Vancouver Isle M 6.6 Earthquake

Interesting little swarm here, on the coast of northwestern Vancouver Island. The convergence of these plates results in the Cascadia subduction zone (CSZ), that extends from Cape Mendocino in the South to at least Haida Gwaii in the North (there was a small subduction zone earthquake swarm there in 2013).
Here is a map that shows the CSZ (plotted as yellow), along with the MMI USGS ShakeMap for the M 6.6 earthquake.

This map shows some historic earthquake epicenters. Note how they align with some plate boundaries (e.g. the Blanco fracture zone, the transform plate boundary plotted as a west-northwest oriented blue line) and in some deformation zones (e.g. the Mendocino deformation zone that extends into the Gorda plate in the south).

The depth of the magnitude M 6.6 earthquake seems to align roughly with where we would expect the subduction zone fault. Today’s magnitude M 6.6 earthquake has a strike slip moment tensor.

Here is a regional view of the shaking intensity from the M 6.6 earthquake.

This map shows the results of the USGS Did You Feel It Program (that was initiated because of the work of Bob McPherson and Lori Dengler).

This shows the attenuation of shaking intensity with distance from the earthquake. Ground Motion Prediction Equations (attenuation relations) have been developed using thousands of ground motion measurements from earthquakes. As time passes, we get more earthquakes and we get more records. With the larger sized databases, the relations between shaking intensity and distance can be perfected for different fault settings, different site settings, and different earthquake magnitudes. Each of these factors can contribute to how the shaking from any given earthquakes is felt.

The estimated losses for this M 6.6 Vancouver Island earthquake are fairly low. Note the Green Designation.

This regional map shows today’s M 6.6 swarm as it relates to the Haida Gwaii earthquake along the Queen Charlotte plate boundary system to the northwest of Vancouver Island.

Here is the moment tensor from the M 7.7 subduction zone earthquake in Haida Gwaii on 10/28/2012

Here is a map that shows the M 7.7 earthquake swarm using a global scale.

This is a map of the Haida Gwaii swarm. Note how the main shock is plotted east of the mapped fault. One may also observe an accretionary prism that is probably related to subduction along this plate boundary. The fault line should really be mapped to the west of where it is drawn here. This fault system has alternately been interpreted as a strike-slip (or transform) plate boundary. If we consider the obliquity of subduction, the strike-slip faulting (aka Queen Charlotte – Fairweather fault system) is possibly a sliver fault (like the Sumatra fault). Also, note that some of the smaller aftershocks appear to align on north-northeast oriented patterns. Perhaps these are northeast striking strike slip faults (or normal faults formed at the spreading ridge, reactivated with strike slip motion) triggered by the subduction zone earthquake.

    The M 7.7 earthquake generated a tsunami. Here is a map showing some model estimates of trans Pacific tsunami heights.

  • Here are two plots of the tsunami record as measured in Crescent City.
  • This first plot shows the modeled wave height compared to the measured wave height.

  • This plot shows the measurement of the tsunami plotted on top of the tidal water surface elevation.

The USGS put together this poster to help explain their interpretations of the M 7.7 Haida Gwaii earthquake swarm.

Another triggered earthquake in the Solomon Islands

This is a very interesting part of the world, where subduction and transform faults interact with each other. The New Britain trench is linked to the New Hebrides trench with a plate boundary fault. Based on plate motions, this could also be a transform fault. Perhaps this series of earthquakes will help resolve these different interpretations (much like the Queen Charlotte fault zone north of Vancouver Island, which is an imperfect analogy). Here is the USGS web page for this M 7.4 earthquake.
Here is a map showing the latest triggered earthquake (not an aftershock). This map shows shaking intensity with Modified Mercalli Intensity contours (look at the legend).

Here is the moment tensor for this large magnitude earthquake, showing a thrust mechanism.

This map shows the moment tensors for the M 7.4 triggered earthquake and the M 7.6 earthquake, probably triggered due to changes in stress on the fault as a result of the M 7.1 earthquake further north.

This map shows the M 7.1 and M 7.6 earthquakes and their moment tensors.

This is the same map, but at a smaller scale and larger spatial extent.

Here is a primer for those interested in looking at how the change in orientation of principal stress can affect faulting on different fault orientations. This is from a pub prepared by the World Stress Map Project.

This is an animation from IRIS that describes well the complicated tectonics in this region, where transform and subduction zones meet. Those in the southern and northern Cascadia subduction zones see analogues in their back doors. Thanks to Rick Wilson for reminding me of this animation! This animation explains so much. The map below is a link to a 10 MB m4 video file. The video may or may not open in your browser. You may want to view the online youtube version.

Solomon M 7.6 aftershocks

The aftershocks continue to light up the earthquake fault that is participating in the part of the three pronged swarm in this region, I mentioned earlier. Two deep swarm regions of compressional earthquakes in the north, with this strike slip swarm in the south. The earthquake magnitudes for the northernmost swarm peak out around M 5.0 and are pretty deep at ~95 km!
This map shows all three swarms with shaking intensity contours for the two mainshocks (M 7.1 thrust and M 7.6 strike slip).

Here is an updated aftershock map.

This map shows where we might place the fault if it is oriented like the moment tensor is oriented.

Here is a little speculation… There are a few aftershocks that appear to align with the conjugate shear orientation (the second possible fault plane solution permitted with the moment tensor). Randomness is an equally valid interpretation for the spatial pattern of these aftershocks that may align with a possible fault conjugate to the mainshock fault. If these aftershocks are actually on a conjugate fault, then they would not be aftershocks, but triggered earthquakes.

Here is a map that shows the age of the oceanic crust in this region. The age contours (black lines) show the relative orientation of the spreading ridges from where the crust originated. I use a kmz file from here.

This is a larger scaled map that reveals hoe complicated the structure of the crust is in this region.

Here is a great gravity anomaly map for this region. Gravity lows are in blue and gravity highs are in red. Here is some background on this gravity anomaly map from UCSD.
SATELLITE DERIVED GRAVITY – Colors and 20 mGal contours on this overlay represent variations in the pull of gravity associated with variations in mass inside the earth or beneath the ocean. Normal gravity is 9.82 m/s**2. These variations mapped into color have a range that is 10,000 times smaller. Over the ocean the variations in gravity are measured by a radar altimeter in orbit. The radar measures variations in the height of the ocean surface relative to an ideal ellipsoidal shape. The similarity between the ocean anomalies and the Google Earth seafloor base layer is not an accident. In areas where no depth soundings are available, the seafloor depth is a scaled version of the gravity anomaly with a scale factor that varies with a number of parameters including sediment thickness and mean ocean depth. An overview of gravity and topography can be found at:
A more detailed description of the gravity model can be found at:

DATA SOURCES – source of ocean data:
source of land data: .
GRAPHICS – Generic Mapping Tools (GMT) was used to generate the colored contour maps for direct input into Google Earth

COMMENTS – David T. Sandwell,

Solomon aftershocks are lining up!

This is an update to some material I posted earlier. The possible orietation of the M 7.6 earthquake may be revealing itself by the spatial pattern of aftershocks.
This map shows some aftershocks.

I placed the moment tensor here also. Then I drew a line that is oriented with a strike aligned to the moment tensor, that is the red line in this second map.

These earthquakes, in the Solomons

Here is the latest in the story this April 2014. Another large magnitude earthquake has occurred in locations associated with the New Britain trench in the Solomon Islands. Here is the USGS website for this earthquake. There is some background information for the regional tectonics on their page.
This is a regional map showing today’s M 7.6 strike slip earthquake. The colored contours are computer model estimates of ground shaking based on the earthquake’s magnitude and location. The epicenter for this M 7.6 earthquake is a large orange dot in the center of the intensity contours. The shaking intensity scale is called the Modified Mercalli Shaking Intensity scale and the USGS legend for that scale is in the upper left hand corner of this map.

This shows the Mww moment tensor for this strike slip earthquake. The earthquake probably ruptured a fault that was oriented either north-northeast or west-northwest.

I plot the USGS subduction zone fault slab contours in this region. Today’s earthquake hypocentral depth is currently set at ~29 km, just north of the trench axis. This earthquake is probably related to the other earthquake swarms to the northwest. They were reverse/thrust earthquakes and this one is strike-slip. I will look at some papers to see what the coulomb stress changes might be expected in this scenario (if there are any analogues).

This is a fascinating plot of the modeled shaking intensity for these two large magnitude earthquakes.

Here is the PAGER loss estimate poster. The probability for fatalities is low, but not non-existent.

Here is a map where I placed blue dots scaled for magnitude overlain upon the USGS summary poster for this region. I also placed the moment tensor for each of these earthquakes. The 4/11 Mw 7.1 earthquake was at a depth of ~54 km, while the 4/12 earthquake was at a depth of ~36 km. The USGS also has a tectonic summary poster for the New Hebrides trench to the south. Which orientation do you think that todays earthquake fault is oriented upon?

Perhaps this earthquake is on a reactivated strike slip fault in the oceanic crust of the downgoing slab. Looking at the bathymetry along the Woodlark Basin, there are north-south striking transform faults. However, the downgoing slab where the M 7.6 earthquake is located was sourced elsewhere, further to the south. The source of the downgoing slab probably looked similar to the Woodlark Basin at some point.
Another possibility is that there is some sort of a forearc sliver fault in the downgoing slab. These are found where plate convergence is not perpendicular to subduction zone faults (“oblique convergence”). Strain and Slip Partitioning processes result in earthquakes accommodating the maximum and minimum orientations of strain with strike slip faults accommodating the minimum strain axis and thrust or reverse faults accommodating the maximum compressional strain. An excellent example of this is the Sumatra fault, interpreted as a forearc sliver fault that accommodates the strike-slip portion of strain exerted due to subduction of the India-Australia plate beneath the Eurasia plate. I need to do more work to find analogues of forearc slivers in downgoing slabs. I might be out in left field about this.
Here is a map that shows plate motion vectors. Note how the plate motions of the Pacific plate and the Solomon Sea plate are not perpendicular to the subduction zone fault.

Here is a primer for the orientations of principle stress in different tectonic regimes. This is from the World Stress Map Project.

Here is a map that is zoomed into these two earthquakes.

Anderson, E.M., 1951. The dynamics of faulting and dyke formation with application to Britain, 2nd ed., Edinburgh, Oliver and Boyd.
Zoback, M.L., 1992. First- and second-order patterns of stress in the lithosphere: The World Stress Map project. J. Geophys. Res., 97, 11,703-11,728.

Where is waldo?

This earthquake swarm is pretty interesting. We clearly have lots to learn about the tectonics of this region. One of the biggest mysteries yet to be solved is what fault the mainshock occurred on. The M 7.1, reported about early this morning, is still placed at ~50 km depth. The problem is that the subduction zone fault at this location is mapped at about 30 km. For some reason not evident to me, the USGS states that this earthquake is on the subduction zone fault in this region. The USGS fails to explain why these earthquakes are completely inconsistent with their fault model. All but a couple of the aftershocks also plot well beneath the subduction zone fault. Some of the aftershocks appear to align along a northeast striking line, possibly a strike-slip fault in the downgoing slab. However, the largest aftershock (a M 6.5) also has a reverse mechanisms (so it is not strike slip). These aftershocks probably just appear to line up for a reason unrelated to a strike slip fault.
Because this earthquake swarm is well below the subduction zone fault, my interpretation of this swarm is that it is occurring within the downgoing slab. Another alternative is that the slab modeled by Hayes is incorrect, by about 20 km. Unfortunately, the aftershocks do not support this second alternative, since they are closer to the trench and remain at depths much greater than the slab modeled by Hayes. There is one M 4.9 that has an epicentral location plotted right at the trench at a hypocentral depth of 47 km. This is not on the subduction zone fault since the subduction zone fault must be between 0 and a few kms at this location.
Here is a map showing the mainshock and larger aftershocks (< M 2.5). The subduction zone fault is represented by the red-orange-yellow contour lines. This fault surface is modeled by using hypocentral depths for earthquakes. There could be a second large fault that is deeper than the one modeled by Hayes.

This is the figure for the slab mapped by Hayes.

This map shows the mainshock and aftershocks > M 2.5 magnitude.

Here is a moment tensor for the M 6.5 aftershock, showing a reverse or thrust earthquake solution.

This map shows the shaking intensity (Modified Mercalli Intensity) with color representing intensity. This shaking intensity is for the M 6.5 aftershock, which was most certainly felt by someone or something on the island.

Here is a solution! The earthquake was not on the subduction zone fault but a splay fault. Below are a series of maps that show a cross section showing the hypocentral depths along a profile just to the northwest of today’s EQ swarm.
Here is a map from the USGS Open File Report on the tectonics of this region. The cross section is for the rectangular region marked D-D’. Note that this region is to the northwest of Bougainville Island. Today’s M 7.1 is just west of the label “1975.”

Here is the D-D’ profile. Remind yourself that the subduction zone fault may change along strike.

Here is the seismic hazard for this region, also from the USGS open file report.

Panguna, Papua New Guinea: subduction zone earthquake

This looks to have shaken people up, just by looking at the intensity map below. These are generated automatically and take some assumptions that simplify the results (so the real shaking is probably not what the shake map intensity maps show). In this region of the Solomon Sea, there is a subduction zone fault that dives to the north beneath Bouganville Island, forming the New Britain trench. The Solomon or Woodlark sub plate is subducting to the north, beneath the Bismark sub-plate, as shown in a map from Greg Corbett.
There probably was a local tsunami, but the magnitude and location are too distant to pose a tsunami hazard in northern California (especially with the configuration of the islands in this region). In 1998, ~1,500 km to the west, there was a landslide triggered by an earthquake that generated a large tsunami.
Here is the USGS website for this earthquake.
This is an early moment tensor, which shows a pure compressional mechanism.

This shows the earthquake as it relates to the regional plate boundaries, islands, and Australia. Shaking intensity uses the Modified Mercalli Intensity (MMI) scale.

This shows some historic earthquakes as grey circles. Note how they align with the plate boundaries.

Here is a regional view of the islands, the epicenter, and the shaking intensity:

This regional scale map shows the subduction zone fault depth contours as modeled by Dr. Gavin Hayes, from the USGS. Hayes and his colleagues used earthquake hypocentral depths to constrain the position of the subduction zone fault. The hypocenter is listed at 50 km deep, which places it well beneath the known location for the main subduction zone fault here, but there may be some complications with the plate boundaries here. The megathrust is only ~30 km in this epicentral location.

This is the Hayes map for this region.

Here is a great map that was posted to a blog. The blogger did not give credit to the person who made the map, but i found it used in this presentation prepared by IRIS (they gave proper credit).

This is the PAGER earthquake shaking and loss estimate page for this earthquake. This earthquake gets a “green” alert. The estimated fatalities and economic losses are low, but not zero. Here is a web page with some information about how these estimates are made.

This is a map from Australian National University showing GPS rates in the region, along with the major plate boundaries.

This map shows the regional tectonic setting in more detail.

This map shows how McCue modeled seismic hazard for this region in 1999. Click on the map for a pdf of his paper.

This map has some of the islands and water bodies labeled.

This map shows the mainshock as a large orange circle and the first major aftershock as a red circle. Here is the USGS page for the M 6.7 aftershock. There was a swarm of earthquakes to the northwest of today’s earthquake and they also have deep hypocenters, deeper than we would expect given the fault geometry modeled by Hayes at the USGS.

Here is the moment tensor from Potsdam GEOFON Extended Virtual Network :
GFZ Event gfz2014hbgl
14/04/11 07:07:23.79
Solomon Islands
Epicenter: -6.61 155.02
MW 7.1
Depth 60 No. of sta: 50
Moment Tensor; Scale 10**19 Nm
Mrr= 4.58 Mtt=-2.65
Mpp=-1.92 Mrt= 0.95
Mrp=-0.89 Mtp= 2.47
Principal axes:
T Val= 4.75 Plg=82 Azm= 46
N 0.21 1 311
P -4.96 8 221
Best Double Couple:Mo=4.9*10**19
NP1:Strike=131 Dip=53 Slip= 91
NP2: 310 37 89
——########## #######——-
——########## T ########——
——-######### #########—–

SCSN animation of the seismic waves from the Puente Hills Thrust fault

The Southern California Seismic Network published this animation. Robert Graves, with the URS corporation, created the simulation. The San Diego Super Computer Center also helped. They are all given proper credit in the animation.
The animation shows the simulated seismic waves propagating through the earth, with ground velocity designated by color. At first I thought this was a new visualization from the recent La Habra earthquake, but it is from earlier.
Here is the 115 MB avi animation (it will open into a new tab):

northern Chile M 8.2 earthquake series animations

As I mentioned in my earlier post, there were some foreshocks and have been aftershocks, related to this M 8.2 earthquake on the subduction zone fault offshore of northern Chile. This story may not yet be over… The aftershocks are located on a section of the fault to the south of the mainshock and all the foreshocks. We would not be surprised if other regions of the fault were to be triggered. However, this likelihood decays with every day. The increased stress to the adjacent fault segments is short lived and not very large, compared to the stress budget of faults. In other words, faults manage themselves in the range of millions of Pascals, but the increased stress from adjacent earthquakes is measured in thousands of Pascals. Increased stress may trigger earthquakes, but those faults need to be “ready to go” in order to be triggered.
Here I used the USGS hypocenters to create some animations of these earthquakes through time. The first one shows the earthquake epicenters in colors representing the age (orange = past day, yellow = past week, so the most recent aftershocks are all in orange. The largest aftershock, an earthquake of magnitude M 7.6, is plotted nearest the highway 16 label. Click on the map and the video will either open in a new tab, or be ready to download it. These are avi video files ~15 mb in size.

Here is the animation with color designating depth. As I mentioned in my page on the M 8.2 mainshock, these earthquakes are mostly aligned with the depths of the subduction zone fault as modeled by Dr. Gavin Hayes with the USGS (based on hypocentral depths of seismicity).