Scientists say some quakes may be predictable
Recent megaquakes in Japan and Chile were preceded by foreshocks and slow slip on underwater faults like the one off the Pacific Northwest.
Seattle Times science reporter
For the past 20 years, getting an earthquake scientist to utter the word “prediction” has been about as tough as getting a biologist to endorse the existence of Bigfoot.
The quest to identify reliable precursors to ground shaking has turned up one dead end after another, from moon phases to radon gas and animal behavior. Many seismologists are convinced that prediction is impossible because the factors that trigger quakes are so complex.
But a pair of experts at the University of California, Santa Cruz, say it’s time to take another look.
In an article Thursday in the journal Science, seismologists Emily Brodsky and Thorne Lay say that recent megaquakes in Japan and Chile suggest that offshore faults — like the one that runs along the coast of the Pacific Northwest — may chatter and creep before they rupture catastrophically.
Called subduction zones, such faults generate the world’s most powerful earthquakes, along with deadly tsunamis.
Brodsky said it was the monster quake and tsunami off Japan’s Tohoku coast in 2011 that made her reconsider her ingrained cynicism about quake prediction.
“I’ve become a real convert,” she said.
A series of small earthquakes started popping off on the seafloor 23 days before the main shock. Foreshocks aren’t unusual; at least half of major earthquakes have them. The problem is that it has always been impossible to distinguish foreshocks from run-of-the-mill quakes that don’t presage anything bigger.
But in the days before Japan’s magnitude 9 quake, the small temblors were on the move, migrating along the fault toward the area that later broke with such force that it jolted the Earth on its axis.
A handful of instruments on the seafloor showed that the fault was slipping slowly during the foreshocks.
“That suggests that maybe the fault really was doing something different and, perhaps, even measurable, for about a month before a very major, magnitude 9 earthquake,” Brodsky said.
A similar pattern of migrating foreshocks appears to have preceded the magnitude 8.1 quake that struck the northern coast of Chile on April 1. Seismologists around the world were glued to their computers, Brodsky said, because the small quakes occurred on a portion of the fault that hadn’t ripped in 137 years.
“As it was happening, we were asking each other: What do you think? What do you think?”
But migrating foreshocks don’t always mean a big quake is imminent, she added. Swarms off Central Chile in 1997 were not followed by anything major.
That’s why other scientists remain skeptical about the prospects for prediction.
“I think they’re being a bit too optimistic,” said John Vidale, director of the Pacific Northwest Seismic Network at the University of Washington.
After decades of research on earthquake prediction came to naught by the 1980s, the emphasis in the United States shifted to what’s called earthquake forecasting. Based on studies of individual faults, scientists calculate the probability of future quakes.
The last megaquake on the Cascadia Subduction Zone, which runs 700 miles from Vancouver Island to Northern California, struck in the year 1700 and measured about magnitude 9. The odds of a repeat within the next 50 years are estimated at between 15 and 30 percent.
The key to understanding the significance of foreshock sequences and slow slip on subduction zones is a dense network of seafloor sensors that can track those motions, Brodsky said.
Japan has one such network, but it wasn’t in the right place to offer any warning of unusual activity leading up to the 2011 quake. And the few sensors that were deployed off the Tohoku coast didn’t provide readings until after the fact.
Now, Japan is spending $400 million to install more than 100 real-time sensor packages in that area to detect small quakes and measure seafloor motion.
“There’s always a temptation to build these things after a large earthquake,” Brodsky said, “and it really would be vastly more useful to do it before.”
Instruments, including a few seismometers, are being installed this summer along a 500-mile loop of fiber-optic cable off the coast of Oregon, but that network is primarily for oceanographic research.
Monitoring motion on the seafloor is very expensive, but such a system would provide valuable information about the Cascadia fault, Brodsky said.
“I don’t think anybody should be saying we could guarantee that such a tool would let us predict earthquakes,” she said. “That would be dishonest.
“But I do think it would allow us to study whether or not these earthquakes are predictable.”
Even if it’s not possible to provide exact predictions, better monitoring of offshore faults could identify periods of heightened danger, Vidale said.
Before Chile’s April megaquake, when a hundred small tremors were hitting daily, Chilean officials couldn’t be sure that a big quake would follow. Nevertheless, they let residents know that the risk was elevated.
But what can people do with such imperfect information?
Geophysicist Kelin Wang, of the Geological Survey of Canada, argues that scientists don’t know enough yet to offer meaningful warnings, and that false alarms would be costly to society and confusing to the citizenry.
“The problem with earthquake prediction is in its practice,” he wrote in an email.
“I cannot think of a way to make short-term prediction useful and practical, at least not in the foreseeable future.”
Sandi Doughton at: 206-464-2491 or firstname.lastname@example.org