Cascadia's Fault - Jerry Thompson [75]
When the first wave from Adak Island finally reached the northern tip of Vancouver Island at Cape Scott, it was only four feet (1.2 m) above mean high tide. When it got down to Neah Bay in Washington, it was only two feet (60 cm) above normal. At Grays Harbor and Willapa Bay, it was less than that. The maximum height of the Adak tsunami was a harmless 1.8-foot (54 cm) slosh by the time it hit the beaches in Hilo, Hawaii. In Japan it was a hissing five inches (13 cm) of foam. By 10:00 p.m. Pacific time that night emergency officials began lifting the evacuation orders. All that scrambling and racing around in the dark had been for nothing. Stephanie and David Fritts still didn’t find each other until later the next day.
Angry and frustrated citizens in dozens of coastal towns called it a false alarm. For others a potential disaster had degenerated into a poor joke. “We gave this big party and nobody came,” quipped Lieutenant Commander Tom Pearson of the Coast Guard in Seattle. A train of five small waves were in fact triggered when the ocean floor heaved upward, so in reality it was not a false alarm. There was simply no way to tell how big the waves would be.
For Stephanie Fritts and others with small children to round up and protect, or for those who got separated from loved ones, family, and friends in the panic of those first few hours, the experience was nothing to laugh about. “This was the first I had ever known that the Pacific Coast could be impacted by a tsunami and I was having visions of the old movie Krakatoa. I didn’t really understand the whole thing and was confounded for the most part! I didn’t realize that tsunamis were real and not an invention of the movies,” she said.
The thing that stuck in her mind was the lack of information. There had to be a better way of detecting and measuring what happens at sea when a subduction event tears the ocean floor apart. The more immediate and dire implications of an Alaska-type catastrophe happening very close to home, just a few miles offshore from Long Beach—a monstrous quake and train of waves from Cascadia—was another whole movie that nobody had explained to Stephanie and her neighbors. She was annoyed and motivated enough to do something about it.
After the conference at Monmouth, conversations between Brian Atwater and Gary Carver moved into new territory. While others were slowly coming to accept the notion that Cascadia’s fault was an active subduction zone, Atwater and Carver and a handful of others had jumped ahead to the next big question. Assuming they had proven the existence of past quakes, how could they find out whether the entire subduction zone had ripped loose all at once—as opposed to rupturing in a series of smaller segments, releasing only part of the accumulated strain each time?
“There were two competing hypotheses,” Carver told me. “We were arguing about what kind of earthquake it was. Not whether it happened.” Looking at how far apart the sunken marshes and forest floors were geographically—all the way from Vancouver Island to Cape Mendocino—and sorting through their calendar of radiocarbon dates, they began to speculate about whether or not Cascadia always ruptured from end to end. Was it a single, Alaska-size event each time, or did some segments of the down-going slab—perhaps the southernmost Gorda plate off Humboldt Bay, for example—rupture separately in smaller events? Meaning magnitude 8 disasters instead of magnitude 9 catastrophes.
“There was the Brian Atwater ‘apocalyptic model,’ which was a magnitude 9 which broke the whole subduction zone,” Carver mocked, “and then I had coined the term ‘decades of terror’ for a series of earthquakes that occurred very close together in time—so when you radiocarbon dated them, you couldn’t tell them apart.” When not trading quips with Carver, Atwater insisted he was “agnostic” about magnitude 9 and that—all kidding aside—they needed some way to