Swept away

Or, Here come the mud (part 2)

ON AUGUST 10, a mixed busload of oceanographers, geologists, emergency-management specialists, and reporters trundled out to a bedraggled clam- digging resort north of Ocean Shores to look at a 6-inch-thick layer of sand buried a yard deep in a salt marsh on the Copalis river.

As a photo op it left a lot to be desired, but there was an eerie thrill contemplating the featureless pale-gray seam between two beds of marsh peat. The sand commemorates the catastrophic day 301 years ago when land along the coast from Neah Bay to Astoria shuddered and sank beneath the sea, only to be inundated a few minutes later by a wall of water more than a hundred feet high.

That tsunami (what used to be called a "tidal wave") was just one of many discussed and evaluated at a recent international symposium on "tsunami hazard mitigation" at the UW. While one scientist crowed about his success in forecasting such a phenomenon in Chile, researchers from New Zealand warned how local conditions—including shoreline shape and seafloor conformation—make hazard planning difficult.

The one thing all agreed on was that a lot more study, costing a lot more money, is needed before places like Ocean Shores, or Poulsbo for that matter, can feel comfortable that everything's been done to minimize loss of life when such a wave of destruction rolls in again, as one is sure to do.

Off the Northwest coast (and off Japan, New Zealand, Chile, and other areas at risk) the Pacific seafloor is gradually pushing under the continent—but not gradually enough for comfort. For centuries at a time pressure builds up and then lets loose all at once, along a front that potentially could extend from Vancouver Island to Cape Mendocino, Calif.

The resulting quake would be unimaginably greater than any we've experienced since European settlement of the area. And because such undersea quakes actually alter the depth of the ocean floor and the height of the shoreline, a great deal of water gets sloshed around before equilibrium returns. Such quakes can also trigger huge undersea mudslides, which can lead to even more sloshing.

On that scale, it doesn't take much of a wave to wipe a low-lying sandspit community like Westport or Long Beach clean off the map. The consequences when such a wave rolls into enclosed waters like Willapa Bay, Grays Harbor, or the Columbia estuary can be just as dire and far harder to predict.

One of the biggest problems facing planners in developing tsunami hazard mitigation programs is that the civic elites of towns in need of such plans don't want to discourage tourists or vacation-home buyers by putting up "tsunami evacuation route" signs everywhere.

In Oregon such signs are already posted in a lot of places, to the considerable irritation of the tourist industry. And you can sympathize with those who point out that it doesn't do much good to tell people where to go to get away from a wall of water if there's no way for them to do so before it hits.

One Oregonian at the conference wryly remarked that when the Big One returns, it would be better for Seaside, Ore., if it avoided doing so on a summer weekend, when up to 30,000 visitors would find themselves with open ocean on one side and two narrow bridges to the mainland on the other.

It wasn't on the agenda—the folks attending, after all, are professionally committed to mitigating hazards, even if there isn't yet any known way to do so—but there is an argument that the money it's going to cost to develop quake- and tsunami-prediction equipment and to deploy effective evacuation programs could be better spent otherwise.

In more than a decade of paddling Washington estuaries, mapping sand exposures like the one on the Copalis, and taking soil cores where exposures are few, U.S. Geological Survey scientist Brian Atwater has established a pretty clear time-line for Pacific tsunamis in these parts. The average interval between such events is around 500 years, plus or minus a couple hundred. This means that the odds of such an event recurring today are rising, but it still might not happen for 400 years. Are we equipped to calculate the cost-benefit ratio of preparing for an event that could well not happen until 2410? Is it possible that society might be better off letting the good people of Ocean Shores (and their insurance companies) take their chances?


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