How likely is ‘the Big One’ earthquake to hit west coast after recent LA tremors?

Low-level quakes have struck Los Angeles this year, but it’s unlikely they’ve upped the chances of a more serious one rocking California

Josh Marcus
San Francisco
Tuesday 29 June 2021 22:56 BST
Los Angeles newsroom shakes live on air as earthquake hits California

California is about 80 years overdue for “The Big One”, the kind of massive earthquake that periodically rocks California as tectonic plates slide past each other along the 800-mile long San Andreas fault.

But recent quakes striking California, including clusters of low-level tremors in Southern California in June and a “swarm” in April, don’t seem likely to increase the chances of this disaster, which experts believe could cause 1,800 deaths and a $200bn bill when it finally strikes.

Los Angeles experiences an average of five earthquakes a year with magnitudes between 3 and 4, putting recent quakes within the normal range of size and frequency.

Overall, the US Geological Survey says there are 31 and 20 per cent probabilities of an earthquake measuring magnitude 7.5, nearly Big One status, occurring in Los Angeles and the San Francisco Bay Area, respectively.

The last Big One occurred in 1906, a 7.9 quake that moved 300 miles of fault, razed large parts of San Francisco, and killed more than 3,000 people in the deadliest earthquake in US history.

When the Big One hits, it will be 44 times stronger than the Northridge earthquake of 1994, which killed 72 people, injured 9,000, and caused $200 billion in damage.

Predicting when earthquakes are going to happen remains difficult, San Jose State University geologist Kimberly Blisniuk told the Los Angeles Times in March.

“The San Andreas fault is one of the best studied faults in the world, and there’s still so much we can do,” she said.

Last year, researchers concluded that a pair of major southern California quakes in 2019, registering 6.4 and 7.1 magnitudes, slightly raised the chances the Big One could strike, though the probability remains low, with about a 1 per cent chance of a major quake along the San Andreas over the next year.

“What’s next is a really tough problem for us,” seismic expert Dr Kenneth Hudnut told The New York Times, following the release of the study. “But it’s what everybody wants to know,” adding, “But just because we can create a plausible scenario does not mean it’s going to happen.”

Some scientists argued even the 1 per cent forecast was making too big a claim.

“The trouble is the uncertainties on all the numbers in this sequence are huge,” Seth Stein of Northwestern University told Science Magazine. “And you’re cascading those all together.”

There’s just not great way to know when exactly The Big One may come, according to seismologist Dr Lucy Jones.

“I’m surprise we haven’t had it yet — we average 150 years between San Andreas events and it’s been 350 on the southernmost part,” she wrote on Twitter in May. “But we could easily go another 50 — or more. The time between big quakes seems to be Poissonian - that means the time since the last quake doesn’t matter.”

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