Over 300 Earthquakes Shake Bay Area: Continuous Seismic Activity Raises Concerns

Over 300 earthquakes have struck the San Ramon area since November. The sequence includes two recent notable quakes, measuring 4.0 and 3.9 in magnitude on Friday and Saturday respectively.

These tremors form a pattern known as a swarm, characterized by numerous small shocks clustered in a localized region without a clear mainshock. Sarah Minson, a geophysicist with the U.S. Geological Survey, explained the earthquakes stem from tiny, complex faults filled with pressurized fluids, not from major fault lines.

This activity is not new to San Ramon; since 1970, at least six similar earthquake swarms have been recorded, including one in 2015. That year’s event involved nearly 90 quakes above magnitude 2 within two months, compared to 71 such quakes in the current swarm.

Experts emphasize that while the shaking can be unsettling for residents, these swarm quakes have not historically triggered significant damage or a larger earthquake. Minson said these small quakes “probably have no way of jumping onto one” of the major faults nearby, such as the Calaveras Fault.

Despite the current swarm, the Bay Area remains at risk for a major earthquake, often referred to as “the big one.” A 2014 scientific outlook estimates a 72% probability that an earthquake of magnitude 6.7 or higher will strike somewhere in the region before 2043.

The Hayward Fault poses the greatest risk and runs about 75 miles through many densely populated cities, including Fremont, Hayward, Oakland, and Berkeley. Roland Burgmann, a UC Berkeley seismologist, noted the fault typically ruptures every 150 years, and its last major quake was over 150 years ago.

The Calaveras Fault, near the San Ramon swarm location, holds the second-highest risk for a large quake in the Bay Area. It extends through Danville, Pleasanton, and San Jose, with its last magnitude 6-plus event occurring in 1984.

Authorities advise residents to prepare for earthquakes by assembling emergency kits and staying informed through resources such as the ShakeAlert app. These measures remain crucial in this seismically active region prone to unpredictable shaking.