An earthquake swarm under the Salton Sea appears to be calming down after a day, but scientists continue to keep a close eye on the region.
By Alka Tripathy-Lang, Ph.D (@DrAlkaTrip)
Citation Tripathy-Lang, Alka, (2020), Salton Sea Swarm quiets down, Temblor, http://doi.org/10.32858/temblor.110
On Sunday, August 9, a magnitude-5.1 earthquake struck North Carolina, surprising the east coast of the United States. Not to be left out, early Monday morning, southern California hosted an earthquake swarm of its own. These mostly small temblors began with a magnitude-3.2 earthquake at 6:33 a.m., with the largest magnitude-4.6 event reported at 8:56 a.m. None of these quakes nucleated along the San Andreas Fault.
On its first day, the Salton Sea Swarm boasted 54 earthquakes according to the U. S. Geological Survey (USGS), whereas Tuesday, the region produced only 10 quakes greater than or equal to magnitude-2.0. Approximately 8 miles (less than 13 kilometers) north of the swarm, the southernmost tip of California’s notorious San Andreas Fault slumbered through these disturbances, and continues to do so as of this writing. Of utmost concern is whether the San Andreas will keep quiet, and also what significance this swarm might hold for the faults to the south of the Salton Sea.
The Salton Sea
To people other than scientists and Californians, the Salton Sea may seem like some distant, rather mysterious body of water. This saltwater lake sits 236 feet below sea level as of January 2018, only 5 feet higher than the lowest point in the United States, and has a peculiar history. The Salton Sea resides in the lower basin of a prehistoric lake—the ancient Lake Cahuilla—that was formerly one of the largest lakes in North America. Lake Cahuilla periodically dried and refilled over the past 2,000 years. In the early 1900s, efforts to tame the Colorado River resulted in a ruptured irrigation canal that caused the great river to switch its course from the Gulf of California into the Salton Basin, where it flooded the formerly dry Imperial Valley lakebed for two years.
The Salton Sea lounges in a valley where Earth’s crust stretches apart. Along the San Andreas Fault, the Pacific plate travels northward at a rate of approximately one inch per year (or 25-35 mm per year) relative to North America, says Caltech seismologist Men-Andrin Meier. This movement becomes more complicated where the San Andreas Fault terminates, somewhere below the water. To the south, Meier says, the plates instead begin to move away from each other, forming the Gulf of California.
The locus of the Salton Sea Swarm lies in submerged faults near the southern end of the San Andreas. Called the Brawley seismic zone, this extensional region connects the San Andreas with the Imperial Fault in southern California. Small quakes along both the Brawley seismic zone and Imperial fault during the past several days indicate that although the behemoth to the north seems to be sleeping, these faults are curiously awake.
Not the first time
A swarm, says Meier, is distinct from a mainshock-aftershock sequence, where both the magnitude and rate of activity decreases with time. With swarms, he says, “instead of starting with a big bang, they start more gradually and then [often] fizzle away.”
The Salton Sea region has been hosting swarms since at least the 1930s, before scientists could detect earthquakes smaller than magnitude-3.0, says Sue Hough, a USGS seismologist. These previous swarms could have been as short as a day, or as long as a few weeks, she says, but the average length is about one week. The underlying driver for such events, she says, could be fluid moving around, or incredibly slow, incremental movement along faults.
The most recent swarm, from 2016, Hough explains, “was interesting because it had three distinct bursts over a couple of days.” Although this 2020 Salton Sea Swarm appears to be quieting significantly after the first day, she says “there is precedence for swarms quieting down and picking back up, so it will be awhile before anyone is comfortable saying ‘it’s over.’”
Most of these swarms appear to jostle the region, and sputter out, says Meier. But he cautions that each and every one of these tremors “could potentially be the one that triggers the big one, and that’s why we’re worried about it.”
Triggering earthquakes
When a smaller quake triggers a big one, Hough says, they need to nucleate in the same area. A tiny temblor in the Salton Sea will not result in a big quake in San Francisco, for example. In the case of the Salton Sea Swarm, she says, these relatively small quakes could trigger bigger events within about ten kilometers of the current zone of activity. The southern tip of the San Andreas falls within that zone, but the magnitude and exact location of the triggering quake plays a role. Based on the activity thus far, the San Andreas appears to be ignoring these annoyances to the south.
If a larger earthquake is (or were) triggered, its magnitude dictates how long its rupture might be, and what parts of California could be affected. For example, she says, “a magnitude-7.0 could extend for 100 kilometers northward [along the San Andreas, getting] close to Indio and Palm Springs.”
To prepare communities for possible larger earthquakes related to swarms, Hough says, the USGS doesn’t use the established template for a typical aftershock forecast because “swarms have their own rules.” Instead, she explains, they use published models for how swarms tend to behave, and they put together a seven-day forecast by thinking about each possible scenario, updating it as necessary. The USGS forecast for the Salton Sea Swarm includes three possible scenarios, each less likely than the previous. Hough says that each scenario’s probabilities are front-loaded, meaning, “if something is going to happen, it’s going to happen quickly.”
In the first, and most likely scenario, Hough says, the swarm would continue, but no earthquake larger than magnitude-5.4 would jolt the region. Minimal localized damage may occur, and people may feel the ground shake if they’re near the epicenter. For the seven-day forecast issued August 10, the USGS assigned this scenario an 80% chance of happening. After the decrease in the number and size of earthquakes on day two, they have since updated the seven-day forecast to 98% likely, meaning that earthquakes less than or equal to magnitude-5.4 are most likely to occur.
The second scenario focused on moderate earthquakes between magnitude-5.5 and 6.9, which would certainly cause local damage. The initial forecast on August 10 gave a 19% chance of such an earthquake striking, but after a day of decreased activity, this chance plummeted to 2% in the updated seven-day forecast for August 11-19.
The third, least likely, scenario of a magnitude-7.0 or above rupturing in the Salton Sea region had a 1% chance of happening on August 10. Though that chance seems diminutive compared to the other scenarios, it is significantly elevated compared to the typical chance of 0.001% for a magnitude-7.0 or greater earthquake on the southern San Andreas Fault, says Hough. However, on August 11, the USGS updated the forecast to say that this scenario has less than a 1% chance of happening.
Of these forecasts and probabilities, Hough says “the time to be on your toes is right after an earthquake happens. That’s always the highest probability of something triggering [a bigger earthquake].”
Big quake scenarios
“One of the scenarios in which the big one could unfold is by starting at the southern end of the San Andreas and rupturing northward,” says Meier. “We know that earthquakes can trigger each other, so if you have a critically stressed fault, you don’t need a lot of perturbation,” he says.
Whether the San Andreas is critically stressed in the Salton Sea region remains an open question. However, Hough says, “We all worry about the San Andreas, but a Salton Sea swarm could also trigger a larger earthquake towards the south.” She points out that almost 700,000 people live in Mexicali, just south of the U.S.-Mexico border.
“We should always be prepared, but many of us are probably not,” says Meier, pointing out that much of our infrastructure isn’t ready. “For most of these swarms, nothing happens afterward, but sometimes, it does,” he says, like the L’Aquilla earthquakes in Italy. In that tragedy, an official made flippant comments about an ongoing swarm in the region that culminated in a magnitude-6.3 earthquake that killed more than 300 people. “Just because most of these swarms don’t result in a bigger earthquake doesn’t mean that it’s not dangerous,” Meier cautions. This means that if you haven’t prepared for a big earthquake and you live in earthquake country, now is the time to get your earthquake kit in order and remember to drop, cover and hold on if you feel the earth start to shake.
Check your earthquake risk at Temblor.
Further Reading
Ebel, J., 2020, Magnitude-5.1 earthquake rattles southeastern U.S., Temblor, http://doi.org/10.32858/temblor.109.
Hauksson, E., Meier, M.-A., Ross, Z.E., and Jones, L.M., 2017, Evolution of seismicity near the southernmost terminus of the San Andreas Fault: Implications of recent earthquake clusters for earthquake risk in southern California, Geophysical Research Letters, v. 44, p. 1293-1301, https://doi.org/10.1002/2016GL072026.
Hobbs, T., 2019, Over 66 million people participate in international ShakeOut; prompts evaluation of personal preparedness, Temblor, http://doi.org/10.32858/temblor.052.
Stein, R.S. and Sevilgen, V., 2016, Italy earthquake leaves seismic gaps that were last filled by three large earthquakes in 1703, Temblor, http://doi.org/10.32858/temblor.006
Wallace, R.E., 1990, The San Andreas Fault System, California; USGS Professional Paper 1515.