Cascadia: The West Coast Fault Line That Is "Nine Months Pregnant"

The fault zone that produced the largest known earthquake in the Lower 48 is "nine months pregnant and overdue." This time line illustrates it.

Ever since the massive 9.0 magnitude earthquake struck Japan, I've been thinking a lot about my friends in Oregon. Why? Because the impending "Big One" that Californians are nervous about is actually a lot more likely to occur off the coast of Oregon—and would be an even "Bigger One" there.

The Cascadia Subduction Zone, which sits off the coast of Oregon, was the site of the largest known earthquake to have ever struck the Lower 48 American states: a magnitude 9 megathrust in early 1700 that sent a tsunami crashing into the Pacific coast, and across the ocean to Japan.

Andy Revkin, whose post immediately after the Japan quake alerted me to the Cascadia earthquake threat, ran this note from Yumei Wang, an Oregon-based earthquake analyst and expert.

The average time between magnitude 8 and larger Cascadia earthquakes is about 240 years (see page 8, Cascadia earthquake timeline, based on Chris Goldfinger’s data, Oregon State University). The last megaquake, estimated as a magnitude 9, occurred in 1700 — that’s 311 years ago. In geologic terms, Cascadia is “9 months pregnant” and overdue.

Even though geologists identified 41 past Cascadia megaquakes, they cannot pinpoint exactly when the next Cascadia earthquake will strike. Nonetheless, engineers can design and build to withstand earthquake shaking. Now is the time to take preparations seriously, safeguard those in harm’s way, and strengthen aging critical infrastructure.


Here's the time line that Wang references:

In case you need a closer look, I'll zoom in on just the last 4,000 years:

The heavier red lines are earthquakes of magnitude 9 or higher. The smaller lines are magnitude 8 or higher. As you can see, it's been over 310 years since the last magnitude 8+ earthquake, and the rhythm and period of massive seismic events along that fault is typically shorter. Nine months pregnant and overdue.

The time line was included in this sobering Cascadia report (PDF) from the Oregon Department of Geology and Mineral Industries, and the time line data came from Chris Goldfinger at Oregon State University. The report is loaded with information about the Cascadia region's vulnerability. I've already emailed it (again, PDF) to all of my friends in the Beaver State, and I can't imagine it would hurt for you to do the same. When you're talking natural disasters, preparedness is everything.

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