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.
"We did not see any indication from the measurements in March that anything was imminent. But, without the year-to-year comparisons, we probably would not have [seen it]."
Some residents of the worst-hit area, though, did measure Monday's quake against the "big one" that hit six decades ago.
Valdez, the seismology center director, called the 1932 tremor the most powerful quake in modern Mexican history.
In the state capital of Colima, businessman Mario Gonzalez said the moment the rumbling ended, hundreds of residents ran to check the city's main cathedral, which was badly damaged during the 1932 quake.
Times staff writer Kenneth Reich in Los Angeles contributed to this report.
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The Mexican Consulate is providing information about those living or vacationing in the coastal areas affected by Monday's quake; (213) 351-6800.