San Jacinto Fault very active

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No telling signs yet; just be prepared

Over the past month, the San Jacinto Fault, which passes under Garner Valley and near Idyllwild, has been the source of dozens of small quakes.

Almost all were magnitude 2.9 or less, although on Nov. 8, tremors with magnitudes 2.9 and 3.2 were only an hour apart in the area. Since Monday, Nov. 13, the U.S. Geological Survey recorded eight quakes greater than 2.0 along this fault.

The San Jacinto Fault intersects the infamous San Andreas Fault in San Bernardino County near Claremont and the Cajon Pass, and runs south almost to the Mexican border. While it is the lesser-known cousin of the San Andreas, the San Jacinto is considered the most active fault in Southern California.

“It has the most quakes, but more are little or modest than on any other fault,” said Dr. Julian Lozos, assistant professor in the Geological Science Department at the California State University, Northridge. He partly attributes this to the San Jacinto’s youth. “It is only about one million years — very young.

Yet, he considers it a very important fault. “It is a very hazardous fault,” he stated. “But I don’t expect anything imminently.”

A 7.5-magnitude major quake occurred on the San Jacinto in 1812, according to Lozos. “If it did happen, it could happen again,” he warned.

Seismologists are quick to allay worry about any immediate major quake and to discourage any connection between the recent serious of quakes and a “big one.”

“At this point, I would not say it indicates anything special,” said Dr. Abhijit Ghosh, an assistant professor in the Department of Earth Sciences at the University of California, Riverside. He added, “There is natural variation, up a bit and down some. I would not read too much into this.”

Nevertheless, he, Lozos, and other seismologists recommend that Hill residents prepare a go bag and have it handy in case either of the faults has a major shake.

Ghosh has done research on the San Jacinto Fault. And one area, the Anza Gap, was particularly interesting to him and his graduate student, Alexandra Hutchinson.

The Anza Gap several miles south of Highway 371, which runs through Anza Valley, has had very little seismic activity in the past few centuries. Other segments of the fault have released quakes greater than 5.5 several times in the past couple of centuries, but not the 12-mile gap south of Anza.

“I believe the fault is slowly slipping under the plate,” he suggested, based on his research. Further, this slippage is very deep and very slow, but it could release energy, which essentially is what an earthquake does.

While not a rare occurrence, seismic research in the two decades has recognized the difference. For example, the stress levels are different depending upon the depth. The Anza Gap is very deep; faults tend to be nearer the ground surface, and very slow. This difference results in less ground vibration, Ghosh said. So, we are less cognizant of the deep slips and shifts.

“[R]egular seismic activity occurs on either side of the Anza gap, whereas there is very little seismic activity within the gap itself,” he wrote in his latest research paper.

But major quakes have occurred in this area, and Ghosh expects another. He just cannot predict when it might occur. Recent research indicates the last quake in the gap area was about 1918. Other areas of the San Jacinto have felt much more recent quakes.

“The San Jacinto is a big player,” Lozos advised. “So, don’t write it off, don’t be afraid; just be aware and have a plan.”

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