Sunday, November 10, 2013

"Is that safe?" Part II: Thinking About Tsunamis


When my friend Jason Jensen (a.k.a. Pirateboy: “Pillage and plunder every treasure, every grain, every day”) set out to hike the OCT in the early 1990s, a friend gave him a huge (like, 18-inch square) Rice Krispies Treat, the idea being that he could use it as flotation in case of a tsunami or, should no tsunami strike, he could eat it. It was a joke, obviously, but he was a little spooked by the possibility. This was just a few years after scientists first discovered that there have, in fact, been huge tsunamis on the Pacific Northwest coast in the past and there will be again. But exactly when, no one knows.

Tsunamis are an infrequent phenomenon on the Oregon coast but one OCT hikers should be aware of. You will most definitely see signs along the highway indicating that you are entering (or leaving) a tsunami zone or indicating what direction to run, should a tsunami be imminent. Most tsunamis worldwide are triggered by earthquakes on the ocean floor. To understand the risk and how to minimize it, it’s essential to understand the two very different types of tsunamis that occasionally strike the Oregon coast.

A distant tsunami is one caused by a very large earthquake hundreds or thousands of miles away: for instance, off the coast of Japan or Alaska. These tsunamis take hours to cross the ocean to Oregon and, by the time they arrive, typically have diminished greatly. Many towns on the Oregon coast have a system of tsunami warning sirens. If you are not near a town, be aware that arriving tsunamis are preceded by a sudden, significant withdrawal of water at the shore. If you notice the ocean suddenly receding far more than normal—and it will be noticeable—simply move farther up or off the beach. Distant tsunamis large enough to be noticeable and do damage in Oregon rare and typically are only dangerous in bays and harbors or near the waterline on the beach. Other than the tsunami following the 2011 Japan earthquake (which, by the time it reached Oregon, was perceptible only in a few harbors), the last distant tsunami large enough to do any damage here was in 1964 and, prior to that, possibly in 1899, both times caused by earthquakes off Alaska.

A local tsunami is a far more dangerous but even less common phenomenon, and the warning sign is unmistakable: it is preceded by a huge earthquake. An underwater fault line (called the Cascadia Subduction Zone) runs just off the Pacific Northwest coast from Vancouver Island south to Cape Mendocino, California (south of Eureka), and every few hundred years it generates a very large earthquake that, in turn, triggers a tsunami that strikes the coast in 15 to 30 minutes (quicker on the southern Oregon coast). If you happen to be on the beach when a large earthquake strikes (it will be unmistakable: four to five minutes of severe shaking), as soon as you are able to walk, run to the highest ground you can reach. The last big Cascadia quake occurred in January 1700; the next one could be any day now—or not for hundreds of years. 
I didn’t know much about tsunamis and tsunami risk when I hiked the OCT in 2008 and 2009; I’ve since learned a great deal (my book on the topic, The NextTsunami: Living on a Restless Coast, is due out from Oregon State University Press in spring 2014). I will say that, knowing what I know now, I would still without hesitation hike the OCT, but I might make a point of not camping at the end of a sand spit. I’m willing to risk the very small chance of being on the beach when the Big One hits, but I would prefer to not have to start running in the dark at the end of a 4- to 5-mile-long spit.

1 comment:

  1. Excellent advice, clearly stated. Where can I get one of those tsunami flotation de-RICES?
    Your book is next on my list.
    Gregory Zeigler—prospective through-hiker, July 2016