Editor's note: On March 11, a massive earthquake struck Japan and sent a tsunami across the Pacific. The earthquake was the worst in Japan's recorded history. This story explains how scientists study earthquakes that weren't recorded in history, and how they use this information to predict and prepare for the next big one.
From This Story
Brian Atwater paddled a battered aluminum canoe up the CopalisRiver, pushed along by a rising Pacific tide. At this point, a 130-mile drive from Seattle, the 100-foot-wide river wound through wide salt marshes fringed with conifers growing on high ground. The scene, softened by gray winter light and drizzle, was so quiet one could hear the whisper of surf a mile away. But then Atwater rounded a bend, and a vision of sudden, violent destruction appeared before him: stranded in the middle of a marsh were dozens of towering western red cedars, weathered like old bones, their gnarly, hollow trunks wide enough to crawl into. “The ghost forest,” Atwater said, pulling his paddle from the water. “Earthquake victims.”
Atwater beached the canoe and got out to walk among the spectral giants, relics of the last great Pacific Northwest earthquake. The quake generated a vast tsunami that inundated parts of the West Coast and surged across the Pacific, flooding villages some 4,500 miles away in Japan. It was as powerful as the one that killed more than 220,000 people in the Indian Ocean in December. The cedars died after saltwater rushed in, poisoning their roots but leaving their trunks standing. This quake is not noted in any written North American record, but it is clearly written in the earth. The ghost forest stands as perhaps the most conspicuous and haunting warning that it has happened here before—and it will surely happen here again. “When I started out, a lot of these dangers were not all that clear,” says Atwater, a geologist for the U.S. Geological Survey (USGS) who specializes in the science of paleoseismology, or the study of earthquakes past. “If you look at what we know now, it beats you over the head.”
In one of the more remarkable feats of modern geoscience, researchers have pinpointed the date, hour and size of the cataclysm that killed these cedars. In Japan, officials had recorded an “orphan” tsunami—unconnected with any felt earthquake— with waves up to ten feet high along 600 miles of the Honshu coast at midnight, January 27, 1700. Several years ago, Japanese researchers, by estimating the tsunami’s speed, path and other properties, concluded that it was triggered by a magnitude 9 earthquake that warped the seafloor off the Washington coast at 9 p.m. Pacific Standard Time on January 26, 1700. To confirm it, U.S. researchers found a few old trees of known age that had survived the earthquake and compared their tree rings with the rings of the ghost forest cedars. The trees had indeed died just before the growing season of 1700.
In the Pacific Northwest, where written records start in the late 1700s, paleoseismologists have spotted many other signs of past disasters, from sands washed far inshore to undersea landslides. In addition to the risk from offshore earthquakes, recent studies show that Seattle and the greater Puget Sound area, with its four million people, is itself underlain by a network of faults in the earth’s surface. They also have ruptured catastrophically in the not-very-distant past. Considering all the geologic evidence, scientists now say a major earthquake strikes the Pacific Northwest every few hundred years—give or take a few hundred years. That means the next one could strike tomorrow.
The study of the past has taken on paramount importance because scientists still cannot predict earthquakes, though not for lack of effort. One important quake-forecasting experiment has taken place since 1985 in tiny Parkfield, California, the self-proclaimed “earthquake capital of the world.” The town sits atop a highly active section of the San Andreas fault, the dangerous crack that cuts the state south to north for 800 miles. Due to underlying geological forces, quakes occur in the same places repeatedly. Until recently, much of modern earthquake theory was based on the idea that intervals between these events were nicely regular. Through most of the 20th century, Parkfield, for example, had one every 22 years or so. But experience now shows that quakes are maddeningly unpredictable. Scientists forecast that a quake would hit Parkfield in 1988, give or take five years. They installed networks of strainmeters, creepmeters, seismometers and other instruments around the town. Their goal was to capture precursors to the expected quake, such as a pattern of subtle tremors, that they could later use to predict when another quake is imminent. The earthquake did come along—in September 2004, with one-twentieth the expected power—and with no warning whatsoever. Looking at all their measurements, scientists still have found no reliable signs that an earthquake is about to strike.
Still, by gathering ever more information about the past, paleoseismologists are becoming adept at mapping danger zones and spreading the warning, even if they can’t say when the next one is due. The information, though imprecise, is useful to engineers, city planners and others who can strengthen building codes and educate the public about how to survive a major quake whenever it comes. Art Frankel, a chief architect of the USGS national seismic hazard mapping project, says such geological “hazard maps” are like charts of the most dangerous traffic intersections; they can’t predict when the next car accident will happen, but they do tell you to watch out.
Due to these studies of past earthquakes, the world is looking ever more inhospitable. Paleoseismology is turning up portentous signs of past upheavals in the U.S. Midwest, eastern Canada, Australia and Germany. “We’re discovering some new hazard every few months,” says Brian Sherrod, a USGS geologist investigating the Seattle faults. The Pacific Northwest may not be the only place harboring such nasty surprises, but it is where the geological signs are most dramatic, the science is moving fast, and a future earthquake would be among the most catastrophic.
The earth’s crust consists of interlocking tectonic plates that float on the hot, pliable interior of the planet, drifting and colliding with one another. The Pacific Northwest coast is such a dangerous place because it rests on a continental plate that meets, some 30 to 90 miles offshore, a seafloor plate. The boundary between the two plates, stretching 700 miles from British Columbia to Northern California, is called the Cascadia subduction zone. Subduction is the process by which an ocean plate nudges under a continental plate, usually by a few inches a year. Grinding between such plates can bring small temblors, but often the parts lock against each other like sticky watch gears, causing the stilladvancing seafloor to compress like a spring and the overlying coastline to warp upward. When the pent-up pressure finally pops, the seafloor lunges landward and the coast lunges seaward, with seaside real estate collapsing. The shifting plates displace seawater in all directions, creating a tsunami that travels up to 500 miles an hour. These subduction-zone quakes are the world’s largest, dwarfing those that take place in the land’s crust. December’s subduction quake in Indonesia, a magnitude 9, was about 30 times more powerful than the 1906 San Francisco event that took place in the continental crust near the city. Other major subduction-zone quakes off Alaska in 1946 and 1964 sent tsunamis all the way to Hawaii and Northern California, killing scores of people.
Downriver of the ghost forest, with heavy rain threatening the tidal estuary of the CopalisRiver, Atwater stepped from the canoe to stand crotch-deep in cold water and mud. He wore hiking boots and chest waders, having learned long ago that tidal mud can suck hip waders right off of you. Wielding an entrenching tool, a military folding shovel, he chopped at the riverbank to view the sedimentary layers, which can yield a great deal of information about past quakes. Every time a seafloor earthquake occurs here, forests and marshes suddenly drop, and are reburied by later sediments washed in by tides and river drainage. Ageologist can dig a hole in search of such buried evidence—or find a riverbank where erosion has done most of the work for him, which was what Atwater had here. His tool kit also included a hunting knife and a nejiri gama, a trowel-size Japanese gardening tool shaped like a hoe.