As of 10:30am this morning local time, 1,022 earthquakes were recorded in southern California in the area around the Salton Sea over the last week, with more than half of those occurring in the last 24 hours as part of an earthquake swarm that continues there. In the last 24 hours, an earthquake has been recorded once every 2-5 minutes.
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.
This weekend’s significant 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.
The strongest of the swarm was a 5.3 earthquake which struck about 6 and a half miles west of Calipatria at a depth of 3.6 miles yesterday. 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.”
California is home to the border between the North American and Pacific tectonic plates, and this plate boundary area is normally rich with seismic activity. The San Andreas fault is the state’s longest fault, considered by experts as the most dangerous too.
The nearby San Andreas fault is overdue for a large earthquake, which USGS scientists say is likely. However, those same scientists at USGS say the exact timing for when such a large earthquake will occur isn’t exactly known and scientists aren’t sure if this weekend’s tremor is a more common swarm or a precursor to something more ominous. Even without factoring in this weekend’s swarm, scientists are concerned about the inevitable large earthquake that will strike southern California at some point in the future . USGS regional scenarios anticipate 1,800 deaths and 50,000 injuries in the event of a major San Andreas earthquake. More than 3 million homes could be damaged, at a reconstruction cost of $289 billion.
Swarms also aren’t a stranger to southern California; over the Memorial Day holiday weekend, another swarm was responsible for shaking in the Disney Land area of southern California.
The USGS has issued an earthquake forecast based on yesterday’s 5.3 earthquake. Their forecast calls for hundreds more earthquakes in the region in the coming days. For now, USGS believes the odds of a destructive larger earthquake are low; the odds of a magnitude 7+ earthquake is 1:500 while there’s only a 2% chance of a 6+ earthquake there. USGS adds there’s a 17% chance of a magnitude 5+ earthquake but a 99% chance of magnitude 3+ earthquakes over the upcoming week. “As many as 30 such earthquakes may occur in the case that the sequence is re-invigorated by a larger aftershock”, USGS cautions.
USGS did include caveats with their earthquake forecast: ” No one can predict the exact time or place of any earthquake, including aftershocks. Our earthquake forecasts give us an understanding of the chances of having more earthquakes within a given time period in the affected area. We calculate this earthquake forecast using a statistical analysis based on past earthquakes. Our forecast changes as time passes due to decline in the frequency of aftershocks, larger aftershocks that may trigger further earthquakes, and changes in forecast modeling based on the data collected for this earthquake sequence.”