David Bekaert is a PhD student based in the School of Earth and Environment at the University of Leeds. David’s research involves using space based remote sensing technologies to study ground motions from slow-slip earthquakes, particularly in Mexico. In today’s guest blog he relates this morning’s earthquake to the broader seismic threat faced by Mexico City.
The earth’s crust is segmented into different tectonic plates. Often large earthquakes occur on the plate boundaries where two plates move against each other. As plates do not move everywhere at the same velocity, some regions are stressed much more than others, i.e. build up more energy. This stress is released by deformation in the crust and earthquakes. The occurrence of earthquakes depends on many parameters like friction, temperature, seismic history, and many more, and for this reason cannot be predicted.
In southern Mexico the heavier oceanic Cocos plate dives beneath the lighter continental North America plate with a velocity between 5.6-6.1 cm/year , Figure 1. This is much higher than average plate velocities. On average plates move as fast as your fingernails grow, about 3.7 cm/year . In 1985, central Mexico (Michoacan) was struck by a magnitude 8 earthquake, killing 9500 people and causing 3-4 billion U.S. dollars worth of damage . After this, a lot of effort was put into designing new building regulations and preparation for future earthquakes.
Today a 7.2 magnitude earthquake (USGS) occurred at 9:27 am (local time) in Guerrero, a state in Southern Mexico, about 120 km west of the coastal city of Acapulco. Initial reports indicate some damages to buildings, electric infrastructure, and roads (Figure 2), but so far no casualties have been reported by media [4,5]. This morning’s earthquake occurred in a region where no earthquake has occurred since 1911, a so-called “seismic gap”. Scientists have estimated that an earthquake of magnitude 8.0-8.4 is needed in order to rupture (remove all stresses in) this seismic gap . And since the earthquake moment magnitude scale is logarithmic this requires an earthquake about 31 times stronger than the earthquake today.
The question is, whether this gap is fully loaded. Something special about the Guerrero gap is the occurrence of a new type of earthquake, which does not shake the Earth, referred to as slow slip events. In Guerrero, they have been observed using GPS about every 4-years, with the most recent event in 2009/2010. A new event is thus due soon. Unlike a regular earthquake that happens instantaneously, slow slip events last for days to months, or even up to a year, as is the case in southern Mexico. This steady sliding removes a lot of stress. In Guerrero, if you would release a slow slip event instantaneously, it would achieve an earthquake-like magnitude up to 7.4, a similar size to today’s earthquake!
While these slow slip events are not dangerous in themselves, they release energy and can change the stresses in the area, which potentially could trigger a devastating earthquake. The earthquake today occurred on the western extend of the Guerrero gap. The question for Mexico remains, how much energy have these slow slip events released in this seismic gap, enough such that no earthquake would occur?
 Singh, S. K., and F. Mortera (1991), Source Time Functions of Large Mexican Subduction Earthquakes, Morphology of the Benioff Zone, Age of the Plate, and Their Tectonic Implications, J. Geophys. Res., 96 (B13), 21,487–21,502.