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Swarm of earthquakes sparks worry about San Andreas’ fault

A swarm of small earthquakes in California, near the Mexican border, is being closely monitored if it could increase the risk 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. in the southeastern part of the Salton Sea, according to the U.S. Geological Survey. It was among a series that began at 06:33 with an earthquake of 3.2. Tremors of magnitude 4 struck at 09:03 and 12:29

This is only the fourth time in 88 years with modern records that such a swarm has occurred in this part of California ̵

1; a region that raises concerns among seismologists for the chance that it could trigger a significant earthquake on the San Andreas Fault.

The southernmost section of the San Andreas Fault has not erupted since 1680 to 1690. Major earthquakes on this part of the southern San Andreas Fault fault averaged 250 years – although there can be large variations in how often they actually occur.

Generally, there is a 20% risk of an earthquake of 7 or greater on this part of the San Andreas Fault over the next 30 years.

The last time a similar swarm happened was almost four years ago. The series of moderate earthquakes – the worst were two in magnitude 4.3 and one in magnitude 4.1 – increased the chance of an earthquake by 7 or greater from 1 to 10,000 to 1 in 100 and received a rare warning from the US Geological Survey .

The warning convinced San Bernardino officials to close City Hall for two days in 2016; the building was months away from being released due to long-standing concerns about its seismic safety. No major earthquake occurred.

Monday’s swarm “slightly increases the risk of a major earthquake on San Andreas,” said USGS research geophysicist Morgan Page. “So it’s definitely something to look at.”

Still, “it’s not necessarily doom and gloom,” given that the last three similar swarms – 2001, 2009 and 2016 – did not result in major, catastrophic earthquakes, Page said. “But every time it happens, we worry that it may be the time it triggers something.”

Computer simulations are underway on Monday to calculate what the increased risk of a major earthquake on the San Andreas Fault could be.

Monday’s swarm is further from the San Andreas fault than the 2016 swarm was, said seismologist Lucy Jones. Researchers have not seen an earthquake that has triggered a major earthquake more than 6.2 miles away, and Monday’s earthquake sequence so far is about 7.5 miles from the San Andreas fault, Jones said.

“So this is probably too far away,” she said before Monday’s earthquakes to trigger a major one in San Andreas. “It’s not too far off to say that it’s impossible. But probably too far away. “

However, the situation would become more urgent if the swarm starts moving north, towards the fault, she said.

San Andreas is one of the state’s most dangerous faults and can, in the worst case, release an 8.2 magnitude earthquake along a stretch from near the Mexican border through Palm Springs, San Bernardino and into Los Angeles’ mountain county, all the way up to Monterey County.

A less powerful Big One in San Andreas, a 7.8 magnitude earthquake from the Mexican border with the San Gabriel Mountains in LA County, could hypothetically kill 1,800 people; damage 5,000; shift 500,000 to 1 million; and hobble the region financially for a generation, according to a USGS simulation called ShakeOut. Such an earthquake would send strong tremors almost simultaneously to Los Angeles, Orange, Riverside, San Bernardino, Kern and Ventura counties.

The outlook has prompted efforts by California’s local governments to strengthen earthquake retrofitting laws. In recent years, Los Angeles and other cities have enacted sweeping laws that require wooden apartment buildings and brittle concrete buildings to be reinforced to avoid catastrophic collapse.

But there are still vulnerabilities. Most cities, including Los Angeles, have not required thorough inspections or retrofitting of potentially vulnerable steel frame structures. 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 retrofitted, despite decades of warnings about the risk to human lives in an earthquake.

Major earthquakes can easily occur without being detected in advance. But California has a history of minor earthquakes that preceded major and catastrophic events.

Southern California’s last mega calf, measuring 7.8 in 1857, was on the San Andreas Fault and was preceded by minor earthquakes at the northern terminal of the Southern San Andreas Fault, in Monterey County.

The first earthquakes occurred about nine hours before magnet 7.8. Then, two hours before the Big One hit, an earthquake of 6.1 struck, and an hour before the main event, an earthquake of 5.6. It started in Monterey County but rushed down to Los Angeles County in about two minutes, dropping trees in Sacramento and eradicating trees near the Grapevine section of Interstate 5.

In 1987, an earthquake of 6.2 more than 11 hours later was followed by an earthquake of 6.6 on the Superstition Hills fault. It injured $ 4 million 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 an earthquake triggered in California occurred in 1992.

A magnitude 6.1 earthquake shook Joshua Tree National Park and triggered a series that migrated north in the following months. On June 28, an earthquake 63 times stronger – a magnitude 7.3 earthquake, with an epicenter more than 250 km northeast of Palm Springs. A sleeping 3-year-old boy died after being hit by a collapsing chimney.

Three hours later, a magnitude 6.3 earthquake struck about 200 km west, just a few miles from Big Bear.

And just last year, the Ridgecrest 6.4 magnitude earthquake followed on July 4 in the Mojave Desert less than 34 hours later. with an earthquake of 7.1. The second earthquake on July 5 was the most powerful to hit California in 20 years, causing billions of dollars in damage, particularly in the city of Trona and China Lake Naval Air Weapons Station, the fleet’s largest base for developing and testing weapons of warfare.

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