Cascadia's Fault - Jerry Thompson [81]
The seismic data showed an almost flat focal plane and the nearly horizontal motion of a thrust fault, agreeing with what Carver could plainly see on the beach—the upper plate had popped loose from the subducting ocean floor and ridden up over it. “It produced coseismic uplift,” he noted, “just like the ’64 one did. Except it was all in miniature.” In essence the continental plate along California’s western shore got massively and permanently deformed during the rupture, the same thing George Plafker had seen in both Alaska and Chile.
“When you analyze exactly where that hypocenter or focus of where the earthquake was, it really coincides very closely with where we think that subduction zone interface is,” said Dengler, equivocating only slightly. “I mean there’s still some debate amongst the scientists as to whether it was really the main subduction zone or a subsidiary fault. And there’s also some debate as to really where the end of the subduction zone is and how complicated things get down there in that Triple Junction region. But I think we all agree that it was a thrust fault and it was clearly related to the subduction zone. And so it became really the first major earthquake to occur on the subduction zone or a very closely related fault.”
“The Petrolia earthquake was a subduction zone earthquake because it broke on the subduction zone,” said Carver, equivocating not at all. “The boundary between the Gorda plate and the North America plate is a very low-angle thrust fault. And it slipped and caused this uplift and subsidence of the coast and generated this earthquake,” he said. “It was a subduction zone earthquake, as far as I’m concerned.”
The point of contention seems to be that the Gorda plate, which has broken off the southern end of the Juan de Fuca plate, seems to move and behave separately from the larger slab of the ocean floor and therefore might not be considered a part of the overall subduction zone. And to some extent Carver agreed with this view because evidence of several events found down in the Eel River valley showed radiocarbon dates for Gorda plate ruptures that were completely separate from the Juan de Fuca quakes.
In other words, temblors like the 1992 Petrolia one seem to have happened several times before, with the Gorda plate breaking loose from the overriding continental plate on its own timetable. “It looks like there are some little end pieces that have a life of their own,” joked Carver, which to me sounded like yet more evidence for the decades of terror scenario.
The interesting thing was that in Petrolia in 1992 there were three separate ruptures, and to me it looked as if the subduction zone had started coming apart underneath the oil town, working its way outward beyond Cape Mendocino under the ocean floor and toward the main subduction zone. “Once the fault started to unzip, why did it stop?” I asked. “Why didn’t it go all the way to Vancouver Island?”
“Yep,” said Carver, “that’s a very good question. And I’ve been puzzling on that since ’92.”
Lori Dengler agreed. “The 1992 earthquake ruptured the southernmost little corner of Cascadia—maybe fifteen miles [24 km] in length. So why did 1992 stop? It’s been a long time since the last event. There’s been a lot of strain put into the system, so why didn’t we have, on April 25th, 1992, a much larger earthquake? We don’t have a simple answer to that. Clearly it stopped because it didn’t have enough energy to make it through the bump or the asperity or the sticky spot that would have allowed it to go further. Does it mean that we’re closer to a larger rupture? Does it mean that we could have another little piece go? Well obviously we’re closer. Every day we get a little bit closer.”
Proving that Cascadia’s fault had started coming apart was only part of the significance of the triple shock that morning in Petrolia. According to Lori Dengler the new data generated by that event caught the eyes