Mention earthquakes in California, and it’s not uncommon for the discussion to involve the “Big One”—a major earthquake expected to occur along the San Andreas fault line at some unknown time.
The recent earthquake in northern California did not occur along the San Andreas fault line, but rather along the West Napa fault, says Harold Magistrale, senior lead research specialist at FM Global. “It’s not the San Andreas proper,” he says, “but it’s part of the San Andreas system. It’s part of the plate boundary between North America and Pacific plates."
He adds, “It has same sense of slip; the same kind of motion that the San Andreas fault has.”
The West Napa fault, says Magistrale, has been previously recognized as active, and the recent quake was certainly no major surprise with respect to where it happened, even if scientists cannot predict when such quakes will occur.
The area, Magistrale says, is near other active faults that are also part of the San Andreas system. The Hayward fault is nearby, he said, and was the site of a big quake in 1848.
“The San Francisco Bay area is pretty much riddled with faults that are all part of the San Andreas system, and this West Napa fault is just one of those,” Magistrale says.
Fallout from the recent quake
With all of these faults, there is a chance that other areas within the San Andreas system could potentially see earthquakes due to the changes in stress conditions. The motion on the fault, Magistrale says, redistributed stresses in the crust of the earth nearby, and that could change stress conditions on other nearby fault lines. “And so that rearrangement of the stresses could indeed trigger other earthquakes,” he says. “It’s been seen to happen in other situations.”
The potential for such an event should not be confused with aftershocks. Magistrale says this earthquake will cause aftershocks near the rupture area that will typically be smaller. He points out the aftershocks could still cause damage to buildings weakened by the main shock.
“But then there’s triggering of what could be larger earthquakes on nearby faults,” he says. “And that all depends on the state of stress on those other faults, and it’s hard to know the state of stress in enough detail to foresee what kind of earthquake, if any, could be triggered.”
While that kind of detail is still not within reach, scientists are learning more about faults and earthquakes. Magistrale says there are a couple of different systems for studying faults.
The first is a network of seismometers run by the USGS. These, Magistrale says, monitor the day-to-day occurrence of small and large earthquakes, and determine earthquake parameters such as where they occur, how deep they are, and their magnitude.
There there is a network of GPS stations, “just like a handheld GPS, but put into ground,” Magistrale says. These send data to a central collection site, and, when the information is processed, the strain from plate-tectonic motion can be monitored on those systems.