A swarm of small earthquakes in California, near the Mexican border, is being closely monitored as it increases 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. below the southeastern part of the Salton Sea, according to the U.S. Geological Survey. It was during a series that began at 6:33 a.m. with a magnitude 3.2 earthquake. Magnets 4 shook at 9:03 and 12:29 p.m.
It is only the fourth time in 88 years of modern records that such a swarm has occurred in this part of California – a region that raises concerns among seismologists for the possibility that it could trigger a major earthquake on the San Andreas debt.
The southernmost stretch of the San Andreas Fault has not been broken since 1680 to 1690. Major earthquakes on this section of southern San Andreas Fault erupt on average every 250 years – although there can be wide variations on how often they actually occur.
Overall, over the next 30 years, there is a 20% chance of an earthquake of 7 or greater on this part of the San Andreas debt.
The last time a similar swarm occurred was almost four years ago. That series of moderate shakes – of which at least two were at 4.3 and one at large 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 due to low-level concerns over years of concerns about its seismic safety. There was no major shaking.
Monday’s swarm “slightly increases the chance of a major earthquake on the San Andreas River,” said USGS geophysicist Morgan Page. “That it’s absolutely something to see.”
Still, “it’s not necessarily doom and gloom,” given that the last three similar swarms – in 2001, 2009 and 2016 – did not result in major, catastrophic shocks, Page said. “But every time it happens, we worry that this may be the time it triggers something.”
In any given week, there is a 1 in 10,000 chance of a magnitude 7 earthquake on the San Andreas Fault. USGS calculations released Monday night said there is now a 1 in 100 chance of such a shelter next week. It is the same increase in risk that was calculated during the 2016 swarm.
Monday’s swarm is further from the San Andreas debt than the 2016 swarm was, said seismologist Lucy Jones. Scientists have not seen a foreshock that triggers a major earthquake more than 6.2 miles away, and Monday’s quake sequence so far is about 7.5 miles from San Andreas’ fault, Jones said.
“That this is probably too far away,” she said, for Monday’s shake-up to trigger a magnitude on San Andreas. ‘It’s not so far away that you say it’s impossible. But probably too far away. ”
However, the situation would become more important as the swarm begins to move north, toward debt, she said.
The San Andreas fault is one of the state’s most dangerous fault and could at least trigger an earthquake with a magnitude of 8.2 along a stretch near the Mexican border via Palm Springs, San Bernardino and in the mountains. from Los Angeles County, all the way to Monterey County.
A less powerful magnitude at San Andreas, a magnitude 7.8 earthquake near the Mexican border to LA County’s San Gabriel Mountains, could have killed 1,800 people hypothetically; 5,000 wounded; expel 500,000 to 1 million; and rock the region economically for a generation, according to a USGS simulation called ShakeOut. Such a shake would send strong shakes to the counties of Los Angeles, Orange, Riverside, San Bernardino, Kern and Ventura almost simultaneously.
The prospectus has encouraged efforts by California’s local governments to strengthen earthquake retrofit laws. In recent years, Los Angeles and other cities have enacted floating laws that require wooden apartment buildings and crumbling concrete buildings to be reinforced in efforts to prevent catastrophic collapses.
But there remain vulnerabilities. Most cities, including Los Angeles, do not require sweeping inspections or retrofits of potentially fragile steel frame buildings. And an analysis by the Los Angeles Times in 2018 identified hundreds of graying bricks in bricks in Riverside and San Bernardino that have been marked as dangerous and have not been reapplied, despite decades of warnings of the risk to human lives in an earthquake.
Large shocks can easily occur without pre-recognized foreheads. But California has a history of minor earthquakes prior to major and catastrophic events.
Southern California’s last megaquake, magnitude 7.8 in 1857, was on the San Andreas Fault and was preceded by smaller shakes at the northern end of the Southern San Andreas Fault, in Monterey County.
The first shakes came about nine hours before the big 7.8. Then, two hours before the Big One hit, a quake hit 6.1, and an hour before the main event occurred, a magnitude 5.6 quake struck. It started in Monterey County, but ran in about two minutes to Los Angeles County, sowing trees in Sacramento and uprooting trees at the Grapevine section of Interstate 5.
In 1987, a magnitude 6.2 earthquake more than 11 hours later was followed by a 6.6 magnitude earthquake. That one caused $ 4 million in damage and injured 94 people in Imperial County, while 3,000 people were temporarily left homeless in the Mexicali area of Baja California.
Perhaps the most famous example of triggered earthquakes in California occurred in 1992.
A magnitude 6.1 earthquake on April 22 in Joshua Tree National Park began a sequence that migrated northward in the following months. On June 28, an earthquake struck 63 times stronger – the great 7.3 Landers shook, with an epicenter more than 25 miles northeast of Palm Springs. A sleeping 3-year-old boy died after she was hit by a collapsing chimney.
Three hours later a quake struck 6.3 miles to the west, just a few miles from Big Bear.
And just last year, the magnitude 6.4 Ridgecrest earthquake struck on July 4 in the Mojave Desert less than 34 hours later. with a magnitude 7.1 shake. That second quake on July 5 was the most powerful that struck California in 20 years and caused billions of dollars in damage, particularly in the city of Trona and the China Lake Naval Air Weapons Station, the Navy’s largest base for development and testing of weapons of warfare.
