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10.13: Tsunamis

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    • Contributed by Miracosta Oceanography 101
    • Sourced from Miracosta)


    A tsunami is a very long and/or high sea wave or coastal serge of water caused by an earthquake or other disturbance. Tsunamis get their name from Japan (where they are fairly common): "Tsu"[ harbor], "nami" [wave].

    Tsunamis are caused by displacement of the earth's crust under an ocean or body of water of any size. They can also be generated by earthquakes, volcanic explosions, underwater landslides, even asteroid impacts. When the solid earth moves, the water above it also moves with it (Figures 10-32 and 10-33). Tsunamis are the result of both the initial shock waves and the following motion of the water readjusting to a stable pool (sea level). Tsunamis can travel great distances throughout the world's ocean. Their energy is dissipated when they approach shorelines where they come onshore as a great surge of water, with or without massive waves crashing onshore. Although most tsunamis are small (barely detectable), some modern tsunamis have reached inland elevations many hundreds of feet above sea level.

    Tsunami diagram
    Figure 10.32. How a tsunami is generated by an earthquake.

    Tsunami surge
    Figure 10.33. Tsunamis move onshore more as a surge than just a wave.

    Tsunami Characteristics:
    • Tsunamis are usually less than 2 feet in the open ocean.
    • In deep ocean, tsunami wavelengths are long, commonly 100’s of miles.
    • Tsunamis always behave like shallow water waves ( d < L/20) because no ocean deep enough!
    • Undetectable by ships in open ocean because wavelengths are so long (slow rise and fall as wave passes).
    • Open ocean tsunami velocity is 400 – 500 mph. So about 4 – 5 hours from Alaska to San Diego (or Hawaii).
    • Wave stacks up on continental shelf, about ½ of the time a trough arrives first (sea recedes from shore).
    • Waves 30 – 100 ft are common – locally run-up can be higher.
    • Highest is thought to be +300 ft., 66 million years ago from asteroid collision in the Gulf of Mexico.