A whopping 408 earthquakes have hit San Ramon, California, in the past two weeks, including 11 in one 24-hour stretch.
This record-breaking earthquake swarm is nothing to fear, however, according to the U.S. Geological Survey (USGS). Periods of tectonic unrest are common in the area and probably don’t presage a larger quake, the USGS said.
Most of the quakes affecting this Bay Area city are tiny. The one that kicked off the swarm at 8:50 a.m. Pacific Time on Oct. 13 was a mere magnitude 0.8. People don’t usually feel earthquakes until they reach a magnitude of 2.5; quakes between magnitude 2.5 and 5.4 can be felt but rarely cause much damage. Most of the quakes occur at depths of 5 to 6 miles (8 to 9.7 kilometers). [The 10 Biggest Earthquakes in History]
A handful of the San Ramon quakes have rattled locals, including a magnitude-3.6 quake that hit on Oct. 19 and a magnitude-3.1 quake recorded on Oct. 27, both causing weak to light shaking in the East Bay (the eastern part of the San Francisco Bay area). Between 11:30 a.m. PT on Oct. 22 and 11:30 a.m. on Oct. 23, 11 quakes hit the area. Five were between magnitudes of 2.0 and 2.9, and one was a magnitude 3.2.
The quakes are hitting on the north end of the , according to the USGS. Swarms of small earthquakes are common in this fault system. In fact, in 1990, a swarm of 351 earthquakes struck over a 42-day period. The largest quake in that swarm was a magnitude 4.2, a level unlikely to cause serious damage.
The most recent earthquake swarm on this fault system was a 30-day stretch in 2003, over which 120 temblors hit a few miles south of the current quakes. Again, the largest quake was a magnitude 4.2.
“Based on the characteristics of the historical earthquake swarms in this region, the current swarm may continue for several more weeks and is unlikely to be a foreshock to a larger quake,” USGS officials said. “East Bay residents may feel shaking from more earthquakes in the magnitude 3-4 range until the swarm abates.”
The chance of a serious quake (of magnitude 6.7 or above) hitting the San Ramon area is a mere 8 percent over the next 30 years, the agency estimates. In comparison, the chance of a quake that size occurring somewhere in the Bay Area over the next three decades is 72 percent.
source: livescience.com by Stephanie Pappas