Cascadia's Fault - Jerry Thompson [68]
In other words, the high impact and long duration of shaking felt in Chile’s 1960 magnitude 9.5 disaster has never happened to a large city with a forest of tall buildings. None of the San Andreas temblors has ever hit magnitude 9; San Francisco had only a few tall buildings in 1906; the high-rise core of downtown Los Angeles has never been shaken by a force as strong as a megathrust subduction quake. Even those devastating shockwaves in Japan occurred before most of the high-rise skyline of Tokyo was built. Now two respected seismologists from Caltech and the USGS were warning that Cascadia’s fault might do a kind of urban damage never before seen in the modern world.
Sullivan decided to underscore the point that Cascadia could be every bit the menace of San Andreas and then some: “One cause for concern, the authors wrote, is a tendency of earthquake tremors from a descending plate to be far more damaging, at distances beyond 30 miles [50 km], than those from horizontal slippage such as that along California faults. Furthermore, they say, oscillation rates are of a nature especially damaging to large buildings.”
Although Sullivan did not mention Mexico City as proof, the images of collapsed apartment blocks certainly flashed into my mind when I read the story. Mexico City may not have been thought of as particularly modern, but its high-rise towers were certainly the focus of harmonic amplification and extreme damage caused by the long wavelength of shockwaves traveling 190 miles (300 km) inland from the 1985 rupture. What Heaton and Hartzell were telling us was that we should imagine the same kind of amplification happening to younger cities with dense clusters of tall buildings that have never been tested in a subduction event—namely Vancouver, Seattle, and Portland.
When the eastward motion of Cascadia’s sea floor is added to the westward movement of the North America plate, the “convergence rate is about 13 feet [4 m] per century,” Sullivan continued. “According to Dr. Heaton and Dr. Hartzell, the key question is whether the sea floor has been smoothly slipping under the continent or is ‘locked’ and accumulating strain. If it has been storing elastic energy for a long time, a sequence of several great earthquakes or a single giant one, comparable to that in Chile, would be necessary to relieve the tension.”
Even though Cascadia has not produced a major jolt in the Northwest since it was “permanently settled by Europeans in about 1810,” Sullivan reported, “there are indications of periodic seafloor landslides and coastal subsidences that could have been triggered by such events in the more distant past.” So finally, there it was in print for a general audience to digest: reference in the New York Times to the turbidite landslide cores from Griggs and Kulm via John Adams, and to the sunken coastal meadows that Brian Atwater had found. The so-called smoking gun evidence of Cascadia’s violent past was now a matter of very public record.
For Brian Atwater the next step was to ask two questions a lot of people were asking him: how big and how often? To find the answers he packed his kit and returned to the coast in the summer of 1987 to conduct a systematic survey by canoe of those three southern Washington estuaries: Copalis River, Grays Harbor, and Willapa Bay. He paddled miles and miles of shoreline and hiked through marsh, muck, and greasy river mud until persistence, and serendipity, paid off again.
In his initial reconnaissance at the Copalis River, he had walked in, venturing only a short distance from the road. “I missed the ghost forest,” he smiled, pointing across the lagoon toward the grove of weathered hulks bathed in