Chapter 17 - Coastal Processes and Tides
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 130 km
in water 3.6 km 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 31 m above
||Figure 17.7 (a)
Hourly positions of leading edge of tsunami generated by a large earthquake
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
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.
- Tsunamis appear to be produced by movement (an earthquake) along a linear
- Tsunamis can travel thousands of kilometers and still do serious damage.
- The first wave of a tsunami is not likely to be the biggest.
- Wave amplitudes are relatively large shoreward of submarine ridges. They
are relatively low shoreward of submarine valleys, provided the features
extend into deep water.
- Wave amplitudes are decreased by the presence of coral reefs bordering
- Some bays have a funneling effect, but long estuaries attenuate waves.
- Waves can bend around circular islands without great loss of energy, but
they are considerably smaller on the backsides of elongated, angular islands.