Los Angeles (CNN) -- "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.
"We are having a swarm," Jones said.