Recent quakes are a peek into San Andreas’ fury
LOS ANGELES – Faults crisscross California, producing deadly earthquakes. But whenever the ground shakes, the first thought always turns to the mightiest and most dangerous fault: the San Andreas.
This is the 730-mile monster capable of producing the Big One, the fault famous enough to be the main character in a hit disaster movie.
Scientists knew almost immediately that two large quakes that hit near Ridgecrest last month did not come from the San Andreas.
But ever since, they’ve been studying whether the quakes could cause more seismic activity from other faults – including the San Andreas nearly 100 miles away.
A new calculation conducted in recent weeks at the U.S. Geological Survey showed that there’s an extremely remote chance the San Andreas could be triggered from the Ridgecrest quakes.
“It’s slim. But it’s the difference between slim and none,” said USGS seismologist Susan Hough. “I don’t think any earth scientists are going to lose sleep that this will cascade on to the San Andreas.”
But the fault remains a source of constant anxiety, especially when ground moves. Previous quakes on the San Andreas were triggered by earlier nearby temblors. The great magnitude 7.8 quake in 1857 that ruptured 225 miles of the fault between Monterey County to the Cajon Pass in San Bernardino County was preceded by a pair of smaller quakes one and two hours earlier.
Earthquakes in Southern California in 1992, 2001, 2009 and 2016 all sparked
concerns from scientists they could trigger a major quake on the San Andreas. In some of those cases, officials even issued a public warning of heightened seismic risk. For example, after the twin quakes in Landers and Big Bear in 1992, state officials announced there was a 50% chance of another big temblor in the coming days.
None, however, caused the Big One on the southern San Andreas.
In the case of the recent temblors, the potential for the San Andreas to be triggered by the Ridgecrest quakes seems to be less of a concern, relatively speaking.
“We know that earthquakes can trigger other faults, even hundreds of miles away,” said USGS research geophysicist Morgan Page. “On one hand, the probability may go up. But it’s not a big increase compared to the risk we have year to year from the San Andreas, living in California.”
The recent calculation is highly theoretical and relies on a model known as the Third Uniform California Earthquake Rupture Forecast, or UCERF3ETAS, an epidemic-type aftershock sequence model that factors how an earthquake in one location can transfer seismic stress to a nearby fault.
Like anything being researched, it’s possible that this calculation about the San Andreas is wrong. A scenario like the one envisioned in the recent calculation – a Ridgecrest-tosan Andreas situation that spans some 100 miles – has not happened in California’s relatively short modern record.
The southern San Andreas is quite dangerous on its own and can rupture without any nudging from a distant Mojave Desert fault. And it’s accumulating seismic stress so fast that even if it did rupture soon, scientists would probably spend the rest of their careers arguing over whether the July quakes had anything to do with it, said USGS research geologist Kate Scharer.