New concerns about big quake fault

By Gary Robbins 09:54a.m. Dec 14, 2014

The San Jacinto is the middle of the three red lines in this map. The San Andreas is located to the east and the Elsinore system is situated to the west.

The San Jacinto fault, a twitchy system that cuts through the East County, could produce larger earthquakes than scientists believed and may rival the San Andreas in power, according to research led by San Diego State University.

"A magnitude 7.5 was generally accepted to be the largest earthquake that would like occur on the fault," said Tom Rockwell, an SDSU seismologist and the study's leader. "We have shown that the central and northern sections of the San Jacinto fault appear to fail together at times, and that would be in the magnitude 7.6 to 7.7 range.

SDSU's Tom Rockwell is one of the world's foremost authorities on the San Jacinto fault. — Gary Robbins

"If it ruptures onto the San Andreas fault, it could approach a magnitude 8.0, although we don't see evidence that that has happened in the past couple of thousand years. The take home here is that earthquakes on the San Jacinto fault potentially rival those on the San Andreas fault."

The San Jacinto system is a 130-mile strike-slip fault that stretches from Imperial County through Anza, Ocotillo Wells and Borrego Springs into Riverside County and the San Bernardino Valley. Rockwell oversaw field studies that examined 21 seismic events that have occurred on the fault over the past 4,000 years. The study, recently published in the journal Pure and Applied Geophysics, was centered at Hog Lake, a spot near Anza.

The San Jacinto is part of the larger San Andreas fault, the dividing line between the Pacific and North American plates. Scientists say that the two faults are responsible for roughly 80 percent of the slippage along the plates. Rockwell has played a major role in revealing the path of the San Jacinto and reconstructing its history. Records show that the fault produced a magnitude 6.5 quake in April 1968. A different strand of the fault, known as Superstition Hills, produced a 6.7 quake in November 1987 that was strongly felt in San Diego.

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