3.2 magnitude earthquake shakes near San Ramon

SAN RAMON, Calif. - A 3.2 magnitude earthquake struck near San Ramon Thursday morning. 

The quake struck just after 7:30 a.m. according to the U.S. Geological Survey.

The San Ramon Valley area has recorded 83 quakes since Nov. 9 wiin a tight one-mile radius.

According to USGS, the earthquakes have ranged from magnitude 0.6 to 3.8, with most too small to be felt.

Scientists say the pattern is a classic earthquake swarm, a burst of numerous small quakes over a short period, rather than a single large quake with aftershocks.

Centered on the Calaveras Fault zone

What we know:

Preliminary analysis indicates the activity is occurring along the Calaveras Fault, one of the most active branches of the greater San Andreas Fault system.

This segment runs directly through the East Bay, beneath San Ramon, Danville, Dublin, and Pleasanton.

The quakes recorded so far appear to be strike-slip events, meaning the two sides of the fault are sliding horizontally past each other. USGS data shows the earthquakes are occurring at depths of roughly 3 to 6 miles beneath the surface.

One of the larger quakes — the magnitude 3.8, struck at a depth of around 5.7 miles. These depths place the activity in the upper crust, shallow enough that residents may feel the shaking, but still within the normal depth range for the Calaveras Fault.

Scientists say nothing about the depth or pattern suggests a larger earthquake is imminent.

Region known for swarms

Dig deeper:

The San Ramon–Danville corridor has a long history of earthquake swarms.

• 2015 San Ramon Swarm: More than 400 small quakes over five weeks, one of the most active swarms in Bay Area history.
• 1990 Swarm: Dozens of small quakes in a similar location.
• Additional small swarms have been recorded throughout the 2000s and 2010s.

Experts say swarms are typical for this fault system, which tends to release stress through frequent, small adjustments deep underground.