California is certainly no strangers to earthquakes, but a swarm of almost 200 small earthquakes–and three slightly less-small ones–has generated a slight stir. Over the course of Monday, September 26, the quakes struck the Salton Sea, a shallow lake in Southern California that sits directly on the San Andreas fault.
The USGS reports that the swarm kicked off in the morning, and by night nine earthquakes had registered over 3.0 on the Richter scale; two temblors late in the day were recorded as having a magnitude of 4.2 and 4.3.
According to the LA Times, the majority of the quakes were centered around the city of Bombay Beach, on the lake’s eastern side, and were felt in surrounding nearby areas.
Salton Sea itself is surprisingly prone to such swarms; in 2009 over 200 quakes were recorded in one day at the lake. This slightly peculiar phenomenon occurs for a few reasons; the Salton Sea is what is known as a rift lake, formed from tectonic shifts. Not only does the San Andreas fault pass beneath the waters, but so too does the San Jacinto Fault, Imperial Fault Zone, and what is known as a “stepover fault” shear zone system.
Which alone would make it a pretty earthquake-hospitable region. But on top of that, the crust beneath the lake is exceptionally thin, meaning that hotter material from deep inside the Earth is able to get closer to the surface than in other areas, and generate temblors.
But while 200 earthquakes in one day might sound alarming, experts don’t appear to be particularly concerned. As Dr. Lucy Jones, a seismologist who previously worked for the USGS explained in a tweet:
M4s near San Andreas increase chance of big EQ by a little bit. But we have swarms without big EQ – most likely nothing more will happen
— Dr. Lucy Jones (@DrLucyJones) September 27, 2016