“A swarm” of several hundred earthquakes, the strongest measuring a 5.5 magnitude, were centered east of San Diego near California’s border with Mexico on Sunday afternoon but caused little damage, seismologists said.

Sunday’s shaking in Southern California’s Imperial Valley was the most activity recorded there since the 1970s, according to U.S. Geological Survey seismologist Lucy Jones.

Only minor damage had been reported, said Robert Graves, a USGS seismologist.

Graves told reporters that about 300 quakes had been recorded by Sunday evening. More than 30 had a magnitude over 3.5, and the biggest “are a little bit larger than what we have seen in these types of processes in the past.”

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