Increased risk of a major earthquake on San Andreas fault, California

Although the swarm of over 300 earthquakes under the Salton Sea in southern California seems to be over, seismologists are warning there still remains an increased risk of a big earthquake on the San Andreas fault. Experts said it’s important to understand that the chance of the swarm triggering a big one, while small, was real. This region is long overdue for a massive quake.

The swarm under the Salton Sea, near Bombay Beach, California, started on September 26, 2016, in the Brawley Seismic Zone, which lies near the southern terminus of the San Andreas fault. By 22:23 UTC on September 30, USGS recorded more than 300 earthquakes there, 3 of them with magnitudes above 4.

It marked only the third time since earthquake sensors were installed there in 1932 that the area had seen such a swarm, and this one had more earthquakes than the events of 2001 and 2009. The event caused an intense interest, and concern, among seismologists, the Los Angeles Times reports. 

A big earthquake in this area on average happens once every 150 or 200 years. Since there hasn’t been a major one in more than 330 years (since about 1680), scientists say the region is long overdue for a big one.

Earthquakes in the Brawley seismic zone as of September 30, 2016 – Salton Sea, California. Credit: USGS

According to the USGS, as of September 27, the chances of a magnitude 7 or greater earthquake being triggered on the southern San Andreas fault over the next seven days were as high as 1 in 100 and as low as 1 in 3 000. Without the swarm, the average chance for such an earthquake striking on any given week is 1 in 6 000.  

“Any time there is significant seismic activity in the vicinity of the San Andreas fault, we seismologists get nervous,” said Thomas H. Jordan, director of the Southern California Earthquake Center, “because we recognize that the probability of having a large earthquake goes up.”

As seismic activity drops, the probability of having a large earthquake also decreases.

As of 19:00 UTC on September 30, 2016, preliminary calculations made by the USGS indicate that there is 0.006% to 0.2% 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 within the next seven days through October 7, with the likelihood decreasing over time.

This range is estimated using several models developed in California to assess foreshock/aftershock probabilities, and the lower bound is about equal to the average chance of a magnitude 7 earthquake occurring on the Southern San Andreas fault in any given week.

These revised probabilities are lower than those made earlier this week, due to decreasing swarm activity. The probabilities may change if the swarm activity increases or decrease.

“The southern San Andreas is actually seismically fairly quiet. It doesn’t really make noise. So to have something right next to the main strand making a little noise, you have to pay attention to how it might be transferring stress onto the main strand of the fault,” said USGS research geologist Kate Scharer.

“The problem with the southern San Andreas fault, the stretch from Monterey County to the Salton Sea, is that when it goes, it’s probably going to go big, such as with a magnitude 7 or higher quake,” Scharer said.

Featured image credit: USGS

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