Remember the rather minor 5.8 earthquake that shook the east coast last year? It was hardly strong enough for many to feel, but still, buildings trembled, and people’s nerves did, too. Southern California experienced a similar small-scale quaking this weekend — except it happened 300 times in a day.
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. So what is an earthquake swarm, exactly?
For one, it’s not an aftershock. Don Blakeman, a geophysicist with the National Earthquake Information Center, explained swarms are better described by what they are not. “Typically when an earthquake occurs, you have a large earthquake followed by a bunch of aftershocks, which a lot more earthquakes in the same spot,” he told TIME. “That’s not a swarm. A swarm usually doesn’t have the large earthquake beginning, but is instead a series of earthquakes of similar low magnitudes that occur in the same region.” , there were roughly 50 earthquakes an hour on Sunday.
Swarm sequences generally last a few days or weeks, said Lucy Jones, a seismologist with the U.S. Geological Survey (USGS). Traditionally, swarms will have many earthquakes within a day or a week before tailing off. After Sunday’s whopping 300 earthquakes in the area around Imperial County, east of San Diego, Monday only had about 100, Jones said.
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The swarm’s quakes generally weigh in at around 2 or 3 on the Richter Scale, but some can range up to the high 4′s, said Blakeman. Jones said that a magnitude of 3 is usually noticeable, while high 2’s are felt if it’s a shallow quake striking near a population center. Generally, earthquakes that small will only cause minor damage such as items falling off of shelves. But the bulk of the damage came during a few outlier temblors. According to the Los Angeles Times, the strongest quakes in Sunday’s swarm were 5.3 and 5.5.
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 [farther north near Death Valley] because of the hotter temperatures, weaker crust, and presence of fluids,” says Jones.
Other areas of high volcanic and swarm activity are Yellowstone National Park and the Mammoth Lakes area of California. Swarms can happen in any area, though. As with all earthquakes, there is no way to predict where or when an earthquake swarm will happen other than through history, Blakeman said. Though swarms do not usually foreshadow a larger, more destructive quake, Jones said it is important to take proper steps during the sequences, like putting away china and filling up a bathtub with water in case water supply cuts off.
After all, swarms are not necessarily harmless. And they sometimes 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.