LOS ANGELES - To the crowd of baseball fans cheering at PETCO Park in San Diego, the jolt in the eighth inning came as a surprise.
But for scientists, Monday night's magnitude-5.7 earthquake was the kind of aftershock they expected from the deadly magnitude-7.2 Easter Sunday quake centered in Baja California two months earlier. Major shakers tend to produce aftershocks for months and sometimes years, scientists say.
"People in the border region should expect more aftershocks and earthquakes in general in future years," said seismologist Egill Hauksson of the California Institute of Technology in Pasadena. "The earthquake history of the region is very rich."
Monday's quake was the largest aftershock yet from the Easter shaker. Its epicenter was 85 miles east of San Diego, near the U.S.-Mexico border.
South of the border in Mexicali, 17 hospitals were evacuated.
More than 5,000 aftershocks have struck around the border since the April 4 Baja quake that killed two people in Mexico and caused $100 million in damage in California alone. A half-dozen of the aftershocks were larger than magnitude-5, but the vast majority were too weak to be felt.
"We are not surprised to see an aftershock of this magnitude 2 1/2 months after the main shock," said geophysicist Paul Earle of the U.S. Geological Survey's National Earthquake Information Center. A USGS report yesterday said the overall aftershock sequence after the 5.7 tremor was three or four times higher than a typical sequence in California.