A little-known section of the San Jacinto Fault in Southern California could erupt with a damaging earthquake a lot sooner than once thought.

A research team at UC Riverside made the discovery using a new technique in seismic detection.

It’s called multi-beam back projection, developed by UC Riverside earth scientist Abhijit Ghosh. He says it’s like radar that scans subsurface movement deep below the earth — in the case of the San Jacinto Fault, about 10 to 15 miles down.

“And it looks for fault slips. It allows you to boost very faint signals that you wouldn’t have seen otherwise,” says Ghosh.

What he and graduate student Alexandra Hutchinson detected on the Anza Gap — a portion of the much longer San Jacinto Fault that snakes through open desert and densely populated cities from Riverside to Imperial County — are tectonic tremors, basically slow, deep earthquakes that can release energy over long periods of time.

“Different from garden-variety earthquakes,” says Ghosh.

“So this is a relatively new kind of earthquake that we have not seen on the (San Jacinto) Fault. That means that the Anza-San Jacinto is slowly slipping,” says Ghosh.

One segment of the San Jacinto fault — the red line running through the pink Peninsular Ranges rock-type section — as it runs from near Hemet north between Riverside and San Bernardino.
One segment of the San Jacinto Fault — the red line running through the pink Peninsular Ranges rock-type section — as it runs from near Hemet north between Riverside and San Bernardino. (U.S. Geological Survey)

“It alters the stress at the shallow level. That may mean that it’s taking the fault a lot closer to the next damaging earthquake.”

Ghosh says research continues to quantify the findings.

He and his team are also conducting similar studies on the Aleutian Islands and along the Pacific Northwest’s Cascadia fault zone, which reaches into Northern California.

Ghosh says there’s no reason for residents living near the San Jacinto Fault to panic, although the recent finding is yet another sober reminder that Californians in particularly earthquake-prone regions should remain vigilant.

“We can get a rough idea when it may happen, where it may happen and what might be the magnitude — but we cannot predict earthquakes,” says Ghosh.

“But what we can do is give you a rough idea of seismic hazard and prepare for it.”

Could the San Jacinto Fault Zone Rupture Sooner Than Expected? 1 November,2017Steven Cuevas


Steven Cuevas

Steven is the California Report’s Los Angeles bureau chief.

He reports on an array of issues across the Southland, from immigration and regional politics to religion, the performing arts and pop culture.

Prior to joining KQED in 2012, Steven covered Inland southern California for KPCC in Pasadena. He also helped establish the first newsroom at KUT in Austin, Texas where he was a general assignment reporter.

Steven has received numerous awards for his reporting including an RTNDA Edward R. Murrow Award for investigative reporting in addition to awards from the LA Press Club, the Associated Press and the Society for Professional Journalists.

Steven grew up in and around San Francisco and now lives in Pasadena just a short jog from the foot of the San Gabriel Mountains.

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