By Beatriz Cosenza

Edited by Angelique Rosa Marín & Jessica Pi

You’ve probably heard about earthquakes happening near the southern coast of Mexico. Why do so many earthquakes happen in this region? Blame plate tectonics! Mexico is on the North American plate and south of its territory, offshore, the Cocos plate moves towards it. When they meet, the Cocos plate gets pushed underneath the North American plate into the Earth’s mantle forming what we call a subduction zone. All along southern Mexico, the Cocos plate is subducted underneath the North American plate. To the west, where the Cocos plate ends, a small plate called Rivera also subducts under the North American plate. All of this forms the Mexico Subduction Zone (MSZ) (Figure 1, left).

The areas where the plates are in contact, pushing against each other, are stuck due to friction: we say they are locked. The amount of deformation and energy accumulating at the area (if you are into physics, think of it as elastic potential energy) depends on how strongly locked the plates are. When the accumulated stress surpasses the friction force, a rupture happens: the area of contact disengages, and the plates catch up with the motion deficit caused by their locking by slipping abruptly, using part of the accumulated energy. This produces seismic waves that travel in all directions through the interior of the Earth and reach the surface: This is an earthquake (see animation).

Several segments of the MSZ (Chiapas, Oaxaca, Guerrero, Michoacan) have had earthquakes fairly recently, but we haven’t heard much about the Jalisco-Colima segment at its southwestern end. In 1932, two very destructive earthquakes only fifteen days apart, with magnitudes of 8.2 and 7.8, ruptured a ~220 km long segment. In 1995 the magnitude 8.0 Colima-Jalisco earthquake occurred and, in 2003, we had the magnitude 7.4 Tecoman earthquake. These ruptured approximately half of the 1932 rupture (Figure 1, right). This means that there is an area that has been accumulating energy since 2003, another one since 1995, and the westernmost part, south of the state of Jalisco (with no earthquakes since 1932), and near all the touristic infrastructure has been accumulating energy for more than 90 years! 

Cosenza et al. (2022) estimated the locking of the subduction zone in that area. They studied the surface deformation in southwestern Mexico by measuring with GPS, for many years, the changes in position of several different points on land. They observed motion patterns that can be associated with the locking at the subduction zone (Figure 2, left). They found that the locking is strong and uniform south of Jalisco, and moderate-to-weak and heterogeneous south of Colima (Figure 2, right). By calculating the accumulated energy due to plate motion, their locking and the time since the last earthquake, they estimated that the zone that hasn’t had an earthquake since 1932 can produce one of magnitude 8.0 if it releases all the energy. They also estimated that the whole Jalisco-Colima area can potentially produce at least one magnitude ~7 per century.

So, if you live in southwestern Mexico, remember that earthquakes can happen any time and be very destructive, so you have to be prepared. Keep an emergency backpack ready to go, make a plan with your family and practice it, and follow the recommendations in case of earthquakes from civil protection agencies in your area. 

References

• B. Cosenza-Muralles, C. DeMets, B. Márquez-Azúa, O. Sánchez, J. Stock, E. Cabral-Cano, y R. McCaffrey. GPS-derived interseismic fault locking along the Jalisco–Colima segment of the Mexico subduction zone. Geophys. J. Int. (2022) 228, 2174–2197, https://doi.org/10.1093/gji/ggab435.

Extra material 

• GPS Measures Deformation in Subduction Zones: Ocean/continent