Subject: Seiches

In simple terms, what is a seiche? Could a seiche counteract the forward energy of a tsunami? How do you pronounce "seiche"?

    Every enclosed body of water has a number of natural resonances. If you sit in a bathtub part full of water and rock back and forth you'll find that at the right period (about a second) you can easily get the waves to grow until they overflow the bath. The resonant oscillation of the water is a seiche. Seiches are often generated in swimming pools by small oscillations from earthquakes - the oscillations happen to be at the right frequency for the swimming pools to "catch" them. During the Northridge earthquake of 1994, swimming pools all over Southern California overflowed. During the great Alaska earthquake of 1964, swimming pools as far away as Puerto Rico were set into oscillation!
    Tsunamis generate seiches too, although we usually do not consider them as seiches. The predominant period of the tsunami that hit Hawaii in 1946 was fifteen minutes. The natural resonant period of Hilo Bay is about half-an-hour. That meant that every second wave was in phase with the motion of Hilo Bay so that the sloshing of the bay built up. We usually think of the damage to Hilo in 1946 as being simply from the tsunami, but it was really a combination of the tsunami and a tsunami-generated seiche.
    Could a seiche counteract a tsunami? I assume here you are asking if a seiche generated by seismic waves could counteract any tsunami generated by the earthquake. Interesting idea! I'm afraid the answer is "no," for two reasons. The first reason is timing. On the deep ocean, tsunamis travel about 800 km/hour (500 mph). That's about 0.2 km/s. Earthquake waves travel much faster, say 8 km/s (i.e., forty times faster). Any seiche excited by earthquake waves will have died down before the tsunami arrives. The second reason is frequency of oscillation. Earthquake waves tend to have most of their energy at periods (the time from one wave crest to the next) of ten seconds to a few minutes. Tsunamis tend to have periods of five minutes to as much as an hour. So a seiche excited by earthquake waves would be at too high a frequency to interact with the tsunami.
    How do you pronounce "seiche?" Sigh-shh. The word was introduced to science by the Swiss seismologist F.A. Forel in 1890. The word had apparently long been used in the German-speaking part of Switzerland to describe oscillations in alpine lakes.

Dr. Gerard Fryer
Hawaii Inst. of Geophysics & Planetology
University of Hawaii, Honolulu, HI 96822

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