Series of small earthquakes in Imperial Valley lightly shakes parts of San Diego County

A series of small earthquakes erupted Saturday on the Brawley Fault System in the Imperial Valley, causing mild shaking in parts of northern and eastern San Diego County, the U.S. Geological Survey said .

The earthquakes began at 3:58 p.m. when a 3.7 earthquake was recorded four miles south-southwest of Brawley, in an area southeast of the Salton Sea where the faults of Brawley and San Andreas.

This earthquake appears to have been a pre-shock to the 3.9 earthquake which occurred in the same area at 4:05 p.m. A 3.7 aftershock followed at 4:13 p.m. followed by a 3.5 earthquake at 4:24 p.m. Aftershocks are expected, but it is somewhat unusual to have ones close in size to the main shock.

Tiny swarms like this are quite common in Southern California and can sometimes last for several days. Scientists say they rarely lead to a much larger earthquake.