Earthquake Swarm Rattles Bay Area

SAN RAMON, CA — Several small earthquakes rattled the San Francisco Bay Area Friday night. The swarm of quakes struck in the San Ramon area of Contra Costa County.

The first quake, a 2.5-magnitude quake, struck at 7:41 p.m. about 3 miles southeast of San Ramon, according to the U.S. Geological Survey. The largest was a 4.0-magnitude quake, which struck at 7:56 p.m. in the same area. They were accompanied by temblors of 2.9 at 7:49 p.m., 3.8 at 7:49 p.m., 3.1 at 7:57 p.m., and a 2.9 at 8:10 p.m.

The quakes were measured at depths of between 4 and 9 kilometers.

Residents reported feeling the 4.0-magnitude quake across the East Bay, San Francisco, and parts of the Peninsula and South Bay, according to the agency's "Did You Feel It?" map.

There were no immediate reports of damage and injuries.

The latest swarm comes after more than 150 earthquakes have rattled a fault in San Ramon over the past month, according to the U.S. Geological Survey.

• Also see: Earthquake Swarm Rattles Bay Area Overnight. USGS Weighs Chances Of Bigger Ones To Come

The city sits along the Calaveras Fault, and several micro-faults along the main fault result in swarms of quakes. When fluids like water or gas move through a complex web of cracks in tiny faults, this can trigger many dozens of small quakes in quick succession.