Is Vashon at Risk for Tsunamis?

Most folks think of an overseas or Cascadia based quake as generating a terrible tsunami – and they are right. However such a tsunami is not likely to cause massive damage to Vashon from what I’ve heard – largely due to the moderating influence of the Straits of Juan de Fuca. (It will be interesting to verify this at our upcoming April 18th community meeting with Washington State experts).

There are some other sources of significant tsunami risks for Vashon however. The main ones are thought to be underwater landslides displacing huge volumes of water after earthquakes along the Seattle or Tacoma faults - which run a couple miles north of Vashon and through lower Vashon respectively.

Puget Sound Tsunami Sources—2002 Workshop Report; A contribution to the Inundation Mapping Project of the U.S. National Tsunami Hazard Mitigation Program

A great overview of potential tsunami impact on Puget Sound is the NOAA OAR Special Report, Puget Sound Tsunami Sources—2002 Workshop Report; A contribution to the Inundation Mapping Project of the U.S. National Tsunami Hazard Mitigation Program. A Joint Special Report, this was published in June 2003 and was sponsored by:

• National Oceanic and Atmospheric Administration
• United States Geological Survey
• Washington State Department of Natural Resources
• Washington State Military Department Emergency Management Division

These graphics, from this report highlight that we are in an area with significant and periodic tsunami risks.

Washington, Tsunami Hazard Mapping Project: Modeling Tsunami Inundation From Tacoma And Seattle Fault Earthquakes

The most specific study on Vashon-Maury specific impacts that I’ve seen is NOAA Technical Memorandum OAR PMEL-132, Tacoma, Washington, Tsunami Hazard Mapping Project: Modeling Tsunami Inundation From Tacoma And Seattle Fault Earthquakes, published in January 2007. The publisher, our local Pacific Marine Environmental Laboratory, has a variety of sources for further exploration into this topic.

Some Vashon specific information – which I’ve taken out of context (and added my own emphasis) includes the following:

Seattle Fault Scenario

The southern wave front splits into two upon striking the northern tip of Vashon Island 4 min after initial deformation. The eastern and more intense front travels down East Passage striking northern Maury Island and Dumas Bay and then reflecting off the Ruston Way waterfront and southern Vashon Island 12 min after generation (Fig. 7). The weaker west front travels down Colvos Passage striking Point Defiance before joining with the stronger reflected wave in Dalco Pass. Maximum wave crests of approximately 3.5 m amplitudes reach Commencement Bay and Gig Harbor approximately 19 min after generation (Fig. 6). Part of the primary front’s reflected wave energy travels north into Quartermaster Harbor and back up East Passage toward Elliott Bay and northern Puget Sound. The remaining wave energy travels into The Narrows and then dissipates in southern Puget Sound. Smaller though still significant waves continue to reflect back and forth within the study region for 3 hr.

High (>1.5 m/s) wave velocities occur within Commencement Bay, Gig Harbor, East Passage, Quartermaster Harbor, The Narrows, and Wollochet Bay. Since the model does not dynamically include tidal currents, the current speeds may be more substantial if these events occurred during a flood tide.

- from Page 10

Tacoma Fault Scenario

Inundation occurs at both State ferry terminals approximately 18 min after tsunami generation. The Tahlequah terminal at the southern point of Vashon Island is hit with an initial 3.3-m wave with speeds ranging from 5–7 m/s and overtopping the dock and staging area. A 3.9-m wave strikes the Point Defiance terminal and marina at speeds of approximately 3 m/s

- from Page 12

Additional Scientific Information

6 minutes after a Seattle Fault Quake	26 Minutes Later…	43 Minutes Later…	Color indicates the water’s height – in Meters

The above graphics from PMEL, show Vashon shorelines could get ~4 meter surges of water above the current tide levels and as much as ~4 meter drops too. It appears worst for Vashon’s inner and outer Quartermaster harbor, but certainly appears to me as potentially strong enough to breach the 100 year old isthmus between Vashon and Maury Island. Recall that these are not just temporary waves, but large masses of water with much greater mass and potential for damage.

http://nctr.pmel.noaa.gov/state/wa/index.html and http://nthmp.tsunami.gov/publications.html has more related info.

http://nctr.pmel.noaa.gov/animate.html has a few good movies of this, especially http://nctr.pmel.noaa.gov/Mov/seattle_png.mov (despite being labeled “Tacoma Inundation [6.1 MB file, .mov]”) which shows over 4M surges!

Still more information may become available in the coming months from the new HAZUS runs being done by FEMA for the Vashon region.