More than 400 earthquakes have rattled the Inland Empire in the past week, but seismologists on Monday said they did not believe they were foreshocks to a larger temblor in the region.
The quake swarm, centered in the Glen Avon area of Jurupa Valley, began on May 25. Since then, hundreds of small tremors have been recorded, according to Robert Graves, a seismologist with the U.S. Geological Survey.
The largest registered as a magnitude 3.2 and struck about 4:30 a.m. on Monday, Graves said. The smallest measured around magnitude 0.8.
“This activity is probably related just to the ongoing tectonic stress that builds up in Southern California and is being relieved on some very small fault structures that do not reach the surface,” he explained.
Most of the quakes are relatively shallow, hitting at a depth of about 1 to 2 miles beneath the surface, which — according to seismologist Lucy Jones — is the reason they’re being felt.
“When the quake Is only 1 mile down, instead of 5 miles or more, you are that much closer to the event,” she tweeted Friday.
Because there are no large or active fault structures in the area, seismologists do not expect the current swarm to lead to bigger, damaging quakes.
“It’s highly unlikely that this type of activity we’re seeing today in this area will lead to a larger earthquake,” Graves said.
While more tremors are anticipating in the coming days, he also noted that swarms typically don’t produce temblors with magnitudes larger than 4.0.
The Glen Avon area is no stranger to seismic activity, having experienced similar swarms earlier this year. The last one produced about 100 quakes between February and March, according to Graves.
Another one occurred in the same region last year between June and July. In fact, the area has experienced a flurry of small seismic activity dating back to the 1980s, he said.
It’s considered to be a “perennial hotspot of small quakes,” Jones said on Twitter. “This is just a place where the earth sputters along instead of letting go all at once.”
Nevertheless, seismologists urged people to prepare for large quakes – something in Southern California that is a matter of when, not if, according to Graves.
Earthquake preparation tips can be found on USGS’s website here.