Mexico's magnitude 7.2 earthquake Thursday was the 10th major temblor on that country's west coast since 1932, part of a series that presents a significant danger to central Mexico.
The region is near a highly active subduction zone--a boundary between great tectonic plates that float slowly over the Earth's subterranean mantle. From the Gulf of California south to Panama, the comparatively small Cocos plate is diving under the massive North American plate, quite close to the coast. The action of these 50-mile-wide "slabs" generates earthquakes and, further inland, volcanic activity.
Many of the earthquakes of the past 63 years, from Mexico's Sinaloa state south to the isthmus between Oaxaca and Chiapas states, have been deeper and stronger than Thursday's quake. Their waves have reverberated sharply into the populous cities of the interior, causing damage far from their epicenters.
This was particularly the case with the 8.1-magnitude quake of Sept. 19, 1985, that killed at least 10,000 people in Mexico City.
Two years ago, Hiroo Kanamori, director of the Caltech Seismological Laboratory, and other scientists wrote in the Bulletin of the Seismological Society of America that another large quake would probably occur in the Guerrero "seismic gap," which includes the resort city of Acapulco, where no major quakes had occurred since 1911.
A seismic gap is an area that lies between the sites of past large quakes and that has not ruptured recently. Kanamori's article warned that "the Guerrero gap is likely to break in the next decade or so" and that, if it did, "some components" of shaking in Mexico City "could be amplified by a factor of 2 to 3" over the 1985 earthquake.
Thursday's earthquake was south of this seismic gap and did not affect those prospects.
Two Caltech scientists, Thomas Heaton and Egill Hauksson, explained Friday why the Mexican subduction zone, under the Pacific Ocean, seems to be the source of more frequent damaging earthquakes than the Cascadia subduction zone off the coast of Washington, Oregon and Northern California.
Heaton noted that the Mexican zone is much closer to the coast than most of the Cascadia zone, meaning that the quakes shake the Mexican interior more severely.
He added that the Mexican zone seems to produce frequent quakes in the magnitude 7 or 8 range, while the Cascadia zone may shake less frequently but does so more powerfully. Some studies have indicated that the Cascadia zone may produce a catastrophic magnitude-9 quake every few hundred years.