This morning (my time) there was a large earthquake in central Honshu. Here is the USGS page for this earthquake. The earthquake hypocentral depth is shallow, giving rise to the likelihood of damage to infrastructure and people.
Here is a map of the region, showing the epicenter in orange, near Nangano. This earthquake occurred along the Itoigawa-Shizuoka Tectonic Line, a convergent plate boundary between the Eurasian/Amurian plate and the North America/Okhotsk plates. This earthquake is in the region where this fault zone flips vergence (in the south it dips to the southwest and to the north it dips to the northeast. This is where the Niigata-Kobe fold and thrust belt meets the mobile belt of the east margin of the Japan Sea.
This is the moment tensor, which is interesting in that it is not evidently parallel to the tectonic boundary of this region. The main tectonic boundary is complicated here and changes strike and also incorporates some strike slip faulting.
Here is an illustration showing the subduction zones and how they meet near Honshu. This image is from here.
This map shows the current tectonic configuration of this region, along with some inherited features from the tectonic past (e.g. green lines). This is from NUMO’s report: “Evaluating Site Suitability for a HLW Repository (Scientific Background and Practical Application of NUMO’s Siting Factors), NUMO-TR-04-04.”
Also from the NUMO report, this shows the Niigata-Kobe fold and thrust belt.
Here is the shaking intensity map as modeled using ground motion prediction equations (attenuation models).
Compare the above map with this one, which is generated by surveys from real people, not models.
Here is the attenuation plot, showing how the shaking intensity drops off with distance from the earthquake. Note how the real data (dots) match with the attenuation curve that was developed using seismicity in California, rather than the curve using central and eastern USA seismicity. Think about why you think this is.
Given the shallow depth for this earthquake, we can see that the PAGER estimate of damage is high.
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.
Here is a regional scale map showing the plate configuration.
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.”
Interestingly, deep focus earthquakes take up ~66% of the deep earthquakes globally. From this paper, we can see that the slab contour may change strike in the region of yesterday’s earthquake.
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.
Here is the USGS Open File poster for the region (Benz et al., 2010). Hypocenters are plotted as cross sections to show the geometry of the subducting slabs.
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