Earthquake Swarm: 150 Quakes In 24 Hours Near Salton Sea

WESTMORLAND, CA — A massive earthquake swarm that began striking near the Imperial County city of Westmorland Wednesday continued Friday, but the U.S. Geological Survey reported that the temblors were occurring at a lower frequency with less shaking.

"The swarm continues to show activity today, with some evidence of slowing down," the agency said Friday. "There were 13 magnitude 3 or larger earthquakes recorded yesterday Oct. 1st, compared to an initial burst of 48 magnitude 3 and larger earthquakes on the evening of Sept. 30th."

The quakes began at 4 p.m. Wednesday, and just five hours later more than 100 temblors had rocked the area near the Salton Sea, with the largest striking at 5:31 p.m. and measuring a magnitude 4.9.

From 12:01 a.m. to 9 p.m. Thursday, however, the USGS reported about 50 quakes in the area — none registering over magnitude 3.9.

The seismic activity has been between the San Andreas fault in the north and the Imperial fault to the south, in an area known as the Brawley seismic zone, according to the USGS.

The agency updated its "swarm forecast" Friday, predicting that one of three scenarios could happen between Oct. 2-9.

"Only one of these scenarios will occur within the next week. These scenarios include the possibility of earthquakes on and off the Imperial and San Andreas Faults," the USGS said.

In scenario one, the earthquakes will continue, possibly including temblors up to magnitude 5.4, but the quakes will continue to decrease in frequency over the week, according to the USGS. The agency predicted there was a 93 percent chance that scenario one would happen.

In scenario two, which is far less likely, a larger earthquake — magnitude 5.5 to 6.9 — could occur within the next 7 days, the agency said, noting that there is about a 7 percent chance of this scenario happening.

In scenario three, which is least likely, a much larger earthquake — magnitude 7 or higher —could occur within the next 7 days. If it happened, aftershocks would be generated, according to the USGS.

The USGS warned Friday that residents should not let their guard down.

"In a typical week, there is approximately a 1 in 3000 chance of a magnitude 7+ earthquake in the vicinity of this swarm," the USGS said Friday. "During this earthquake swarm, the probability of larger earthquakes in this region is significantly greater than usual — about 1 in 500. The swarm continues to evolve, and we expect to update this forecast with more specific probability information as we collect more data."

Longtime desert residents have felt swarms before: A 1981 Westmorland swarm, which included a magnitude 5.8 earthquake, and the 2012 Brawley swarm, which included a magnitude 5.4 quake.

"Past swarms have remained active for 1 to 20 days, with an average duration of about a week," the USGS reported.

The USGS is advising everyone to be aware of the possibility of future earthquakes, especially when in or around vulnerable structures such as unreinforced masonry buildings.

"This swarm may lead to larger and potentially damaging earthquakes in the future, so remember to: Drop, Cover, and Hold on if you feel shaking or receive an earthquake alert powered by the ShakeAlert earthquake early warning system," the agency said. "When there are more earthquakes, the chance of a large earthquake is greater, which means that the chance of damage is greater."

No injuries or serious damage were immediately reported as a result of the swarm activity in and around the small city of Westmorland that is part of the El Centro Metropolitan Area.

The quakes were being felt as far away as San Diego County, Riverside County, and Mexico, according to the USGS.