Winds, Tides and Weather

Bill Trandum

Bill Trandum

Where are the tsunami evacuation signs for the KP?

An undersea earthquake is the most common cause for a tsunami or tidal wave. Do we need to worry about it? According to Seattle author Sandi Doughton, in her book “Rip 9.0 The Next Great Pacific Northwest Earthquake,” it not only could happen here, it will. We just don't know when.

To give you an idea of the mechanics, consider this example from her book. Imagine holding a single playing card in one hand. Your fingers hold one edge, your thumb the other. It is bent into the shape of an upside down “U” and is kind of springy.

The card is the North American tectonic plate. It reaches 700 miles from Cape Mendocino in California to Vancouver Island. It is anchored inland by mountain ranges and the fruited plain.

Its other edge is deep under the Pacific Ocean, about 70 miles offshore. It is held by the undersea Juan de Fuca plate that is stuck, but keeps trying to creep its way under the North American plate.

Now imagine the Juan de Fuca plate lets go, fails to hold the bottom edge of the card, causing it to spring upwards. Imagine the volume of ocean water that would be displaced by that action.

Doughton's research, and that of geologists in North America and Japan, concluded that the last time it happened was on Jan. 28, 1700, when it sent a tidal energy pulse to Japan, traveling faster than a modern jet plane. It developed into a roiling burbling wall of seawater destruction 100 feet high.

It didn't spare the U.S. Pacific coast. The energy pulse radiated in a circle and inevitably found its way through the Strait of Juan de Fuca and into Puget Sound. Today, simulation models show the maximum tidal flood height at around 30 feet on the Pacific coast, arriving 20 minutes after the earthquake.

The wave would surge east through the Strait of Juan de Fuca then spread south toward Seattle, where wave heights would likely be around 16 feet. By the time the pulse traveled through the Narrows, that height would be dampened even more. The Key Peninsula would likely be affected only by the backwash receding from Nisqually. Whew!

But wait, in 1949 during an earthquake, 11 million cubic yards of the hillside just north of Salmon Beach tumbled into the Tacoma Narrows, causing a 7- to 8-foot tsunami in and around the Narrows.

Something close to that could happen again. Geologists warn that most likely a major earthquake could strike along the Seattle fault line, which they say could produce a tsunami in central Puget Sound with wave heights from 15 to 25 feet. Models show devastation in and around Seattle, Bremerton, Bainbridge Island and Tacoma.

But, the tsunami simulation models consider the Key Peninsula to be so relatively far away, they don't even mention us. Bear in mind that tide levels around our peninsula run from about minus-4 to around plus-16 feet, a span of about 20 feet. So if an event happened at the lowest tide and were “only” 16 to 20, feet we’d be fine. Even so, to the experts, we're so danged remote, even a cataclysmic tidal wave won't bother with us.

And that, my children, is why there are no Tsunami Evacuation Route signs on the Key Peninsula.

But just in case there was one, you wouldn’t want to exit the Peninsula via the Purdy Spit or via SR-302 alongside Case Inlet to Allyn. Most of the Key Peninsula Highway is more than 100 feet above sea-level. Exceptions are at Longbranch, Home and Purdy. The best high-ground way to get off the Key would be to take the KP Highway to SR-302, head west to Wright-Bliss Road, turn north and head for the Port Orchard Sedgwick Road interchange at Highway 16. From there, you could head south across the Narrows to Tacoma, take I-5 north to the airport, then fly to Denver where you'd be a mile high.

Or you could go to any Key Pen fire station. Every one of them –– Longbranch, Key Center, Wauna and Wright Bliss –– is more than 100 feet above sea-level. And there you'd find teams of Fire District 16 first responders with disaster recovery plans.

Bill Trandum is a retired U.S. Navy Captain, a resident of the Key Peninsula, president of the KeyPen Parks Commission, and a student of all things winds, waves, tides and weathers.