September 2017 saw a pair of weird earthquakes in Mexico. A magnitude 8.2 on September 8 just offshore the state of Chiapas was followed by a magnitude 7.1 on September 19—this time much closer to Mexico City, causing considerable destruction there. While the two earthquakes were not connected, they were the same type of earthquake, which is unusual in the region.
Mexico’s western coast is a tectonic plate boundary, where the Pacific plate collides with and dives beneath the continent. That means that earthquakes along that boundary are typically the result of compressive force that squeezes rock to slide up the plane of the fault. Both September earthquakes were the result of stretching, however, which is almost as much of a head-scratcher as finding that part of your car’s engine was pulled apart during a head-on crash.
The explanation here is that as the oceanic plate disappears beneath the continent, it sinks downward into the mantle. First of all, the bending of the plate downward causes stretching, just as the skin over your elbow has to stretch. Second, the sinking plate pulls downward as it “hangs” from the part of the plate that is still up at the surface—another stretching force. Occasional earthquakes within this bending, sinking plate can reflect that stretching.