Mention earthquakes in California, and it's not uncommon for thediscussion to involve the “Big One”—a major earthquake expected tooccur along the San Andreas fault line at some unknown time.

The recent earthquake in northern California did not occur alongthe San Andreas fault line, but rather along the West Napa fault,says Harold Magistrale, senior lead research specialist at FMGlobal. “It's not the San Andreas proper,” he says, “but it's partof the San Andreas system. It's part of the plate boundary betweenNorth America and Pacific plates.”

He adds, “It has same sense of slip; the same kind of motionthat the San Andreas fault has.”

The West Napa fault, says Magistrale, has been previouslyrecognized as active, and the recent quake was certainly no majorsurprise with respect to where it happened, even if scientistscannot predict when such quakes will occur.

The area, Magistrale says, is near other active faults that arealso part of the San Andreas system. The Hayward fault isnearby, he said, and was the site of a big quake in 1848.

“The San Francisco Bay area is pretty much riddled with faultsthat are all part of the San Andreas system, and this West Napafault is just one of those,” Magistrale says.

Fallout from the recent quake

With all of these faults, there is a chance that other areaswithin the San Andreas system could potentially see earthquakes dueto the changes in stress conditions. The motion on the fault,Magistrale says, redistributed stresses in the crust of the earthnearby, and that could change stress conditions on other nearbyfault lines. “And so that rearrangement of the stresses couldindeed trigger other earthquakes,” he says. “It's been seen tohappen in other situations.”

The potential for such an event should not be confused withaftershocks. Magistrale says this earthquake will cause aftershocksnear the rupture area that will typically be smaller. He points outthe aftershocks could still cause damage to buildings weakened bythe main shock.

“But then there's triggering of what could be larger earthquakeson nearby faults,” he says. “And that all depends on the state ofstress on those other faults, and it's hard to know the state ofstress in enough detail to foresee what kind of earthquake, if any,could be triggered.”

Studying earthquakes

While that kind of detail is still not within reach, scientistsare learning more about faults and earthquakes. Magistrale saysthere are a couple of different systems for studyingfaults.

The first is a network of seismometers run by the USGS. These,Magistrale says, monitor the day-to-day occurrence of small andlarge earthquakes, and determine earthquake parameters such aswhere they occur, how deep they are, and their magnitude.

There is a network of GPS stations, “just like a handheld GPS,but put into ground,” Magistrale says. These send data to a centralcollection site, and, when the information is processed, the strainfrom plate-tectonic motion can be monitored on those systems.