Thinking about tsunamis here in the Northwest

Reports about the loss of life and devastation in Japan are overwhelming — and yet most experts seem to consider Japan as the best prepared for earthquakes among all countries in the world.

I’ve been covering Northwest earthquake science for more than 25 years. When I heard that the Japanese quake was around magnitude 9 and sending a tsunami across the ocean toward the U.S. West Coast, I thought about an earthquake that occurred off the Washington Coast more than 300 years ago.

That earthquake sent a wall of water across the ocean, washing up on the shores of Japan. Because of that tsunami, researchers have been able to calculate the time of that quake to about 9 p.m. on Jan. 26, 1700.

I wrote a story for Saturday’s Kitsap Sun making some general comparisons between Friday’s earthquake in Japan and the last great Cascadia earthquake of 1700.

In broad-brush terms, “the two earthquakes are very similar,” John Vidale, director of the Pacific Northwest Seismograph Network, told me. “As a first guess, what might happen here is what happened there.”

For Saturday’s piece, written for a general audience, I decided to avoid some of the technical details about the two earthquakes, so allow me to offer some additional information here:

  • The Cascadia earthquake occurred on the interface between the North American and Juan de Fuca plates. The Japanese earthquake occurred on the interface between the Pacific and North American plates. (Some people prefer to pinpoint the quake on the Okhotsk microplate, a kind of subdivison of the North American plate.) Please check out the plate map at the bottom of this page.
  • The fault rupture in the 1700 Cascadia quake was about 1,000 kilometers (621 miles) long, compared to about 500 km for the Japanese earthquake. Although the fault zone is shorter in the Japanese quake, its 200-km width appears to be much wider.
  • Slippage along the fault was somewhat less in the Cascadia earthquake, according to Vidale. The Japanese quake has been estimated at more than 18 meters maximum slippage. I’ve seen similar estimates for the Cascadia quake.
  • Coastal areas generally drop after a subduction earthquake
    Graphic courtesy of PNSN
  • The 1700 tsunami that hit the West Coast (then occupied by Native Americans) was huge. I can’t find an estimated height. But even after crossing the Pacific Ocean, the Cascadia quake may have been 15 feet high when it hit Japan. This latest tsunami has been reported with wave heights as much as 30 feet in Japan, but only one to two feet on the West Coast.
  • One reason for the smaller waves on the West Coast from the Japanese quake may relate to the angle of the fault in Japan, which directed the waves more southward than the Cascadia earthquake did, Vidale said. Depths of the quake and the angle of slippage are other factors.
  • Los Angeles Times reporter Eryn Brown said researchers figure that the land near the Japanese coast dropped about two feet in the recent earthquake as the coast shifted seaward up to 12 feet. The Cascadia earthquake of 1700 resulted in subsidence between three to six feet according to estimates.

Although a tsunami generated from a subduction quake off the West Coast would have a relatively small effect in Puget Sound, we still need to be concerned about landslides along the shoreline that could be broken loose from the shaking. Landslides can create a small, but damaging, tsunami.

We also have ongoing risks from surface faults, which could set off 10-foot waves in Puget Sound that could strike local shores within minutes, if not seconds. When I wrote about the tsunami risk for Kitsap County in 2001, less was known about the Seattle, Tacoma and South Whidbey faults, as well as others.

The best advice I have heard is to plan ahead, so you know what to do after an earthquake. Think about what you would do if you live on the water or just happen to be there during an earthquake. Consider whether you should move immediately to higher ground as soon as the shaking stops, for there may not be time to listen for emergency sirens or news broadcasts. There may not be time to think.

Finally, I’d like to share a story I wrote for the Kitsap Sun in 1999, in which I describe a remarkable time in the history of earthquake research in the Pacific Northwest. That and other earthquake stories are listed on a special web page that we put together last year.

The page automatically updates recent earthquakes in Washington state and around the world. It has been adding new Japanese earthquakes rapidly the past few days.

The West Coast shares both the North American and the Pacific plates with Japan. Japan also is at the intersection of the Eurasian and Filipino plates.
Map courtesy of Wikimedia Commons

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