California Hit by 400 Earthquakes in Swarm on San Andreas Fault, USGS Warns Bigger Quakes Could Strike

A swarm of more than 400 earthquakes has hit California in the area between the San Andreas fault and the Imperial fault, with further seismic activity and potentially larger earthquakes set to follow over the next week.

The biggest earthquake that has been recorded in the swarm so far was a magnitude 4.9, which hit at 5.31 p.m. local time on September 30, but bigger quakes are a possibility.

"In a typical week, there is approximately a three in 10,000 chance of a magnitude 7+ earthquake in the vicinity of this swarm,' the U.S. Geological Survey (USGS) said in a statement. "During this earthquake swarm, the probability of larger earthquakes in this region is significantly greater than usual. Currently, the swarm is rapidly evolving, and we expect to update this forecast with more specific probability information as we collect more data."

The most likely scenario is that the rate of earthquakes in the swarm will decrease over the next seven days, though moderately sized as they may be—magnitude 4.5 to 5.4 is the estimation—they could still be significant enough to cause localised damage.

The second most likely scenario is that a larger earthquake, between magnitude 5.5 and 6.9, could hit within the next seven days.

"Earthquakes of this size could cause damage around the area close to the earthquakes that have already occurred and would be followed by aftershocks that would increase the number of smaller earthquakes per day," the USGS said.

This happened in 1981, when a magnitude 5.4 earthquake hit the region during a swarm.

Another possibility, though the weakest of the three, is that a much larger earthquake, magnitude 7 or higher, could hit within the next seven days.

The USGS says the probability of this happening is "very small." However, if such an earthquake were to occur it would have "serious impacts on communities nearby." It would also be followed by aftershocks that would increase the number of smaller earthquakes each day.

While there's no way to predict the time and place of an earthquake, statistical analysis of past earthquakes has shown that the chance of large earthquakes is elevated during earthquake swarms.

Around half of the earthquake swarms recorded in the area are over within a week, and the USGS will continue to monitor seismic activity throughout the region.

salton sea california earthquakes
CALIPATRIA, CA - JANUARY 01: Mud seen on land that was under the Salton Sea, near where the earthquake swarm was detected, on January 1, 2019 near Calipatria, California, United States. David McNew/Getty Images