SAN RAMON, Calif. -- Another set of earthquakes hit San Ramon Friday night, just days after two separate swarms on Monday and Tuesday.
Five earthquakes struck San Ramon Friday night, according to the U.S. Geological Survey. The first, 2.5, occurred at 7:41 pm and at 7:49, hit 4.0. Just four seconds later, he hit a 3.8, followed by a 3.8 at 7:56pm and a 3.1 at 7:57pm.
Residents across the East Bay, San Francisco, parts of the South Bay and the Peninsula reported feeling a magnitude 4.0 light, according to the US Geological Survey's "Did You Feel It" map.
Last month alone, San Ramon experienced nearly 150 earthquakes, its second highest number in nearly 50 years, according to the San Francisco Chronicle.
The city lies along the Calaveras Fault, and many small faults along the main fault trigger swarms of earthquakes. When fluids such as water or gas move through a complex network of cracks in small faults, it can trigger dozens of small earthquakes in quick succession.
"It is also possible that these small earthquakes are triggered by the movement of fluids across the Earth's crust, which is a natural process, but the numerous faults in the region may facilitate these micro-movements of fluids and smaller faults," said Anne-Marie Baltay, a research geophysicist at the USGS.
According to the USGS, there were similar clusters very close in 1970, 1976, 2002, 2003, 2015, and 2018.
The strongest earthquake in the Alamo was in 1990, when 177 earthquakes with a magnitude of 4.4 shook the region over a period of 42 days.
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