3.21.11
An unfounded scientific assertion by a nonscientist has swept across the Web like a tsunami over the past few days. In an article in Newsweek, writer Simon Winchester claimed that the 9.0-magnitude Japan earthquake, following close on the heels of recent quakes in New Zealand and Chile, has ratcheted up the chances of a catastrophic seismic event striking in California.
In his article, "The Scariest Earthquake Is Yet to Come," Winchester pointed out that all three of those recent earthquakes occurred along faults on the edge of the Pacific Plate — the giant tectonic puzzle piece under the Pacific Ocean — and that this also butts up against the North American plate along the San Andreas Fault.
"[A] significant event on one side of a major tectonic plate is often … followed some weeks or months later by another on the plate’s far side," he wrote. "Now there have been catastrophic events at three corners of the Pacific Plate — one in the northwest, on Friday; one in the southwest, last month; one in the southeast, last year. That leaves just one corner unaffected — the northeast. And the fault line in the northeast of the Pacific Plate is the San Andreas Fault, underpinning the city of San Francisco."
Winchester claimed that the geological community is "very apprehensive" about these earthquakes triggering a massive California quake. Life's Little Mysteries, a sister site to LiveScience, checked that claim with a panel of geophysicists.
"There is no evidence for a connection between all of the Pacific Rim earthquakes," Nathan Bangs, a geophysicist who studies tectonic processes at the University of Texas Institute for Geophysics, told Life's Little Mysteries. "I don't know what the basis is for the statements and implications in the Newsweek article, but there is no evidence that there is a link."
U.S. Geological Survey (USGS) earthquake geologist David Schwartz, who heads the San Francisco Bay Area Earthquake Hazards Project, concurred. "Simon Winchester is a popular science writer, not a scientist," Schwartz said. "I'm not saying we won't have an earthquake here in California at some point in the future, but there really is no physical connection between these earthquakes."
Schwartz explained that earthquakes can indeed cascade, with one setting off another — but only locally. "When an earthquake happens, it changes the stress in the vicinity around it, and if there are other faults nearby, this increase in stress can trigger them and produce more earthquakes. In other places, it relaxes the crust and puts earthquakes off," he said.
In New Zealand, for example, a 7.1-magnitude earthquake that rumbled 20 miles northwest of the city of Christchurch in September triggered the much smaller 6.3-magnitude that occurred closer to the city in February. The 1906 San Francisco earthquake, on the other hand, relaxed nearby faults, which has placed the region in a relatively quake-free "stress shadow" for the past 100 years. "But these static stress changes occur in a relatively restricted region," Schwartz said. The effects of the stress changes aren't just anybody's guess, either: Scientists can produce very accurate computer models of the local stress transfer.
Rich Briggs, a USGS geologist whose work focuses on how earthquakes happen, explained another way in which earthquakes can cascade. "The other way earthquakes affect their neighbors is that when a fault ruptures, it sends out seismic waves that in the case of large earthquakes can even circle the globe. In some cases, this 'dynamic stress transfer' increases seismicity," Briggs told Life's Little Mysteries. "But that only happens as waves go by, in the minutes that it takes the waves to travel out from the fault zone."
The dynamic stress transfer induces aftershocks immediately after the initial seismic event — not days, months, or years after. Because the 9.0-magnitude earthquake that hit Japan can only alter regional faults, the dynamic stress transfer process is the only way to set off a similar reaction in California. If that were the case, though, the earthquake would have hit already.
So when will a major earthquake strike California? "Based on models taking into account the long-term rate of slip on the San Andreas fault and the amount of offset that occurred on the fault in 1906, the best guess is that 1906-type earthquakes occur at intervals of about 200 years," Robert Williams, USGS seismologist, wrote in an email. "Because of the time needed to accumulate slip equal to a 20-foot offset, there is only a small chance (about 2 percent) that such an earthquake could occur in the next 30 years."
"The real threat to the San Francisco Bay region over the next 30 years comes not from a 1906-type earthquake, but from smaller (magnitude about 7) earthquakes occurring on the Hayward fault, the Peninsula segment of the San Andreas fault, or the Rodgers Creek fault," Williams wrote.
Schwartz agreed that the Hayward fault, located just east of the San Francisco Bay, is more likely to slip than the San Andreas. But the bottom line is that, "if a fault slips, it will do so on its own, not because of something 5,000 miles away."
"I think the idea of saying the earthquake hazard is real is good, because it hopefully gets people to prepare. It's hard to get people to prepare," Schwartz said. "But to scare people by saying the earthquakes are jumping around and the next place one will jump is here – that's just bad science."
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