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California earthquake swarm isn’t an omen of the Big One, experts say

By Rong-Gong Lin II, Los Angeles Times –

LOS ANGELES — Ever since hundreds of earthquakes began rippling through southeastern California over the weekend, many asked the question: Could this be a precursor to the Big One?

The answer: Probably not — at least, if this swarm of quakes follows past patterns.

Certainly, the weekend’s quakes were troubling for Imperial County, which is located in one of California’s most earthquake prone regions. More than 400 earthquakes have been detected since Saturday evening, according to the U.S. Geological Survey. One local family felt 15 quakes in 2 ½ hours.

But for all the ground movement, experts said there is no evidence the earthquake swarms were a precursor to much larger quakes on longer, more dangerous faults. And scientists don’t see any immediate signs of added pressure to the San Andreas Fault, which is not far from the location of the earthquake swarm.

That makes this weekend’s swarm different than what occurred after the 2010 Easter Sunday quake that shook up the California-Mexico border. The 7.2 quake appeared to have directed tectonic stress northward, toward populated areas in Southern California. Three months after the Mexicali quake, a 5.4 quake that centered south of Palm Springs rattled the region.

Scientists said the Easter Sunday quake and its aftershocks triggered movement on at least six faults, including the Elsinore and San Jacinto faults, which run close to heavily populated areas in eastern Los Angeles County and the Inland Empire region.

For now, there is no evidence that this weekend’s swarm will trigger quakes elsewhere, U.S. Geological Survey seismologist Lucy Jones said.

No deaths have been reported from the weekend’s swarm, but the shaking was sharp enough to postpone what was to be the first day of the school year in Brawley. Local officials reported 20 mobile homes shifted from their foundations and cosmetic damage to downtown buildings in the city of 25,000.

The swarming of earthquakes has occurred before in this largely agricultural, desert region near the Mexican border. The so-called Brawley seismic zone, about 100 miles east of San Diego, has endured earthquake swarms in the 1930s, ‘60s, and ‘70s, but was quiet between 1981 to 2000, according to a report on the Southern California Seismic Network.

In fact, some swarms in the ‘60s and ‘70s included “many thousands” of earthquakes, but the largest quakes during those sequences topped out at a magnitude 5.

“Swarms are fairly typical for this region,” U.S. Geological Survey geophysicist Elizabeth Cochran said. The last significant swarm occurred in 2005, when the largest quake was a 5.1. After a few days of quakes, the shaking tapered off.

Before this weekend’s swarm, in which the top magnitudes were a 5.5 and 5.3 on Sunday, the most powerful swarm to hit the region was in 1981, when the most powerful quake reached 5.8.

There are a couple of reasons the Brawley seismic zone is prone to earthquake swarms.

The area sits at the crossroads between two different types of faults, Cochran said.

To the region’s northwest is the more familiar type of fault, where the Pacific Plate wants to grind past the North American plate, with one plate moving northwest and the other southeast.

But south of the border, the two plates are seeking to pull away from each other. (That movement is what created the Gulf of California, which separates Baja California from the rest of Mexico, Cochran said.)

Sitting at the crossroads of the different types of faults makes the area particularly volatile, Cochran said.

Another reason is the relative thinness of the Earth’s crust in that region, which allows naturally occurring heat from subterranean rock to flow closer to the surface, increasing instability.

By Monday, the swarm appeared to be decreasing in frequency, Cochran said, although she didn’t rule out the pace picking up again.

Previous earthquake swarms have gone on for days.

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