DANVILLE, California - A cluster of small quakes struck near Danville Friday with the largest one reaching a magnitude of 3.6 in the early afternoon, according to the U.S. Geological Survey.
At 12:21 p.m., there was a 2.8 magnitude, and two minutes earlier, there was a 3.6, the USGS reported. Before that, there were several early morning quakes from 2:30 to 5:30 a.m. ranging from quakes with magnitudes of 2.9 to 3.5.
Greg Sawdey was in downtown Danville and said he felt a "serious jolt." KTVU anchor Dave Clark felt it ,too. "It felt like a rough elevator ride," he said.
For the early-morning cluster, BART engineers quickly checked the tracks in the early morning hours and while they cleared them, the agency said there might be residual delays.
U.S. Geological Survey Geophysicist Annemarie Balty says, unlike larger individual earthquake events, swarms tend to be much smaller in magnitude with many minor quakes occurring over a period of days or weeks.
"There's no clear start or stop, necessarily, and there's no clear order to the magnitudes of the earthquakes," Balty said. "These swarms have lasted as long as from two to 42 days. So, it's hard to say right now exactly what the full extent of this swarm is going to look like."
In the last half century, there have been a half dozen other significant earthquake swarms -- right in the area where the current one is, on the Mount Diablo Thrust Fault. "This event is typical. It supports all the information we put in there. It supports our models and it doesn't change anything. It's not unusual," said geophysicist Balty.
As each year goes by, we have better technology to gain even more insight into when swarms occur and how they work. So, expect more activity out here that will, one day, end without notice.
Earlier this week, there was another small cluster of quakes. In 36 hours between Monday and Tuesday, there were a cluster of seven small quakes.