New Britain trench swarm

There have been a few aftershocks. I have outlines the region of the subduction zone that appears to have slipped in this March 2015 swarm. Here was my first post about the main shock and the tsunami notifications. I have also outlined the regions that slipped in 2010 and 2014. As I stated earlier, this 2015 swarm is in the same region as a swarm from 2000. Here is my post where I place the moment tensors from these three periods (2000, 2010, and 2014) in relation to this 2015 swarm. I also list the USGS web pages for all these earthquakes on that page.


In the above map, notice the Mw 5.9 earthquake. This earthquake (here is the USGS web page for this earthquake) is at a depth of ~196 km. This earthquake is at a depth that places it at or near the plate interface, the subduction zone fault. However, we may note that it has an extensional moment tensor. Thuse it is probably in the downgoing Solomon Sea plate (either from extension in the plate due to slab pull of due to bending of the plate). The map below shows the slab contours (based on seismicity, Hayes et al., 2012). Click on the map to see it at higher resolution.


As a review from my earlier posts, here is a map modified from Hamilton (1979). This shows the relative motion between the Solomon Sea plate and the South Bismarck plate. There are some right-lateral transform faults in the South Bismarck plate (the upper plate), but today’s moment tensor and hypocentral depth support the hypothesis that this was on the megathrust.


This is a cross section that goes along with the above map. This shows the configuration of the megathrust. I got both of the graphics from Oregon State University, CEOAS here.


Here is a primer for those who would like to understand focal mechanisms better (from the USGS). Normal earthquakes are extensional, reverse earthquakes are compressional, and strike-slip earthquakes are the result of shear.

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
• Hamilton, W., 1979, Tectonics of the Indonesian region: U.S. Geological Survey Prof. Paper 1078.