there were earthquakes about 2 weeks prior to the M 7.8. These “foreshocks” (not really foreshocks since they are on different faults) are thrust or reverse earthquakes (the result of compression from the convergent plate boundary). The M 7.8 and triggered M 6.2 are, instead, normal earthquakes (the result of extension).
I have included a cross section on the map (also include this below). The cross section B-B’ is in a great location compared to the M 7.8. Note the purple line, as this coincides with the B-B’ cross section on seismicity in the inset figure. I placed a red circle on the inset, approximately where the M 7.8 hypocenter would plot. This M 7.8 is the deepest earthquake in this part of the subduction zone, but on cross section E-E’ shows an earthquake with a similar hypocentral depth. I also show where the 1944 M 8.1 Tonankai and 1946 M 8.3 Nankai subduction zone earthquakes are.

    Here are the USGS web sites for the earthquakes with moment tensors plotted on the above map.

  • 2015.05.10 M 5.3
  • 2015.05.10 M 5.6
  • 2015.05.30 M 6.2
  • 2015.05.30 M 7.8

Something that is interesting, possibly, is that the M 7.8 earthquake is at a much deeper position than is suggested by the slab model from Hayes et al. (2012). Based upon these slab contours (the depth contours for where the subduction zone fault may be, based upon seismicity), the slab is at a depth of 200-220 km in the region of the M 7.8 earthquake, yet this earthquake has a USGS hypocentral depth of 677 km. These slab contours are free to download and then they can be plotted in Google Earth (or ArcGIS). However, if we examine the cross section on the inset figure (or below), we will see that the M 7.8 fits well with the earlier seismicity. Perhaps the Hayes et al. (2012) slab model could be updated for this subduction zone.

The M 7.8 was felt broadly in Japan.

This shows how the seismic energy attenuates (diminishes) with distance from the earthquake. Note how the observations are at such a great distance.

Here is the cross section figure that is an inset on the above map. This comes from Dr. Matt Fouch from Arizona State University. Here is some text from the wiki page.
“Map view of bathymetry and seismicity in the IBM subduction zone using the earthquake catalog of Engdahl, van der Hilst & Buland 1998. Circles denote epicentral locations; lighter circles represent shallower events, darker circles represent deeper events. Black lines denote cross sectional areas depicted in 6 profiles on right, organized from N to S. Black circles represent hypocentral locations in volume ~60 km to each side of the lines shown on the map at left. Large variations in slab dip and maximum depth of seismicity are apparent. Distance along each section is measured from the magmatic arc.”
“B) Central Izu Bonin region. Slab dip is nearly vertical; seismicity tapers off from ~100 km to ~325 km but increases in rate and extends horizontally around 500 km, and terminates at ~550 km.”


  • Engdahl, E.R.; van der Hilst, R.D.; Buland, R., 1998. Global teleseismic earthquake relocation with improved travel times and procedures for depth determination. Bulletin of the Seismological Society of America 88: 722–743.
  • Hayes, G. P., D. J. Wald, and R. L. Johnson (2012), Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524

Earthquake in Izu-Bonin!

Early this morning (luckily I was not awake at the time) we had a really deep (~677km) M = 7.8 extensional earthquake along the Bonin Trench in the western Pacific. Here is the USGS page for this earthquake.
Here is a map that shows the epicenter in red (due to the depth), along with the moment tensor for this earthquake. I also plot the general location of the 1944 Tōnankai and 1946 and Nankai Earthquakes. I also include a cross section for this subduction zone (also from the wikipedia page).

Here I have plotted the slab contours (Hayes et al., 2012).

Here is a regional tectonic map from the wikipedia site on this region.

There is more about the regional tectonics on the USGS page here.
Here is a map showing the general locations of the 1946 Tōnankai and 1944 Nankai Earthquakes.

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.


  • Hyndman and Wang, 1995
  • Plafker, 1972

Gorda earthquake triggered a Gorda earthquake?

We just had an earthquake along the nodal plane of the Gorda plate earthquake earlier today. Here is the USGS web page for the Gorda M = 4.3 earthquake and here is the USGS web page for the Gorda plate M = 2.9 earthquake. The M = 2.9 earthquake plots at ~30 km depth, which is deeper than the megathrust fault, based upon the McCrory et al. (2006) slab contours.
Here is a map showing these two epicenters. The fault that ruptured during the M = 4.3 earthquake is quite possibly the same fault that ruptured in 2010. However, it seems improbable that this fault would be straight enough to align with the nodal plane in the transparent focal mechanism shown in this figure.

Here is a map from my M = 4.3 earthquake post from earlier today.

I post more material about the Cascadia subduction zone on this 315th anniversary page.


  • McCrory, P. A., Blair, J. L., Oppenheimer, D. H., and Walter, S. R., 2006, Depth to the Juan de Fuca slab beneath the Cascadia subduction margin; a 3-D model for sorting earthquakes U. S. Geological Survey

Gorda plate earthquake: 2010 zone strikes again

Early this morning we had a small rumbler offshore of northern California, generally in the same location as the 2010.01.10 earthquake. I remember the 2010 earthquake very well as the ground shaking lasted over 20 seconds in Manila. Here is the USGS website for the M 4.3 earthquake. I missed today’s earthquake, but probably would have been awaken by it.
Here is a map that I put together that has some moment tensors for some of the significant earthquakes since 1980. Note how the earthquakes that are in the Gorda plate (read more below) are all left-lateral (sinistral) earthquakes. I also interpret today’s M 4.3 as a left-lateral strike slip earthquake. I discuss more about the deforming Gorda plate below.

This is a close up view of my Moment Tensor / Focal Mechanism (MT/FM) explanation illustration. Moment Tensors and Focal Mechanisms are two ways of depicting the orientation of fault motion based upon seismologic observations. They are calculated differently, but the graphical depiction of their solutions is the same (the beach balls). The illustration below shows how each MT/FM is depicted from map view (as if looking down on Earth from outer space), as well as from side-view (in cross section). These are lower hemisphere projections of spherical data. Note how there are two potential fault planes for each MT/FM. We must use other knowledge about the region in order to be able to best interpret which is the correct fault plane. For the strike-slip example, the fault plane solution is either a left-lateral (purple arrows) or a right-lateral (green arrows) earthquake. Likewise, the normal (extensional, orange arrows) and thrust (compressional, blue arrows) earthquakes are either dipping to the right or to the left.

Here are a couple maps that show the ground shaking intensity estimates for this earthquake.
The first one (intensity map) is based upon computer modeling of ground motions based upon a point source and general attenuation relations (as seismic waves propagate from an earthquake rupture, their energy dissipates as it expands across the region, and their energy is absorbed within Earth materials). Due to this attenuation, the energy content in seismic waves diminishes with distance from the earthquake. Both of these maps use the Modified Mercalli Intensity scale (a relative scale of ground shaking intensity based upon observations made by people).

Here is a map (DYFI map) that is based solely upon the reports provided by people who actually felt the earthquake. These observations can be reported to the USGS via their “Did You Feel It?” web page.

Here is a plot showing the attenuation relations used for the intensity map. There is one attenuation relation model shown (orange line), along with real data shown as green dots. Note how the orange line, which plots intensity on the vertical axis) diminishes with distance (horizontal axis).

    Here are the USGS websites for the earthquakes with moment tensors on the above map.

  • 1980.11.08 M 7.2
  • 1991.08.17 M 7.1
  • 1992.04.25 M 7.2
  • 1994.09.01 M 7.1
  • 2005.06.15 M 7.2
  • 2010.01.10 M 6.5

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.

This is also from Chaytor et al. (2004) and shows moment tensor solutions for earthquakes in the Gorda plate. Note how they could predominantly be interpreted as northeast striking strike-slip faults.

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).

This map shows an earthquake swarm from 2014, which appears to align along another northeast striking strike-slip earthquake fault in the Gorda plate. This swarm is related to a Mw 6.8 earthquake. Check out my pages about the mainshock and the aftershocks. I made some animations of these earthquakes here.

Here is a primer for those who want to learn more about focal mechanisms. This is from the USGS, where you can read more about them. Moment tensors are calculated differently, but their graphical representation is very similar to that of focal mechanisms.

    Here are some posts of mine for recent earthquakes in the Gorda plate.

  • 2013.12.17 M 4.1
  • 2014.03.10 M 6.8
  • 2014.03.11 M 6.8 aftershocks
  • 2014.03.13 M 5.2 (near the Gorda Ridge)
  • 2015.01.28 M 5.7 (Mendocino fault)


  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
  • Nelson, A.R., Kelsey, H.M., and Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone: Quaternary Research, doi:10.1016/j.yqres.2006.02.009, p. 354-365.
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.

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. This Mw = 6.3 earthquake occurred northwest of St. Helena, where Napoleon spent his last years of his life (and experienced a large earthquake which is known as Napoleon’s Earthquake on 1796.10.22).
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 compressional (blue 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

Sandwich Islands Earthquake… mmmm

We just had a M 5.8 earthquake in the volcanic arc region of the Scotia subduction zone. The moment tensor (currently) shows an extensional mechanism. I will post more on this as I am about to hit the road. Here is the USGS web page for this earthquake.
Here is the map that I just put together. I have a number of posts about earthquakes in this region. I will add these links later tonight.

Santa Cruz (western Pacific) Earthquake Swarm!

The past couple of days have brought us a series of large magnitude strike-slip earthquakes along a plate boundary between the Solomon Trench and the New Hebrides Trench. I posted about the first earthquake here and here. This swarm of earthquakes, along with the other recent seismic activity along this system, supports the hypothesis that this is a strike-slip plate boundary (nor a subduction zone as plotted on some maps).
Here is a map that I put together. I plot the epicenters of the earthquakes, along with the moment tensors for the three largest magnitude earthquakes. I also place a transparent focal mechanism over the swarm, showing the sense of motion for this plate boundary fault. Technically, transform plate boundaries are strike-slip (shear) plate boundary fault systems that connect spreading ridges. SO, I would like to call this a transform plate boundary fault system, but need to see if it really satisfies the definition that people use… I’ll get back to us on this…
I also note that these three largest earthquakes happen in a time order from east to west, unzipping the fault over three +- days. I label them in order (1, 2, 3) and place an orange arrow depicting this temporal relation). Very cool!

    Here are the USGS web pages for the three largest earthquakes in this series:

  • 2015.05.20 Mw 6.8
  • 2015.05.22 Mw 6.9
  • 2015.05.22 Mw 6.8

Here is an updated regional map with all of the epicenters from this swarm plotted. Below I link to an earlier map with more notations on it.

Earthquake in the Solomon Islands!

Following yesterday’s seismic swarm along the plate boundary that connects the Solomon Trench with the New Hebrides Trench, we have a M = 6.0 compressional earthquake in the Solomons. Today’s earthquake is in the region of the possibly subduction zone earthquakes from 1977 (April 20, 21) and 1984 (February 7). Given the M = 6.8 strike-slip earthquake yesterday was left-lateral (dextral), we would expect that this would increase stress upon the subduction zone fault (and synthetic faults) along the southern Solomon Trench. Here is my post about the M = 6.8 strike-slip earthquake.
UPDATE 4:45 PM PST: Magnitude has been updated to M = 5.7. Also, it is pretty shallow at 5.0 km! It is possible that the seismometer coverage is not very good, which could contribute to a poorly resolved hypocentral depth.
I have also plotted the epicenters of the New Britain Swarm (mainshock 2015.05.05 M = 7.5) and the Solomon Isles Swarm (mainshock 2015.05.07 M = 7.1). I include generic focal mechanism diagrams for these regions. Note how the New Britain swarm is along the subduction zone, so is in compressional strain, while the Solomon Isles swarm is in the Solomon Sea plate and in extensional strain.
I plot the moment tensors for some of these earthquakes. These moment tensor plots are a graphical depiction of a physical/mathematical based solution regarding the forces released during an earthquake based upon seismologic observations. Focal Mechanisms are also a representation of these stress releases, but are calculated differently. Both moment tensors and focal mechanisms are plotted with the same convention: black quadrants represent compression and the white quadrants represent tension. I include a legend that shows how moment tensors and focal mechanisms represent thrust-reverse/compressional, normal/extensional, and strike-slip/shear faults. Find out more about focal mechanisms at the USGS website here and find out more about moment tensors at the USGS website here.

In 2014 there was a strike-slip earthquake about 120 nautical miles to the west of the 2015.05.20 M = 6.8 earthquake. Here is my post about that earthquake. Here is a map showing the Modified Mercalli Intensity contours for the M 7.6 earthquake, as well as MMI contours for a swarm with a mainshock with a magnitude of M = 7.1 which is located to the northwest (Solomon Islands). Note how the M 7.1 earthquake swarm is east of the swarm that I label as the Solomon Isles Swarm (also with a mainshock earthquake of magnitude M = 7.1). The Solomon Isles swarm earthquakes have extensional moment tensor solutions, while the M 7.1 earthquake was compressional. These swarms are probably related! I discuss these earlier earthquakes more in this post.

Here is a map showing the slab contours (Hayes et al., 2012), 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. Historical seismicity of earthquakes that are magnitude greater or equal to M = 7.0 are plotted as grey circles. More on the historic seismicity of this region is found on a summary poster from the USGS (Benz et al., 2011).

Santa Cruz Islands Earthquake: Update #1

The USGS has presented their moment tensor solution for this M = 6.8 earthquake and it indeed appears to be a strike-slip type of an earthquake, supporting our interpretation that this is a transform plate boundary. Here was my first post about this earthquake. This is the USGS webpage for this earthquake.
Here is my first map showing the epicenter of the M = 6.8 earthquake as a red dot. I also highlight recent activity along the Solomon Isles (New Hebrides Trench) and the New Britain region (Solomon Sea plate), in addition to the Molucca Sea.

Here is a map showing the last month’s seismicity, with the M = 6.8 earthquake still as a red dot. I place a strike slip focal mechanism over the plate boundary to show how left lateral motion would produce such a focal mechanism. The black quadrants represent compression and the white quadrants represent tension. More can be found at the USGS site here. Note how the New Britain earthquakes are compressional and the Solomon Sea earthquakes are extensional. The New Britain earthquakes are most likely the result of slip on the subduction zone (a convergent plate boundary), while the Solomon Sea plate earthquakes are probably due to the bending of the downgoing plate (causing extension in the plate).

I posted a map that shows two earlier swarms here, the New Britain swarm and a Solomon Sea swarm. Here is one of the maps from that page:

There was also a swarm of activity in the Molluca Sea region in November of 2014. Here I posted about the mainshock, an compressional earthquake with magnitude M = 7.1. Here I post about some of the aftershocks, each matching east-west compression. This is a map showing the mainshock and these aftershocks:

To no surprise, this earthquake is not exposing many people to strong ground shaking (it is in the middle of the ocean). The nearest islands are 2 degrees longitude away (less than 120 nautical miles). Here is the V1 PAGER report. This is an estimate of damage to people and their belongings generated by a computer model.

Here is a map showing these Modified Mercalli Intensity scale contours:

In 2014 there was a strike-slip earthquake about 120 nautical miles to the west. Here is my post about that earthquake. Here is a map showing the Modified Mercalli Intensity contours for the M 7.6 earthquake, as well as MMI contours for a swarm with a mainshock with a magnitude of M = 7.1. Note how the M 7.1 earthquake swarm is east of the swarm that I label as the Solomon Isles Swarm. The Solomon Isles Swarm earthquakes have extensional moment tensor solutions, while the M 7.1 earthquake has compressional. These swarms are probably related! I discuss these earlier earthquakes more in this post.

This map shows the moment tensor for the M = 7.1 earthquake and the M 7.6 earthquake.