Monthly Archives: February 2015

another earthquake in the east Java / Timor region of Indonesia

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We had another earthquake in this region today. This is an interesting area of the world since this is a place where there is a transition between subduction to the west and collision to the east. The Island of Timor is interpreted to be part of Australia and has continental rocks that are found in Australia. While there is a large compressional plate boundary fault south of Timor, Timor was actually more part of Australia than it is part of the volcanic arc of Indonesia to the west (e.g. Java, Bali, etc.). Here is my post about the Mw 7.0 earthquake from a couple of days ago.
Here is a map showing the M = 5.0 earthquake as it relates to the Mw = 7.0 earthquake from a couple days ago. In the map, there is a large thrust fault that strikes east-west, north of the archipelago in this region. This is a back thrust (the Wetar Backthrust) as interpreted by Audley (2011) below.


Hall et al. (2012) provides a tectonic reconstruction for our enjoyment. Note how the Island of Timor is part of Australia back to 150 Ma.


Snyder et al. (1996) uses gravity data to interpret the plate boundary fault in this region.


Here is Snyder et als. (1996) interpretation of the structure of the plate boundary in this region.


Finally, here is the Audley (2011) cross section showing how the backthrust relates to the subduction zone beneath Timor.


This shows the Audley (1986) interpretation of the faulting in this region. One may see that the ISland of Timor is being popped up like an onion skin (compare with the cross section above^^^).


Here is a paper by my colleague Beau Whitney regarding the tectonics immediately to the south of Timor (Hengesh and Whitney, 2014). Here is their map showing the region of their work:


References:

  • Audley-Charles, M.G., 1986. Rates of Neogene and Quaternary tectonic movements in the Southern Banda Arc based on micropalaeontology in: Journal of fhe Geological Society, London, Vol. 143, 1986, pp. 161-175.
  • Audley-Charles, M.G., 2011. Tectonic post-collision processes in Timor, Hall, R., Cottam, M. A. &Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 241–266.
  • Hall, R., Audley-Charles, M.G., Banner, F.T., Hidayat, S., and Tobing, S.L., 1988. Basement rocks of the Halmahera region, eastern Indonesia: a Late Cretaceous-early Tertiary arc and fore-arc in: Journal of the Geological Society, London, Vol. 145, 1988, pp. 65-84.
  • Hangesh, J. and Whitney, B., 2014. Quaternary Reactivation of Australia’s Western Passive Margin: Inception of a New Plate Boundary? in: 5th International INQUA Meeting on Paleoseismology, Active Tectonics and Archeoseismology (PATA), 21-27 September 2014, Busan, Korea, 4 pp.
  • Snyder, D.B., Milsom, J., and Prasetyo, H., 1996. Geophysical evidence for local indentor tectonics in the Banda arc east of Timor in Hall, R. & Blundell, D. (eds), 1996, Tectonic Evolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 61-73.

Indonesia Strikes Again

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A deep earthquake has occurred probably within the downgoing Indo-Australia plate. The moment tensor shows oblique extension (part strike-slip, part normal). The depth aligns with the depth of the plate given the Hayes et al. (2012) slab models (which ends immediately adjacent to this earthquake, probably due to the more complicated double collision/subduction zone here).


Here is the same map with historic earthquakes plotted as well.


Randy Web has a blog about the tectonics of this region here.
Here is his map showing where he interprets the continental crust to be (which matches what I know of it, based on the literature I have read).


Here is another view of the local tectonics from Darman (2012).


References

  • 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.
  • Hayes, G.P., and Wald, D.J., 2009. Developing framework to constrain the geometry of the seismic rupture plane of subduction interface a priori – a probabilistic approach, Geophys. Jour. Int., 176, 951-964
  • Hayes, G.P., Wald, D.J., and Keranen, K., 2009. Advancing techniques to constrain the geometry of the seismic rupture plane on subduction interfaces a priori – higher order functional fits, Geochem. Geophys. Geosyst., 10, Q09006, doi:10.1029/2009GC002633.

another northeast Japan earthquake in the Tohoku-Oki earthquake region

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We have had another small aftershock. Here is the USGS web page for this earthquake. This time it is in a different region than the swarm of the past couple of weeks.
There are several report updates, listed here:

Here is a map showing today’s earthquake with a couple of the larger earthquakes also plotted. See my prior posts about these.
The USGS websites for these earlier earthquakes are here:



This map shows the USGS (Hayes et al., 2012) slab depth contours. These contours represent the depth to the top of the downgoing plate (i.e. the subduction zone fault interface location). The depth is currently set at ~56 km. This is a little deep for the slab model, but the slab model is constrained by seismicity (which has considerable depth variation, so cannot be considered precise).

Gorda plate earthquake 2015.02.24

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Where else would we get a focal mechanism or moment tensor for an earthquake of such a small magnitude! We are so lucky. This is the USGS web page for the earthquake. We get Gorda plate earthquakes frequently and most of them (if not all) are probably aligned with the northeast striking strike-slip faults.
Here is a map showing the epicenter with the USGS focal mechanism.


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).


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 soem 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.


References:

  • 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.

Tohoku-Oki aftershocks are in a region of low slip during main events in 2011

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Here is what I was getting at in my last post. I have taken the Ammon et al. (2011) slip model and placed it into a real world reference frame (rubbersheeted it in ArcGIS). Then I exported it as a kml file and placed it into google Earth where I can also plot the recent seismicity.
There are several report updates, listed here:

Here are the USGS web pages for the thrree largest magnitude earthquakes I plot in the map below:

Here is a map showing the three largest magnitude earthquakes in this recent seismic swarm. Check out my previous post here to see other slip models, estimates of stress change due to the 2011 March 11 Tohoku-Oki earthquake, and how these relate to historic slip models.


Here is the Ammon et al. (2011) figure.


These hypocenters continue to align with the probable location of the subduction zone fault based on the Hayes slab model.


References:

Tohoku-Oki Aftershocks Continue

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The aftershocks continue offshore northeast Japan. The recent swarm appear to align along a northwesterly strike (trend). Here is the USGS web page for the earthquake from today. This is the USGS web page for the largest magnitude earthquakes in this recent swarm. Here is my post from earlier in the week about this most recent swarm. Here are one and two follow up posts about this most recent swarm.
There are several report updates, listed here:

One would wonder if there were some northwest striking structure to which these earthquakes were responding. The Cascadia subduction zone, as others like Sumatra, has strike slip faults in the downgoing plate that have sisters in the overlying accretionary prism. Due to the seismogenic coupling along the fault, when the downgoing plate deforms along these faults, the accretionary prism also deforms. Chris Goldfinger has published about these faults in Cascadia.
This is probably not the case here in Japan. These earthquakes have compressional moment tensors, not strike-slip solutions. It is more likely (?) that this swarm is simply the subduction zone fault slipping in an across-strike direction (these epicenters are up-dip and down-dip of each other, i.e. they are all in a narrow along-strike segment). I have some figures below that I will use to discuss this. But first, here is a map showing this recent swarm of seismic activity.


At first I thought that this swarm was in a location that did not slip in 2011 since the slip models did not cover this segment of the fault. However, looking at the aftershock maps, there were aftershocks in this along-strike segment of the fault.

Slip Models and Aftershock Maps

Gusman et al., 2013

This one from Kosuga et al, 2011 show the aftershocks in 2011 as they relate to some historic earthquakes and their aftershocks. This most recent swarm appears to plot just to the south of the 1994 swarm.

This shows various mainshock and aftershock slip patches from Nishimura et al., 2011:

This Hirose et al. (2011) map shows focal mechanisms for select fore- and after-shocks, as well as the main shock. There were some large magnitude afteshocks in the segment that has swarmed in the past ~1 week.

This Toda et al. (2011) map also shows focal mechanisms for related earthquakes, along with their slip model.

Lets take a look at some of the earthquake slip models for the 2011 March 11 Tohoku-Oki M 9.1 subduction zone earthquake. There are dozens of them. Note where the slip was in 2011 and where this recent swarm is located. There are different sources of information that have been inverted for these source slip models. Some only use one type of data, others use multiple types of data (seismologic data, GPS data, tsunami wave form data, etc.). The best inversions include as many sources as possible. I posted these slip models first when I was reporting on some aftershocks on 2015/02/20 here.
Earthquake Slip Inversions
Ammon et a., 2011 show their slip inversion (GPS) along with the epicenters and their source time function plot.

Fujitsu et al., 2011, inverted from tsunami wave forms.

Gusman et al., 2013, inverted from tsunami wave forms.

Iinuma et al., 2012 shows the seafloor displacement used in their slip model.

Koper et al., 2011 using seismic and GPS data:

Lay et al., 2011 b, showing their seafloor deformation using seismic data.

Lay et al., 2011 a, using seismic, GPS, and tsunami wave inversions.

Lay et al., 2011 c, joint inversions os seismic and GPS data:

Lee et al., 2011:

This shows how the Lee et al. (2011) slip model incorporates the three main sub event slip patches through time (with the source time function plotted as well).

Newman et al., 2011, based on a GPS inversion:

Satake et al., 2013, based on tsunami wave form inversion:

Simons et al., 2011 seafloor deformation for their tsunami wave inversions:

Simons et al., 2011 slip inversion from high frequency seismic data:

Wang et al., 2013 showing inversions from six different sources:

Wang et al., 2013 and their joint inversion:

Here is one of the more comprehensive figures from Yagi et al., 2011. They plot their slip model (seismic data), historic slip patches, source time functions for the main sub-events, and the seismic waves, outlining the 3 main sub-events.

This is Yagi et al.’s (2012) final slip distribution, the source time function for two patches, and the spectra of the slip rate.

This is from Yamazaki et al., 2011 using GPS and tsunami wave inversions.

This is from Yamazaki et al., 2011 showing the sea floor deformation used for their inversion.

Yue and Lay, 2013 show their joint slip model (GPS):

Shao et al., 2011. Slip models prepared by inverting teleseismic body and surface waves, followed by their figure caption as a blockquote.

Comparison of surface projections of slip model I, II, III by joint inverting teleseismic body and surface waves. Yellow line highlights the 5-m slip contour. Red star indicates the epicenter location and red dots are the aftershocks within first 6 days from JMA catalog. White arrow shows the Pacific plate motion relative to the North America plate (Demets et al., 1994). (a) Model I. (b) Model II. (c) Model III. (d) Comparison of moment rate functions of UCSB Model I, II, and III.

Lets now compare the 2011 earthquake with historic earthquake slip regions. Lets also look at the regional slip deficit and how this recent swarm may fit into the bigger picture of this part of this subduction zone.
Slip Compared to historic earthquakes and pre- and post-seismic slip, etc.
Ito et al., 2011 2011 slip compared to historic earthquake slip patches.

Ikuta et al., 2012, 2011 slip compared to historic slip estimates:

This Ikuta et al. (2012) plots shows the slip deficit for this part of the subduction zone. Basically, this is a way of viewing how much plate convergence might be expected to contribute to earthquake slip over time. In this case, we see how the smaller earthquakes took up some of the slip adjacent to the 2011 slip patch (think about where today’s swarm took place compared to the region that slipped in 2011).

What about how the 2011 earthquake changed the stress on the fault in regions adjacent to the 2011 earthquake?
Here are some plots showing the coulomb stress changes due to the 2011 earthquake.
Basically, this shows which locations on the fault where we might expect higher likelihoods of future earthquake slip.
Lay et al., 2011 b:

Lay et al., 2011 c, note how the segment to the north of the 2011 slip region experiences an increased stress:

Terakawa et al., 2013, also note how the segment to the north of the 2011 slip region experiences an increased stress:

Toda et al., 2011 shows stress changes on certain receiver faults:

Yagi and Fukahata, 2011 shows the stress drop calculated from their slip distribution.

This is a series of seismicity forecast maps from Nanjo et al. (2012; another way of looking at how the stress changed following the 2011 earthquake).

Here is a cool plot showing Uchida et al.’s (2011) view of where the asperities are in this part of the subduction zone. There are two definitions for asperity. (1) the regions of higher slip and (2) the regions of the fault that store strain (like sea mounts?). They are related, but defined differently (most people conflate the two). This figure refers to type 2. How does this most recent swarm fit into this view of subduction zone faults?

Here is the Yomogida et al. (2011) view of the segmentation of the subduction zone in 2011 Japan, 1964 Alaska, and 2004 Sumatra. Note how this recent swarm is in the northernmost segment (areas of lowest slip in 2011).

This Orzawa et al. (2011) map is exciting because it shows the slip distribution for several earthquakes in this region… The 2011 in the south and the 2003 in the north (along with the 1994 slip in the center). Where was this most recent swarm?

    References:

  • Ammon et al., 2011. A rupture model of the 2011 off the Pacific coast of the Tohoku Earthquake in Earth Planets Space, v. 63, p. 693-696.
  • Fujitsu et al., 2011
  • Gusman et al., 2012. Source model of the great 2011 Tohoku earthquake estimated from tsunami waveforms and crustal deformation data in Earth and Planetary Science Letters, v. 341-344, p. 234-242.
  • Hirose et al., 2011. Outline of the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) Seismicity: foreshocks, mainshock, aftershocks, and induced activity in Earth Planets Space, v. 63, p. 655-658
  • Iinuma et al., 2012. Coseismic slip distribution of the 2011 off the Pacific Coast of Tohoku Earthquake (M9.0) refined by means of seafloor geodetic data in Journal of Geophysical Research, v. 117, DOI: 10.1029/2012JB009186
  • Ikuta et al., 2012. A small persistent locked area associated with the 2011 Mw9.0 Tohoku-Oki earthquake, deduced from GPS data in Journal of Geophysical Research, v. 117, DOI: 10.1029/2012JB009335
  • Ito et al., 2011. Slip distribution of the 2011 off the Pacific coast of Tohoku Earthquake inferred from geodetic data in Earth Planets Space, v. 63, p. 627-630
  • Koper et al., 2011. Frequency-dependent rupture process of the 2011 Mw 9.0 Tohoku Earthquake: Comparison of short-period P wave back projection images and broadband seismic rupture models in Earth Planets Space, v. 63, p. 599-602.
  • Kosuga et al, 2011. Seismic activity around the northern neighbor of the 2011 off the Pacific coast of Tohoku Earthquake with emphasis on a potentially large aftershock in the area in Earth Planets Space, v. 63, p. 719-723.
  • Lay et al., 2011 a. The 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake: Comparison of deep-water tsunami signals with finite-fault rupture model predictions in Earth Planets Space, v. 63, p. 797-801.
  • Lay et al., 2011 b. Possible large near-trench slip during the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake in Earth Planets Space, v. 63, p. 687-692.
  • Lay et al., 2011 c. Outer trench-slope faulting and the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake in Earth Planets Space, v. 63, p. 713-718.
  • Lee et al., 2011. Evidence of large scale repeating slip during the 2011 Tohoku‐Oki earthquake in Geophysical Research Letters, v. 38, DOI: 10.1029/2011GL049580.
  • Newman et al., 2011. Hidden depths in Nature, v. 474, p. 441-443.
  • Nishimura et al., 2011. The 2011 off the Pacific coast of Tohoku Earthquake and its aftershocks observed by GEONET in Earth Planets Space, v. 63, p. 631-636.
  • Orzawa et al., 2011. Coseismic and postseismic slip of the 2011 magnitude-9 Tohoku-Oki earthquake in Nature, v. 000, p. 1-4.
  • Satake et al., 2013. Time and Space Distribution of Coseismic Slip of the 2011 Tohoku Earthquake as Inferred from Tsunami Waveform Data in Bulletin of the Seismological Society of America, v. 1032, p. 1473-1492.
  • Shao et al., 2011. Focal mechanism and slip history of the 2011 Mw 9.1 off the Pacific coast of Tohoku Earthquake, constrained with teleseismic body and surface waves in Earth Planets Space, v. 63, p. 559-564.
  • Simons et al., 2011. The 2011 Magnitude 9.0 Tohoku-Oki Earthquake: Mosaicking the Megathrust from Seconds to Centuries in Science, v. 332, p. 1421-1425.
  • Terakawa et al., 2013. Changes in seismic activity following the 2011 Tohoku-oki earthquake: Effects of pore fluid pressure in Earth and Planetary Science Letters, v. 365, p. 17-24.
  • Toda et al., 2011. Using the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake to test the Coulomb stress triggering hypothesis and to calculate faults brought closer to failure in Earth Planets Space, v. 63, p. 725-730.
  • Uchida and Matsuzawa, 2011. Coupling coefficient, hierarchical structure, and earthquake cycle for the source area of the 2011 off the Pacific coast of Tohoku earthquake inferred from small repeating earthquake data in Earth Planets Space, v. 63, p. 675-679.
  • Wang et al., 2013. The 2011 Mw 9.0 Tohoku Earthquake: Comparison of GPS and Strong-Motion Data in Bulletin of the Seismological Society of America, v. 103, p. 1336-1347.
  • Yagi and Fukahata, 2011. Rupture process of the 2011 Tohoku‐oki earthquake and absolute elastic strain release in Geophysical Research Letters, v. 38, DOI: 10.1029/2011GL048701
  • Yamazaki et al., 2011. Modeling near‐field tsunami observations to improve finite‐fault slip models for the 11 March 2011 Tohoku earthquake in Geophysical Research Letter,s v. 38, DOI: 10.1029/2011GL049130
  • Yomogida et al., 2011. Along-dip segmentation of the 2011 off the Pacific coast of Tohoku Earthquake and comparison with other megathrust earthquakes in Earth Planets Space, v. 63, p. 697-701.
  • Yue and Lay, 2013. Source Rupture Models for the Mw 9.0 2011 Tohoku Earthquake from Joint Inversions of High-Rate Geodetic and Seismic Data in Bulletin of the Seismological Society of America, v. 103, p. 1242-1255.

Japan earthquake may be aftershock to 2011 Tohoku-Oki earthquake

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The USGS depth has been updated to 23.3 km. Now we may interpret this to be an interface earthquake. Here is my first post about this earthquake and here is a post about some aftershocks.
There are several report updates, listed here:

Here is a map showing the depth contours from Hayes et al. (2012). The depth contours are at 20 km intervals, the eastern most contour is 20 km. The USGS hypocentral depth is 23.3 km, which is between 20 and 40, consistent with this Hayes et al. (2012) slab model. The slab contours (the depth to the top of the down going Pacific plate, i.e. the subduction zone fault interface) are located using seismicity. Seismicity has considerable spatial variability, so the slab depths are not too well constrained in this way. Therefore it is reasonable to interpret this earthquake as an interface (subduction zone fault) earthquake (rather than a slab or crustal earthquake, not on the subduction zone fault).


There is an article online that quotes Japan Meteorological Agency (JMA) seismologist Yasuhiro Yoshida. Yoshida tells reporters that “This quake is an aftershock of the 2011 quake that hit the Tohoku region”
There are several different ways to estimate how long after an earthquake we might expect aftershocks. They each rely on unknowns, so include considerable uncertainty. The two main “laws” are Omori’s Law and the Gutenberg–Richter [relation] Law. The b-value controls the decay of aftershocks. In most cases, we do not have enough seismologic data to have a good estimate for the b-value of any given fault (One needs many many earthquake cycles for a fault to know this constant. Paleoseismology is perhaps the single most important way to evaluate the b-value, since our seismologic data are only about a century old, far too young to capture the seismologic variation along any given fault.).
Here is a plot of the aftershock decay (G-R Law) using different b-values, from this source.


Here is a list of foreshocks and aftershocks associated with the 2011 Tohoku-Oki M 9.0 earthquake. This is the fourth largest earthquake ever recorded with modern seismometers. It has been almost 1.5 years since we had an aftershock near the magnitude of today’s earthquake.
References:

  • 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.

NE Japan aftershocks continue offshore the Sanriku coast

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There continue to be aftershocks in the M 4-5 range. These are aftershocks and triggered earthquakes (if they are generally on different faults) from the Mw 6.7 earthquake. Here is my page on this mainshock and here is the USGS page for the mainshock. Here is the USGS page for a triggered earthquake (unlikely on the same fault), a M 5.8 earthquake.
There are several report updates, listed here:

Here is a map showing the most recent assemblage of these aftershocks.


This M 5.8 earthquake is located closer to land, but it is a much smaller earthquake. These two factors combine to have similar shaking intensity at the coast. Each step in magnitude (e.g. from M = 5 to M = 6) the energy released (strain) is 33 times more. So a M = 6 earthquake releases 33 times more energy than a M = 6.
Here is a map for the larger earthquake.


Here is a map for the smaller earthquake.


Here is the attenuation plot for the larger earthquake. Note how these reports match well with the western US attenuation (GMPE) model.


Here is the attenuation plot for the smaller earthquake.


Further demonstrating how magnitude and distance relate between these two different earthquakes are the pager model results. These are modeled estimates of damage to people and their belongings (roads, houses, etc.).
This is the pager report for the larger earthquake.


This is the pager plot for the smaller earthquake. Note how the probability estimates for damage are similar.

Sandwich Earthquakes in Scotia, mmmmm

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We have had a swarm of earthquakes near the Sandwich Islands along the Scotia subduction zone, an oceanic subduction zone. Here is the USGS web page for this earthquake.
Here is a map showing the regional tectonic setting with relative plate motion arrows.


Here is a map that shows the global tectonic setting.


There were a number of large earthquakes along the transform plate boundaries in the last year or so, west of today’s earthquake swarm.

This shows the slab contours in this region.


This shows the historic earthquakes in this region.


This map shows an interpretation of the regional tectonics from here.


Also, here is a short explanation (from the USGS) of the graphical solutions for focal mechanisms. While moment tensors (shown in my interpretation figure above) and focal mechanisms are determined differently, their graphic representation of the deformation from earthquakes is the same.

Earthquake offshore of the Sanriku Coast

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We just had a good sized shallow compressional earthquake in the northern part of the 2011 Tohoku-Oki earthquake slip region. The USGS web page lists the magnitude at Mw 6.7. Here is a great summary of the tectonics of this region. Based on the magnitude, this earthquake is not likely to generate a tsunami that would affect the continental US. The initial magnitude was 6.9.
There are several report updates, listed here:

Here is a map that shows the earthquake as it relates to Japan and the Japan trench. I have labeled the different tectonic plates and the relative plate motions. While the plate boundaries between the Pacific-Okhotsk, Pacific/Philippine sea, Philippine Sea/Eurasia plates are well known, the plate boundary between the Okhotsk/Eurasia plates is lesser well known nor understood.


This map shows the region along with the historic epicenters. I have plotted a moment tensor from the 2001 earthquake, as well as the moment tensor from today’s earthquake.


This map shows the slab contours (the depths to the top of the downgoing Pacific plate). Today’s earthquake has an hypocentral depth of 10 km. In the epicentral location, the slab is probably at about 75 km depth, so today’s earthquake is clearly in the accretionary prism of the upper plate. UPDATE! The depth has been updated to 23.3 km (the 10 km was an automatic depth). So, now we may interpret this as an interface earthquake (on the subduction zone fault). See my updated post here.


Also, here is a short explanation (from the USGS) of the graphical solutions for focal mechanisms. While moment tensors (shown in my interpretation figure above) and focal mechanisms are determined differently, their graphic representation of the deformation from earthquakes is the same.