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:
http://topex.ucsd.edu/sandwell/publications/119.pdf
A more detailed description of the gravity model can be found at:
http://topex.ucsd.edu/sandwell/publications/122_JGR_gravity.pdf


DATA SOURCES – source of ocean data:
http://topex.ucsd.edu/WWW_html/mar_grav.html
source of land data:
http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/anomalies_dov.html .
GRAPHICS – Generic Mapping Tools (GMT) was used to generate the colored contour maps for direct input into Google Earth
http://gmt.soest.hawaii.edu/

COMMENTS – David T. Sandwell, dsandwell@ucsd.edu.

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.

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.