San Ramon earthquake swarm continues, but USGS says no records broken

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Wednesday, October 28, 2015

SAN RAMON, Calif. (KGO) -- With temblors arriving at a consistently rapid pace in San Ramon and nearby, the quake swarm would seem to be smashing records set by previous swarms -- but the U.S. Geological Survey said today it likely isn't.

As of noon today, the USGS has recorded 477 earthquakes since Oct. 13 in an area that covers a few miles along the San Ramon-Danville border. And the pace of their arrival isn't slowing, with more than 40 of those quakes striking from Tuesday to noon today.

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.

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