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.