Cascadia's Fault - Jerry Thompson [34]
Geologists and oceanographers knew from mapping the underwater terrain (the bathymetry) that the ocean floor looked like a broken dinner plate. From what they could see with the earliest, relatively low-tech echo-sounding equipment, the bottom of the Pacific Ocean had cracked in several places. The recently “rediscovered” (and newly named) Juan de Fuca Ridge rose from the deeps off the northern California coast, fracturing the sea floor in a northwesterly direction toward Vancouver Island, more or less parallel to the coast.
A convection cell of hot magma from the earth’s mantle had apparently broken the Pacific plate apart, shoving a slab of oceanic crust (the Juan de Fuca plate) east underneath the oncoming (westward-moving) North America plate. This would eventually become known as the Cascadia Subduction Zone. On closer examination, researchers discovered that the Juan de Fuca plate itself had been fractured. The southern end of it was broken off and appeared to be moving independently.
It turned out there was a separate, smaller ridge—another seafloor spreading zone called the Gorda Ridge—that looked like a southern extension of the Juan de Fuca system. It appeared to be pushing another chunk of oceanic real estate, the Gorda plate, beneath the California coast. There was also a heaved-up fracture zone running east to west across the larger Pacific plate. All these cracks and broken slabs, including the San Andreas, converged offshore at Cape Mendocino. Geologists decided to call this tectonic wreck the Mendocino Triple Junction.
The ongoing and extremely slow-motion convergence of plates had fractured, bent, and folded rocks along the shore and hoisted up the beaches in several places, creating terraces we could now see from the air at a place called Singley Flat. They were, on a smaller scale, similar to the sections of heaved-up coastline George Plafker had discovered in Alaska after the 1964 disaster. If I had not been told what to look for, I would never have guessed these grassed-over benches of coastal farmland were the bent fenders of a continental crash, evidence that Cascadia’s fault had caused numerous earthquakes over the years.
Farther north we shot pictures of the ruptured earth at places called Little Salmon River, Mad River, and McKinleyville. These were even harder for our untrained eyes to notice because today they are camouflaged by a veneer of human civilization, the streets and homes, schoolyards and shopping malls of Eureka and Arcata, California. Who would notice that the nice little house on what looks like a landscaped terrace is in fact perched on the edge of an active, still-moving fault, a fractured wedge of crust that is being shoved upward by the force of plate tectonics?
Native people who have lived in beachside villages along this coast for thousands of years tell stories they learned from their elders of horrific ground shaking on a winter’s night long ago, followed by a killer wave that wiped out entire communities. For the most part, though, the tide of white settlers who began arriving here in the 1850s to homestead and log the redwood forest were unaware of, or simply uninterested in, the local knowledge of Aboriginal people. After 1906, they knew—the whole world knew—about the deadly San Andreas, but the concept of plate tectonics and the fact that a moving block of ocean floor could cause an even larger shock was still unknown.
The scientists and engineers who in the early 1960s drew up plans for an atomic power plant—California’s first commercial reactor—to be built on the shore of Humboldt