Chapter 17 - Coastal Processes and Tides

17.2 Tsunamis

Tsunamis are low-frequency ocean waves generated by submarine earthquakes. The sudden motion of seafloor over distances of a hundred or more kilometers generates waves with periods of around 12 minutes (Figure 17.7). A quick calculation shows that such waves must be shallow-water waves, propagating at a speed of 180m/s and having a wavelength of 130km in water 3.6km deep (Figure 17.8). The waves are not noticeable at sea, but after slowing on approach to the coast, and after refraction by subsea features, they can come ashore and surge to heights ten or more meters above sea level. In an extreme example, the Alaskan tsunami on 1 April 1946 destroyed the Scotch Cap lighthouse 31m above sea level.

Figure 17.7 (a) Hourly positions of leading edge of tsunami generated by a large earthquake in the Aleutian Trench on April 1, 1946 at 12h 58.9m GMT. (b) Maximum vertical extent of tsunami on Oahu Island in Hawaii and the calculated travel time in hours and minutes from the earthquake epicenter. (c) & (d) Tide gauge records of the tsunami at Honolulu and Valparaiso. From Dietrich, et al. (1980).

Figure 17.8 Tsunami wave-height four hours after the great M9 Cascadia earthquake off the coast of Washington on 26 January 1700 calculated by a finite-element, numerical model. Maximum open-ocean wave-height, about one meter, is north of Hawaii. From Satake et al. (1996).

Shepard (1963, Chapter 4) summarized the influence of tsunamis based on his studies in the Pacific.

1. Tsunamis appear to be produced by movement (an earthquake) along a linear fault.
2. Tsunamis can travel thousands of kilometers and still do serious damage.
3. The first wave of a tsunami is not likely to be the biggest.
4. Wave amplitudes are relatively large shoreward of submarine ridges. They are relatively low shoreward of submarine valleys, provided the features extend into deep water.
5. Wave amplitudes are decreased by the presence of coral reefs bordering the coast.
6. Some bays have a funneling effect, but long estuaries attenuate waves.
7. Waves can bend around circular islands without great loss of energy, but they are considerably smaller on the backsides of elongated, angular islands.

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