Cascadia's Fault - Jerry Thompson [44]
In October 1972, Robert Crosson, a seismologist and professor at the University of Washington, wrote a paper suggesting that this had probably already happened. Based on data from the Pacific Northwest Seismograph Network, a newly installed, six-station, high-sensitivity telemetry system capable of pinpointing even the smallest tremors, Crosson and his colleagues had shown that nearly all the jolts in the Puget Sound region around Seattle and Tacoma were the result of north–south compression. There was no evidence of eastward pressure from the Juan de Fuca plate at all, as far as they could tell.
The vast majority of the recent Puget Sound earthquakes had been relatively shallow ruptures in the upper crust of the North American continental plate. The absence of a down-sloping zone of much deeper shocks (known as a Benioff zone) along the eastward-dipping plate boundary and the lack of any recent volcanic activity in the Cascades (in 1972) could be seen as further evidence that the Juan de Fuca plate had stopped moving or was in its final phase of subduction. Without that constant eastward shove from the Juan de Fuca Ridge, the dominant tectonic pressure would have become northerly—and that’s indeed what the new seismographic data in Washington State seemed to confirm.
Just across the border in British Columbia, however, two scientists working for the Geological Survey of Canada had looked at their data and come to exactly the opposite conclusion. Robin Riddihough and Roy Hyndman argued in August 1976 that subduction was still happening. They pointed to the “significant eastward dip” of the ocean floor and to the layers of sediment—two and a half miles (4 km) thick, lying on top of the Juan de Fuca plate—that had been dragged sideways into a shallow, less obvious trench at the edge of the continental shelf where they were crumpled, folded, and fractured, all relatively recently.
The continental shelf itself had been recently deformed and uplifted, just like those terraces (former beaches) hoisted up near Cape Mendocino in California. They cited a higher than normal heat flow from the inland Cascade Range that was probably caused by upwelling magma from the melting oceanic slab. All of these were classic symptoms of active subduction, according to Riddihough and Hyndman, although they agreed it would be hard to tell if and when a plate had stopped moving in the recent past. So there remained a degree of uncertainty about whether or not Cascadia posed a clear and present danger.
Another possible explanation for the lack of large earthquakes came in June 1979, when Masataka Ando of the U.S. Geological Survey and Emery Balazs of the National Geodetic Survey suggested that the Juan de Fuca plate was still subducting, but doing it aseismically—without earthquakes. Given the lack of large ruptures over the 140 years since white settlers had arrived and written records had been kept, two things were possible. Silence along the boundary zone could either mean the two plates were now locked together by friction and strain was building up for a major rupture, or that big temblors simply didn’t happen in this subduction zone. Which brings us back to the idea that Cascadia is somehow a special case.
“In some subduction zones, such large earthquakes do not occur,” wrote Ando and Balazs. They had a hunch that friction between the Juan de Fuca and North America plates was too low for the rocks to get