By Dan Bartelheimer / For The Herald
If you heard a tsunami alarm, would you recognize it and know your preplanned escape route? Have you observed posted tsunami escape routes in your community? Most of us have no knowledge and appear to have little concern of a tsunami ever happening within the Puget Sound region.
Two scientists, one from the University of Washington, wrote a bulletin in April 2001 titled “Geologic evidence of earthquakes at the Snohomish delta, Washington, in the past 1,200 years.” Within the bulletin it states, “Radiocarbon ages show that the tsunami and liquefaction date from ca. A.D. 800 to 980, similar to the large earthquake on the Seattle fault, 50 kilometers to the south.”
There is a lot of evidence that points toward a very significant tsunami that occurred within the Puget Sound, creating waves that may have exceeded 50 feet and sent water above 150 feet on the surrounding hills. The tsunami reshaped the San Juan Islands and the surrounding landscape. Large portions of the coast and the islands within the sound were washed away, leaving steep, bare bluffs. An earthquake or a meteorite may have caused the tsunami.
As the tsunami reverberated back and forth within the sound much of the vegetation was removed from the landscape. As the waters raced up the river valleys and low coastal plains, it snapped off trees and destroyed almost everything in its path.
Centuries later, as the lowlands were cleared and drained for farming, a number of things have become evident. Farmers started unearthing large woody debris, some of the trees were over 4 feet in diameter. They had been broken off at their roots. The wood remained very solid, indicating it was completely buried and sealed within its tomb within a short period. As the valleys were cleared, drained and farmed, the soil’s organic matter decomposed at an accelerated rate. As it decomposed, the soil elevation subsided, leaving the woody debris exposed.
As the tsunami exploded up the Snohomish River valley, it deposited thousands of trees in the area west on Monroe along U.S. 2. The trees are all laying in the same direction within a few feet of each other. They all appear to have been brokne off at their roots.
The tsunami also brought a lot of sand and sediment. Much of the impacted areas received enough to completely bury and preserve the woody debris. The river’s estuary may have received 5 to 10 feet of sediment while a less amount was left upstream. Flooding over the centuries has deposited finer silt over the tsunami sediment. The course sediment from the tsunami allows water to flow freely through it. As the lower river tributaries are diked, the up pressure from the river during high tides and flooding can cause small “boils” within the open ditches or in the tilled fields behind the dikes.
In some areas where the fine sediment has been disturbed or removed, the tsunami sediment is like quick sand. There are reports that some farmers lost tractors when they cleared the land or got stuck. The tractors just continued to sink and were completely consumed. A small railroad engine jumped the track and was lost on Ebey Island a century ago. It was later located one hundred feet below the surface.
Large rafts of vegetative debris, some up to a half mile in diameter, were also pushed up the valleys. As they were grounded, the ensuing tsunami left silt around these islands, leaving them in depressed bowls filled with water. The upper layer decomposed over time but the lower portion remained submersed in water and decomposed at a very slow pace. Small sticks and leaves can still be identified. These high organic areas have been classified as Mukilteo muck series soils.
Along the hills east of Marysville there are disturbances in the soils at or near 150 feet. The topsoil has been removed along a thin band. This was probably the height of the tsunami.
If a similar tsunami occurred today, it would be a devastating catastrophe killing hundreds of thousands of people. Many of the cities within the Puget Sound would be destroyed. It would be one of the greatest natural disasters in the history of the world.
The current tsunami warning system, where present, is inadequate. The tsunami happened a long time ago, but it did happen, and it can occur again. The public needs to be informed, a tsunami warning system needs to be strengthened, and evacuation routes identified.
Dan Bartelheimer is president of the Snohomish County Farm Bureau.
Tsunami warning system
A test of the Washington’s tsuanmi warning sirens is scheduled for 10:21 a.m. Oct. 21 as part of the Great Washington ShakeOut.
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