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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.”

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-- Abby Sewell and Kate Mather

 
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