Southern California is an epicenter for earthquakes, with an estimated 10,000 small temblors hitting the region every year. Now, there’s a discovery of a new, potentially significant fault line located 120 miles east of San Diego. It runs parallel to the notorious San Andreas fault, said Neal Driscoll, professor of geology and geophysics at Scripps Institution of Oceanography.
“It’s located in the Salton Sea along the eastern margin,” Driscoll said. “This fault has a component of what we call ‘dip-slip.’ It has a component of extension, where the plates are pulling apart, as well as a component of strike-slip, like the San Andreas,” Driscoll explained.
Driscoll and his team recently mapped out the Salton Trough Fault after spending hours, for years, in a boat and along the shore of the Salton Sea, collecting and analyzing data with a host of high-tech imaging instruments.
“It allowed us to see these deformation patterns that constrain the location of this new fault," Driscoll said. "But we need further work. We don’t know how it interacts with the San Andreas, either in time or depth.”
The fault could be as long as 40 kilometers (about 25 miles), he said, enough to produce a large-magnitude quake.
“It appears geologically active. It has deformation in the sentiments right up to the seafloor in the Salton Sea,” Driscoll said. “We don’t have any historical earthquakes on this fault, but faults of this length could engender a magnitude 6, maybe even a 7."
The new fault discovery comes as a swarm of more than 200 small-to-moderate quakes in the same vicinity triggered an alert last week from USGS of a potential major rupture on the San Andreas. Driscoll said there’s no correlation between the new fault and the swarm.
“It looks like any triggering mechanism is starting to abate,” he said.
“If we saw the size of the earthquakes increasing back up to magnitude 4s or 5s, that would be something that we’d want to keep an eye on,” Driscoll said.
The last major temblor to hit the San Diego County was 216 years ago. A magnitude 6.5 quake jolted the region on Nov. 22, 1800.
Since then, shaking has been relatively quiet, with the exception of some distant quakes, including the Easter earthquake in Mexico in 2010.
The last time the southern section of the San Andreas Fault ruptured was in 1680, more than 330 years ago. Scientists estimate a big rupture usually occurs once every 150 or 200 years.
The county is surrounded by several active faults capable of unleashing large quakes up to a magnitude 7 or higher, including the San Jacinto, Elsinore and offshore faults.
Driscoll said it’s a good reminder to put together an emergency kit with food, water and supplies.
“We do live in earthquake country," Driscoll said. "But remember, be prepared, not scared.”
