Earthquake Swarm Rattles San Ramon For The Third Time This Week

SAN RAMON, CA — Yet another swarm of earthquakes hit San Ramon Friday night, just days after two separate swarms Monday and Tuesday.

Five earthquakes struck San Ramon Friday night, according to the U.S. Geological Survey. The first, a 2.5, took place at 7:41 p.m. At 7:49, a 4.0 struck. Just four seconds later, a 3.8 struck, followed by a 3.8 at 7:56 p.m. and a 3.1 at 7:57 p.m.

Residents across the East Bay, San Francisco, and parts of the South Bay and Peninsula reported feeling the 4.0 lightly, according to USGS's "Did You Feel It" map.

In the past month alone, San Ramon has seen nearly 150 earthquakes, its second highest number in nearly 50 years, according to the San Francisco Chronicle.

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.

“It is also possible that these smaller earthquakes pop off as the result of fluid moving up through the earth's crust, which is a normal process, but the many faults in the area may facilitate these micro-movements of fluid and smaller faults,” said USGS research geophysicist Annemarie Baltay.

According to the USGS, there have been similar clusters very close by in 1970, 1976, 2002, 2003, 2015, and 2018.

The strongest was in Alamo in 1990, when 177 earthquakes as strong as a 4.4 rattled the area over a period of 42 days.

SEE ALSO:

• Earthquake Swarm Rattles Bay Area Overnight. USGS Weighs Chances Of Bigger Ones To Come
  
• Morning Quake Rattles The East Bay
  
• Why Does San Ramon Get So Many Earthquakes?
  
• 5 Things Most People Don't Know About Earthquakes
  
• Are You Living on a Fault Line?
  
• Strong Earthquake Prompts Preparedness Reminders