San Ramon experienced a quake swarm within the last 24 hours - by Wednesday morning, the small East Bay city had experienced 20 or so small earthquakes.
The largest ranked a magnitude of 3.0 reported about 8 a.m. Many were centered near the Crow Canyon Country Club. Five of the quakes were 2.0 or higher.
Senior US Geological Survey research geologist David Schwartz said the swarm is not all that unusual, noting that San Ramon Valley is at the "center of earthquake swarm activity" in the Bay Area. He recalled a swarm in 2003 of 120 earthquakes over 31 days in San Ramon, with the largest clocking in at a magnitude of 4.2.
The biggest swarm on record in the same general area, Schwartz noted, was in 1990 in Alamo. That's when 351 earthquakes struck over 42 days, with the largest recorded at a magnitude of 4.4.
Schwartz, who coincidentally lives nearby in Danville, was shaving in the bathroom when the largest quake recorded at a magnitude of 3.0 struck. "I thought at first a truck was passing by," he said.
As for why the swarms occur? "We just don't understand the structural geology of the swarms, why they turn on and shut off quickly," he said. "We just don't understand."