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Science for Society:

Impact of tsunamis on Oregon coastal communities


Dedicated to John Paul van Son, an inquisitive young mind from Ilwaco, WA

Background

Scientific findings of the last several years have shown that the Oregon coast is vulnerable to great (M 8-9) earthquakes that can occur on the offshore Cascadia sudbuction zone (CSZ) fault system. CSZ earthquakes are rare events: the last has occurred about 300 years ago, and they are believed to occur every 200 to 600 years. While rare, these events can be highly destructive. A major manifestation of CSZ earthquakes are tsunamis, large water waves generated by the seismic deformation of the sea floor. As these waves reach the coast, severe flooding can occur, resulting in loss of life and property.

Predicting the impact of tsunamis anywhere in the world is a process fraught with uncertainty. The rarety of CSZ tsunamis compounds the uncertainties, and has forced scientists to blend computers, fundamental laws of physics, and information from prehistoric earthquakes and tsunamis in a fairly unique way.

One of the outcomes are computer models that simulate the propagation of CSZ tsunamis. These models provide insight into what happened in the past and what might happen in the future, should a great subduction zone earthquake and tsunami strike the Pacific Northwest coast. To partially account for uncertainties, several scenarios of sea floor deformation have been investigated.


The Cascadia Subduction Zone (CSZ) extends from southern British Columbia to northern California. The length of the crosshatched area is the approximate length of the postulated fault rupture for a magnitude 8.8 earthquake on the Cascadia subduction zone.

Acknowledgement: Image reproduced from an original of the Oregon Department of Geology and Mineral Industries

Scenarios of Sea Floor Deformation

The uncertainties regarding the definition of CSZ sea floor deformations led us to nvestigate several alternative scenarios of deformation. The figures below illustrate four major such scenarios. Each figure shows isolines of predicted uplift and subsidence from hypothetical subduction zone earthquakes. Redish colors indicate uplift, and blueish colors indicate subsidence. The darker the color, the larger the sea floor deformation.

Scale of sea floor deformation, in meters Map Scale, in kilometers

Scenario 1
Used by Paul Whitmore, of the Alaska Tsunami Warning Center, as the basis for his 1993 predictions of theoretical wave height for the Oregon Coast. Whitmore's predictions have been used by local communities for tsunami hazard planning, and his sea floor deformation scenario was included in our research as a reference. An extensive subsidence zone is visible off the coast of Oregon and Washington, leading to subsidence at the shoreline that does not match coseismic subsidence inferred from studies by Curt Peterson of buried wetland soils.

Scenario 2
A first attempt to account for Roy Hyndman's and Kelin Wang's information on crustal temperature in the Cascadia Subduction Zone. The subsidence region starts further offshore than in Scenario 1, and does not match the coseismic subsidence inferred from buried wetland soils.

Scenario 3
Accounts matches geological information from paleotsunamis. The uplifts region is broader than for other scenarios, and most subsidence is onshore.

Scenario 4
Is a second attempt to more accurately accounting for information on crustal temperature, while providing a fairly good match to the data from buried wetland soils. Note that the broader uplift region is higher than for the other scenarios and, like in Scenario 3, the subsidence is mostly on or close to shore.



Multi-media The text of this page contains links to several supporting images and computer animations. For a list and explanation of images and animations, select the proper icon below.

Images

Animations


Current understanding and uncertainties The characteristics of the tsunami wave, both offshore and along the coast, will depend primarily on the sea floor deformation. Predicting the exact characteristics of that deformation is beyond current scientific capabilities. Scenarios of sea floor deformation that have been used in our research are based on and generally consistent with both geological information from prehistoric tsunamis and known seismic and geological caractheristics of the CSZ fault system. However, it is unlikely that any of the scenarios will mimic exactly the characteristics of future events.

Most scenarios that can reasonably be constructed show the generation of two waves, one propagating towards the coast, and the other towards the deep ocean. The latter will cross the Pacific basin, and will eventually impact distant coasts: Hawaii, Japan, etc. The former will reach in a matter of minutes the coasts of Oregon, Washington, southern British Columbia, and northern California.

Maximum wave run-up heights may reach close to 12m (~36ft) for some scenarios. Local impact will be stronger in certain regions of the coast than in others. For a given event, the offshore and nearshore coastal topography determine the regions where energy will concentrate. High waves will have only a serious impact, however, if the shoreline is low enough to be susceptible of flooding. Communities that are particularly exposed in the Oregon coast include Seaside, a city built near sea level in a region where, possibly because of Tillamook Head, there is a tendency for strong concentration of energy. Long Beach, WA and Crescent City, CA are examples of high-risk communities in neighboring states.


Society-Driven Outcome Our research has assisted the Oregon Department of Geology and Mineral Industries (DOGAMI) to develop detailed inundation maps for Siletz Bay and coarse-scale inundation maps for the entire State of Oregon.
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