Quake swarm near California-Mexico border: Why no cause for concern this time?

The California city of Brawley rang in the New Year with a swarm of more than 250 small earthquakes near the Mexico border.  

The strongest earthquake to shake the rural city, located 170 miles southeast of Los Angeles, was magnitude 3.9, according to the Los Angeles Times. Most of the tremors were too weak to be felt, although the combination of quakes and New Year’s Eve fireworks spooked some local dogs who went missing, according to media reports.

The Brawley Seismic Zone, in the desert region of Imperial County, is close enough to the major San Andreas and Imperial faults that it could theoretically trigger a domino effect of quakes through the more populous Riverside, San Bernardino, and Los Angeles counties, some experts say.

But not this time, say seismologists.

"This area may have produced the most earthquakes in the entire state of California, but they are small," Lucy Jones, a Caltech seismologist who has been called "the Beyoncé of earthquakes," told CNN. "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 earthquakes in the southern end of the Brawley Seismic Zone registered magnitudes between 2.5 to 3.9, according to the US Geological Survey. The Brawley Seismic Zone extends for about 30 miles, from the city of 26,000 across the southern half of the Salton Sea.

Earthquake swarms along the Brawley Seismic Zone are fairly common, but seismologists are still decoding the physics behind them.

"Compared to the rest of California, it's thinner crust, and hotter, and [has] more fluids," Dr. Jones told The Christian Science Monitor after another swarm in 2012. She described the Imperial Valley as "one big bowl of sediment."

"Those would likely be reasons that contribute to this," she said. "But we don't yet have the details of the physics."

The Brawley area, 100 miles east of San Diego, is "like the very end of a ridge that's trying to spread open. It's actually probably giving up on spreading open," Jones said.

In 2012, the swarm of about 300 earthquakes included some with magnitudes 5.5 and 5.3. In a 2005 swarm, the strongest tremor was magnitude 5.1, according to a Caltech report.

Seismic activity in the area hasn't always limited itself to rattling windows. In 1979, a magnitude 6.5 earthquake shook nearby El Centro, injuring 91 and damaging buildings beyond repair, according to the Los Angeles Times. In 1940, a magnitude 7.1 earthquake shook the same area, leading to nine deaths, with tremors felt as far as Los Angeles.

Eight years ago, researchers simulated what would happen if an even stronger earthquake – registering 7.8 magnitude – plowed from the Salton Sea up the San Andreas Fault. They estimated that it could kill as many as 1,800 people and cause $200 billion in damage, reported the Los Angeles Times' Rong-Gong Lin II in September, just after another swarm shook Brawley.

But swarms happen all the time. A series of 600 small earthquakes jiggled the Mammoth Lakes area in the Sierra Nevada mountains in 2014, while other swarms have occurred in Idaho and Alabama.

As Jones indicated on Twitter Saturday, swarms appear to be more of an internet sensation than a seismic one.