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There’s a New Fault Line in CA—Right Next to the San Andreas. Here’s What That Could Mean.

[DIGEST: USA Today, Time, San Francisco Gate, Christian Science Monitor, IFL Science]

California seismologists have discovered a new fault line running along the northeastern edge of the Salton Sea, parallel to the infamous San Andreas Fault. The findings were published last week in the “Bulletin of the Seismological Society of America” by seismologists from the University of California, San Diego and the University of Nevada, Reno.

New fault lines are rarely discovered. California, in particular, is very seismically active and has been extensively surveyed. The Salton Trough Fault, however, was undiscovered until now because it was underwater. “The location of the fault in the eastern Salton Sea has made imaging it difficult and there [are] no associated small seismic events, which is why the fault was not detected earlier,” said principal investigator Neal Driscoll, a geologist at the University of California, San Diego.

The study was published in the wake of nearly 200 mini earthquakes in the same area at the end of September. The confluence of quakes temporarily increased the risk of a major quake (magnitude 7 or higher) in the region to about 1 in 100. “Any time there is significant seismic activity in the vicinity of the San Andreas fault, we seismologists get nervous, because we recognize that the probability of having a large earthquake goes up,” said seismologist Thomas H. Jordan, the director of the Southern California Earthquake Center. The risk has now dropped back down.

A geologic setting of the study region. (Credit: Source.)

Valerie Sahakian, a researcher with the U.S. Geological Survey and lead author of the study, said the Salton Trough Fault was not connected to these earthquakes.

Yet there is much the researchers still do not know about the newly-discovered fault. More research needs to be conducted to understand the fault’s full length and location. Once more information is known about the fault, scientists can then better assess the risk of

earthquake. “Things like locations of the fault and expected magnitude at the fault are what’s used to predict the maximum ground shaking a region can expect,” said Sahakian.

Researchers are also still investigating the impact the newly-discovered fault may have on the nearby San Andreas fault.

The article’s researchers speculated that the Salton Trough Fault might be postponing a devastating earthquake, like the long-predicted “big one,” in Southern California. The San Andreas Fault typically erupts in a major quake every 175-200 years. The last major earthquake from the Salton Trough fault was 300 years ago. Because the newly-discovered fault is located so close to the San Andreas Fault, it may be taking on some of the region’s strain, thus increasing the time until its next major rupture.

“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. “Based on the deformation patterns, this new fault has accommodated some of the strain from the larger San Andreas system.”

Credit: Source.

On the other hand, having two fault lines so close together may have the opposite effect: the region may succumb to “double fault” earthquakes. In this scenario, the release from one fault could trigger the other fault to rupture. Scientists believe this is what happened during a major 1812 earthquake, when the San Jacinto fault jutted forward, inducing a rupture of the San Andreas Fault shortly after.

The discovery of the fault is a major first step in understanding the risk of earthquakes in the region. Said Sahakian: “To aid in accurately assessing seismic hazard and reducing risk in a technically active region, it is crucial to correctly identify and locate faults before earthquakes happen.”