"Since Nov. 9 there have been 83 earthquakes in a roughly 1-mile radius around the town of San Ramon. The earthquakes, which geologists call a swarm, are small, ranging in size from .6 magnitude to 3.8. Six, including one Monday night at 10:47 pm were 3.0 or above - generally large enough for most people to feel. Residents have definitely noticed, leaving some to wonder if a bigger quake is on the way."
"Normally, earthquake swarms happen near volcanoes or geothermal fields. But the geology under the San Ramon Valley - which runs roughly from Walnut Creek to Dublin along Interstate 680 - is a complex mix of small faults, many of them without names, between the Calaveras Fault and Mount Diablo that combine to occasionally trigger the flurries of small quakes, scientists say."
"And although quake swarms can increase the chance of a larger quake, it's only by a very small amount, scientists said Tuesday. Most important, in none of the previous cases has a major earthquake been triggered. "These things turn on and turn off," said David Schwartz, a geologist and scientist emeritus with the U.S. Geological Survey who lives in Danville. "They typically have not led to larger earthquakes. But they do scare the heck out of people.""
Since Nov. 9 a swarm of 83 earthquakes occurred within roughly a one-mile radius around San Ramon, with magnitudes ranging from 0.6 to 3.8 and six events at or above magnitude 3.0. Several of the quakes were large enough for residents to feel and prompted public concern. The San Ramon Valley overlies a complex network of small, often unnamed faults between the Calaveras Fault and Mount Diablo that can produce intermittent flurries of small quakes. Five prior swarms have occurred since 1970, some lasting up to a month. Scientists say swarms slightly raise but do not typically trigger major earthquakes.
Read at The Mercury News
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