Cascadia's Fault (12 page)

Only a few years later, two geologists hiking the north woods of the California coast would quietly shift the focus back to the atomic power plant at Eureka. Gary Carver, a freshly minted professor at nearby Humboldt State University, and Tom Stephens, one of his senior thesis students, were conducting field research on a forest company's land in the upper Mad River drainage basin east of Arcata when they discovered some “very large and previously unknown and unmapped faults.”

The Simpson Timber Company had kept hundreds of miles of access roads closed to the public during the 1950s, '60s, and '70s while they logged off huge stands of old-growth redwood forest. None of the land had been geologically mapped and Carver and Stephens didn't quite know what to expect. “We were able to go back in where geologists hadn't been for a very long time,” Carver told me.

Now, with the big trees gone, the ground was nearly naked and rock formations were easier to see. Still, Carver and Stephens had to hike for miles and miles along steep switchbacks, mapping and following a web of fractures from the mountains all the way west and downhill to the intertidal and beach zones along the coast. They noticed a distinctive angularity, what they called a “rhombohedral fracture” pattern, which to a non-geologist's untrained eye would look like nothing more than
“tiny little cracks in the sand” of cutbanks sliced through the wilderness by road builders for the logging crews.

This rhombohedral pattern, Carver explained, was how faults propagate through unconsolidated sand deposits. This was loosely packed sand left behind when this part of the coast was under water, a wedge of ocean sediment that had been shoved against the continent and now stood well above the high-tide line. “Instead of a nice, clean, one-plane fault in which two pieces of the earth's crust move past each other, it becomes hundreds or thousands of little tiny faults all closely spaced together,” he elaborated. “I thought this was really neat.”

Each of the fractures Carver and Stephens found cut through geologically young terrain, suggesting the cracks were relatively recent. That meant whatever tectonic force had caused the fractures might still be an ongoing threat. Carver was pretty sure the rhombohedral fractures had been caused by plate convergence and compression. While none of the individual cracks had a huge amount of movement, taken as a whole the offset was significant.

“We realized that these little tiny fractures we were seeing in many places were parts of faults that had very large amounts of displacement on them,” said Carver. The displacement added up to several miles in total. “And again—you're sittin'right there on the edge of the mapped subduction zone and you see those big folds in young sediment,” said Carver, “and you can't help but think that that subduction zone is still active.” All of this within a few miles of the nuclear plant at Humboldt Bay.

That's when Carver decided to fly north to Alaska for a first-hand look at what had happened there. He needed to “see what a big earthquake looked like” in all its mangled glory so that he could better understand what he was seeing on the ground in California.

CHAPTER 7

Proving the Doubters Wrong: The Chile Connection

Gary Carver spent an entire summer in Alaska looking at the aftermath of subduction. He flew the entire length of the '64 rupture, every mile of broken shoreline. He also met George Plafker, who was more convinced than ever that the primary fault that had caused the beaches and bays to heave, buckle, and subside could not have been vertical. Plafker had recently returned from Chile and was eager to tell anyone who'd listen that the two biggest earthquakes in recorded history had caused exactly the same kinds of physical damage to the landscape.

Not only that, but some very prominent senior scientists were apparently coming around to Plafker's point of view. Frank Press, who had so famously disagreed about the angle of the fault, sat in the audience at the 1968 meeting of the American Geophysical Union in Washington, DC, and listened to Plafker's presentation of his paper on Alaska. The main theme of that year's convention was “The New Plate Tectonics,” and here was Plafker telling the science establishment that the Alaska quake had been caused by two huge slices of the earth's crust converging almost horizontally, getting stuck together, and then snapping apart.

Plafker told me that after the speech Press cornered him and unloaded. “He came up and he was real mad,” Plafker recalled. “He said, ‘You know, I've written a lot papers and I've seldom been proven wrong. But you did it to me this time!' He told me I had caught him in the biggest mistake he made in his career,” said Plafker. “His views on the mechanism of this earthquake had changed and he was man enough to say so.”

But not everyone was convinced. Clarence Allen, another senior scientist who'd heard Plafker's talk, still needed convincing. And he threw down a challenge that Plafker simply could not resist. How, he asked, could this underthrusting of the ocean floor be happening only in Alaska? Did Plafker think that's what happened in the Chile earthquake as well? Plafker said yes—even though he didn't know for sure—and so Allen arranged the funding necessary to send him south.

Plafker spent two months scouring the Chilean coastline by car along the mainland and by chartered boat in the islands of the southern archipelago, measuring areas of heaved-up and down-dropped land. “There again in Chile, in the southern part, vegetation grows right down to the shorelines,” he said. “You could see the effects of subsidence from the drowned and dead trees and brush.” He found a zone of “tectonic warping, including both uplift and subsidence,” that was 125 miles (200 km) wide and roughly 625 miles (1,000 km) long. It affected an area of at least 50,000 square miles (130,000 km
²
) in southern Chile.

The two-day series of temblors in 1960 had included two main shocks, thirty-three hours apart, along with fifty-six large aftershocks. The sequence of ruptures and the tsunamis they triggered killed 2,000 people in Chile and 230 more in Japan, Hawaii, and the Philippine Islands.

The main finding of Plafker's paper, however, refuted the previous conclusion that Chile's wreckage had been caused by a nearly vertical strike-slip fault (like the San Andreas) because it was based on “incorrect” data that were “clearly incompatible” with his newer evidence of
tectonic movement. He wrote that the Chilean main shock “resulted from a complex rupture on a major thrust fault or zone of thrusting roughly 1,000 km long that dips at a moderate angle from the continental slope beneath the continental margin.” In other words, the fault was more horizontal than it was vertical, just like in Alaska.

Plafker estimated that to cause such widespread upheaval and deformation of the landscape, there must have been at least 65 feet (20 m) and perhaps as much as 130 feet (40 m) of horizontal slip once the fault broke. This might seem “surprisingly large,” he wrote, but “not excessive” if compared to the horizontal thrust of roughly 65 feet he had seen in the 1964 Alaska earthquake. The bottom line appeared to be that the events had both been caused by the same process: two pieces of the earth's crust crashing together.

So his gambit in Chile had been a success; he was able to prove plate convergence. “It was very straightforward, once you know what you're looking for,” he said, putting the apparent mistakes of the first scientists on the scene in Chile into some kind of context, “but you know, it's just like anything else. If you've done it once before, it's a cinch. And if you haven't, you don't know what to do.”

Nevertheless, when Plafker met Gary Carver a few years later and they compared notes on Alaska, Chile, and northern California, the similarities were hard to miss. “It's pretty clear to me,” said Plafker, “that the southern end of Cascadia is very much like the eastern end of the Aleutian Arc and the area where the '64 earthquake occurred. We have the same type of continental margin.”

 

When I asked Gary Carver why he thought it took so long for most geologists to come around to the view that Cascadia was a threat, he could remember clearly one paper—written by Masataka Ando of the U.S. Geological Survey and Emery Balazs of the National Geodetic Survey—that stood out. They believed the subduction zone had “foundered” and was no longer active. “They related the idea that the rise
[the Juan de Fuca Ridge where the sea floor was spreading apart] was so close to the trench that the plate was too hot to go down,” Carver explained. “It couldn't go down, so therefore, subduction had stalled.” This would presumably explain why there had been no large subduction earthquakes in all of recorded history.

But Carver was now infected by Plafker's enthusiasm. He was sure those cracks in the sandstone meant something significant. Back at work in California he and Tom Stephens continued their research on the fracture zones along the northern California coast. They mapped each individual rupture and gave it a name—the Big Lagoon, Trinidad, McKinleyville, Mad River, and Fickle Hill faults. “As far as I know,” said Carver, this was “the first recognition of the existence of large, active thrust faults north of the Mendocino Triple Junction.” It was also the first onshore evidence of tectonic motion on the southern end of the Cascadia Subduction Zone.

The discovery of unknown crustal cracks on logging roads in the hills behind Arcata made geology a hot topic for students and other scientists working in the area. One of those drawn to a series of talks that Gary Carver gave in 1974 was Tom Collins, a geologist working for the U.S. Forest Service, based in Eureka at the Six Rivers National Forest office. When Collins saw slides of the “rhombohedral fractures” and heard Carver speculate about the relation between the faults in the hills and the big subduction zone offshore, he wanted to find out more about it.

Collins went exploring on his own. He knew about the Little Salmon fault, which had been partially mapped back in 1953 by a local geologist named Bud Ogle, and perhaps because it was the one closest to where he worked in Eureka, Collins decided to have a closer look. Across Highway 101 from the nuclear power plant, he wandered into a recently excavated sand quarry at the base of Humboldt Hill. There he discovered, completely by accident, more of those rhombohedral fractures that Gary Carver had talked about in his lectures.

A day or two later Collins phoned Carver who agreed to join him at the sand pit for a quick recon. “We recognized the Little Salmon fault extended further north than Ogle had mapped,” said Carver. Here again “young material” had been torn, meaning the shockwaves that had caused those distinctive fractures had occurred not so long ago in geological time. Even more worrisome, it looked like the crack probably continued right underneath the highway and onto the 143-acre (58 ha) site where PG&E had built the Humboldt Bay reactor.

So Collins wrote up his discovery and, as a concerned citizen, sent it to the Nuclear Regulatory Commission (the new name for the Atomic Energy Commission) in Washington. It was the first in a long and increasingly political chain of events that galvanized local antinuclear activists who had formed the Redwood Alliance to do battle with PG&E. It also, as an unintended consequence, accelerated the scientific research that would finally confirm the true nature of the Cascadia Subduction Zone.

 

A magnitude 5.2 earthquake shook the town of Ferndale on June 7, 1975, causing repeat damage to a town that had barely survived the pounding of 1906. The shockwaves also hit Humboldt Bay to the north of Ferndale, and in the aftermath fresh cracks were discovered in the concrete pavement of the road leading into the nuclear reactor site. A team of engineers from the University of California at Berkeley was called out to study the “ground motions and structural response” at the power station. The concrete caisson, with walls four feet (1.3 m) thick and an outside diameter of 60 feet (18.3 m), dug 85 feet (26 m) into the ground, appeared to be okay. But PG&E decided to err on the side of caution and ordered a thorough examination just in case.

The Berkeley report confirmed that there had been no significant damage to the reactor. The summary page, however, spoke volumes. “The regulatory requirements led to an
adequate but not excessively conservative
margin of safety based on the motions recorded in this event”
(my emphasis). In other words, PG&E had followed all the rules and nothing bad had happened this time, but if there was any chance of larger earthquakes, then all bets were off.

Roughly a year later, in July 1976, when the reactor was shut down for routine refueling, the seismic safety questions were red-flagged by the Nuclear Regulatory Commission. The NRC decided to keep the plant closed until the Little Salmon fault and the new system of fractures discovered by Gary Carver and Tom Stephens could be checked and the seismic hazard issues dealt with.

PG&E hired several consulting firms to conduct field studies to find out whether any of the faults were still active. A sixteen-station array of seismographs was installed in the surrounding mountains and along the northern California coast to get a more detailed picture of all the tectonic motion. In addition, the NRC decided to send its own team of scientists into the field to follow up on the work done by Carver and Stephens.

They created a timeline of earthquakes in the region. With backhoes they dug trenches across the Little Salmon and Mad River faults for close-up looks at where and how often the various layers of soil and rock below ground had been torn apart. Taking samples of woody debris, dead plants, and the remains of tiny sea creatures contained in the layers disrupted by quakes, they used radiocarbon dating to figure out when the ruptures had happened.

In the fall of 1980 the geologists concluded that the Little Salmon fault was indeed active and that it probably ran underneath or very close beside the reactor. The bottom line according to Woodward-Clyde Consultants, hired by PG&E, was that the seismic issues could be dealt with but the job would be neither cheap nor easy.