Hundreds of earthquakes have rattled Imperial County since Sunday morning as an earthquake swarm continued.
But experts say the swarm does not necessarily indicates a larger temblor is on the way.
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.
PHOTOS: Earthquake swarm damages Imperial County buildings
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
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.
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.
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.
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.
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.
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.
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