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Two things for the Portland metro area. There is a state geo agency that published a detailed series of earthquake hazard maps. If you are considering a home purchase, look at these first (sorry, realtors!). I was surprised to see that in Beaverton there is a large area of soil that will cause major structural problems; this is far from the fault line that runs through downtown PDX.
Second: No, a wave will not wash over the coast range. If there is a tsunami headed straight for the coast it will cross the Columbia bar. What will the effect be of that wave as it travels inland, and what could be the expected distance of travel inland?
I've been told that even if the tsunami wave itself doesn't reach Portland, there will be a back-up of the Columbia, as flow to the ocean is blocked by the incoming wave. Picture that you could put a curtain wall across the Columbia at, say, Longview WA, for one minute, and all the water flowing down the rivers has nowhere to go but will spread out as the water backs up. Consider the Cowlitz R. (elevation 3 feet where it joins Columbia), what will happen to Longview WA? And the Wmt R. is elevation 10 feet where it joins the Columbia, Sauvie Island might be under water? Vancouver? PDX Airport and the levee at Marine Drive? How far does the backup go? So, while the coastal impact of a tsunami has been studied ad infinitum, I have yet to see any answer to the river back-up question. Or is there nothing to worry about? Could we see water 5, 10 feet above the flood stage in Portland and Northwest Indistrial zones? What will happen to the Columbia Bar, will the sand bar be altered enough to bring a halt to shipping traffic as they sort out the new channel and dredge the shifted river sediment? And Seattle, wow. Hasn't anyone tried to model the tsunami wave action in the Puget Sound? The ports? What happens when a wall of water travels south and ends up in Tacoma and Olympia? As the water travels to narrower channels, it piles up higher.
I've seen no answers from any authority. Grad students: thesis topic and computer models, anyone?
posted 3 years ago
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