Photo: The San Andreas fault (top center to lower right) divides the San Gabriel Mountains from the Mojave Desert.
If people in California are feeling worried and stressed out right now, it's not just because of the presidential election.
An unusual swarm of 200 earthquakes that rattled the Salton Sea area last week prompted the state government's board of seismic experts to conclude that there's a slightly elevated risk of a major earthquake on the massive San Andreas Fault, which stretches for much of the state's length.
News headlines about an "earthquake advisory" or a "heightened earthquake alert," probably haven't helped anyone's nerves. Statistically, the risk of the Big One, as Californians call it, remains small, and emergency officials don't advise any precautions other than the ones California residents already should be taking, given the state's extensive networks of faults, not all of which may even have been mapped, and its history of earthquakes.
California Earthquake Prediction Evaluation Council members studied seismic data from the swarm, which began on Sept. 26 and took place in the Brawley Seismic Zone, a section of cross-faults in a network that connects the southernmost end of the San Andreas with the Imperial Fault.
Most of the quakes were magnitude 2.0 or less -- too small to be felt by people, though they show up on instruments. One quake was measured at magnitude 4.3 and another at 4.1, which can be felt but causes minor damage, at most. The earthquake that destroyed San Francisco in 1906, by comparison, has been estimated to be between magnitude 7.7 and 8.3. (Here's an explanation of earthquake magnitude.)
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"CEPEC believes that stresses associated with this earthquake swarm may increase the probability of a major earthquake on the San Andreas Fault to values between 0.03 percent and 1.0 percent for a (magnitude) 7.0 or larger earthquake occurring over the next week," an Office of Emergency Servcies press release explained on Sept. 27. The period of elevated risk ends on Tuesday.
The U.S. Geological Survey, which did its own seismic modeling, described the risk in slightly different terms. USGS said there was a 0.006 percent to 0.2 percent chance (less than 1 in 10,000 to 1 in 500) of a magnitude 7 or greater earthquake being triggered on the Southern San Andreas Fault between Sept. 27 and Oct. 7. That risk decreases gradually as the end date approaches.
The lower end of that range "is about equal to the average chance of a magnitude 7 earthquake occurring on the Southern San Andreas Fault in any given week," USGS said.
Swarms have occurred in that area in the past, and aren't necessarily an indication that a bigger quake is coming. Nevertheless, scientists were concerned, because some of the cross-faults are oriented so that they could add stress to the San Andreas.
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Despite all this, California officials advised state residents and local government agencies to focus on the big picture, which is that they prepare for the eventual possibility of a big quake. USGS has calculated that over the next 30 years, there's somewhere between a 0.1 and a 1 percent chance of an earthquake greater than 6.7 on the San Andreas. But the risk of a large quake is lower in northern California, because the 1906 San Francisco quake relieved some of the stress in that area.
If the ground beneath you does start shaking, use the "drop, cover and hold on" tactic to protect yourself from injury.
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