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Swarm of Earthquakes Near San Andreas Fault Triggers Fear of a Big One

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Is the San Andreas Fault about to unleash The Big One? Southern Californians were on heightened alert this week after a swarm of small quakes in the Salton Sea near the fault triggered officials to issue an earthquake alert for residents in the area.

Aerial photo of San Andreas Fault looking northwest onto the Carrizo Plain with Soda Lake visible at the upper left.John Wiley / User:Jw4nvc

The swarm began just after 4 a.m. on Sept. 26, starting earthquakes three to seven miles deep underneath the Salton Sea, The Los Angeles Times reported. The biggest earthquakes hit later that morning—a 4.3—and then a pair later at night, another 4.3 followed by a 4.1. There was another burst of activity the following night. The U.S. Geological Survey (USGS) said 96 earthquakes above magnitude 2 were reported by Sept. 30.

This is only the third time this area has seen a swarm like this since earthquake sensors were installed in 1932, and it had more earthquakes in it than swarms in 2001 and 2009, LiveScience said.

Earthquakes in the Brawley seismic zone as of the evening of 09/30/2016.U.S. Geological Survey

Based on this activity, on Sept. 27, the USGS said the risk of an earthquake of magnitude 7.0 or greater in Southern California increased to between 1 in 3,000 and 1 in 100 over the the next seven days. However, as of Friday, Sept. 30, the likelihood was decreasing.

"Preliminary calculations 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 Oct. 7," the updated warning said. "These revised probabilities are lower than those made earlier this week, due to decreasing swarm activity."

The average chance for such an earthquake striking on any given week is 1 in 6,000.

The risk assessment from USGS prompted California Gov. Jerry Brown to sign legislation Sept. 29 ordering the development of a statewide warning system that would inform Californians of impending earthquakes through their cellphones, radios and other devices, the Associated Press reported.

San Bernardino, which is on the San Andreas Fault, even closed down city hall through today over concerns about how the structure would fare in a big quake. A 2007 report revealed how the building, constructed in 1971, was not built to withstand a major earthquake and would cost about $20 million to get retrofitted for such an event.

"We haven't had an alert like this," Mark Scott, San Bernardino's city manager, told the Los Angeles Times. "We're not trying to suggest that the alert is an impending catastrophe. We're just trying to use an abundance of caution. We care about the safety of the public and our employees."

The city already has plans to vacate the building within the next few months.

Scientists estimate that the last time the San Andreas Fault's southernmost stretch ruptured was in 1680—more than 330 years ago. A big earthquake happens on average in this area once every 150 or 200 years, making the region long overdue for a major quake. The largest historical earthquakes that occurred along the fault were those in 1857 and 1906.

The general location of the San Andreas fault and several other major faults in California.U.S. Geological Survey

The area where this recent swarm occurred was near a set of north-northeast trending cross-faults beneath the Salton Sea, some which are oriented as such that they would add stress to the San Andreas Fault and the San Jacinto Fault system, according to USGS. However, the agency said swarm-like activity in this region has occurred in the past, so this week's activity, in and of itself, is not necessarily cause for alarm.

While this news is somewhat reassuring, residents in Southern California shouldn't breathe too big a sigh of relief, Morgan Page, a research geophysicist with the USGS in Pasadena, told LiveScience.

"We live in earthquake country and we should be prepared for an earthquake at any time," Page said. "Events like this can change the probabilities from week to week but it never goes to zero."

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