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A seiche is a standing wave in an enclosed or partly enclosed body of water. Seiches are normally caused by earthquake activity, and can affect harbors, bays, lakes, rivers and canals. In the majority of instances, earthquake-induced seiches do not occur close to the epicenter of an earthquake, but hundreds of miles away. This is due to the fact that earthquake shock waves close to the epicenter consist of high-frequency vibrations, while those at much greater distances are of lower frequency which can enhance the rhythmic movement in a body of water. The biggest seiches develop when the period of the ground shaking matches the frequency of oscillation of the waterbody.
In 1891, an earthquake near Port Angeles caused an eight foot seiche in Lake Washington. Seiches generated by the 1949 Queen Charlotte Islands earthquake were reported on Lake Union and Lake Washington. The 1964 Alaska earthquake created seiches on 14 inland bodies of water in Washington, including Lake Union where several pleasure craft, houseboats and floats sustained minor damage.
Inland areas, though not vulnerable to tsunamis, are vulnerable to seiches caused by earthquakes. Large lakes such as Washington, Sammamish and Union have many water craft, houseboats, docks, piers, houses and buildings located on or close to their waterfronts. Lake Washington’s floating bridges may also be damaged by a seiche. Additional vulnerabilities include water storage tanks, and containers of liquid hazardous materials that are also affected by the rhythmic motion.
Seiches create a "sloshing" effect on bodies of water and liquids in containers. This primary effect can cause damage to moored boats, piers and facilities close to the water. Secondary problems, including landslides and floods, are related to accelerated water movements and elevated water levels.
