Tsunamis and Seiches

Key Points

Tsunami

  • Definition: Tsunamis are waves caused by earthquakes, volcanic eruptions or landslides. In deep water they have long wavelengths and travel very fast. They slow down but can build to great heights as they enter shallow water near shore.

  • Damage is caused by the wave's kinetic forces and the flood that follows.

  • Tsunamis generated in the open ocean would not have a great effect in Seattle because a tsunami wave has to make a 90 degree turn to reach the city and the complex coastline of Puget Sound acts as a restraint. The most likely effect of a tsunami from the Pacific in Seattle would be strong currents.

  • Tsunamis can be generated within Puget Sound by landslides or earthquakes. The most frequent cause of Puget Sound tsunamis is landslides.

  • The 1949 Olympia earthquake triggered a landslide into the Tacoma Narrows that causeds a 6 - 8 foot tsunami three days after an earthquake. An earthquake on the Seattle Fault about 1,000 years ago produced a 16ft tsunami.

  • The Seattle Fault, which runs through Seattle's midsection and through Puget Sound to Bainbridge Island, caused a tsunami around 900 AD.  

  • If a tsunami like the one in 900 AD happened again it would be devastating. The tsunami would hit immediately after the ground stopped moving. People along the shore would have little time to escape. It would destroy buildings along the shore and flood areas up to a mile inland.

  • The 900 AD tsunami was probably a worst case. It is more likely (but not certain) that the next Seattle Fault tsunami will be smaller.

Seiche

  • Definition: Seiches are standing waves in water bodies caused by most often by seismic waves or atmospheric pressure. They can occur at great distances (100s or 1000s of miles) from an earthquake epicenter. Because they are standing waves they move vertically more than horizontally.

  • Lake Union is especially prone to seiches due to its shape. The east and west sides are roughly parallel and the V-shaped northern end focuses waves. There is a historical report of a seiche or tsunami on Lake Washington, but it is not clear how large seiches on Lake Washington could be.

  • Seiches have occurred multiple times in Seattle, but they have not caused extensive damage so far. Large seiches are a danger to the I-90 and SR520 bridges. A large seiche could strain cables anchoring the bridges. The new State Route 520 bridge is designed to take about 12 feet of upward motion and 8 feet of downward motion from a seiche. Based on models the most damaging seiche would probably be caused by a Cascadia subduction zone earthquake.

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