Cascadia's Fault by Jerry Thompson

convection cell pushing new magma up through the East Pacific Rise: the segment of the baseball seam running parallel to the coast of South America. Upwelling magma had pushed the smaller Cocos plate eastward underneath Central America.
    Farther south, researchers learned that another broken slab of sea floor was being thrust under the coast of Chile. To the north another was punching its way down beneath the beaches of Alaska. The same was happening under the coast of Japan and in many other places around the Pacific Rim—all because of seafloor spreading.
    Anywhere you looked, broken plates were pushing against one another. At each one of these collision points were large mountain ranges, violent earthquakes, and active volcanoes. The Pacific was circled by a “ring of fire” caused by lumps of the earth’s crust crashing together, melting and erupting.
    While scientists around the world were busy piecing it all together in their minds and on paper, the earth itself was providing physical proof of what was really at stake. In 1960, the broken chunk of ocean crust jammed beneath Chile’s continental shelf finally reached its breaking point and snapped loose in the largest earthquake ever recorded. Scarcely four years later, George Plafker was collecting evidence that the same kind of horizontal fault had caused the 1964 earthquake in Alaska. Even though the old guard had still not accepted the idea of plate tectonics, Plafker was pretty sure he was right.
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    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