Cascadia's Fault - Jerry Thompson [87]
The first thing that jumped out from this updated, multilayered mass of evidence was the idea that the built-up strain and deformation along the western edge of the continent had to be “transient.” The strain was obviously getting released from time to time. The rates at which the coastline was being lifted and the mountains tilted—but especially the speed at which the ground was being squeezed horizontally—were considered “geologically unreasonable.” If the hoisting and crunching had continued at this rate with no interruptions for a million years, the mountains along the west coast would be several miles higher than they are now and would look much like the Himalayas.
The fact that the peaks of southern Vancouver Island, the Olympic Peninsula in Washington, and the Coast Range in Oregon and California are not towering piles like Mount Everest could be taken as one more level of proof that the nonstop pressure of subduction must have been released every few hundred years by very large earthquakes. The point of this latest study was to find the edges of the locked part of the zone and figure out how much real estate was likely to slip sideways when the two plates come unstuck next time.
Roy Hyndman and Kelin Wang, coauthors with Dragert and Rogers, had been working for several years on the idea that temperature could tell the story of where these two plates of rock were bonded together by friction and where they were sliding along smoothly. Wang told me that the dangerous part of a subduction zone—the area that can generate an earthquake—cannot be very far below the surface. “It doesn’t go very deep,” he said, “because when they go deeper and the temperature is too high, the rock becomes too soft to produce earthquakes.”
Rocks go from brittle to ductile as they heat up. The brittle ones will grind against each other and friction will lock them together. Hotter, softer rocks won’t stick together. The deeper the slab of ocean floor slides during subduction, the warmer and softer the surfaces become. So at a certain depth and temperature, two tectonic plates theoretically would slide past each other without the risk of rupture. For these reasons Hyndman and Wang thought measuring heat flow could show them where the seismogenic danger zone started and stopped.
First they studied published results of laboratory experiments on rock friction and figured out how warm the ocean floor and continental crust would have to be to stick together and get locked. They knew there was a wild card in the equation—something unusual about the Cascadia Subduction Zone—that would complicate their calculations. The accretionary wedge of sand and clay piled along the edge of the fault was getting dragged down with the slice of ocean floor. It was full of seawater. How did that affect the temperature on the surface of the subduction zone? Kelin Wang had a hunch that this sediment played a major role in the size and severity of subduction zone quakes.
“There’s a tendency for the rupture to be very long at these subduction zones,” he said. Plates can slip along hundreds of miles of rock surface—from the epicenter, ripping along the “strike” of the fault at more than a mile (2–3 km) per second—all in one earthquake. “I think the amount of sediments cause faults like this to be very smooth,” Wang continued. “When the fault is smooth, the rupture has a better chance to propagate for a long way.” It’s not that the sediments lubricate the movement, he clarified. “It just makes the fault property more uniform.” Wang’s idea was that instead of the fault having a million little asperities—rough spots, cracks, and jagged edges—the ocean sediment probably