To the crowd of baseball fans cheering at San Diego's Petco Park, 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."
A similar-sized earthquake in 1992 — the magnitude-7.3 Landers quake that rattled the Mojave Desert northeast of Los Angeles — has unleashed about 10,000 aftershocks in a sequence that continues today.
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.
There were no serious injuries or damage, but the game between the Toronto Blue Jays and San Diego Padres was briefly interrupted until the shaking stopped. South of the border in Mexicali, 17 hospitals were evacuated.
Aftershocks are smaller quakes that follow a main shock and usually occur in the same area. After a big quake, the Earth continually readjusts itself in the form of aftershocks.
More than 5,000 aftershocks have struck around the border since the April 4 Baja California 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 2½ months after the main shock," said geophysicist Paul Earle of the U.S. Geological Survey's National Earthquake Information Center.
A USGS report Tuesday said the overall aftershock sequence after the magnitude-5.7 tremor was three or four times higher than a typical sequence in California.
Over the next seven days, there is a 26 percent chance of one or more quakes of magnitude-5 or greater, the report said, noting that the probabilities would change if the sequence slowed to a more normal rate.