In analyzing the seismological impact of Monday's tremor, U.S. and Mexican scientists said it was the latest in a series of "subduction zone" earthquakes on or near the Pacific coast. Carlos Valdez, director of the National Seismology Center at the National Autonomous University of Mexico (UNAM), said the quake was caused when two relatively small tectonic plates--the Rivera Plate and the Cocos Plate--shifted underneath the larger North American Plate.
American scientists at the National Earthquake Information Center in Golden, Colo., who measured the temblor at 7.6--or twice the power of the 1992 Landers quake in Southern California--also pinpointed it in the region where a much more powerful tremor devastated the area in 1932.
In March, U.S. and Mexican seismologists at Caltech, UC Berkeley, the University of Wisconsin and UNAM collaborated in installing 15 monitoring stations in the area to study earth movements between Manzanillo and Guadalajara, the Jalisco state capital.
"One of the reasons we are selecting this area is that it was close to the big 1932 quake," said Joanne Stock, associate professor of geology and geophysics at Caltech. "The area northwest of Manzanillo has been a problematic zone, alternately locking and slipping.