But the sheer amount of temblors recorded doesn't necessarily mean it's a larger swarm than those in decades past, as a reevaluation of data posted online today by USGS confirmed.
The San Ramon area adjacent to the northern Calaveras fault has long been a locus of earthquake swarms, USGS officials said.
Five notable swarms have occurred near there since 1970, the most significant of which was a swarm centered on Alamo that began in 1990 and produced 351 earthquakes over 42 days, according to the USGS.
The recent swarm would appear to have surpassed that record by 126 earthquakes in less than half the time.
But USGS officials said it's like comparing apples to oranges.
USGS officials said seismic activity detection equipment wasn't nearly as advanced back then. Earthquakes below 1.0-magnitude, like many of those recorded in the recent swarm, weren't picked up at all.
When looking at temblors above 2.0-magnitude, the 1990 Alamo swarm had 40 percent more than the recent swarm in the first two weeks, according to the USGS. The recent swarm had 74 compared to the previous record's 106.
Right now, the recent swarm looks closer in scale to what was the second-largest swarm on record, which occurred in 1970 in Danville over the span of 15 days. That swarm had 69 recorded earthquakes of 2.0-magnitude or more over the first two weeks.
And, with its largest earthquake being a 3.6-magnitude shaker on Oct. 19, this recent swarm hasn't beat any of the previous five swarms in the size of its biggest quake. The 1990 Alamo swarm brought a pair of 4.4-magnitude earthquakes.
But if this swarm lasts as long as that Alamo swarm, there could be another two weeks or more of potential shakers for the area, according to the USGS.
Update, Friday October 23
BCN: A preliminary 3.2 earthquake struck about a mile from San Ramon this morning, the latest in a string of hundreds of small quakes in the area, U.S. Geological Survey officials said. The quake hit at 6:48 a.m. about half a mile east northeast of San Ramon and about 3 miles southeast of Danville, USGS officials said. More than 200 quakes have struck the San Ramon area in the last week.
Original Post:
Tuesday's first earthquake in San Ramon happened at 2:05 a.m. The next at 4:10 a.m. That one was followed by shakes -- each one deep enough and light enough to be nearly imperceptible unless you were sitting right on the epicenter -- at 4:55 a.m., 5:31 a.m., 7 a.m., 7:02 a.m., 8:45 a.m., 10:18 a.m., 10:38 a.m., 1:01 p.m., 1:40 p.m.
Those are the latest in a swarm of temblors, about 240 through Tuesday afternoon, that began rattling San Ramon a week ago. A total of four have been magnitude 3.0 or greater, with the strongest shake, a 3.6, occurring Monday at 4:21 p.m. About 50 more of the quakes, occurring near the Calaveras Fault (with many directly under the Crow Canyon Country Club), have registered between magnitude 2.0 and 2.9.
Brad Aagaard, a research geophysicist with the U.S. Geological Survey, told KQED's Amanda Font the series of quakes is atypical in that they haven't followed a strong main shock and trailed off into a series of aftershocks.
"In contrast to many times where we'll have a magnitude 3 or 4 and then some aftershocks that decrease in number with time, in this particular sequence as in others we call swarms, there's a much more constant rate of earthquakes going on," Aagaard said.
Aagaard added that the current swarm isn't unprecedented for the San Ramon-Alamo-Danville area.
"We've seen large swarms like this in the past," he said. "One of the more significant ones was in 1990 under Alamo, where we had 350 earthquakes in 42 days."
What everyone wants to know, naturally, is whether a swarm like the one shaking up San Ramon right now is a precursor to, you know ... something bigger and scarier.
"In terms of historical events, when we've had a swarm like this, it has not been followed by a large earthquake," Aagaard said. "The most likely scenario is that the swarm lasts for some period of time -- it could last up to a month. Many of these have lasted 30 or 40 days."
He said the chance for a big quake in the area is probably about the same as if the swarm had not occurred.