Fault that caused Anza quake has destructive potential

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As though Californians needed more natural disasters to contend with on top of an ongoing pandemic, a magnitude 4.9 earthquake centered near rural Anza rattled much of the southern part of the state Friday, April 3.

While the quake was not powerful enough to cause any damage, mild-to-moderate shaking could be felt across a wide swath of Southern California, not just in Riverside County but in heavily populated Los Angeles, San Diego and Orange counties as well.

Many Californians have probably heard about the San Andreas fault – the one responsible for the 1906 San Francisco earthquake – for much of their lives. But what about the fault responsible for the April 3 quake in Anza, the San Jacinto fault?

Thomas Rockwell, a paleoseismologist and structural geologist at San Diego State University, said the San Jacinto fault, too, is capable of large earthquakes, and it has caused them in the past.

If you’ve ever looked at a map of the Hemet-San Jacinto area before, you might have noticed that there’s an almost straight line running along the eastern edge of the valley, sharply dividing the valley floor and the mountains. That, in fact, is the San Jacinto fault. That fault line runs from the San Bernardino region all the way down into the mountains of eastern San Diego County.

Rockwell said the fault, if it were named today, would probably be more accurately called the western branch of the San Andreas fault.

The San Jacinto fault is “potentially dangerous,” Rockwell said.

The segment near Anza does frequently produce small quakes, in the magnitude 3.0 range, as well as moderate ones like the April 3 one.

“These 5.0s, there’s been at least five of them in the last 20-25 years,” Rockwell said. “They’re relatively common.”

The San Jacinto fault can also produce much larger seismic events.

“In the southern part of the fault, we had relatively large earthquakes in 1968 and 1987, with the Borrego Mountain and Superstition Hills earthquakes,” Rockwell said. “These are in the magnitude 6.5 range. So we know the fault is capable of producing moderately large quakes rather frequently.”

Historically, it’s responsible for quakes even larger than that. 

“It has sustained a number of moderately-large earthquakes historically,” he said.

Some of the most notable ones in the last couple hundred years include an estimated magnitude 6.9 quake in 1918 that ruptured the fault line from Anza toward Hemet, a magnitude 6.5 earthquake that struck on Christmas Day, 1899 and a magnitude 7.3 quake in 1800.

“The worst-case scenario is a repeat of the earthquake of 1800,” Rockwell said. “That ruptured the fault from at least Hemet all the way to the southern end of the Clark strand” – meaning Clark Lake, close to 50 miles southeast of Hemet near Borrego Springs.

That 1800 earthquake is known to have cracked walls at the San Diego de Alcalá and San Juan Capistrano missions, but other than that, it didn’t cause much in the way of serious damage – because there wasn’t much to damage. Even 50 years later, after California was admitted to the union, the 1850 census recorded only about 92,000 people living in the entire state.

Few people were around in Southern California to be affected by the 1800 quake. Today, more than 2 million people live in Riverside County alone.

How likely is it that there actually will be a repeat of the 1800 quake in the coming decades?

Nothing is certain, Rockwell said, but it is a possibility we should be prepared for.

The average return time for large earthquakes on the San Jacinto fault is about 200-270 years, he said.

“We’re already coming into the window where it could happen tomorrow or it would wait 100 years,” Rockwell said.

There are other lingering nearby threats as well. In addition to the San Andreas fault, which experts have long said is overdue for a major earthquake in its southern section, the Elsinore fault – which runs between the Corona area and Temecula – is also capable of producing a serious earthquake. The fault’s Glen Ivy segment near Lake Elsinore produced a magnitude 6.2 earthquake in 1910, while a southeastern extension of the fault in Baja California has caused two earthquakes in the magnitude 7.0 range in the last century and a half, most recently on Easter Day 2010.

It hasn’t caused an earthquake in its Temecula segment recently, and there’s not thought to be any immediate threat of that happening, but the possibility is there.

“The Elsinore ruptured along the Temecula segment about three to four hundred years ago, and it produces these larger earthquakes every four to six hundred years, so the likelihood is not high but it’s certainly capable,” Rockwell said.

The important thing, he said, is just to be prepared for the potential of a large earthquake, an ever-present threat that Californians may end up taking more seriously at the end of the coronavirus pandemic, another unpredictable threat that has always been within the realm of possibility.

“I think the take home is we should all be prepared for the potential of a large earthquake,” Rockwell said. “I think this virus situation is teaching us that we should have a store of food and water.” 

Will Fritz can be reached by email at wfritz@reedermedia.com.