A Landscape Created by the Earthquake of 1700

Dead western red cedars in salt marsh

The Washington State coast is located along a convergent plate boundary. The Juan de Fuca ocean plate is being subducted beneath the North American Continental plate. In other locations around the world some of the largest earthquakes ever recorded have taken place along similar convergent plate margins. However, since arrival of Europeans and Americans, no big quakes have been recorded along the coast. Tom Heaton and Garry Rogers proposed that the reason no large earthquakes had been recorded along the Washington coast was that the two plates were jammed and that when a quake did happen it would be very big.

 

Brain Atwater, a geologist with the USGS, went on a hunt for prehistoric evidence of large quakes. He found lots of evidence and the above photo is one piece of our landscape that looks the way it does because of a massive subduction zone earthquake.

Western red cedar does not like salt water. These trees grew in a fresh water environment until the land subsided and the area they were growing became a salt water tidal marsh. The trees have been age dated and died in approximately 1700. A tsunami recorded in Japan on January 27, 1700 matches very well with the evidence compiled by Atwater.

The standing cedars are a dramatic sight and cover a large area. Not as obvious, but equally impressive are that the banks of the tidal stream are lined with the stumps of drowned Sitka spruce. Sitka spruce can tolerate some salt, but not daily saturation. The spruce rot faster than the cedar so the only standing trees remaining are the cedar. A layer of sand blankets the old soil horizon from the tsunami wave that impacted the area immediately after the quake.

Sitka Spuce stump along stream bank

Atwater's work has explained a landscape that can be seen throughout the tidal marshes of the Pacific coast of Washington. But that sleuthing led to another landscape change along the Washington coast. Radio towers and warning sirens as well as tsunami escape route signs are now part of the landscape along our coastal areas.