Wednesday, November 6, 2013

Tsunamis in the Strait of Juan de Fuca

A photo of probable "tsunami sands" in the Salt Creek salt marsh, Strait of Juan de Fuca. Photo by Sarah Sterling.

Its well known that what are called "far field" tsunamis impact the Strait of Juan de Fuca...heck, one of my early posts to this blog was of a tsunami wave propagating into Port Angeles Harbor after the Tohoku earthquake and tsunami that hit Japan in March 2011. And within the academic and emergency management community it is also well known that we are at risk from very large "near field" tsunamis - one generated near to our coast. In particular a so-called "mega-thrust" earthquake on the Cascadia subduction zone could generate a large and potentially catastrophic earthquake and tsunami that could impact much of the Pacific Northwest coast. The Washington Emergency Management Department, for example, is very focused on tsunami outreach and risk reduction throughout the coastal areas of Washington State.

However, since there hasn't been a large earthquake or tsunami in this part of the world since the "historic" period started its hard to really accept that we have a risk, especially in the relatively protected waters of the Strait of Juan de Fuca. Communities on the outer coast of Washington State seem to have embraced their risk, and are taking various steps to reduce that riks, either by building vertical evacuation structures, practicing evacuation drills, or even

Increasingly, though, the traces of tsunamis that have impacted the shoreline of the Strait of Juan de Fuca are being uncovered. Today I learned of Ian Hutchinson, Curt Peterson and Sarah Sterling's latest paper, documenting their discovery of sand layers in the Salt Creek salt marsh, just west of Port Angeles. They detail the evidence that leads them to conclude that these sand layers were almost certainly deposited by tsunamis occurring 1000-2000 years ago (find the paper here, starting on page 12). This is added to some of the original work on the U.S. side of the Strait of Juan de Fuca that shows multiple tsunami sand beds in Discovery Bay, Curt Peterson's recently published work from Neah Bay, and tsunami sand beds documented on Whidbey Island. Taken together, it is clear that numerous sites on the Strait of Juan de Fuca appear to record repeated relatively large tsunamis.

What is at risk? Quite a bit, as documented here, and some communities are taking notice (see Clallam County's tsunami fact sheet and this story about the Lower Elwha Klallam Tribe's real evacuation after the Tohoku tsunami). Any time new information like Hutchinson et al, 2013 comes out, though, should be a moment that we reconsider our preparations, and redouble our efforts to plan and prepare.

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