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