Aftershocks inch toward big faults
After major quakes of July 4 and 5, USGS says there’s a 1-in-300 chance of a powerful temblor being set off.
Aftershocks of the magnitude 7.1 earthquake near Ridgecrest, Calif., have been creeping into areas close to two major earthquake faults, a development that is generating interest and some concern among seismologists over whether it could trigger another huge temblor.
Both faults are capable of producing new earthquakes of magnitude 7 or greater. The U.S. Geological Survey says the chance of an earthquake of magnitude 7 or greater from the July 5 earthquake is 1 in 300 — “possible, but with a low probability.”
Some aftershocks have rumbled northwest of the Searles Valley earthquake, approaching the Owens Valley fault. That fault triggered an earthquake of perhaps magnitude 7.8 or 7.9 in 1872, one of the largest in California’s modern record.
Felt as far away as Los Angeles and Sacramento, the 1872 earthquake killed 27 people — 1 out of every 10 people in the mining camp of Lone Pine — and destroyed 52 of 59 houses there.
The Ridgecrest aftershocks have also headed southeast toward the Garlock fault, a lesser-known fault capable of producing an earthquake of magnitude
8 or more.
The fault along the northern edge of the Mojave Desert can send shaking south and west into Bakersfield and Ventura and Los Angeles counties.
“Those are places we would be more concerned,” U.S. Geological Survey research geophysicist Morgan Page said. “Little earthquakes are telling us where big earthquakes are more likely.”
No one can predict exactly when and where the next big earthquake will occur in California. Some quakes can trigger seismic activity on nearby faults, but it’s not a given.
Triggered temblors
Perhaps the most famous example of triggered earthquakes in California occurred in 1992.
An April 22 magnitude 6.1 earthquake in Joshua Tree National Park began a quake sequence that migrated north in the coming months.
Then on June 28, an earthquake 63 times stronger ruptured — the magnitude 7.3 Landers earthquake with an epicenter more than 25 miles northeast of Palm Springs. A sleeping 3-yearold boy died after being struck by a collapsing chimney during a sleepover.
Three hours later, a magnitude 6.3 quake struck about 20 miles west, just a few miles from Big Bear.
“We always worry when seismicity picks up very close to a major fault or if it’s at the end of a major fault — whether it’ll push it enough to start a major rupture,” Caltech seismologist Egill Hauksson said.
Sometimes fears of seismic triggering don’t materialize.
The Easter Sunday magnitude 7.2 quake of 2010 directed tectonic stress toward Southern California from Mexico.
There was concern about a potential triggered quake on the Elsinore fault, capable of a magnitude 7 quake. It extends into Orange County and the Los Angeles area and could produce devastating damage to the region.
But seismic activity eventually ended before it reached that fault, Hauksson said.
There’s a rule to seismology that will probably come as a disappointment to many — earthquakes don’t actually reduce the risk of future quakes; they increase them.
“Every earthquake actually increases the probability of more earthquakes,” Page said.
In fact, earthquake scientists actually model quakes like disease epidemics. “It’s based on the idea on how a contagion spreads to a population,” she said. “Earthquakes are like that … in general, if there are a lot of earthquakes going on, it’s more probable for a large earthquake to go on.”
Although earthquakes do relieve stress to some areas around them, we become less safe after earthquakes because they “redistribute the stress and can push other faults in the area to failure,” Page said.
One big observation so far has been that there’s now a line of seemingly missing earthquakes between the northern end of the July earthquakes and the southern end of where the Owens Valley fault finished rupturing in 1872.
“That’s a kind of thing seismologists can get nervous looking at. It’s got to be filled in,” said USGS seismologist Susan Hough, who has researched the Owens Valley fault extensively. “There’s certainly room to put another earthquake.”
A seismic boundary
Both the July quakes and the 1872 Owens Valley quake lie in one of California’s great seismic zones, the Eastern California Shear Zone, which generates earthquakes as a result of the southwestern part of California sliding up northwest, toward Alaska, compared with the northeastern part of the state. (Yes, that does mean eventually, Los Angeles will be right next to San Francisco a long time from now.)
The San Andreas fault gets the most attention because it’s the main boundary between the Pacific and North American plates. But “there’s other fault systems that slice California into ribbons,” Hough said, including the Eastern California Shear Zone, which carries a good chunk of the earthquake burden needed to accommodate that tectonic plate movement.
The zone covers a swath of California from Palm Springs to the Owens Valley east of the Sierra Nevada, “and we know there have to be big earthquakes eventually everywhere across this zone,” she said.
But it’s far from certain that the next big earthquake will happen on either of these two faults.
The Eastern California Shear Zone isn’t just one single through-going fault; there’s a bunch of faults there that slip over time. And it’s possible that other faster-moving faults might be better candidates to move in big quakes next, Hough said.
One might be the Garlock fault. A simulation of a hypothetical magnitude 7.7 earthquake on that fault would bring severe shaking to towns across the Mojave Desert and send strong shaking to Santa Clarita and the San Fernando Valley.
Another fault might be one underneath the valley sandwiched between Owens Valley and Death Valley — the Panamint Valley fault, according to analysis conducted in recent weeks, Hough said.
But there are other plausible scenarios as well — earthquakes lighting up south of the July earthquakes and north of the 1992 Landers quake.
Or the next big quake could strike somewhere with no connection to the July quakes at all, say, a devastating
‘The bottom line is we don’t ever have a crystal ball. The next earthquake may be something that no one sees is coming.’ — Susan Hough, seismologist for the U.S. Geological Survey
temblor on the San Francisco Bay Area’s Hayward fault.
“The bottom line is we don’t ever have a crystal ball,” Hough said. “The next earthquake may be something that no one sees is coming.”
Aftershock clues
Despite the limitations in what scientists can glean from tracking aftershocks, it’s still worth doing.
“Most big earthquakes have foreshocks. It’s extremely common,” Page said. One notable example was small quakes before the magnitude 6.3 Big Bear quake of 1992. “It’s lighting up areas where there’s more stress to be relieved.”
The reason the Garlock fault is one that scientists are concerned about is that the seismic strain on it accumulates at one of the faster rates in California. It’s in a category a notch below the most worrisome faults in the state — the San Andreas, San Jacinto and Hayward.
The Garlock fault hasn’t ruptured in a big way in the modern historical record, but paleoseismic work suggests that the average time between earthquakes of at least magnitude 7 on the central part of the fault is about every 1,200 years, Page said.
But there’s huge variation in that average. Sometimes, only 200 years can pass between major quakes there; other times, 2,000 years can go by before a repeat performance.
The last time a big earthquake is believed to have hit the Garlock fault is roughly 465 years ago, give or take about a century.