Scientists Examine San Ramon Quake Swarm
Tue Nov 26, 7:44 PM ET

Tony Russomanno

The earthquakes (news - web sites) along a new fault line near San Ramon show no sign of letting up.

Tuesday, quakes measuring 3.7 and 3.1 rumbled through the area. Many people assume that the smaller quakes that have been shaking San Ramon are aftershocks. But they're not. Geologists call the phenomenon a quake swarm, and say this one has included at least 75 [95] earthquakes since Sunday.

But could this quake swarm trigger a larger, more dangerous earthquake? An innocuous little mark on a neighborhood street just off San Ramon Valley Boulevard is helping scientists assess the potential danger of the Calaveras Fault. It's a survey point for instruments used to measure how fast the fault is creeping.

"If the fault moves even as much as a millimeter -- which is 1/25th of a inch -- we can tell," said Jim Lienkaemper, geophysicist with the United States Geological Survey.

Lienkaemper has been taking measurements at the same point every two months for the past ten years. By coincidence, it's right where the current earthquake swarm started Sunday morning. And that's not the only coincidence. The survey team was taking their most recent measurements while the swarm was underway.

"We were measuring... one of the regular sites Sunday immediately after the swarm and it showed no creep," Lienkaemper said.

More measurements will be taken next weekend. Scientists do not know if the swarm will affect the creep rate, or if it's the other way around -- if the creeping triggered the swarm.

"Swarms by themselves in the San Ramon Valley in the past have had no effect on the Calaveras," said Dr. David Schwartz, USGS (news - web sites) geologist. "But they've been physically removed from the Calaveras. In this particular case, the fault that's producing it probably actually intersects the Calaveras."

A similar earthquake swarm in Alamo in 1990 lasted 40 days and produced more than 300 quakes. But another one in Danville in the 1970s ended after only three days.

"We really can't say now exactly how long it's going to last," said Schwartz.

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• Scientists Examine San Ramon Quake Swarm