An “earthquake storm” continued to rattle Imperial County on Sunday, with the region experiencing about 300 temblors throughout the day but causing little damage.
The majority of the quakes were low-intensity, but two reached magnitudes above 5.0 -- 5.3 and 5.5 respectively -- early in the afternoon.
"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.
Such earthquake storms are not unprecedented or unusual in that 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. A previous swarm in 1981 reached a magnitude of 5.8.
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 cause of the swarm phenomenon is not entirely understood, Graves said, although he noted that the Brawley area is a transition between known active earthquake faults, including the Imperial fault to the south and the San Andreas fault to the north.
The data collected during this event may give clues to what causes the phenomenon, he said.
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 two and a half 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.”
The USGS' "Do You Feel It" system shows the quakes were felt as far away as San Diego, Temecula and San Clemente. The 5.3 quake was also felt in Moreno Valley, Indio, National City and Palm Desert.
Lucy Navarro, who works at the front desk at the Vacation Inn in El Centro, said she felt two of the afternoon’s quakes, including the one that reached 5.3 magnitude at 12:30 p.m.
“The first one was pretty strong," she said. "That one scared me because it really shook me. I could see the building move."
The first quake — a magnitude 3.8 temblor — occurred at 10:02 a.m. about three miles northwest of Brawley, and was followed by a series of other quakes in the same general area, according to the U.S. Geological Survey.
The agency's data have changed throughout the day, but the two largest quakes measured at magnitudes of 5.5 and 5.3.
Seismologist Lucy Jones of the USGS said the activity was not uncommon for the area, a region known as the "Brawley Seismic Zone" that sits between the San Andreas and Imperial faults. Similar swarms occurred in the area in the 1970s, she said, the most recent in 1981.
"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.
"We've never seen a Brawley swarm followed by a big earthquake on another fault," she added.
The current swarm could last for days, Jones said.
La Times
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