A “swarm” of earthquakes that touched off Sunday morning in southern California was still rolling along Monday afternoon, registering more than 300 small to moderate quakes that could be felt from Arizona to San Diego. The swarm is unusual and rare.
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 Julie Dutton, a geophysicist with the U.S. Geological Survey, diffuse clusters like these are far less common than earthquakes that arrive as one big shake followed by a series of smaller aftershocks. The result is a near-constant trembling that’s called an “earthquake swarm,” which began in southern California on August 26 with hundreds of the tiny temblors. And residents can expect more over the next several days.
The most-impacted areas, particularly the inland Imperial County just north of the Mexican border, endured an especially bad earthquake swarm this weekend that led to power outages, damaged buildings, and several displaced families and hospital patients, the Los Angeles Times reported.
Analyzed on their own, the quakes are usually of low-enough magnitude to cause no damage. But when dozens, even hundreds, rock the ground over the course of a day, it’s sure to bring a bit of uneasiness to nearby residents.
But even earthquakes of that magnitude aren’t a huge cause of concern for Californians, who are used to the ground shaking on occasion. But 300 in a single day? As seismologists put it, the Imperial Valley area is a sweet spot for swarms due to the region’s high volcanic activity. “You’re more likely to see swarms in volcanic areas like the Imperial Valley and Owens Valley (just north of Death Valley) because of the hotter temperatures, weaker crust, and presence of fluids,” says Jones.
But the question remains: is this just the beginning of something big to come?
Most seismologists say no. But sometimes, earthquake swarms show no sign of stopping. Kate Hutton, a staff seismologist at Caltech, recalled a record-breaking sequence in Japan. That swarm, the Matsushiro Swarm, began in August 1965 and lasted more than three years. Now that’s a fabled tale to make Californians tremble.