The quakes were centered near Westmorland in the Imperial Valley, just southeast of the Salton Sea. That's roughly 180-200 miles southeast of Los Angeles and about 30 miles north of the Mexico border.
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By 8 p.m. the USGS had recorded at least 240 quakes, with the largest hitting a magnitude of 4.9 at 5:31 p.m.
Seismologist Lucy Jones said the quakes are happening in the Brawley Seismic Zone, a network of small faults that connect the San Andreas and Imperial faults.
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Historically, she said, the largest quake in that zone measured 5.8.
She said the zone is a common source of swarms and far enough from the San Andreas fault so that a quake closer to the Los Angeles area is not any more likely.
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