Earthquakes: Reckoning With ‘The Big One’ in California—and It Just Got Bigger

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For years, scientists believed the mighty San Andreas—the 800-mile-long fault running the length of California where the Pacific and North American plates meet—could only rupture in isolated sections.

But a 2014 study by federal, state and academic researchers showed that much of the fault could unzip all at once, unleashing a rare, singular catastrophe. Now, a firm has used that research to come up with a new analysis of the damage that could be caused by statewide break of the San Andreas.

The analysis, by CoreLogic Inc., a real-estate analytics firm in Irvine, Calif., lays out an alarming scenario of destruction.

As many as 3.5 million homes could be damaged in an 8.3-magnitude quake along a roughly 500-mile portion of the fault—compared with 1.6 million homes damaged if only the northern part of the fault were to break, or 2.3 million if the southern piece ruptured.

The damage to homes alone could total $289 billion, compared with a previous range of $137 billion on the southern portion of the fault and $161 billion in the north, according to the CoreLogic analysis.

Researchers say a statewide quake above 8.0 would likely hit the Golden State once at least every 2,500 years. “We are talking about very rare earthquakes here,” said Maiclaire Bolton, a seismologist and senior product manager for CoreLogic.

But officials of the California Earthquake Authority, a nonprofit seller of quake policies, said the statewide threat could put upward pressure on pricing of earthquake policies by an undetermined amount.

The CoreLogic report could also help encourage more people to take out earthquake insurance in the state, where just 10% of homeowners have coverage, said Glenn Pomeroy, chief executive officer of the state-managed authority.

“When reports like these do come out, it does serve as a reminder there is a tremendous amount of uninsured exposure,” Mr. Pomeroy said.

Before 2014, when the U.S. Geological Survey, Southern California Earthquake Center and California Geological Survey conducted their updated forecast to show the possibility of a single statewide quake, seismologists didn’t think an earthquake could occur along such a long portion of the San Andreas.

That is because sections of the fault in the northern and southern parts of the state are locked in place as pressure from plate movement builds. The portion of the fault in Central California creeps along almost imperceptibly, they say, providing a slow release of some of that pressure.

Some of California’s largest earthquakes have rocked Southern California and Northern California, where seismic pressure has built up the most.

In the 2014 study, researchers determined a quake that starts at either end of the San Andreas could ripple along its length—producing a rupture extending hundreds of miles, such as the 9.0 temblor that devastated Tohoku, Japan, in 2011.

“Scientists weren’t really sure if you could have a rupture through the creeping section of the San Andreas,” said Morgan Page, a USGS research geophysicist who participated in the 2014 study. “Now we think it’s not very probable, but it is possible.”

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