Earthquake Swarm Rattles Bay Area Overnight. USGS Weighs Chances Of Bigger Ones To Come

SAN RAMON, CA —Nearly 150 earthquakes have rattled a fault in San Ramon over the past month, including a magnitude 3.1 tremor that struck overnight, according to the U.S. Geological Survey.

San Ramon sits directly atop the Calaveras Fault, helping explain these frequent earthquake swarms. As fluids move through a complex web of fractures beneath the city, numerous small quakes are triggered, creating the activity reported above.

Earthquake experts define a “swarm” as a series of small-magnitude earthquakes occurring over a period of time.

U.C. Berkeley's Seismology Lab says there have been 24 small earthquakes in San Ramon in the past week, but six times as many since November — about 150 earthquakes since early November, according to Robert de Groot with the USGS.

"There have been other swarms in the area as recent as 2015. There were something like 644 earthquakes," de Groot told reporters. "None of these events resulted in a bigger earthquake - something we consider to be damaging. There's really about a 5 percent chance that this would lead to something bigger," de Groot said.

"It's pretty scary when people feel earthquakes on a regular basis like this," said Richard Allen, Director of the U.C. Berkeley Seismology Lab, according to reports. "They are of small magnitude, but they come with a strong jolt which people are feeling in San Ramon."

Tuesday's trembler jolted San Ramon residents.

"Earthquake Central here at The Tiki - three shakers today, each with epicenters in San Ramon! Pictures flying, scared canine, little sleep," one resident said.

One woman, Ana Gomez, said, "3.0 I saw our Xmas tree move !"

"Yep. Startled me awake just like the one at 5:53 this morning. I’m ready for them to stop already," another neighbor said.

Residents worry that the series of tremors is a warning sign of a much larger quake to come.
The Calaveras Fault last produced a notable earthquake in October 2007, the 5.4-magnitude Alum Rock earthquake, which ruptured to the south, according to the USGS.

Historically, the southern half of the central segment of the Calaveras Fault has been the most seismically active segment of the fault.

It produced the 6.2 Morgan Hill earthquake in 1984. The 5.9 Coyote Lake earthquake in 1979 ruptured slightly to the south of these other earthquakes. But USGS researchers expect the fault is not capable of an earthquake having a magnitude much larger than the 1984 Morgan Hill trembler.

The chances of anything much higher in the next 30 years were estimated at 11 percent by the 2003 Working Group for California Earthquake Probability.

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