As if worrying about whether or not the massive San Andreas Fault would erupt after a swarm of 200 earthquakes hit the nearby Salton Sea wasn't enough for Californians, a new fault line was discovered this week in the same area. Researchers are calling it the Salton Trough Fault (STF).
The fault, which lies along the eastern edge of the Salton Sea and runs parallel to the San Andreas Fault, had eluded seismologists up until this point because the sea's bottom has been difficult to map.
"The location of the fault in the eastern Salton Sea has made imaging it difficult and there is no associated small seismic events, which is why the fault was not detected earlier," principle investigator Neal Driscoll, a geologist at the University of California San Diego, said.
To locate the STF, the research team used instruments—including multi-channel seismic data, ocean-bottom seismometers, and light detection and ranging—to precisely map the deformation within the various sediment layers in and around the sea's bottom.
Writing in the Bulletin of the Seismological Society of America, the authors say "the presence of this fault could alter the current understanding of stress transfer and rupture dynamics in the region, as well as community fault models and ground‐motion simulations on the San Andreas Fault." Meaning, seismologists will have to reassess their earthquake risk models for the greater Los Angeles area.
As the authors explain, recent research has revealed that the region has experienced magnitude-7 earthquakes roughly every 175 to 200 years for the last 1,000 years. But it's been about 320 years since the last major earthquake prompting seismologists to theorize that maybe the STF has something to do with it.
"The extended nature of time since the most recent earthquake on the Southern San Andreas has been puzzling to the earth sciences community," said Nevada State seismologist Graham Kent, a coauthor of the study and former Scripps researcher. "Based on the deformation patterns, this new fault has accommodated some of the strain from the larger San Andreas system, so without having a record of past earthquakes from this new fault, it's really difficult to determine whether this fault interacts with the southern San Andreas Fault at depth or in time."
The discovery of this new fault line will no doubt have researchers busy as they try to determine its character, the hazards it poses and how it interacts with the San Andreas Fault.
"The patterns of deformation beneath the sea suggest that the newly identified fault has been long-lived and it is important to understand its relationship to the other fault systems in this geologically complicated region," Valerie Sahakian, a Scripps alumna and lead author of the study, said.
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.
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.
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.
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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 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."