More Earthquakes Likely After Swarm Jolts SoCal, Experts Say: 5 Things To Know

BRAWLEY, CA — Hundreds of tremors were felt over the weekend, leaving Southern California on edge over the possibility of powerful aftershocks to follow. But seismologists say the chance of California being rocked by a more powerful earthquake this week is unlikely.

According to the Southern California Seismic Network (SCSN), as of 6:24 a.m. on Monday, 411 earthquakes occurred near Brawley in Imperial County. The strongest was a 4.7-magnitude quake that struck around 12:10 a.m. on Sunday, about 2 miles west-southwest of Brawley about 9 miles below the surface.

There were 138 recorded "aftershocks" recorded as of 6:47 a.m. on Sunday, ranging from magnitudes of 1.1 to 3.3.

Gabrielle Tepp, a seismologist for Caltech, said swarms in recent years have typically only lasted a few days or less in that region, particularly for the larger earthquakes. However, she added, it's "certainly possible" that the seismic activity could pick up again.

RELATED: Hundreds Of Earthquakes Strike SoCal Community In Seismic Swarm

Seismologist Dr. Lucy Jones attributed the swarm over the weekend to the Brawley Seismic Zone, "a band of conjugate faults that extends from the southern end of the San Andreas to the northern end of the IV fault."

The Brawley seismic zone is near several fault lines, including Imperial fault, San Jacinto fault zone, Superstition Hills fault and the San Jacinto fault zone, Wienert fault.

"In the history of these swarms, the largest magnitude we have seen has been in the high M5s. Usually, they stay below M5," Jones wrote on Bluesky.

"The Imperial Valley has seen dozens if not hundreds of these sort of swarms over the century we have recorded," she added.

More seismic activity was expected, but seismologists say it's not likely to be significant.

RELATED: Massive Swarm Of Quakes Shake Healdsburg, NorCal: Did You Feel It?

Here are five key things to know about seismic swarms:

How long does seismic swarm activity last?

"Swarm activity in Southern California can continue for hours, days, weeks, months or years," according to SCSN.

How common is the swarm of earthquakes SoCal experienced on Saturday and Sunday?

"Swarms are a common occurrence in Imperial Valley, especially in the Brawley Seismic Zone, due to the geology in the region. It's basically a very 'broken' area with lots of smaller faults that connect the San Andreas Fault to the north and the Cerro Prieto fault to the south (the main plate boundary faults). It's also geothermally active. Over the past several years, we've usually recorded at least 1 or 2 swarms in this region per year, though not all of them included M4+ earthquakes. In 2021, there was a swarm south of the Salton Sea that produced an M5.3 as well as several M3s and M4s," said Tepp.

How common are aftershocks after a seismic swarm?

"Aftershocks are defined as smaller magnitude events that follow a large magnitude 'mainshock' -- smaller earthquakes occurring before the largest are foreshocks. These definitions can only be applied after a sequence is underway. In contrast, the definition of a swarm is a group of earthquakes with similar magnitudes, or at least no clear 'mainshock.' In other words, no single event is notably larger than the rest, so the term 'aftershock' can't really be applied for these. Swarms can last from hours to weeks," said Tepp.

Which areas in Southern California have experienced the most seismic activity?

"Most seismicity occurs in or near major active fault zones. The Imperial Valley is frequently active, and we're still recording small earthquakes around the faults involved in the 2019 M7.1 Ridgecrest earthquake. We also regularly see earthquakes around the San Jacinto fault zone and some areas of the Elsinore and San Andreas fault zones, in addition to earthquakes in other parts of southern California," said Tepp.

Is "the big one" still a possibility in California?

"Yes, there will be a 'big one' in Southern California at some point, but we can't say where exactly or when. There is usually a heightened probability of a larger earthquake after a moderate-sized earthquake for a few days or weeks, but the overall probability is still quite low," said Tepp.

The California Office of Emergency Services advises residents to have a plan in place for an earthquake. Californians are encouraged to access free earthquake preparedness resources, where they can find survival tips and download the MyShake App for earthquake alerts.