Preliminary data suggest Sunday's 7.2-magnitude quake originated on the Laguna Salada fault, which stretches 43 miles along the U.S.-Mexico border. The last time it unleashed a similar-sized quake was in 1892. Since then, the fault has produced occasional magnitude-5 temblors.
In recent days, Baja California's wine-growing region west of the epicenter has been rattled by small quakes between magnitudes 3 and 4.
Whether they were foreshocks to the deadly magnitude-7.2 that struck 38 miles south of Mexicali is not yet known.
"It's such a chaotic system" of faults that needs more researching, said Erik Pounders, a geologist with the U.S. Geological Survey.
The Baja quake appeared to have ruptured about 30 miles of the fault, stopping at the border. Dozens of aftershocks were recorded on both sides of the border within hours of the quake with the largest registering 5.4.
"At this point, the aftershocks clearly form a nice tight cluster along that trend," said USGS seismologist Lucy Jones, adding that final confirmation will come from field inspection.
Sunday's quake occurred southwest of the San Andreas Fault at the boundary where the Pacific and American plates grind. Jones said quake's size means there's the possibility of strong aftershocks in the coming days.
"The chances are pretty good for a magnitude-5 plus aftershock," she said.
At least one man in Mexico was killed when his house collapsed and others were feared trapped in their homes. There were no reported deaths on the U.S. side, but older buildings in the border city of Calexico sustained structural damage.
Scientists are debating whether the Baja quake triggered smaller events around California far from the epicenter. Soon after the jolt, a magnitude-3 struck the Santa Monica Bay west of Los Angeles and a smaller one hit the Geysers region in Northern California, long known to be seismically active.
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