17.2: Tsunamis
- Page ID
- 30176
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Tsunamis are low-frequency ocean waves generated by submarine earthquakes. The sudden motion of sea floor over distances of a hundred or more kilometers generates waves with periods of 15–40 minutes (figure \(\PageIndex{1}\)). A quick calculation shows that such waves must be shallow-water waves, propagating at a speed of 180 m/s and having a wavelength of 130 km in water 3.6 km deep (figure \(\PageIndex{2}\)). The waves are not noticeable at sea, but after slowing on approach to the coast, and after refraction by sub-sea features, they can come ashore and surge to heights ten or more meters above sea level. In an extreme example, the great 2004 Indian Ocean tsunami destroyed hundreds of villages, killing at least 200,000 people.
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 fault.
- 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 the coast.
- 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.
Numerical models are used to forecast tsunami heights throughout ocean basins and the inundation of coasts. For example, NOAA’s Center for Tsunami Research uses the Method of Splitting Tsunami most model (Titov and Gonzalez, 1997). The model uses nested grids to resolve the tsunami wavelength; it propagates the wave across ocean basins, and then calculates run-up when the wave comes ashore. It is initialized from a ground deformation model that uses measured earthquake magnitude and location to calculate vertical displacement of the sea floor. The forcing is modified once waves are measured near the earthquake by seafloor observing stations.