An outbreak of earthquakes continues to rattle southern California, with an ongoing swarm located near the Salton Sea southeast of Los Angeles and north east of San Diego. While the frequency of earthquakes remains high, their potency is dropping off, with most earthquakes registering at a 2.5 or less magnitude.
There are some subtle changes with the swarm though: while the swarm over the weekend was located primarily over land just south of the Salton Sea, the earthquakes today have been centered under the central and southern portions of the body of water there. This northward migration of earthquake activity is somewhat similar to other north/south movements of earthquake swarms here.
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 ongoing earthquake swarm is occurring near the Salton Sea, a shallow landlocked lake with a high salt concentration in Riverside and Imperial counties of California. The Salton Sea sits near the San Andreas Fault at the southern end of the state of California. In this region, the Earth’s crust is being stretched. Most of the recent earthquakes are being tied to submerged faults near the southern end of the San Andreas Fault. Called the Brawley seismic zone, this extensional region connects the San Andreas with the Imperial Fault in southern California. Specifically, it appears much of the swarm activity is centered around the Westmoreland fault that runs through the area among dozens of fault lines that run through California.
Two weekends ago, a 5.3 earthquake struck about 6 and a half miles west of Calipatria at a depth of 3.6 miles. More than 700 residents from Los Angeles to San Diego to El Centro reported experiencing the quake to the USGS, including many that reported “strong shaking.” While the swarm continued for days, it took a pause, with no/limited activity until this past weekend when the swarm resumed.
Scientists say that while swarms aren’t rare to this area, the latest activity is somewhat out of character for the region. Dr. Debi Kilb from the Scripps Institution of Oceanography at San Diego’s University of California, took to Temblor to ponder whether the seismic events there are part of an earthquake swarm or an earthquake sequence. “Upon a deeper dive into the earthquake data, however, the latest series of quakes may actually be a mainshock/aftershock sequence. What that means for the residents of this shaken region doesn’t change: The chance of an earthquake exceeding magnitude-7 within the following month remains less than 1% . For researchers, though, these events offer a new perspective on the source physics of these earthquakes,” Dr. Kilb wrote.
While scientists like Dr. Kilb pour through seismic data to understand what is happening, why it’s happening, and what could happen next, she offers wise advice: “The take-home message here is that we live in earthquake country. We need to be prepared and have a protection and response plan in place. Building codes protect our structures, and regular quake drills sharpen our awareness. Be prepared, not scared.”