Mexico city earthquake
Mario Vaszque | AFP | Getty Images
September 19, 2017: A magnitude 7.1 earthquake has struck the town of Raboso, Mexico, a small town located 65 miles southeast of Mexico City. This earthquake is substantially closer to the Mexican capital than the magnitude 8.1 earthquake that struck 10 days ago off the coast of Chiapas, Mexico. Preliminary reports indicate significantly more widespread damage to structures in Mexico City from the event today, due to its proximity.
The event today occurred on the 32nd anniversary of the devastating 1985 Mexico City earthquake that resulted in thousands of fatalities and widespread destruction. Mexico City is particularly vulnerable to earthquake damage because large portions of the city are built on lakebed sediments that are more prone to strong shaking than solid rock.
The seismic record shown here was recorded on the seismometer located on Carnegie’s Broad Branch Road campus. These seismometers are installed to record ground motion near sensitive equipment while nearby construction projects are ongoing, but they also serve to record ground motions from large events that occur anywhere on the planet.
Unlike the 1985 earthquake, this earthquake did not occur at a plate boundary. Most of the world's largest earthquakes (like the ones in Japan, Sumatra, Chile, and Alaska) happen where one plate is colliding with another plate. These "convergent" plate boundaries result in one plate being pushed down underneath the other plate. The plates do not slide smoothly past one another, though. They typically get "hung up" due to friction and the uneven surfaces on both plates. However, that doesn't mean the plates stop moving... no, there's much too much force and momentum driving them. Instead, the plate boundaries bend to accommodate the moving plates. This bending tends to cause the coastlines and the seafloor to bulge upwards. When these faults fail, both the coast and the seafloor drop as the plates snap past each other. This results in the double whammy of a lowered coastline and an incoming tsunami... on top of the significant shaking of the earthquake itself.
But this is not what happened on September 19, 2017. Instead, we have to look more closely at the plate that is being shoved down into the Earth's mantle. In most places, the plate that gets pushed down continues to sink at a relatively steep dip angle deep into the Earth's interior. These plates are much colder than the rock around them, and until they reach about 700 km depth, they produce earthquakes that occur entirely within that one sinking plate. These earthquakes are caused by a combination of pushing, pulling, and bending forces experienced by the plate as it sinks downward, together with the high temperatures and pressures it experiences.
In the case of the most recent Mexico City earthquake, the sinking plate in question is the "Cocos" plate. The Cocos plate at this latitude does not sink normally into the mantle. Instead, it only sinks to about 50 km depth - just far enough to get past the North American crust - and then it flattens out and moves horizontally for hundreds of miles before bending downward and sinking into the Earth. Normally, earthquakes that happen in sinking plates are either offshore, or too deep, to cause significant shaking in populated areas. With a flat slab, however, it's possible for relatively shallow slab earthquakes to be located directly beneath (or at least very close to) major population centers. That's what happened on September 19. See the figure below from the USGS website...
The big red dot is the location of the 19-September 2017 event. The black lines show the approximate location of the sinking Cocos plate. Notice how this red dot is right at the corner where the plate bends down. From studying the shape of the seismic waves that arrive at many stations, we can tell that this earthquake was caused by pulling forces - extension. That makes sense - think about the Cocos plate as a Snickers bar. If you bend the end of a Snickers bar downward, the thick chocolate at the top will pull apart and break. The same is true for the crust of the Cocos plate - it has to stretch out to accommodate this sharp bend.
Much of my research involves figuring out the exact shape of slabs like the Cocos plate that do not sink normally. There are other similar flat slabs in Chile, Peru, and Colombia, all of which I have spent time studying. My most recent focus has been on the Colombian flat slab. I'm currently working on a proposal to the National Science Foundation to deploy nearly 100 seismometers around the edge of the Colombian flat slab to learn more about it's geometry, seismicity, and history. For more information about my other projects, either contact me or check out my research page or publications.