Cascadia's Fault by Jerry Thompson Page A

Dengler at Humboldt State University had supplied Doug and me with a list of sites to photograph, complete with GPS coordinates that made them much easier to find. She referred to this section of California coast as a “fold and thrust belt,” the crumpled edge of a tectonic subduction zone.
    The first thing we photographed was a cleft in the hills directly behind Shelter Cove, the northernmost mapped trace of the San Andreas fault. Through the open door of a JetRanger it looked like just another deep shadow among the redwoods. It was bizarre to think this darkish line was in fact a crack in the earth’s crust, the constantly creeping boundary zone between the Pacific and North American plates.
    In the earthquake of 1906 the San Andreas tore itself apart to the north and south of San Francisco. The northern segment of the rupture ran 250 miles (400 km) from the Golden Gate all the way up the coast to Cape Mendocino—westernmost point of land in the lower 48 states—leaving this visible crack in the hills behind Shelter Cove. Even at such a distance from the ruined city, shockwaves here were strong enough to crack walls, break windows, and topple chimneys in the nearby farming towns of Ferndale and Eureka.
    At Cape Mendocino itself, the San Andreas disappears offshore. In the aftermath of the San Francisco disaster, scientists began to speculate and disagree about where the fault goes from there. Does it continue north toward Alaska? Or does it veer west out to sea? The routing or northern extension of California’s most famous and deadly fault remained a mystery for decades. By the mid-1960s, however, the emerging theory of plate tectonics seemed to promise a better understanding of how this amazingly complex system of cracks worked.
    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