Cascadia's Fault - Jerry Thompson [61]
In essence Heaton and Kanamori compared Cascadia (back then it was still being referred to as the Juan de Fuca Subduction Zone) to all the other active quake-prone subduction zones along the coasts of Chile and Alaska and to the Nankai Trough off the coast of Japan. They found more similarities than differences. Bottom line: if giant ruptures could happen there—in Chile, Alaska, or Japan—the same would probably happen here, in the Pacific Northwest. Their paper, published in the Bulletin of the Seismological Society of America in June 1984, argued that the absence of quakes in recent history didn’t mean they wouldn’t happen in the future.
Like the other seismic danger zones around the Pacific Rim, Juan de Fuca’s spreading ridge was so close to the edge of the continent that the new slab of ocean floor pumped out from the bowels of the earth was still relatively warm, thin, and buoyant. Put another way, the seafloor plate had not had much time to cool before it got jammed underneath the landmass of North America. Because it was warm and buoyant, the plate was in all likelihood scraping under the continent at a very shallow, almost horizontal angle. And because it was relatively smooth it was probably sticking to the upper plate the same way wide, flat-surfaced tires known as racing slicks build friction and stick to pavement.
Heaton and Kanamori found that the biggest megathrust events were directly related to young, buoyant plates being strongly coupled to the overlying landmass at shallow angles—which fit the description of the Juan de Fuca Subduction Zone perfectly. All the other extreme danger zones had shallow trenches full of thick sediments that had been piled up against the outer coast and compressed, causing folds, faults, and uplift of the overriding plate. Same with Juan de Fuca.
They found that weakly coupled subduction zones like the Marianas Trench had much steeper angles of dip and much deeper trenches, which caused the down-going oceanic plate to melt sooner and therefore not get stuck to the upper plate. Without being “strongly coupled” to the upper plate, the subduction process does occur smoothly, creeping along aseismically, with no big jolts. But the Juan de Fuca Subduction Zone was nothing like the Marianas Trench. Therefore, Juan de Fuca was probably not aseismic.
At least that’s how Heaton and Kanamori saw it. Compared to all the other places around the Ring of Fire, the Juan de Fuca zone looked just as capable of deadly temblors as the worst of the lot. “This 500-kilometre gap in seismic activity is one of the most remarkable to be found anywhere in the circum-Pacific seismic belt,” they wrote. Their concluding paragraph must have sounded like a call to arms for other geologists: “The Juan de Fuca and North American plates appear to be converging at a rate of between 3 and 4 cm/yr. The Juan de Fuca subduction zone shares many features with other subduction zones that are strongly coupled and capable of producing very large earthquakes. Although the shallow part of this subduction zone shows little presentday seismicity and no significant historical activity, we feel that there is sufficient evidence to warrant further study of the possibility of a great subduction zone earthquake in the Pacific Northwest.”
The problem, as Tom Heaton explained it to me, was that he did not have direct physical evidence of earthquakes. All the comparison studies in the world could not prove unequivocally that Cascadia’s fault had ruptured in the past. The compression evidence from Jim Savage’s survey markers near Seattle, like those squeezed-together mountaintops Herb Dragert was studying on Vancouver Island, were symptoms of stress, not proof that big quakes had already happened here. And that’s what Heaton was urging others to find—the proof.
CHAPTER 11
Quake Hunters: Finding Cascadia’s Ghost Forest
Fog drifted