Risk of big earthquake on San Andreas fault rises after quake swarm at Salton Sea
The rumbling started Monday morning deep under the Salton Sea. A rapid succession of small earthquakes — three measuring above magnitude 4.0 — began rupturing near Bombay Beach, continuing for more than 24 hours. Before the swarm started to fade, more than 200 earthquakes had been recorded.
The temblors were not felt over a very large area, but they have garnered intense interest — and concern — among seismologists. It marked only the third time since earthquake sensors were installed there in 1932 that the area had seen such a swarm, and this one had more earthquakes than the events of 2001 and 2009.
The quakes occurred in one of California’s most seismically complex areas. They hit in a seismic zone just south of where the mighty San Andreas fault ends. It is composed of a web of faults that scientists fear could one day wake up the nearby San Andreas from its long slumber.
The San Andreas fault’s southernmost stretch has not ruptured since about 1680 — more than 330 years ago, scientists estimate. And a big earthquake happens on average in this area once every 150 or 200 years, so experts think the region is long overdue for a major quake.
The swarm actually increased the likelihood of a much more major quake in Southern California, at least temporarily.
According to the U.S. Geological Survey, as of Tuesday, the chances of a magnitude 7 or greater earthquake being triggered on the southern San Andreas fault over the next seven days were as high as 1 in 100 and as low as 1 in 3,000. Without the swarm, the average chance for such an earthquake striking on any given week is 1 in 6,000.
“Any time there is significant seismic activity in the vicinity of the San Andreas fault, we seismologists get nervous,” said Thomas H. Jordan, director of the Southern California Earthquake Center, “because we recognize that the probability of having a large earthquake goes up.”
As seismic activity drops, the probability of having a large earthquake also decreases.
Experts said it’s important to understand that the chance of the swarm triggering a big one, while small, was real.
“This is close enough to be in that worry zone,” seismologist Lucy Jones said of the location of the earthquake swarm. “It’s a part of California that the seismologists all watch.”
The swarm began just after 4 a.m. Monday, starting earthquakes three to seven miles deep underneath the Salton Sea.
The biggest earthquakes hit later that morning, a 4.3, and then a pair later at night, another 4.3 followed by a 4.1. There was another burst of activity Tuesday night.
The earthquakes hit in a sparsely populated area, less than four miles from Bombay Beach, population 171, sitting on the edge of the Sonoran Desert. When swarms hit this area — the northern edge of the so-called Brawley Seismic Zone — it’s enough to give earthquake experts heartburn. And there’s reason for that.
Just 12 hours after a 6.3 earthquake hit south of the Salton Sea in 1987, an even larger temblor, a 6.6, ruptured six miles away — the Superstition Hills earthquake.