Cascadia's Fault - Jerry Thompson [45]
The first precise leveling survey of Washington’s roads had been done back in 1904. By 1974 new surveys had been carried out on ten different sections of highway, some of which ran east–west across the Coast Range mountains. In the time between the first and second surveys, the surveyors’ data showed that the outer coast had been lifted upward and the inland areas east of the Coast Mountains had subsided. In other words, the entire mountain range was tilting slightly toward the east and this had to be a result of active, ongoing subduction because it had happened in the past seventy years, not millions of years ago in geologic time.
However, another important detail made Cascadia different, according to Ando, who had recently studied strain accumulation in the Shikoku area along the east coast of Japan. There the Philippine Sea plate is thrusting under the Asian plate—beneath the islands of Japan—along the Nankai Trough. The geologic setting is very similar to the Juan de Fuca Subduction Zone. The dip angle of both subducting plates is a shallow twenty degrees and both oceanic plates are relatively thin. The significant difference is that the outer coastal landmass in Japan is tilting down toward the ocean rather than leaning inland as it appears to be doing in Cascadia.
Bending the outer edge of the coast downward as the ocean floor scrapes underneath it is a sure sign the plates are locked together by friction and building strain for a large quake, according to Ando’s analysis. Once the rocks along the locked portion reach their breaking point—when friction between them is no longer enough to keep the two plates stuck together—the strain is released in a massive shockwave. As the two plates rip apart in a typical or “normal” subduction zone like the Nankai Trough, the outer coast snaps free from the down-going oceanic plate and springs back upward. The area slightly inland from the coast subsides at the same time. This is exactly what happened in previous large quakes in Japan, Alaska, and Chile.
In the aftermath of these giant jolts, as the overlying continental plate settled back down to its more or less normal position, some of the coastal uplift remained. In other words, the beach never quite got back to where it used to be because the underthrusting oceanic plate was still down there, still moving below the continent, still causing a certain amount of residual deformation. The three-step sequence, according to Ando and Balazs, starts with coastal down-warping just before the quake, followed by heaving upward during the rupture, and then a certain amount of residual uplift of the beach zones in the aftermath.
Cascadia, however, seemed to be doing something entirely different. If the aseismic hypothesis were true, then the Juan de Fuca plate would be just creeping down underneath the continent, slowly and continuously, lifting and tilting the Coast Range mountains to the east, and doing so without getting completely stuck and without accumulating enough strain to cause a major rupture. To me this sounded like the good news. The bad news came in the concluding paragraphs.
Studies of other aseismic zones had revealed that temblors are still possible even if the two plates are not completely locked together. Hiroo Kanamori at Caltech found that if you look at the total distance—how much long-term horizontal movement there had been along the subduction zone in the Kuril Islands, for example—and compared that to the movement that happened during large thrust earthquakes, the ruptures accounted for one-quarter of the total slip. In northern Japan another study showed that