Earthquake Swarm in California Town Gets Attention from Scientists

January 2, 2017

Los Angeles Times - A swarm of more than 250 small earthquakes have struck since New Year’s Eve near the California-Mexico border, causing unease among residents and attention from scientists.

The strongest earthquake in the sequence was magnitude 3.9, striking directly underneath the town of Brawley, about 170 miles southeast of Los Angeles.

The earthquakes struck in the southern end of the Brawley Seismic Zone, a seismically active region where tectonic plates are moving away from each other and the Earth’s crust is getting stretched out “and basically adding land,” said Caltech seismologist Egill Hauksson.

The Brawley Seismic Zone is particularly important to watch because it is the region that connects the San Andreas and Imperial faults, both of which can produce damaging earthquakes. The seismic zone extends for about 30 miles from the city of Brawley, across the Salton Sea’s southern half, and ends near Bombay Beach.

The Brawley Seismic Zone, which stretches between Brawley to an area near Bombay Beach, is important to watch because it is a region that connects the San Andreas and Imperial faults, which can produce major earthquakes. (Los Angeles Times)

Hauksson was closely monitoring the swarm that began Saturday, as there was a chance that an earthquake of magnitude 5 or larger could be triggered.“There’s always reason to be concerned for a bigger earthquake,” Hauksson said. But by Sunday night, the possibility of the swarm triggering a larger event had largely receded.

The southern Brawley Seismic Zone is close in proximity to the Imperial fault. The Imperial fault has caused two major earthquakes in recent decades.

In 1979, a magnitude-6.5 earthquake sent violent shaking into El Centro, injuring 91 and causing so much damage to the concrete Imperial County Services Building that it had to be demolished.There was major damage to the irrigation system in the Imperial Valley, a desert region that is a prolific producer of salad vegetables during the winter. Levees lining the All-American Canal, which funnels water from the Colorado River, collapsed along an eight-mile stretch.A 1979 earthquake damaged the brittle concrete columns of the Imperial County Services Building in El Centro.

The magnitude-7.1 earthquake that hit El Centro in 1940 claimed nine lives and swayed buildings as far away as Los Angeles. Irrigation systems were damaged, and railroad tracks were left warped where they crossed the fault.Earthquake swarms that occur in the other end of the Brawley Seismic Zone — to the north — could trigger a major event on the San Andreas fault, one of California’s most dangerous, that could send catastrophic shaking into Riverside, San Bernardino and Los Angeles counties.

In late September, one such swarm began in the northern Brawley Seismic Zone, with three measuring above magnitude 4. That event led the U.S. Geological Survey to warn that chances of a magnitude 7 or greater earthquake on the San Andreas fault had risen as a result of the swarm.

Another swarm of small earthquakes, topping out at magnitude 3.5, struck the town of Niland near the eastern shore of the Salton Sea on Halloween.

The last major earthquake to hit Brawley was in 2012, registering at magnitude 5.4.

Brawley Mayor Sam Couchman said the earthquakes have placed the city of 26,000 on edge since Saturday afternoon. The combination of the earthquakes and New Year’s pyrotechnics spooked some of the town’s dogs, who went missing, he said.

“We’re just kind of listening to it, and when you can hear it coming, it’ll rattle things,” Couchman said. “Last night, we had the rain, the earthquakes, and the fireworks.

“All we needed were frogs and locusts.”

January 1, 2017

CNN - Dozens of earthquakes helped bring in the New Year around Brawley, California, but they are more of a curiosity than a concern, experts tell CNN.

Caltech seismologist Lucy Jones said earthquake "swarms" aren't unexpected around Brawley -- there also was a swarm in 2012 -- because the fault-riddled region called the Brawley Seismic Zone lies between the large San Andreas Fault and the Imperial Fault.

More than 100 earthquakes have hit the region this weekend, but most are too weak, under 2.5 magnitude, to be noticed by people, said Donyelle Davis, spokeswoman for the United States Geological Survey.

Weak quakes can trigger a bigger, more dangerous quake, but Brawley, a city of about 25,000 people near the Mexican border, is too far from the San Andreas Fault for that to be much of a risk, Davis said.

"This area may have produced the most earthquakes in the entire state of California, but they are small," Jones said. "If they happened a mile away we would be concerned, but these quakes are about 30 miles from the San Andreas Fault."

Since Saturday at least 24 quakes have been 2.5 to 3.9 magnitude, according to the Geological Survey. Earthquakes of 3.9 magnitude or less generally create little or no damage.

They are common in California. As of noon Sunday, the state had had 191 earthquakes in the past 24 hours and 7,707 in the past 365 days, with the largest measuring 6.5 in Ferndale, according to the website earthquaketracker.com.

Catastrophic earthquakes seem to strike along the southern San Andreas Fault about once every 150 years, the Geological Survey said, citing studies examining the past 1,400 years. The last time an enormous temblor on the fault struck Southern California was in 1857.