Modeled Tsunami Inundation from a Cascadia Subduction Zone Earthquake

The greatest tsunami flooding is expected to occur at Swantown Marsh on Whidbey Island and on the southern shores of Padilla Bay. Elsewhere, tsunami flooding is expected only in the immediate vicinity of the shoreline where evacuation to higher ground would be an easy matter. Large areas of inundation occur in areas of low topography surrounding Samish Bay, Padilla Bay, and the Swinomish Channel. Though not part of the modeling study, inundation also occurs within the vicinity of Fir Island. These areas are protected by dikes that were not resolved in the grid used for the modeling, but the height of the dikes suggests they would be overtopped by the modeled tsunami and so inundation shown there is probably appropriate

Current velocities are shown in three zones-less than 1.5 m/s (~3 mph), which is the current speed at which it would be difficult to stand; 1.5-5 m/s, and greater than 5 m/s, which is a modest running pace. Within this zone, computed velocities locally exceed 20 m/s (~40 mph) in confined channels.

The red dots show simulated tide gage records at eight locations. These records show fluctuations of water surface elevation and also the time history of the waves. The initial water disturbance is a trough or drop in sea level of about a meter at about 11/2 hours after the earthquake at the westernmost end of Whidbey Island and about half an hour later in the narrow channels to the north.

The first crest or rise in sea level arrives between 2 and 21/2 hours after the earthquake, again earlier at Whidbey Island and later in Bellingham and Guemes Channels and at Padilla Bay. At about 21/2 hours after the earthquake, another trough of about a meter occurs in the south, but water piles up in Padilla Bay and the crest remains for two cycles, indicating a prolonged period of flooding.

The model run is referenced to mean high water and does not include the influences of changes in tides. The tide stage and tidal currents can amplify or reduce the impact of a tsunami on a specific community. In Padilla Bay, the mean spring tide range is about 81/2 ft and can be as much as about 13 ft). This means that, while the modeling can be a useful tool to guide evacuation planning, it is not of sufficient resolution to be useful for land-use planning.

Enlarged map of Guemes Island.

°The phenomenon we call tsunami (soo-NAH-mee) is a series of traveling ocean waves of extremely long length generated primarily by earthquakes occurring below or near the ocean floor. Underwater volcanic eruptions and landslides can also generate tsunamis. In the deep ocean, the tsunami waves propagate across the deep ocean with a speed exceeding 800 km/hr (~500 mi/hr), and a wave height of only a few tens of centimeters (1 ft) or less.

Tsunami waves are distinguished from ordinary ocean waves by their great length between wave crests, often exceeding a 100 km (60 mi) or more in the deep ocean, and by the time between these crests, ranging from 10 minutes to an hour.

As they reach the shallow waters of the coast, the waves slow down and the water can pile up into a wall of destruction tens of meters (30 ft) or more in height. The effect can be amplified where a bay, harbor or lagoon funnels the wave as it moves inland. Large tsunamis have been known to rise over 30 m (100 ft). Even a tsunami 3 to 6 m high can be very destructive and cause many deaths and injuries.

From Tsunamis-The Great Waves by the U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service, Intergovernmental Oceanographic Commission, and International Tsunami Information Center

Information courtesy of Division of Geology and Earth Resources, Ron Teissere - State Geologist, NOAA TIME Center, Pacific Marine Environmental Laboratory, Seattle, Washington, WASHINGTON DIVISION OF GEOLOGY AND EARTH RESOURCES. Complete document [4.4mb PDF]

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