Another Aftershock in the Molucca Sea!

This earthquake happened really close to the M 7.1 from earlier (2014/11/15). Here is my page that has more information about the regional tectonic interpretations. Here is the USGS page for this M 6.8 earthquake.
Here is the updated map with moment tensors plotted for four earthquakes.

This map shows the modeled estimate of shaking intensity for this earthquake.

Here is a map from the USGS that shows pre-instrumental seismicity for this region. There was a large magnitude earthquake in this same region in the 19th century.

Earthquake in Central Honshu!

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.

Another Earthquake in the Molucca Sea

We just had another large earthquake in this region. Here is the USGS page for this M 6.5 earthquake. This earthquake hypocenter is much deeper than what we might expect for a subduction zone interface earthquake in this region. The M 7.1 earthquake from about a week ago was downdip of this earthquake (deeper along the subduction zone fault) than today’s earthquake. Here is my page on the M 7.1 earthquake.
This is a map of the region, showing the three largest magnitude earthquakes from the last couple of weeks.

Here is a generalized tectonic map showing how these plate boundaries are configured from here.

Here is a more detailed tectonic map with a cross section showing how these two opposing subduction zones are configured. This is from a Geological Society of America Special paper here.

Earthquake along the Halmahera Arc

There was a Mw 7.1 thrust earthquake along the Halmahera Arc (a subduction zone) in the Molucca Sea, Indonesia. Here is the USGS web page for this earthquake. There was a small amplitude tsunami observed in the region of the earthquake: 9 cm at Jailolo, Indonesia (see map below). There was a M 7.5 earthquake to the south of this 2014/11/15 earthquake, also a thrust earthquake here is the USGS page.
This is a map that shows the epicenters on a global scale map.

This map shows the epicenters on a regional scale map.

This is the regional map with Modified Mercalli [shaking] Intensity Contours.

Here is a map I put together that shows the moment tensors for the largest magnitude earthquake and the largest magnitude aftershock. The plate boundaries have relative plate motion vectors. This is a region where there are two opposing ocean-ocean subduction zones. I edited this map since the plate boundaries plotted by USGS are in the incorrect location (the thin lines).

Here is a generalized tectonic map showing how these plate boundaries are configured from here.

Here is a more detailed tectonic map with a cross section showing how these two opposing subduction zones are configured. This is from a Geological Society of America Special paper here.

This earthquake was not located too close to land (i.e. people), so the probability of damage to people and their infrastructure was low (as evidenced by the pager report shown here). This can be downloaded from the USGS web page for this earthquake.

While the above maps of shaking intensity (MMI) are generated using models of ground shaking (GMPEs), this map shows the shaking intensity based upon peoples’ direct observations.

This plot shows the attenuation of ground motion with distance from the epicenter. The Ground Motion Prediction Equation (GMPE) models are plotted in orange and green. The orange model is based upon ground motion attenuation relations for the western US. The green model is based on relations for the eastern US. The western US is mostly accreted terrain, so this model matches the observational data (plotted as dots) probably because the Halmahera region is also composed of accreted terrain (albeit different from the western US).

Lassen Earthquake Swarm

There is currently an ongoing earthquake swarm in the region immediately south of Mt. Lassen. The largest magnitude is a M 3.9 (USGS). The California Volcano Observatory (CalVO) is monitoring these earthquakes. The earthquakes do not appear to be related to volcanism, but most likely related to the fault systems that strike through that area. These faults are an extension of the Walker Lane fault system, which is a sister of the San Andreas plate boundary. The Walker Lane fault system is a right lateral (dextral) fault system that extends the east side of the Sierra Nevada Mountains. We do not currently know how this part of the plate boundary ends, if it feeds into Cascadia, etc. We have some ideas, but I will leave this a mystery as many of these ideas are unpublished and I do not want to scoop anyone. There are no USGS active faults mapped in this region, but there has been earthquake activity in the Lake Almanor area in the last year or so (see map below from 5/23/13). This region may be an extension of the Butt Creek fault zone (the red faults directly to the southeast of this swarm.
Here is a map of the epicenters in the region of Mt. Lassen.

The CAVO is monitoring these earthquakes, but they clearly state this swarm is related to plate tectonics in the region and not volcanism. Here is a plot of seismicity for this region.

Here is the Did You Feel It map for the M 3.9 earthquake.

The M 3.9 earthquake has a focal mechanism that shows NE-SW extension.

Here is a primer for those who would like to understand focal mechanisms better (from the USGS.

Here is a map I made about some of these Almanor earthquakes.

Earthquake Swarm in NW Nevada!

There has been an earthquake swarm in northwest Nevada. There was also a swarm in this region last August. The largest magnitude EQ so far is a magnitude 4.6. Here is a magnitude 4.0 EQ that initiated my interest in posting about this swarm. These epicenters plot along the Warner Valley and Guano valley fault zones.
This map shows the recent EQs in orange, yellow, and white, in increasing age. The grey circles are part of the August 2014 swarm. The faults are from the USGS active fault and fold database.

Here is the extensional moment tensor for the magnitude 4.6 EQ:

Here is the extensional moment tensor for the magnitude 4.0 EQ:

Here is the shake map for the M 4.6 EQ:

Here is the shake map for the M 4.0 EQ:

Here is the USGS 2% probability of exceedance in 50 years map of peak ground acceleration. One can observe that this region of Nevada has a low probability of ground acceleration.

I will post more on this swarm later today. check back!

Earthquake in the Fiji Region!

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