SCIENCE: Nov 05, AU Edition
A CASE OF THE SHAKES
New research says that earthquakes may be contagious, reports Sandi Doughton
Geologists used to answer with an emphatic “No” when asked if mega-earthquakes like the one that hit Southeast Asia last December can trigger temblors on the other side of the globe. Today, some experts are not so sure.
Evidence is mounting that large earthquakes can rattle geologic formations thousands of kilometres away – and perhaps even set off volcanic eruptions days, months or years later.
There’s also an intriguing hint that major earthquakes might occur in clusters: Nearly a third of the biggest quakes of the past century struck during a 20-year span between 1950 and 1970.
After three decades of relative quiet, two massive quakes came in quick succession late last year: the magnitude 9 in Sumatra and a little-noticed magnitude 8.1 off the coast of New Zealand three days earlier.
Do monster earthquakes beget more monster earthquakes? Could the two recent events signal the start of a new destructive cycle? And is it possible the Sumatran quake jolted other geologic plates enough to hasten the day when they let loose, unleashing what geologists predict will be comparable catastrophes? No one knows the answers to the first two questions, which are hot topics of research and scholarly debate.
But scientists are fairly certain people don’t have any more to worry about now than they did six months ago.
“I would venture to say there’s a minimal effect, if any at all, on our region from the Sumatra earthquake”, comments Herb Dragert, a research scientist for the Geological Survey of Canada, surveying the seismic risks on his side of the Pacific.
Dragert and his American counterparts operate a network of GPS sensors throughout the region. The instruments can detect even slight movements of land masses, reflecting changes in the amount of stress at the Cascadia subduction zone – a 900-kilometre-long offshore region where the ocean floor is diving under the continental plate.
The measurements show no troublesome blips as a result of the Sumatran quake, Dragert says.
“If we suddenly had a very large earthquake in Alaska, which is much closer, and I saw displacement in my GPS instruments, then I would begin to worry.”
However, there could be ample cause for concern around Indonesia. When the undersea plates there snapped apart, triggering the earthquake, the dislocation almost certainly increased stress and strain on adjacent geologic faults and plate boundaries. Geologists call the pheno- menon “contagion” because it raises the odds of subsequent earthquakes like an influx of germs raises the risk of infection.
“It’s very expected and quite dangerous”, explains Brian Atwater, a U.S. Geological Survey researcher stationed at the University of Washington. “It gives a certain sense of urgency to efforts to get a warning system going around the Indian Ocean.”
Scientists have long known about the contagion effect, which can extend for 100 miles or so from the epicenter of a major quake. It’s the phenomenon that’s responsible for the aftershocks that follow many major quakes.
But most experts were stunned in 1992 when a magnitude-7.2 quake struck the Mojave Desert in Southern California and was almost immediately followed by more than a dozen quakes as far away as Wyoming. A similar thing happened in 2002, when a magnitude-7.9 earthquake in Denali, Alaska, triggered earthquakes and rearranged the plumbing of geyser fields in Yellowstone National Park – 3,000 kilometres away. The same event spawned a couple of small earthquakes under Mount Rainier and set up sloshing waves that swamped houseboats on Lake Union in Seattle and Lake Pontcha- rtrain in Louisiana.
“As people around the world look more carefully, they’re seeing more examples of this kind of (long-distance) effect”, says David Hill, a USGS geophysicist stationed at Menlo Park, California. “At this point there’s really no doubt that it happens.”
Generally, the triggered earthquakes are smaller than the original, though there’s no reason to believe that larger earthquakes couldn’t be kicked off this way as well, says Hiroo Kanamori, a geophysicist at the California Institute of Technology.
The effect seems to be caused by seismic waves that radiate out from the epicenter of an earthquake, along the surface of the ground. Imperceptible to people, these waves cover a lot of distance.
“The Earth ends up ringing like a bell”, Dragert explains. “You have a surface wave that travels around the globe for hours after the event, and if it passes through an area that is already critically stressed, it can, indeed, trigger an earthquake.”
That is, a fault or plate boundary must already be on the verge of slipping or breaking for the surface waves to push it over the edge.
There’s still no detailed explanation for the way that happens, though, Hill says.
“In a way, it’s frustrating to be doing research on this,” he adds, “because we can’t do it in the lab and repeat the experiment. We’ve got to wait for the Earth to do it, and then have good recording networks in the field.”
There’s even less concrete data to show that distant earthquakes can trigger volcanic eruptions, though the circumstantial evidence is growing, Hill says. One analysis found a high number of volcanic eruptions within a day or two of large earthquakes. Several volcanoes around the world, including Pinatubo in the Philippines, have erupted within weeks or months of major earthquakes.
Indonesia has many volcanoes, none of which has yet erupted in the aftermath of the earthquake – but scientists will be watching closely.
After the Boxing Day tsunami, the Washington Post reported lava was spewing from a volcano on an island in the Andaman and Nicobar Islands, an Indian archipelago off the coasts of Myanmar (formerly called Burma) and Indonesia. Previously, the crater had emitted only gas.
Theories linking distant earthquakes to eruptions and other earthquakes remain controversial. It’s almost impossible to prove what triggered an earthquake or eruption, Kanamori points out.
Researchers look mainly at the timing of events, then do statistical analyses to show that they’re probably linked, not just random coincidences.
Hill does collect some hard data from strain meters buried in 600-foot boreholes in California’s Long Valley Caldera near Mono Lake. The sensitive devices detect changes in the pressures pushing and pulling on the rock, and have clearly shown effects from distant earthquakes, he says.
The statistical jury remains out on the question of whether the apparent cluster of major earthquakes in the middle of the century is significant or simply a phantom.
It certainly looks compelling, Atwater says. Most of the events are clustered around the Pacific Rim, from Alaska to Russia’s Kamchatka Peninsula to Chile.
However, the Cascadia subduction zone off the Northwest coast of the United States was not triggered during that period, he pointed out. The last earthquake there was a magnitude 9 in 1700.
Garry Rogers, a seismologist at the Geological Survey of Canada in British Columbia, says major earthquakes are far too rare and the historical record far too short to be able to draw any conclusion about clusters or large-scale connections.
“In any random process, you will get clusters”, he says.
Hill believes that more data will eventually solve the mystery – and will probably reveal patterns and links no one understands today.
“My own hunch is that there are lots of instances of clusters that are, in fact, related physically. We just don’t know yet what the details might be.”