A geologist explains how the Salton Sea quake swarm could trigger ‘the big one’

CALIPATRIA, CA - JULY 3: Foundations of buildings of an early 20th century resort that was used before the rising waters of the Salton Sea turned this hill into Mullet Island, one of the four Salton Buttes, small volcanoes on the southern San Andreas Fault, are seen on July 3 near Calipatria, California. Mullet Island, the only place for many thousands of island nesting birds to breed at the Salton Sea, will become vulnerable to attacks by predators such as raccoons and coyotes if the water level drops just a couple more feet. Scientists have discovered that human-created changes effecting the Salton Sea appear to be the reason why California's massive "Big One" earthquake is more than 100 years overdue and building up for the greatest disaster ever to hit Los Angeles and Southern California. Researchers found that strands of the San Andreas Fault under the 45-mile long rift lake have have generated at least five 7.0 or larger quakes about every 180 years. This ended in the early 20th century when authorities stopped massive amounts of Colorado River water from periodically flooding the into this sub-sea level desert basin. Such floods used to regularly trigger major quakes and relieve building seismic pressure but the last big earthquake on the southern San Andreas was about 325 years ago. Dangerous new fault branches that could trigger a 7.8 quake have recently been discovered under the Salton Sea. (Photo by David McNew/Getty Images)

A quake swarm in the Salton Sea that lasted over 24 hours from Monday into Tuesday has created a short-term increase in the risk of waking up the San Andreas fault.

“A swarm is when you have, in a tight geographic region, a cluster of earthquakes happen in rapid succession," says Kate Scharer, a research geologist with the U.S. Geological Survey. "It’s an area that often just has generic background seismicity but, for some reason, sort of pipes up for a while and lets off earthquakes.”

By the time all was said and done, over 200 earthquakes had been recorded in the more than day-long period.

The reason, Scharer says, is because the quake swarm happened below the southernmost tip of the San Andreas fault, an area that hasn’t produced a quake since 1680. They say that the odds of a magnitude-7 quake or larger happening in the southern area of the fault is as high as 1 in 100 and as low as 1 in 3,000.

“That’s related to the general statistics of how these swarms happen," says Scharer. "Some of the folks at the USGS collect data on this type of behavior, the swarm behavior, and then try to understand and model what you can expect from the future. So, in general, they would have similar forecasts, given that the magnitude distribution is fairly similar.”

The good news is that these odds decrease over time, but researchers say that it’s still a real enough risk to worry, and that a quake swarm like the one earlier this week are worth watching because, as Scharer notes, these smaller quakes could be what sets off a larger quake along San Andreas.

“Worldwide, you can see lots of cases where you have an earthquake on a subsidiary fault that’s not on the main strand that then is followed in short order by a larger rupture. The earthquakes that they had in Christchurch in New Zealand happened this way, or in kind of a bigger way, the Denali Fault rupture was kind of like that. We know that there’s some transfer of stress from these faults onto the main San Andreas due to their orientation, so there is some communication between them.”

Seismologists say the San Andreas fault is long overdue to rupture since a large-scale quake happens in the area every 150-200 years.

Guest:

Kate Scharer, research geologist for the U.S. Geological Survey