Cascadia's Fault - Jerry Thompson [39]
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.