Cascadia's Fault (13 page)

At this point two other factors may have entered the equation for Pacific Gas and Electric. On March 28, 1979, while the Woodward-Clyde team was still documenting the gritty details of the Mad River area and how it might affect the reactor at Humboldt Bay, things went
alarmingly wrong at a nuclear power station called Three Mile Island in Pennsylvania. A relief valve got stuck open, allowing large amounts of radioactive coolant to be released into the atmosphere. The reactor core overheated and barely survived a partial meltdown.

The accident, while not as catastrophic as it might have been, helped turn the tide of public opinion against nuclear power. By some masterstroke of luck or serendipity, a Hollywood movie called
The China Syndrome
had been released only twelve days before the Three Mile Island accident. The eerily prescient film became an instant box office hit and probably did much to seal the fate of nuclear power in the United States. After months of investigation and analysis, the Nuclear Regulatory Commission issued a new set of far more stringent safety rules that would apply to all reactors, including the one at Humboldt Bay.

Add to this the legal, political, and financial implications of California's own new seismic zoning law, the Alquist-Priolo Earthquake Fault Zoning Act, which was passed in the aftermath of the Sylmar temblor, and the job of retrofitting and upgrading the reactor at Humboldt Bay became too expensive to be economically feasible for PG&E. Four years later the utility applied for permission to decommission the reactor permanently.

In the aftermath, an official report to the U.S. Geological Survey described the twenty-five-mile (40 km) Little Salmon fault as “part of a broad, compressional fold and thrust belt developed in the accretionary wedge above the Cascadia subduction zone.” An accretionary wedge is formed by the sediment and pieces of seafloor crust piled up in a trench where two tectonic plates collide. Think of the North American continent drifting west like a snowplow across the sea floor, scraping up muck and compressing it into rock.

In most cases the wedge is found under water. From Vancouver Island all the way south to the Oregon–California border, this folded and buckled sedimentary wedge is piled up against the continental shelf dozens of miles offshore, where it's difficult and expensive for scientists
to study. Only in northern California was it piled up right in plain sight and on dry ground. The towns of Eureka and Arcata were built on top of it, which is why Gary Carver and others at Humboldt State University were able to draw such a revealing picture of what Cascadia's fault was actually doing. They took advantage of a unique geological setting to make an important discovery.

If the Little Salmon fault was active, then the Gorda plate—which had caused the cracks—had to be active as well, pushing its way underneath California while North America plowed west. The subduction along Cascadia's fault had not “foundered,” and the plates had not stopped moving. At least that was the conclusion I drew from reading the science papers and from interviewing both Plafker and Carver.

Taken as a whole, the Humboldt Bay power project had a significant but unintended consequence. Building a reactor on top of a crack in the crust—a crack directly related to the Cascadia Subduction Zone just offshore—generated the new science that provided the first physical evidence that the northern section of the California and Pacific Northwest coast faced the same kind of tectonic disaster as the ones that happened in Alaska and in Chile.

If I'd been a journalist in California back in the 1970s, I like to think I would have turned this story into headline news. But the immediate impact of these discoveries confirming continental drift was almost nil. The story of Cascadia's fault got lost in the controversy over nuclear power. Fortunately the scientists on the ground knew they were working on significant stuff and refused to quit.

It was the heady, meaningful kind of research that made it an exciting time to be a geologist—especially in the Pacific Northwest. Frank Press may have changed his mind, but many others in the science community still refused to buy the new geology. Even when the top half of a mountain in southern Washington State exploded, only a handful of researchers recognized the distinct sound of Cascadia's smoking gun.

CHAPTER 8

Mount St. Helens: Cascadia's Smoking Gun?

Even though geologists and volcanologists saw it coming, there was no way to prepare for the impact of watching a mountain explode at close range. Mount St. Helens—roughly ninety miles (145 km) south of Seattle and fifty miles (80 km) northeast of Portland—blew steam and dust for two months as a bulge of hot rock sprouted like a giant goiter on its north face. At the same time, the ground trembled and shook. People in downtown Portland turned the prelude into a spectator sport.

Government officials issued repeated warnings to evacuate the hills and valleys around the volcano as the frequency of tremors began to increase. Almost everybody did leave, except for an eighty-three-year-old recluse named Harry Truman who had lived in the woods near the mountain for more than fifty years and decided to stay close to his cabin. The media fell in love with him, a tragic hero in the making. A thirty-year-old volcanologist named David Johnston was collecting data until the very last minute. His final words, “Vancouver! Vancouver! This is it!” were shouted into a walkie-talkie and received at the USGS volcano observatory in Vancouver, Washington, across the Columbia River from Portland, only moments before the eruption. Neither man was seen again.

At 8:32 a.m. on Sunday, May 18, 1980, the volcano started coming apart. A magnitude 5.1 earthquake caused the bulging north side of the mountain to collapse where a new lava dome had been growing. The collapse caused the largest landslide of rock and ice and volcanic mud ever recorded in the continental United States—9,600,000 cubic yards (7,340,000 m
³
) of boulders, muck, trees, and other debris was swept 17 miles (27 km) downhill into the Columbia River. With the face of the mountain suddenly exposed to cool air, the volcano exploded, flattening or burying more than 230 square miles (595 km
²
) of forest and farmland under a blanket of mud and ash that shot 12 to 16 miles (19–26 km) into the sky.

Even though most residents had fled the area, the explosion still killed 57 people, destroyed or severely damaged more than 250 homes and businesses, wrecked 185 miles (298 km) of highway and 15 miles (25 km) of railway track, punched out 47 bridges, and killed more than 7,000 big game animals (deer, elk, and bear) and an estimated 12 million fish at a nearby hatchery.

Before the eruption Mount St. Helens had a spectacular, nearly symmetrical, cone-shaped peak that stood 9,677 feet (2,950 m) high—a stratovolcano. It collected 140 inches (356 cm) of rain and up to 16 feet (5 m) of snow every year, making it look a lot like those famous pictures of Mount Fuji in Japan. After the explosion, the top thousand feet of the mountain had vanished, leaving a horseshoe-shaped crater two miles wide and a half-mile deep (3.2 km by 0.8 km). As the eruption continued for nine hours, the ash plume drifted east at an estimated 60 miles (100 km) per hour, dumping a thick layer of abrasive grit across eastern Washington and Oregon, coating cars as far north as Edmonton, Alberta, as far east as the Dakotas, and as far southeast as Colorado and New Mexico.

The first warning signs had come as early as March 20, when a mild tremor (magnitude 4.2) rattled the mountain. Steam vents began to spew a week later. By the second week of April, scientists had alerted
the media. Walter Sullivan of the
New York Times
touched on the explanation of Mount St. Helens' deep tectonic origin in his story “The West Is Alive with the Sound of Volcanoes.”

The violence of volcanoes like this, according to the
Times
story, was a direct result of the collision of North America with the Pacific Ocean floor. It was the Juan de Fuca and Gorda plates grinding down along Cascadia's fault that had created the Cascade Arc of eighteen major volcanoes from Mount Shasta and Lassen Peak in California to Mount Adams and Mount St. Helens near Portland, to Mount Rainier near Seattle, Mount Baker, about fifteen miles (25 km) south of the Canada–U.S. border, and to Mount Garibaldi, north of Vancouver. Sullivan's feature explained what would probably happen—and why—a full month before Mount St. Helens blew: “The Cascades, part of the Pacific Ocean's necklace of volcanoes, its ‘ring of fire,' are the product of ‘sea floor subduction' . . . Typically, the sea bed bends down as it nears a continent, forming a trench. A sloping zone of earthquakes marks its path into the earth's interior. When the sea floor slab reaches a depth of about 75 miles [120 km], part of it apparently melts and, lighter in weight than the overlying material, forces its way up to produce volcanoes.”

When I thought about the timing and content of Sullivan's story, it struck me that the eruption of Mount St. Helens should have been Cascadia's smoking gun—clear, unequivocal, physical evidence that continental drift was real, that the Gorda and Juan de Fuca plates were on the move and dangerous. This should have been the big wake-up call or tipping point for everyone involved in the geophysical sciences and emergency preparedness, a stark statement that the coast of northern California, the Pacific Northwest, and southwestern British Columbia were every bit as threatened by megathrust subduction disasters as were the coasts of Alaska and Chile. I was wrong. For reasons unclear to me, the alarm bells did not ring.

The Mount St. Helens disaster happened right in the middle of the investigation of those thrust faults that threatened the nuclear reactor
at Humboldt Bay, yet it had no discernible impact on the reluctance of some scientists to accept the Cascadia subduction story. Experts at the top of their fields would still doubt the seismic potential of Cascadia for another eight or nine years.

The spectacle of Mount St. Helens was riveting, no doubt about that. It was also a distraction, such a stunning assault on the senses that few, if any, stopped to think about what this eruption might mean in a larger perspective. Rereading Walter Sullivan's story, I then found a clue to why some scientists were able to separate the volcano itself from the much wider threat a magnitude 9 megathrust quake spread across five major cities would pose.

The scientists Sullivan consulted for his pre-eruption feature story had told him how the Cascadia Subduction Zone was thought to be a special case, different somehow from all the other continental collisions around the Pacific Rim. Cascadia's volcanoes do form a line roughly parallel to the coast about a hundred miles (160 km) inland, and the mountain cones are spaced about forty-five miles (70 km) apart, like most other subduction zones with volcanic arcs. But in the minds of skeptics, that's where the similarities ended.

Cascadia is not typical, Sullivan wrote, because “no coastal trench cuts in to the sea floor” at the point where the two tectonic plates converge and “no sloping zone of earthquakes” marks the descent of a seafloor slab beneath the coast. According to Sullivan's sources, the Cascade volcanoes seem to have been created by an east-driving portion of the Pacific floor that had somehow run out of steam. The subduction process, wrote Sullivan, “is no longer vigorous enough to sustain a coastal trench and cause frequent earthquakes.”

So Cascadia's smoking gun had run out of ammunition. No deep trench offshore and no deep quakes along the plate boundary—all because the movement of the eastbound Juan de Fuca plate had slowed down or even stopped. At least that's what some experts thought at the time.

Trying to imagine how these huge plates float and slide over the curved surface of an imperfectly spherical planet, geophysicists came up with a sequence of events—a long geologic history—that seemed to fit the observable facts. As the floor of the Pacific Ocean spread apart along the Juan de Fuca Ridge, pushing the Juan de Fuca plate eastward, the rest of the Pacific plate (out on the western side of the ridge) was not moving due west but rotating in a more northerly direction toward Alaska.

At the same time, the North America plate was pushing westward and riding up over top of the eastbound Juan de Fuca slab. Around ten million years ago, so the theory went, the Juan de Fuca Ridge and plate started rotating clockwise, almost as if it were being spun by the angular movement of the two larger plates on either side of it. Think of a car going eastbound through an intersection when a northbound car passes just behind it, clipping the back fender. That northbound motion would make the eastbound car spin to the right, just as it got hit head on by a big westbound truck.

Five million years later, the Explorer plate had broken off the northern end of the Juan de Fuca, and some thought it might have fused or welded itself to the larger continental plate just north of Vancouver Island. There was further speculation that the Olympic Peninsula, on the northwest corner of Washington State, might also have been a piece broken off the Juan de Fuca plate and that it too had been fused to the continent, pressed against the outer edge of Vancouver Island, forcing up the Olympic Mountains in the process.

Two and a half million years later, on this hypothetical timeline, the North America plate had “disposed of ” (subducted or recycled) a huge portion of the original Juan de Fuca plate. Down at the southern end, meantime, the smaller Gorda plate was breaking away as well and the spreading ridge offshore—the entire undersea mountain range—had rotated or been spun even farther to the right. At some point, according to this scenario, the relentless westward movement of North America
would completely override what was left of the Juan de Fuca plate—and its spreading ridge offshore—just as it had apparently already done farther south in California.

Eventually, with the Juan de Fuca ridge and plate system gone, the boundary between the North America and Pacific plates would become a much simpler structure, an almost straight-line fault starting with the San Andreas in southern California, extending north along the coast, and connecting with the Queen Charlotte fault system all the way to Alaska. Then the dominant tectonic force in the Pacific Northwest would be a more straightforward, northerly compression caused by the northbound drift of the Pacific plate—just the way it is now along the San Andreas in southern and central California. The eastward subduction of the Juan de Fuca and Gorda plates underneath the continent would become ancient history and Cascadia's tectonic threat would be rendered harmless.