California earthquake swarm is strongest in 30 years, officials say
The earthquake swarm that continued in Imperial County on Monday appears to be the strongest in three decades, officials said.
The region is know for quake swarms, even ones in the 5.5 magnitude range recorded on Sunday.
Seismologist Lucy Jones of the U.S. Geological Survey said the region is known as the Brawley Seismic Zone and sits between the San Andreas and Imperial faults. Similar swarms occurred in the area in the 1970s, she said, and again in 1981.
PHOTOS: Earthquake swarm damages Imperial County buildings
"This is a classic Brawley Seismic Zone swarm," she said. "It's relatively hot."
Experts can't predict what size temblors could come, but Jones said they have never seen a Brawley swarm produce anything larger than a magnitude 5.8 quake. That rattler was part of the 1981 swarm.
"Obviously, all this activity is related or interconnected, but it doesn't really follow the typical main shock, aftershock activity," said Rob Graves, a seismologist with the U.S. Geological Survey.
The Southern California Earthquake Data Center said the zone can produce a 6.0 magnitude quake every few decades:
The Brawley fault zone is a complex set of faults that is intricately connected to the Imperial fault zone. That connection exists, apparently, due to transfer of right-lateral slip from the Imperial fault zone to the Brawley fault zone. The area is made even more unusual by virtue of its high heat flow -- essentially, the subsurface is hotter (and thus, less brittle) due to the local thinness of the crust. Due to this and the rapid rate of slip, faults in the area are probably prone to a seismic creep. Because of the complexity of the fault system at work, it is also prone to earthquake swarms.
Such earthquake swarms are not unprecedented or unusual in the region. The most recent, Graves said, also centered near Brawley, was in 2005, when the area was shaken by hundreds of earthquakes, the largest measuring magnitude 5.1.
"We've never seen a Brawley swarm followed by a big earthquake on another fault," Graves added.
A number of families were displaced and hospital patients evacuated as a result of the most recent earthquakes.
No deaths or critical injuries were reported from the quakes, the largest of which measured magnitudes 5.3 and 5.5.Some buildings were damaged by the quakes, including 20 mobile homes that shifted from their foundations, according to the Imperial County Office of Emergency Services. The office was working with the American Red Cross to set up a shelter for displaced families at the Imperial Valley College gymnasium.
The quakes caused scattered power outages, including at Pioneers Memorial Hospital, which lost power for about three hours. Assistant hospital administrator Art Mejia said generators immediately kicked in, but officials decided to evacuate patients in case the facility had suffered structural damage.
"We decided to err on the side of caution," he said.
Patients in critical condition were transferred to other hospitals in the area or in San Diego and Riverside counties, while others were either discharged or moved across the street to a medical office building.
Mejia said Sunday evening that hospital staff and state regulators were walking through the hospital to assess the damages, but so far, the damages appeared superficial, such as fallen ceiling tiles.
He said they hoped to be able to return patients to the hospital in a matter of hours.
Officials were urging residents to conserve water Sunday, and some schools were planning to close Monday, including Brawley Union High School, schools in the Brawley Elementary School District, Del Rio Community School and Mulberry Elementary. County officials said updates would be posted on the Facebook pages of the Imperial County Emergency Medical Services Agency and the Imperial County Public Health Department, and residents with questions about issues such as school closures and water issues could call (760) 351-2686 .
The earthquakes caused cosmetic damage to at least three buildings dating to the 1930s in downtown Brawley, said Capt. Jesse Zendejas of the Brawley Fire Department. Crews were still assessing other areas of the city, he said, but no injuries had been reported.
The succession of quakes rattled Brawley resident Alfonso Alvarez, who has a business renting bounce houses and other party supplies. Alvarez, 28, said he and his family had felt 15 quakes over 2 1/2 hours and, after the biggest one, had relocated to the front yard.
"It's been pretty bad. Some of them are slow and then they get intense," he said. "We're so anxious right now we can't sit still."
http://latimesblogs.latimes.com/lanow/2012/08/california-earthquake-swarm-is-strongest-in-30-years-officials-say.html~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Earthquake swarm isn't an omen of the Big One, experts say
Swarms like the ones last weekend are typical for the Imperial County area, one of the state's most quake-prone regions, seismologists say.
 Broken glass covers the sidewalk in front of a Brawley furniture store after an earthquake Sunday. (Eric Miller/ Imperial Valley Press / August 26, 2012) |
Related photos »
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 21/2 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.
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 or serious injuries 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 this 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 is 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 grinds 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 rise 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.
http://www.latimes.com/news/local/la-me-earthquake-swarm-20120828,0,740536.story
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