Quake threat lurking in ‘Anza Gap,’ UC Riverside research says

A relatively inactive segment of the San Andreas Fault system known as the “Anza Gap” in south central Riverside County could set loose a moderate to severe earthquake due to deep tectonic plate shifts that aren’t fully understood, according to research published by a pair of UC Riverside scientists.

Professor Abhijit Ghosh and graduate student Alexandra Hutchinson, both affiliated with the College of Natural & Agricultural Sciences, zeroed in on tremors occurring in the Anza Gap, occupying a 12-mile space within the San Jacinto Fault zone, where fault lines course between Hemet and Idyllwild.

According to Ghosh and Hutchinson, the area has been tranquil for 200 years, with no quakes of any consequence originating from the location. However, using amplified sensors that involve “multi-beam backprojection,” developed by Ghosh, the scientists were able to identify a “spontaneous tectonic tremor,” which could suggest that plates eight to 14 miles below the surface are in motion, releasing stress yearly — or even daily.

“Many experts suspect that this area is ripe to produce a damaging earthquake,” Ghosh said, adding that a magnitude-5.5 or greater temblor is possible anytime.

“While relatively little is known about tectonic tremors, in part because they have historically been difficult to detect, we know that these tremors are being caused by slow slip deep in the fault, and that when the deep part of the fault slips, it adds stress to the shallow part,” the professor said. “This may ultimately help to cause a damaging earthquake.”

The San Jacinto Fault zone encompasses the San Jacinto Valley, running north into San Bernardino, as part of the San Andreas Fault system, which is regularly identified as the likely source of the future “Big One” in California.

Ghosh said his and Hutchinson’s research — which was published in the most recent edition of the Bulletin of the Seismologic Society of America — points to the need for further study of “deep slow slip and damaging earthquakes closer to the surface.”

In so doing, “it may be possible that tectonic tremors will enable us to forecast major earthquakes in the future,” he said.

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