Cascadia's Fault - Jerry Thompson [88]
Realizing that sediment must play an important role, they had to figure out how it affected the overall temperature of the locked subduction zone. The bottom layer—the subducting slab of ocean floor—started out warm because it was so young, having been created relatively recently (in geologic time) by the hot volcanic furnace of the Juan de Fuca Ridge. Wang and Hyndman figured the layer of sediment probably acted as an insulating blanket, trapping heat in the lower plate.
As the ocean slab moved slightly deeper and the sedimentary wedge dried out, the temperature would rise to 300 degrees Fahrenheit (150°C), which laboratory experiments and research at other subduction zones had suggested was usually the minimum necessary to cause two plates to stick together and generate earthquakes. Farther down, where the temperature rose to 660 degrees Fahrenheit (350°C), the lab studies showed that rocks became too soft to stick together. With this in mind, Wang and Hyndman took measurements of heat flow in the earth, marked the starting point where the thermometer topped 300 Fahrenheit and the stopping point at 660 and drew a new set of wavering gridlines on the map of the West Coast.
They found that the “fully locked zone” capable of generating quakes was about forty miles (65 km) wide, running roughly north–south parallel to the coast in the offshore region beneath the outer continental shelf. The inner edge of the transition zone barely reached the western beaches of Vancouver Island. The locked zone was slightly wider in Oregon and almost twice as wide along Washington’s Olympic Peninsula.
The report issued by the Canadian team pointed optimistically to the fact that the energy released in Cascadia’s next big event would be restricted to this relatively narrow band of rock beneath the continental shelf west of Vancouver Island. “This reduces the expected ground motion and amplitude at the major coastal cities,” they wrote. With the city of Vancouver ninety miles (150 km) east of the locked zone, the maximum magnitude of the coming shockwave might also be somewhat less than some pessimists were expecting. So that was the good news.
The bad news was that the rupture would be shallow and therefore have a greater potential to generate large tsunamis. And the long duration (three minutes or more) of shaking would not be reduced by distance from the zone. The quake would still cause major damage, especially to tall buildings, and “events well over magnitude 8 are still possible,” according to the report. A separate paper published a short time later by Garry Rogers also reminded readers that being ninety miles away from the locked zone didn’t mean cities like Vancouver would get an exemption from Cascadia’s effects. “Anchorage is about the same distance from the down-dip end of the Alaska seismogenic zone as Vancouver is from the Cascadia seismogenic zone,” he wrote.
The same kinds of cautionary words would also apply to Victoria, Seattle, Tacoma, Portland, and other cities as far south as Sacramento. Yes, the locked zone is a fair distance from the major urban areas—not directly underneath—but don’t forget that Mexico City was roughly 125 miles (200 km) away from the 1985 epicenter and look what happened there. So “the zone” had been defined as a swath about forty miles (65 km) wide, locked and ready to rip.
Years later, in 2009, a study led by Timothy Melbourne of Central Washington University would suggest the locked part of the fault was even closer to the big urban areas—within fifty miles (80 km) of Seattle, Tacoma, and Portland. But the question of whether the entire plate boundary would break all at once from end to end remained unanswered. If Cascadia’s fault broke today, would it start as a magnitude 8.8 in northern California and continue northward with several more huge quakes over the next decade? Or would it slip all at once in a magnitude