According to the United States Geological Survey (USGS), an earthquake swam has kicked-off in southern California today, producing 240 earthquakes as of 8pm local time. The largest of the swarm was a 4.9 magnitude quake that struck earlier this evening. The swarm is centered near the Salton Sea, an inland body of water located northeast of San Diego and southeast of Los Angeles. Palm Springs is located north of the Salton Sea.
An earthquake swarm, according to the USGS, is a sequence of mostly small earthquakes with no identifiable mainshock. Swarms are usually short-lived, but they can continue for days, weeks, or sometimes even months. They often recur at the same locations. Most swarms are associated with geothermal activity. Swarms are usually not tied to aftershocks. Aftershocks are a sequence of earthquakes that happen after a larger mainshock on a fault. Aftershocks occur near the fault zone where the mainshock rupture occurred and are part of the “readjustment process” after the main slip on the fault. Aftershocks become less frequent with time, although they can continue for days, weeks, months, or even years for a very large mainshock.
The USGS is evaluating seismic activity throughout southern California to better understand what is happening; it is possible earthquakes will increase in intensity or fade away.
Right now, the USGS says that the most likely scenario is that the rate of earthquakes in the swarm will decrease over the next 7 days. However, they warn some additional moderate-sized earthquakes, with a magnitude range of 4.5 to 5.4, may occur. Smaller magnitude earthquakes of 3.0 or so may also be felt by people close to the epicenters.
The USGS says it is also possible that a larger earthquake will occur. However, the USGS says this is a less likely scenario. In this case, a larger earthquake upwards of a 6.9 could rock the area. Earthquakes of this size could cause damage around the area close to the swarm and would be followed by aftershocks that would increase the number of smaller earthquakes per day.
Another scenario the USGS is investigating is whether or not this swarm is an early indication warning of a much larger earthquake. The USGS says this is the least likely scenario, compared to the other two scenarios, but it is possible the ongoing swarm could trigger an earthquake significantly larger of 7.0 or more. The USGS warns that while this is a low probability, if such an earthquake were to occur, it would have serious impacts for nearby communities.
In a statement Tweeted tonight, USGS says, “No one can predict the exact time or place of any earthquake, including aftershocks or swarms. Our forecasts give us an understanding of the chances of having more quakes within a given time period in the affected area. One uncertain aspect of this swarm is how long it will last.”
The USGS added that the chance of large earthquakes will remain elevated as long as the swarm continues.