An impressive earthquake swarm continues to rattle southern California, making locals and scientists wonder what is next. Several hundred earthquakes have struck an area near the Salton Sea in the last 36 hours, with more than 150 2.5 or greater magnitude earthquakes in just the last 24 hours. The strongest of the swarm so far a a 4.9 that struck yesterday evening at 5:31 pm local time. The swarm continues today, albeit at a lower rate, with 4 magnitude 3 earthquakes recorded in the first 7 hours of today, compared to 40 magnitude 3 and larger earthquakes recorded in the last 7 hours of yesterday.
These earthquakes are located in an area of diffuse seismic activity between the San Andreas fault in the north and the Imperial fault to the south. This area has also seen swarms in the past –notably the 1981 Westmorland swarm, which included a 5.8 earthquake, and the 2012 Brawley swarm, which included a 5.4 earthquake. According to the USGS, past swarms have remained active for 1 to 20 days, with an average duration of about a week. The current swarm is occurring about 25 miles to the south of the swarm that occurred near Bombay Beach in August 2020.
In a typical week, there is approximately a 1 in 3,000 chance of a magnitude 7+ earthquake in the vicinity of this swarm. During this earthquake swarm, the probability of larger earthquakes in this region is significantly greater than usual – about 1 in 300. The swarm continues to evolve, and we expect to update this forecast with more specific probability information as we collect more data.
The USGS believes there are three possible scenarios that could unfold between now and October 8: earthquakes will continue, including a quake up to a 5.4, an even larger quake will strike within the next 7 days (of 5.5 to 6.9), or a major earthquake greater than 7.0 could strike.
According to USGS, the most likely scenario is the first: earthquakes continue, possibly including earthquakes up to magnitude 5.4. This same scenario also believes the rate of earthquakes in the swarm will decrease over the next 7 days. While the rate would decrease, there could be some additional moderately sized earthquakes (4.5 to 5.4), which could cause localized damage, particularly in weak structures. Smaller magnitude earthquakes (3.0+) may be felt by people close to the epicenters. The USGS believes there’s a 90% chance that this is the scenario that’ll unfold.
For the second scenario, the USGS says there could be a larger earthquake with a magnitude of 5.5-6.9 sometime over the next 7 days. However, USGS says there’s only a 10% chance of this happening. If this type of larger earthquakes did occur, it 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. This scenario occurred in a previous swarm in the area – in 1981, when a swarm in this region included a magnitude 5.8 earthquake.
The third scenario is the least likely but the most violent. In this scenario, where there’s a 1 in 300 chance of it occurring, a much larger earthquake (magnitude 7 or higher) could occur within the next 7 days. This much less likely scenario, compared with the previous two scenarios, is that the ongoing swarm could trigger an earthquake significantly larger than the 4.9 that occurred yesterday, with a magnitude 7.0 or greater earthquake possible. While this is a very small probability, if such an earthquake were to occur, it would have serious impacts on communities throughout the area.
Coincidentally, California launched today a “Don’t Get Caught Off Guard” campaign to raise awareness of California’s earthquake warning system which detects the start of a quake and sends alerts so that people can protect themselves before shaking arrives. This campaign has been in the works well before this swarm. Another preparedness event happens in 2 weeks: the annual Great California ShakeOut, in which millions of people practice the “drop, cover, hold on” response to earthquakes, is scheduled for October 15.