"If the Garlock happens, yes, we will be saying the San Andreas is at increased risk," Jones said. "What do you do when there's an earthquake that could be a foreshock to the San Andreas? What do you say? What do you do?"
The study is the latest suggestion of a plausible scenario in which last summer's earthquakes in a remote part of California might have started a chain of events that could result in a devastating earthquake on the San Andreas fault that has not been seen in Southern California in 163 years.
At its closest, the San Andreas fault comes within 35 miles of downtown Los Angeles.
"Now, you can think of the Ridgecrest earthquake as being so far from Greater Los Angeles ... that it is nearly harmless," said Stein, an earthquake scientist emeritus of the U.S. Geological Survey and adjunct professor of geophysics at Stanford University.
"But the problem is that ... the Ridgecrest earthquake brought the Garlock fault closer to rupture. If that fault ruptures -- and it gets within about 25 miles of the San Andreas -- then there's a high likelihood, maybe a 50/50 shot, that it would immediately rupture on the San Andreas," Stein said. Stein's co-author on the study is Shinji Toda, of Tohoku University in Japan.
If the Garlock fault did rupture close to the San Andreas fault -- but the San Andreas did not immediately rupture -- Los Angeles would face the prospect of having a metaphorical sword of Damocles hanging over its neck, Stein said, with the prospect of L.A. facing a larger risk of a San Andreas quake within a matter of months, or perhaps decades.
"In a way, if the fault ruptures all at once, life is simpler. It's done," Stein said. "But if it doesn't -- if it hangs, and plenty of faults do hang -- that would put the city in a really difficult ... position."
A hypothetical magnitude 7.8 quake on the San Andreas could cause more than 1,800 deaths, injure 5,000, displace some 500,000 to 1 million people from their homes and hobble the region economically for a generation. A quake of that magnitude produces 45 times more energy than the 1994 magnitude 6.7 Northridge quake.
Out of the many faults in California, the San Andreas is singularly poised to be the one that unleashes a megaquake in our lifetime because it is the main tectonic plate boundary between the Pacific and North American plates, and because of how fast the fault accumulates seismic strain.
Another troubling scenario Jones has mentioned before was a hypothetical magnitude 6 earthquake at the Cajon Pass north of San Bernardino. It's a particularly vexing scenario because such a quake could trigger large quakes on three major faults: the San Andreas, the San Jacinto and the Cucamonga.