A "swarm" of more than 250 earthquakes have rattled the border between California and Mexico since New Year's Eve, with the strongest one (magnitude 3.9) striking the town of Brawley, which is 170 miles southeast of Los Angeles.

While that may sound like a lot of earthquakes (and it is), it's not the biggest swarm to hit the area. In August 2012, more than 300 small to moderate quakes could be felt from Arizona to San Diego, though this one also originated outside the small farming town of Brawley. And in September 2016, about 200 earthquakes hit near Bombay Beach, California, which as the LA Times notes is "one of California’s most seismically complex areas ... just south of where the mighty San Andreas fault ends."

The biggest quake in the 2012 swarm measured 5.5 on the Richter scale, according to Julie Dutton, a geophysicist with the U.S. Geological Survey, who says swarms with that magnitude range arrive in Southern California at the rate of one or two per decade. She points to a 2005 swarm, which topped out with a 5.1-magnitude event. (It's worth noting that 5.5 is the cut-off magnitude at which seismologists expect to start seeing casualties in developed countries, according to USGS geophysicist Paul Caruso.)

Such swarms are unusual, but they're not as rare as you might think.

What's an earthquake swarm?

Earthquakes in California Earthquakes that arrive as one big shake followed by smaller aftershocks are more common than earthquake swarms. (Photo: Crystal Eye Studio/Shutterstock)

During an earthquake swarm, an affected area experiences a rapid-fire series of temblors that are all similarly proportioned, so that no one shock emerges as the obvious source of the rest. According to Dutton, diffuse clusters like these are far less common than earthquakes that arrive as one big shake followed by a series of smaller aftershocks.

Dutton estimates that the USGS records about 30 to 40 notable swarms a year, compared with 20,000 to 30,000 total earthquakes. Because swarms are rooted in the same kind of plate movements and stresses that cause more traditional quakes, she thinks that a large part of the phenomenon's apparent scarcity is based on semantics.

Swarms “are really hard to characterize," she told Life’s Little Mysteries. "It's all the same mechanisms. It's just a different way of finding equilibrium in the environment."

The types of plate activity that might bring about a swarm as opposed to a concentrated quake are not yet well understood by scientists, and there's no way of knowing how long a swarm will last (the 2005 swarm kept up for three weeks). But Dutton says there's no reason to believe that the Brawley swarm forebodes the arrival of a "big one."

"It's not something that we can definitely discount," she said. "But typically it's not something that happens when you have a swarm like this."

In volcanically active areas, earthquake swarms often indicate imminent eruptions, Dutton said. The only potential candidates for volcanic eruptions near Brawley are the Salton Buttes, five small lava domes that flank the Salton Sea and haven't erupted in close to 10,000 years.

Editor's note: This story was originally written in August 2012 for Life's Little Mysteries and is republished with permission here. It has been updated with new information. Copyright 2012 Life's Little Mysteries, a TechMediaNetwork company.