5.4.3: The Bad News
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The new discoveries went against the long-held view that the Cascadia Subduction Zone was not a seismic hazard. Most subduction zones around the world are shaken by frequent earthquakes, some of magnitude 9 or greater. But not Cascadia, which has been as seismically quiet as Kansas. At first, it was believed that the apparent absence of recorded earthquakes might be because the Juan de Fuca Plate is no longer subducting beneath North America. The eruption of Mt. St. Helens on May 18, 1980, was a dramatic demonstration that subduction is still going on.
Then, it was suggested that the absence of recorded earthquakes is due to relatively few seismographs in the Pacific Northwest. But in the last thirty years, the University of Washington, the USGS, and the Geological Survey of Canada have deployed an extensive network of seismographs throughout the region. This sophisticated network recorded many earthquakes in the continental crust and within the subducting Juan de Fuca Plate, but almost none precisely on the subduction zone itself (Figure 4-13, left). Are land-based seismic networks sensitive enough to record smaller earthquakes on the subduction zone?
In the past decade, the U. S. Navy has opened access to the hydrophone arrays it had established to monitor enemy submarines by recording the sound waves from their engines. These hydrophone arrays record not only submarine-engine noise but also record volcanic activity at sea-floor spreading centers, whale calls—and earthquakes. Research by Bob Dziak and his colleagues at NOAA shows that SOSUS arrays reveal unprecedented details about the seismicity of the Juan de Fuca spreading ridge and other seafloor features. But even at those listening levels, the subduction zone remains quiet (Figure 4-13, right). Why?
For a long time, it was thought (perhaps “hoped” is a better word) that the absence of earthquakes meant that the slippery subduction zone slides smoothly beneath the continent. Subduction without earthquakes was still being suggested in a paper in a major scientific journal as recently as 1979. But in 1980, as mentioned in the preceding chapter, Jim Savage of the USGS and his colleagues began repeated measurements of networks of surveying benchmarks around Seattle, in Olympic National Park (Figures 3-19, 3-20), and the Hanford Nuclear Reservation. Their conclusion: these networks show that the crust is being slowly deformed in a way that is best explained by elastic strain building up in the crust, like the examples in Figures 3-2 and 3-3. The obvious source of this strain: the Cascadia Subduction Zone. The reason that there have been no earthquakes on the subduction zone is an ominous one: the subduction zone is locked. Completely locked! If this is the case, then strain must ultimately build up along the subduction-zone fault, inexorably, at 1.6 inches per year until, eventually, the subduction zone will rupture in a massive earthquake.
At about the same time (as already reported in the Introduction), John Adams, then of Cornell University, was studying the resurveys of highway benchmarks and discovering that the coastal regions of Oregon and Washington are being slowly tilted eastward. A few years later, Heaton and Kanamori showed that the geophysical setting of Cascadia is like that of southern Chile, where the largest earthquake of the twentieth century struck in 1960. A short time after that, in 1986, Brian Atwater paddled up the Niawiakum River estuary in his kayak and discovered the submerged marshes and forests of Willapa Bay (located in Figure 4-10).
For about ten years, starting in the late 1970s, the argument raged among scientists about whether or not the Cascadia Subduction Zone poses an earthquake threat, triggered by a major economic and political issue: was it safe to build and operate nuclear power plants in western Washington and Oregon and northern California? Proponents of the big-earthquake hypothesis were led by scientists of the USGS, influenced by geodesists such as Jim Savage who were re-surveying benchmarks, and later by geologists like Brian Atwater. As described in the Introduction, it was only in 1987 that the controversy was finally resolved at the Oregon Academy of Sciences meeting in Monmouth, Oregon, where it was recognized that the paradigm change had occurred, at least among scientists. Most of the leading Cascadia earthquake researchers agreed at this meeting that the Cascadia Subduction Zone does indeed pose a major earthquake threat. When a paradigm change takes place, particularly for a topic that has such an impact on society, scientists take on a new mission: to inform the general public of the consequences and implications of this new discovery. This book is part of that mission.
Even though there was general agreement that there would be huge earthquakes on the Cascadia Subduction Zone, a new debate began over how big the expected earthquake would be. Would it be of magnitude 8 or 9? This difference is not, as some have suggested, analogous to being struck by a tractor-trailer or a compact car!
Although there is agreement that the absence of seismicity on the subduction zone cannot be used as evidence for an absence of subduction-zone earthquakes, the question arises: why does Cascadia lack any seismicity on the subduction zone itself? Figure 4-13 shows that the northern and southern ends of the subduction zone in the Explorer and Gorda plates have seismicity, probably due to internal plate deformation. In addition, the southern end of the subduction zone was struck by an earthquake of magnitude 7.1 on April 25, 1992. Only the central part between southern Vancouver Island and northern California lacks seismicity on the subduction zone. Ivan Wong, a consulting seismologist with Oregon roots, examined this problem and pointed out that the only part of the subduction zone that lacks earthquakes contains a huge slab of basalt crust many kilometers thick called Siletzia. This thick slab serves as an insulator against the high temperatures of the young incoming crust of the Juan de Fuca Plate. Is this why there are no earthquakes on the subduction zone in this part of Cascadia? This problem needs further work, and I return to it later in the chapter.