Cascadia's Fault - Jerry Thompson [33]
One could argue that this should have been the dawning of awareness of the megathrust earthquake threat to British Columbia, the Pacific Northwest, and California as well. A 1965 paper by Tuzo Wilson pointed to the existence of what he called the Juan de Fuca Ridge. The name was chosen because the upper end of the ridge lay due west of the Strait of Juan de Fuca, which runs between Vancouver Island and Washington State. Here was an undersea mountain range that had previously been discovered and then dismissed as an insignificant, amorphous hump of rock running parallel to the coast. But if Wilson and the young turks of plate tectonics were right, the Juan de Fuca Ridge was in fact another part of that fiery seam of volcanic mountains running through the oceans.
If this ridge turned out to be spreading apart sideways, powered by a cauldron of hot magma, it must also be thrusting a slab of sea floor underneath the edge of British Columbia, Washington, Oregon, and California. Presumably some kind of trench would be located where the two plates met, a “convergent plate boundary” just like the ones off Chile, Alaska, and Mexico. If so, giant earthquakes must surely follow.
CHAPTER 6
Nuke on a Fault: Early Clues in Humboldt Bay
Flying south from the Oregon line in search of tectonic damage, the helicopter finally angled west over the last wall of mountains and down through a hole in patchy clouds to the California shore. We found the Shelter Cove runway, a cracking strip of sun-baked asphalt surrounded on three sides by a golf course on a bench of land just above the sea. My first thought was—how quintessentially West Coast. Fly in for a quick round of golf, go whale watching in the afternoon, have a barbecue on the beach, then fly home at sunset. Just try not to think about that monster earthquake hiding beneath the surf.
On this particular summer morning in 2007, cameraman Doug Trent and I had planned to shoot aerial pictures for a new documentary called ShockWave on the communities closest to the Juan de Fuca Ridge and fault: the farms, ranches, and small towns from Cape Mendocino north to Humboldt Bay, Eureka, Arcata, and Crescent City, near the Oregon border. There was a reasonable chance the morning fog would burn off by midday, giving us the low-angle light we needed to highlight a half-dozen surface-level fractures where ancient tectonic ruptures had heaved up beaches and hillsides along the foreshore.
Geology professor Lori 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