There was a large earthquake in the Fiji region yesterday (Saturday my time). The epicenter is far from the major convergent plate boundary. Here is the USGS page for this earthquake.
Here is a map at the global scale. The epicenter is marked by a red circle, just South of Samoa and East of Fiji. This earthquake is related to the subduction zone associated with the Tonga Trench. The moment tensor shows an east-northeast striking compressional solution, due to principal axis compression in the n-nw/s-se direction. Prior to looking at the moment tensor, I was expecting to see an extensional solution at this depth. The cross section of seismicity is sourced from earthquakes designated by the purple line.
This is a local scaled map showing the complexity of the spreading ridges and transform faults to the east of this earthquake swarm. The Modified Mercali Intensity Contour 3.5 is plotted as a light blue circle. Epicenters are plotted by color in relation to depth.
Here is the regional map showing the slab contours. The depth of this earthquake (434 km) is close to, but above, the slab depth (500 km). If one looks at the cross section of historic seismicity, it appears that the slab is possibly bending upwards. Perhaps there is some compression in the upper plate here, causing the compressional moment tensor.
Here is another view of the slab, generated using P-wave tomography. Doug Weins discusses his work in this region. “Red and blue colors denote slow and fast velocities, respectively, and the velocity perturbation scale is shown at the bottom.”
Richards et al., 2011 also show bends in the downgoing slab. There is some controversy about the configuration of the slab in this region. They show a detached slab just above the main port (more Star Wars), above the main slab.
The New Hebrides subduction zone dips to the east and turns into a transform fault just west of yesterdays earthquake. This map shows the profile for the above cross section from Richards et al. (2102)
This figure shows Richards et al. Figure 4, that displays their interpretation of how the plates came to be configured here. The Australia plate detached and collided with the Pacific slab about 4 million years ago.
As one might expect, an earthquake at this depth, given this magnitude, would not generate strong ground motions at the surface. The pager, an estimate of human and infrastructural losses, reflects this low likelihood of damage.
Here is a primer about Focal Mechanisms from the USGS.
- 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.
- Benz, H.M., Herman, Matthew, Tarr, A.C., Furlong, K.P., Hayes, G.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011, Seismicity of the Earth 1900–2010 eastern margin of the Australia plate: U.S. Geological Survey Open-File Report 2010–1083-I, scale 1:8,000,000.
- Richards, S., Holm., R., Barber, G., 2011. When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region, Geology, v. 39, pp. 787-790.
- Yu, W. and Wen, L., 2012. Deep-Focus Repeating Earthquakes in the Tonga–Fiji Subduction Zone, BSSA, v. 102, no. 4, pp. 1829-1849