The Cascadia Subduction Fault -- What Would Happen?

Discover magazine has an article about what would happen if a large undersea fault off the West Coast--the Cascadia Subduction Zone--where to slip. It potentially could cause an earthquake larger and more deadly than that which devastated Japan last year.

Now scientists are calling attention to a dangerous area on the opposite side of the Ring of Fire, the Cascadia Subduction Zone, a fault that runs parallel to the Pacific coast of North America, from northern California to Vancouver Island. This tectonic time bomb is alarmingly similar to Tohoku, capable of generating a megathrust earthquake at or above magnitude 9, and about as close to Portland, Seattle, and Vancouver as the Tohoku fault is to Japan’s coast. Decades of geological sleuthing recently established that although it appears quiet, this fault has ripped open again and again, sending vast earthquakes throughout the Pacific Northwest and tsunamis that reach across the Pacific.

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The Cascadia subduction zone is a crack in the Earth’s crust, roughly 60 miles offshore and running 800 miles from northern Vancouver 
Island to Northern California. This fault is part of the infamous Pacific Ring of Fire, the impact zone where several 
massive tectonic plates collide. Here, a slab of the Pacific Ocean floor called the Juan de Fuca plate slides eastward and downward, “subducting” underneath the continental plate of North America.

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Together, Atwater, Yamaguchi, Satake, and their colleagues had sleuthed out precisely when Cascadia had last yawned open. Atwater’s tsunami sands gave a carbon date some time between 1690 and 1720. Rings from the cedar trees narrowed the date to the winter of 1699–1700. Finally, Satake’s written records of a tsunami hitting villages all along eastern Japan nailed the date: Cascadia’s last monster quake happened on January 26, 1700, at 9 p.m. They had cracked the case—except in this detective story, the culprit would almost certainly strike again.

The evidence amassed since then suggests that in fact, Cascadia has generated powerful earthquakes not just once or twice, but over and over again throughout geologic time. A research team led by Chris Goldfinger at Oregon State University (OSU) used core samples from the ocean floor along the fault to establish that there have been at least 41 Cascadia events in the last ten thousand years. Nineteen of those events ripped the fault from end to end, a “full margin rupture.”

It turns out that Cascadia is virtually identical to the offshore faults that devastated Sumatra in 2004 and Japan in 2011—almost the same length, the same width, and with the same tectonic forces at work. Cascadia’s fault can and will generate the same kind of earthquake we saw last year: magnitude 9 or higher. It will send a train of deadly tsunami waves across the Pacific and crippling shock waves across a far wider geographic area than all the California quakes you’ve ever heard about.

Based on historical averages, the southern end of the fault—from Cape Mendocino, California, to Newport, Oregon—has a large earthquake every 240 years. For the northern end—from mid-Oregon to mid-
Vancouver Island—the average “recurrence interval” is 480 years, according to a recent Canadian study. And while the north may have only half as many jolts, they tend to be full-size disasters in which the entire fault breaks from end to end.

With a time line of 41 events the science team at OSU has now calculated that the California–Oregon end of Cascadia’s fault has a 37 percent chance of producing a major earthquake in the next 50 years. The odds are 10 percent that an even larger quake will strike the upper end, in a full-margin rupture, within 50 years. Given that the last big quake was 312 years ago, one might argue that a very bad day on the Cascadia Subduction Zone is ominously overdue. It appears that three centuries of silence along the fault has been entirely misleading. The monster is only sleeping.

Read the whole thing.