Mysterious New Fault Discovered Right Next To San Andreas Fault Line

A Landsat image of a section of the San Andreas Fault, clearly visible making its way through California. NASA

Last week, around 200 closely timed earthquakes known as a “swarm” appeared in the Salton Sea, near Bombay Beach in California. For a brief time on September 26, the threat of a magnitude 7 earthquake in the region – which includes the San Andreas Fault – was about 1 in 100, although this has since dropped off again.

Now, a coincidentally-timed study reveals that a newly discovered fault has been mapped in the same area using a range of radar and seismic detection techniques. The researchers have named it the Salton Trough Fault, and although they cannot yet give any confident indication as to whether or not it presents the area with a major hazard, it does indeed happen to be almost precisely where the recent swarm took place.

“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 in a statement.

Writing in the Bulletin of the Seismological Society of America, the authors highlight that the fault is incredibly close to the southern section of the San Andreas Fault. In fact, this new fault may have been taking on some of the accumulating strain in the region, meaning that it’s diverting some from the San Andreas Fault, and delaying the occurrence of the next “big one.”

It’s because of this risk that there’s always a high level of concern over activity around the San Andreas Fault, and any research linked to it gets plenty of attention. At some point in the future, part or all of the main fault line will rupture, and it will be unflinchingly devastating, particularly to the nearby Los Angeles metropolis.

As earthquakes tend to – but don’t always – occur along fault lines, identifying new ones is vital. More than anything, this new study underscores just how much about the fault complex around San Andreas researchers still don’t know.

STF, the new fault line, running very close to the Southern San Andreas Fault (SSAF). Sahakian et al./BSSA

Worryingly, the section of the San Andreas Fault near the Salton Sea hasn’t experienced a huge event since the 17th century. However, these recent swarms and the discovery of a new fault section beside it doesn’t necessarily make the long-term prospects for a large earthquake any more or less likely.

True, the quake swarm did temporarily increase the chance of a major earthquake happening in the region fairly significantly for a day or two, but now that we know there’s a new fault in the region, perhaps any “big event” would be localized to this new relatively small one anyway.

Then again, the region is thought to suffer from so-called “double-fault” earthquakes, where the stress release from one could trigger a connected fault to jut forwards. It’s likely this happened during an 1812 event wherein the San Jacinto Fault jutted forwards, which induced part of the San Andreas Fault to rupture immediately afterwards. Could this new, minor fault’s future rupture set off San Andreas?

In sum, there’s a lot left to learn. We only recently found out that the Moon of all things is causing tens of thousands of incredibly small quakes within the San Andreas Fault every single year, after all. The big one will happen, and with each day that passes, it becomes more likely that it will be all the more destructive.

The best plan of action is to reinforce LA and San Francisco’s infrastructure, while paying attention to these sorts of studies delving deep into the secrets of San Andreas. Now seismologists have a new fault to keep an eye on.

Downtown Los Angeles awaits the big one. ESB Professional/Shutterstock

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