Quake raises concern over future danger

The moderate quake that struck near Lafayette Thursday night has geologists weighing in on the potential for bigger earthquakes in the area.

The magnitude 4.2 earthquake could be a sign that larger quakes are possible or it could indicate that smaller earthquakes are the norm in this area. The epicenter of the Lafayette quake was around 10 miles below the surface, so the nature of the fault is unclear.

The location of Thursday's quake, which was felt in the Vallejo-Benicia area, is near the trace of a relatively unknown fault called the Reliez Valley fault which crosses under Highway 24 and the BART tracks at the Pleasant Hill Road overpass near Acalanes High School.

Engineering Geologist Keith Kelson of William Lettis & Associates Inc. in Walnut Creek thinks the Reliez Valley fault is part of a broader zone of four small faults between Lafayette and Walnut Creek that his team has dubbed the Contra Costa shear zone.

Kelson has been studying these faults for five years and found evidence that suggested they are active faults. Thursday's quake supports this theory, Kelson said.

Though the Reliez Valley fault, which appears to be the biggest fault in the shear zone, probably isn't capable of producing large Century 21 Distinctive Properties Inc. earthquakes, Kelson suspects the faults may be connected at depth and could act as a link between the northern end of the Calaveras fault which appears to peter out in the Danville area and the southern end of the West Napa fault near Vallejo.

If he is correct, at some time the small faults could move together to create a quake of magnitude 6 or more that could dislocate BART tracks and cause damage to buildings.

There are other examples of shear zones connecting faults in California. Some scientists think the Hayward fault may be connected to the Rogers Creek fault to the north through a zone that lies beneath San Pablo Bay. If those two faults were to rupture together, the result could be a magnitude 7 or greater earthquake.

But geologist David Schwartz of the U.S. Geological Survey in Menlo Park says the locations of about a dozen small aftershocks after Thursday's earthquake don't match up with the north-northwest trending Reliez Valley fault.

"As aftershocks started to occur, they progressed in a west-southwest direction," Schwartz said.

If Schwartz and colleagues at the USGS are right, the fault that hosted Thursday's quake is likely a relatively harmless one in the block of earth between the larger Calaveras and Hayward faults that isn't capable of bigger shocks.

"We're pretty confident that it was this west-southwest trending patch of a fault. It's very hard to argue with what the aftershocks and their orientation shows," Schwartz said.