Cascadia's Fault - Jerry Thompson [80]
The main shock was felt as far south as San Francisco, as far east as Reno and Carson City, Nevada, and across most of southern Oregon. In Sacramento, 202 miles (325 km) to the southeast, a curious thing happened. Lori Dengler did some checking and found that most people living at ground level felt almost nothing. The farther up they were in apartment blocks and condo towers, however, the stronger the motion tended to be.
“If they were in the sixteenth, seventeenth, eighteenth floor of a high-rise building, not only did they feel it—they felt it so sharply that it drove them to run down the stairs and evacuate the building,” she said. “By the time you get above the twentieth floor of some of these buildings, more than half the folks evacuated.” The astonishing thing was that no physical damage had been done in Sacramento, yet the kind of ground motion generated by the undersea fault off Cape Mendocino was able to travel a long distance inland and cause certain tall buildings to resonate with the frequency of the shockwaves. Shades of Mexico City.
But it was Dengler’s cautionary note that really caught my attention. “This was an earthquake that was a thousand times weaker than what we’re talking about in terms of the amount of energy in a Cascadia earthquake,” she said. Put another way, whenever Cascadia does finally rip loose with a magnitude 9, the results will be off the scale. That’s simply the difference between magnitude 7 and magnitude 9.
Several things did, however, make the Mendocino temblor different and indeed significant. It generated a small, three-foot (1 m) tsunami that hit nearby Crescent City, scene of so much destruction and a dozen deaths in 1964. And it lifted up a fourteen-mile (23 km) section of land along the beach at Cape Mendocino. “It turned out that the North American plate that we’re sitting on was shoved up and over the Gorda plate, which is subducting beneath us,” Dengler explained. In other words, this earthquake was apparently generated by tectonic movement along Cascadia’s fault. It was not just another small slip along one of the vertical cracks near the surface.
With a total of ninety-eight people injured and a damage estimate of only $66 million, it may have looked to the outside world like a relatively insignificant lurch compared to the Loma Prieta event on the San Andreas in 1989. There were no dramatic helicopter shots of cars falling through a trap door on the Bay Bridge, no double-decker freeways crushing dozens of cars. But for those living in Humboldt County and those focused on Cascadia, this was a turning point in terms of putting the aseismic subduction argument to rest.
Gary Carver’s memory of that deceptively sunny day was just as vivid. The shockwaves hit while he was driving to his office on the Humboldt State campus in Arcata. After the ground stopped moving he quickly rang home to make sure everything was okay there, then rounded up a bunch of graduate students and headed straight for Petrolia. By about noon the USGS had told them where to find the epicenter. They began scouring the countryside looking at landslides and other physical damage—without ever finding a surface fault.
Because the ocean was high and rough that day Carver and company failed at first to see the quake’s most geologically significant wreckage. Several days after the jolt, however, the HSU crew stopped for lunch in a Petrolia café, where they overheard local residents in the next booth talking about how much the shoreline had changed. Offshore rocks normally submerged were now high and dry and the place “smelled like fish stew.”
So Carver and his colleagues headed back toward the beach and sure enough a big swath of California landscape had been hoisted up sharply into new marine terraces. For the next several days the HSU team hiked the shoreline, documented the