The US Geological Survey (USGS) has identified a swarm of more than 240 earthquakes that have occurred in California, between the San Andreas fault and the Imperial fault, in what could be the prelude of a great earthquake to happen in the next days.
USGS, Situation Prediction for the Westmoreland Swarm Commencing September 30, 2020.
On September 30, 2020, southeast of the Salton Sea Salt Lake, in the Brawley seismic zone, a swarm of earthquakes began , the largest of which had a magnitude of 4.9. It happened at 5:31 PM PT on September 30th.
This earthquake and its associated swarm are located in a zone of diffuse seismic activity between the San Andreas Fault in the north and the Imperial Fault in the south:
There have also been swarms in the area in the past – notably the Westmoreland swarm in 1981, which included the M5.8 quake, and the Brawley swarm in 2012, which resulted in the M5.4 quake. Past swarms remained active for up to 20 days with an average duration of about a week. The current swarm occurs about 40 kilometers (25 miles) south of the swarm that occurred near Bombay Beach in August 2020.
In a typical week, the probability of a 7+ earthquake in the immediate vicinity of this swarm is about 1 in 3000. During this swarm of earthquakes, the probability of larger earthquakes in this region is significantly higher than usual – it leaves 1 in 300. The swarm continues to evolve, and we expect to update this forecast with more specific probability information as more data is collected
The following three scenarios describe the possibilities of what could happen from October 1-8.
Only one of these scenarios will happen during the next week. These scenarios include the possibility of earthquakes within the Imperial and San Andreas Fault, as well as beyond.
1. Scenario one (most likely: 90%): earthquakes continue, possibly including earthquakes with a magnitude of 5.4.
The most likely scenario is that the frequency of earthquakes in the swarm will decrease over the next 7 days. Some additional earthquakes of moderate magnitude (M4.5 to 5.4) may occur, which can cause localized damage, especially in weak structures. Earthquakes of lower magnitude (M3.0 +) can be felt by people close to the epicenters.
2. Scenario two (less likely: 10%): a larger earthquake (magnitude 5.5 to 6.9) may occur within the next 7 days.
A less likely scenario is that a slightly stronger earthquake (up to M6.9) could occur. Earthquakes of this size can wreak havoc on an area close to the quakes that have already occurred and will be followed by tremors that increase the number of smaller earthquakes per day. This scenario took place during the previous swarm in the area, in 1981, when a swarm in the region triggered a magnitude 5.8 earthquake.
3. Scenario three (least likely: about 1 in 300): A much larger earthquake (magnitude 7 or higher) could occur over the next 7 days.
A much less likely scenario, compared to the two previous scenarios, is that the ongoing swarm could trigger an earthquake significantly stronger than M4.9 that occurred on September 30th (i.e. M7.0 and higher). Although it is very unlikely, if such an earthquake occurs, it will have a serious impact on nearby communities and will be accompanied by tremors that will increase the number of small earthquakes per day.
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