A magnitude 7.1 earthquake just occurred south of Mexico city. More information from the USGS is available on it here. What is unusual about this earthquake is its depth and the type of faulting. The earthquake occurred at ~50 km depth in a continental setting. Typically, within the interior of continents earthquakes do not occur at depths greater than ~10-20 km. Below that, it is so hot that rocks will flow and stretch instead of break. Also, this earthquake is extensional, which means the earth is being pulled apart, which is not common when subduction is occurring nearby offshore. So what happened? I think some previous work by researchers from Seoul National University and Caltech gives us some insight. To the right are two images from their paper published in 2013 in EPSL. The first is a map showing their seismic stations and a black line showing the location of their cross section (A-A'), which is shown the second figure. I've marked the approximate location of today's earthquake with a blue star. The second is their cross section of seismic velocity perturbations from the western edge of the continent to the Gulf of Mexico. This can be thought of as a CAT scan of the earth; the different velocities representing different materials and/or temperatures. I've added another blue star showing approximately where this earthquake occurred in the cross section. The black lines in this figure are their interpretation. The solid black lines are interpreted as the crust of the oceanic Cocos Plate subducting beneath Mexico. From these images it looks like the earthquake occurred in the subducting plate just where the subduction angle is steepening and the slab is bending. This bending places the top of the slab in extension is probably why the earthquake is extensional. The fact that the earthquake occurred at this depth is probably because subducting slabs are cold and are releasing water, both of which makes the earth break more easily. This is just my best guess of what happened here and future work will prove or disprove this idea.
Figures from: Kim, Y., Clayton, R. W., Asimow, P. D., & Jackson, J. M. (2013). Generation of talc in the mantle wedge and its role in subduction dynamics in central Mexico. Earth and Planetary Science Letters, 384, 81-87.
Late last night, southern Mexico was hit by an earthquake that occurred offshore near the border with Guatemala. According to the USGS, the earthquake was within the Cocos Plate and was at a depth of ~70 km. In this region, the Cocos Plate is subducting beneath the North American Plate. This was an unusual earthquake for this tectonic setting given its depth and the way the earth ruptured. Typically, earthquakes in subduction zones are caused by compression of two plates colliding and they usually occur at shallow depths. The seismic waves from this earthquake show that the earth was extending during the earthquake and that this earthquake happened at a relatively deep depth. This earthquake will keep seismologists busy for a while.
I've attached a couple of seismograms recording the North Korea nuclear test this past weekend. The top was recorded at the seismic station WUAZ, located just outside of Flagstaff, AZ. The bottom is a figure made by Andy Frassetto overlaying the 2016 and 2017 tests. Earthquake seismology plays an important role in understanding these tests and assessing the yield of the explosions. Further, these images demonstrate the sensitivity of seismic instruments in detecting these types of tests. The station in Flagstaff is ~6000 miles from North Korea. More information on the test and Andy's original figure are available here: ds.iris.edu/ds/nodes/dmc/specialevents/2017/09/03/2017-north-korean-nuclear-test/