The area east of San Diego near California's border with Mexico was hit by "a swarm" of several hundred earthquakes on Sunday, with the strongest clocking in at a magnitude 5.5, but no major damage was reported.
Still, the shaking in the area known as the Imperial Valley was the most significant activity on record since the '70s, U.S. Geological Survey seismologist Lucy Johnson said.
Of the 300 quakes that were recorded by Sunday evening, more than 30 were magnitude 3.5 or greater.
Only minor damage had been reported, said Robert Graves, another 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."
Graves said the area is a "zone of transition" between the Imperial and San Andreas faults and tends to have more frequent earthquake activity than other parts of California. He said that previous swarms -- some of which were recorded as early as the 1930s -- weren't precursors to a bigger quake, but "that's not to say it won't happen."