Monday's swarm is farther from the San Andreas fault than the 2016 swarm was, said seismologist Lucy Jones. Scientists have not seen a foreshock that has triggered a larger earthquake more than 6.2 miles away, and Monday's quake sequence so far is about 7.5 miles from the San Andreas fault, Jones said.
"So this is probably too far away," she said, for Monday's quakes to trigger a large one on the San Andreas. "It's not so much too far away that you say it's impossible. But probably too far away."
The situation, however, would become more concerning if the swarm starts moving north, toward the fault, she said.
The San Andreas is one of the state's most dangerous faults and, in the worst-case scenario, is capable of unleashing a magnitude 8.2 earthquake along a stretch from close to the Mexican border through Palm Springs, San Bernardino and into the mountains of Los Angeles County, all the way up to Monterey County.
A less-powerful Big One on the San Andreas, a magnitude 7.8 earthquake from near the Mexican border to the San Gabriel Mountains of L.A. County, could hypothetically kill 1,800 people; injure 5,000; displace 500,000 to 1 million; and hobble the region economically for a generation, according to a USGS simulation called ShakeOut. Such a quake would send strong shaking almost simultaneously into Los Angeles, Orange, Riverside, San Bernardino, Kern and Ventura counties.
The prospect has fueled efforts by California's local governments to strengthen earthquake retrofit laws. In recent years, Los Angeles and other cities have passed sweeping laws requiring wooden apartment buildings and brittle concrete buildings to be strengthened in efforts to avoid catastrophic collapses.
But there remain vulnerabilities. Most cities, including Los Angeles, have not required sweeping inspections or retrofits of possibly vulnerable steel-frame buildings. And a Los Angeles Times analysis in 2018 identified hundreds of aging brick buildings in Riverside and San Bernardino counties that have been marked as dangerous and have not been retrofit, despite decades of warnings of the risk to human lives in an earthquake.
Large quakes can easily happen with no detectable foreshocks beforehand. But California has a history of smaller earthquakes preceding large and catastrophic events.
Southern California's last megaquake, a magnitude 7.8 in 1857, was on the San Andreas fault and was preceded by smaller quakes on the northern terminus of the southern San Andreas fault, in Monterey County.
The first quakes came about nine hours before the magnitude 7.8. Then, two hours before the Big One hit, a magnitude 6.1 quake struck, and an hour before the main event, a magnitude 5.6 quake hit. It started in Monterey County but rushed down to Los Angeles County in about two minutes, sinking trees in Sacramento and uprooting trees near the Grapevine section of Interstate 5.