A swarm of small earthquakes in California, close to the Mexican border, is being closely monitored as to whether it might raise the chance of a much larger event on the San Andreas fault.
The largest earthquake on Monday was a magnitude 4.6, reported at 8:56 a.m. under the southeastern part of the Salton Sea, according to the U.S. Geological Survey. It was among a series that began at 6:33 a.m. with a magnitude 3.2 earthquake. Magnitude 4 quakes struck at 9:03 a.m. and 12:29 p.m.
It's only the fourth time in the 88 years of modern records that such a swarm has occurred in this part of California -- a region that raises concern among seismologists for the chance that it can trigger a significant earthquake on the San Andreas fault.
The San Andreas fault's southernmost stretch has not ruptured since 1680 to 1690. Big earthquakes on this section of the southern San Andreas fault rupture on average every 250 years -- although there can be wide variations as to how often they actually do occur.
In general, there's a 20% chance of a magnitude 7 or larger quake on this part of the San Andreas fault over the next 30 years.
The last time a similar swarm happened was nearly four years ago. That series of moderate quakes -- the worst of which were two at magnitude 4.3 and one at magnitude 4.1 -- prompted a rare warning by the U.S. Geological Survey.
The warning convinced San Bernardino officials to close City Hall for two days in 2016; the building was months away from being vacated over longstanding worries about its seismic safety. No bigger quake occurred.
Monday's swarm "does increase the chance of a big earthquake on the San Andreas somewhat," said USGS research geophysicist Morgan Page. "So it's definitely something to watch."
Still, "it's not necessarily doom and gloom," given that the last three similar swarms -- in 2001, 2009 and 2016 -- did not result in larger, catastrophic quakes, Page said. "But every time it happens, we do worry that this could be the time that it triggers something."
In any given week, there's a 1 in 10,000 chance of a magnitude 7 earthquake on the San Andreas fault. USGS calculations released Monday night said there's now a 1 in 100 chance of such a quake in the next week. It's the same increase in risk that was calculated during the 2016 swarm.
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.
In 1987, a magnitude 6.2 earthquake was followed more than 11 hours later by a magnitude 6.6 quake on the Superstition Hills fault. That one caused $4 million in damage and injured 94 people in Imperial County, while leaving 3,000 people temporarily homeless in the Mexicali area of Baja California.
Perhaps the most famous example of triggered earthquakes in California occurred in 1992.
An April 22 magnitude 6.1 earthquake in Joshua Tree National Park began a sequence that migrated north in the following months. On June 28, an earthquake 63 times stronger ruptured -- the magnitude 7.3 Landers quake, with an epicenter more than 25 miles northeast of Palm Springs. A sleeping 3-year-old boy died after being struck by a collapsing chimney.
Three hours later, a magnitude 6.3 quake struck about 20 miles west, just a few miles from Big Bear.
And just last year, the magnitude 6.4 Ridgecrest earthquake on July 4 in the Mojave Desert was followed less than 34 hours later by a magnitude 7.1 quake. That second quake on July 5 was the most powerful one to hit California in 20 years and caused billions of dollars in damage, especially in the town of Trona and the China Lake Naval Air Weapons Station, the Navy's largest base for developing and testing weapons of warfare.
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