Quake rocked deep, undetected fault
The earthquakes that shook Northern California over the past two days were a surprise to geologists in only one way — one of them ruptured an area in Contra Costa County where no faults had been detected before.
A 4.5 earthquake struck at 10:33 p.m. Monday near Pleasant Hill and a 3.4 quake struck in the same area at 7:11 p.m. Tuesday. Also Tuesday, a 4.7magnitude quake struck at 12:42 p.m. about 10 miles southeast of Tres Pinos, near Hollister, in San Benito County, the U.S. Geological Survey reported. There were no immediate reports of serious injuries or damage but the cause of an explosion at a fuelstorage facility in Contra Costa County was under investigation.
The temblor south of Hollister happened in a mountainous area off Highway 25 on the San Andreas Fault. Keith Knudsen, deputy director of the USGS Earthquake Science Center, said it was the 10th earthquake with a magnitude of 4.0 or greater in the area in the past 20
years.
But it was the first Pleasant Hill quake, felt by tens of thousands of people, that got the geologists’ attention because it was on a fault 9 miles below the surface that was previously unknown.
David Schwartz, a retired geologist still active with the U.S. Geological Survey, said the rupture in Contra Costa County occurred on one of thousands of small cracks that snake outward from a halfdozen or so major California faults, such as the San Andreas and Hayward.
“These big faults can be locked, and the stress builds up to the point until the rock can’t hold anymore and it slips. But between these faults there are all these little fractures,” Schwartz said. “No active structure has been identified at the surface in the area of the most recent earthquake, but 2 miles to the east of that is the Concord Fault, one of the big structures in the Bay Area.”
Schwartz said Monday night’s rupture in Pleasant Hill was about a mile long. That’s tiny compared with the major faults, like the San Andreas, which had a 300milelong fracture in 1906, he said.
It is a different kind of undocumented crack, called a “blind thrust fault,” that geologists fear because they have been responsible for damaging quakes in the past. Schwartz said that in thrust faults one side of the Earth’s crust moves up over the other side, creating a dangerous thrust motion.
The 6.5magnitude Coalinga earthquake in 1983 and the 6.7magnitude Northridge earthquake, which hit the San Fernando Valley in 1994, both occurred on blind thrust faults.
The Loma Prieta earthquake, which occurred on a relatively quiet patch of the San Andreas Fault, also partly involved a blind thrust fault, Schwartz said. Loma Prieta, which occurred 30 years ago Thursday, was a magnitude 6.9 and took 63 lives.
The common element in all of these large quakes is that the fault was below ground, did not show a mark on the surface and had not been detected previously.
“Earthquakes like Coalinga really alerted us to the hazards of blind thrusts that have the potential to produce significant earthquakes,” Schwartz said, adding that the USGS has dedicated a significant amount of time and technology trying to identify previously unknown thrust faults.
The small and large quakes are the result of the Pacific and North American tectonic plates slipping past each other at a rate of about 40 millimeters a year, causing cracks in the crust as pressure builds.
Small quakes may have little impact, but if they occur on mapped or unmapped faults, they should serve as a warning, Schwartz said.
“Any earthquake of this magnitude can occur anywhere in the Bay Area at any time,” he said. “I think this was a nice, small example of the larger event that we have in our future. It’s a reminder for people that you can run, but you can’t hide. We are going to have a big earthquake in the Bay Area.”