CA’s ‘Big One’ May Look Like No Other Quake In History Or Forecast Models, Researchers Warn

A massive earthquake that struck Myanmar in Southeast Asia in March ruptured a much longer section of a fault line than scientists had previously thought possible.

The 7.7 to 7.9 magnitude quake originated in central Myanmar near Mandalay but ruptured some 317 miles along the Sagaing fault, causing damage as far away as Ho Chi Minh City in Vietnam and Ruili, China. It killed at least 3,791 people in Myanmar and another 63 in Thailand.

The Sagaing fault shares striking similarities with California’s infamous San Andreas fault. Could the Myanmar quake offer clues about the likelihood of “The Big One” in California?

Not exactly, according to a new study published Monday in the Proceedings of the National Academy of Sciences. The research was coauthored by scientists at Caltech in Pasadena.

“Earthquakes never come back exactly the same way,” Solene L. Antoine, the study’s lead author and a postdoctoral fellow at Caltech, told the Los Angeles Times.

For example, the Mandalay earthquake reawakened a section of the Sagaing fault that had not ruptured since 1839 — an area already considered a high-hazard zone.

On California’s San Andreas fault, one stretch has a very different behavior. The so-called “creeping section” between the cities of Parkfield and Hollister, spanning the Central Coast to the Bay Area, moves slowly over time without producing significant shaking, and no large quake has ever been recorded there.

Still, “The Big One” looms large in the collective imagination of Californians — the colloquial name for the anticipated, catastrophic earthquake expected to rupture along a major fault line like the San Andreas.

Although there have been other studies, podcasts, disaster films and articles attempting to prepare Californians for what The Big One could look like, scientists of this latest study say there’s no way to know what that temblor will look like.

We don’t know if it would resemble the cataclysm of 1857, when a quake as powerful as 7.9 tore open the San Andreas from Monterey County clear through Los Angeles County. Nor can we be sure it would mirror the devastation of 1906, when the San Francisco earthquake struck just offshore and ripped the fault in two directions — one tearing north toward Humboldt County, the other south toward Santa Cruz.

The study’s authors acknowledge that the Big One could share certain traits with past quakes, but say it’s unlikely to be an exact repeat.

“Large earthquakes often occur on faults that were known to have produced destructive events in the past. However, anticipating the characteristics of these earthquakes and their impacts remains a great challenge,” the study reads.

But that doesn’t mean nothing can be done to predict and prepare for future temblors.

Jean-Philippe Avouac, coauthor of the study and a professor of geology and mechanical and civil engineering at Caltech, told the Times that the next step will be to build a model that simulates San Andreas fault earthquakes over thousands of years.

“It’s not going to come soon, because it’s quite a heavy calculation,” Avouac said, adding that such simulations would create “all possible scenarios so that we have a better view of the range of possible ruptures that could happen.”

In an interview with Patch in July, a United States Geological Survey expert said a major earthquake striking California within the next 30 years is not just possible, it’s likely.

There’s a 72 percent chance of a magnitude 6.7 or larger earthquake striking the Bay Area in that timeframe, said Sarah Minson, a research geophysicist with the USGS Earthquake Science Center, located at NASA Ames in Mountain View. There’s also a 51 percent chance for magnitude seven or larger, a 20 percent chance for seven and a half or larger, and a 4 percent chance for a magnitude eight.

In Los Angeles, that probability is slightly lower, at 60 percent. But for even larger events — magnitude 7.5 or greater — the roles reverse: there’s a 31 percent chance in Los Angeles compared to 20 percent in the Bay Area.

Minson warned that focusing on the abstract and less likely “major” earthquakes may be misguided.

“I think we have a tendency to focus on, the disaster, ‘The Big One,’ the magnitude eight, that does a lot of damage in a lot of places,” Minson said. “But in fact, most of the hazards come from [events] like that magnitude six earthquake in 2014 near South Napa.”

She pointed to that quake as an example of a so-called moderate earthquake with severe consequences.

Similarly, the authors of the Caltech-led study said the San Andreas fault could rupture in smaller, separate earthquakes — or it could become a magnitude 8 quake that ruptures from Monterey all the way to the Inland Empire and Imperial County.