Tsunamis

A tsunami is a series of ocean waves that are generated by the sudden displacement of water. Although earthquakes are the most common cause, tsunamis may also be generated by landslides, volcanic explosions, meteor impacts, nuclear explosions, and glacier calving. Unlike a wind-generated sea wave, a tsunami wave has an extremely long wavelength. A very large wind-generated wave might have a wavelength of 200 meters (650 feet), but a typical tsunami has a wavelength of 200 kilometers (120 miles). Incredibly, tsunamis can travel at 800 kilometers per hour or kph (500 miles per hour or mph) in the open ocean.

Figure 10.6: San Andreas Fault and associated seismic zones.

Although tsunamis have extremely long wavelengths, while at sea they have only minimal height. In fact, ships in the open ocean may never notice the passing of a tsunami wave. As the wave approaches shore, however, the wavelength decreases and the wave height (amplitude) increases (Figure 10.7) as the water “piles up” upon reaching shallower depths. Even a tsunami with a moderate wave height can do great damage, however, because the great wavelength means it may take a single wave 20 to 30 minutes to sweep up the shore. Such waves resemble a wall of water more than a simple breaking wave (Figure 10.8).

The tallest wave ever recorded occurred in Lituya Bay, located along the Alaskan Panhandle. On the night of July 9, 1958, a M8.5 earthquake initiated a landslide. Thirty million cubic meters (forty million cubic yards) of rock plunged into the bay, creating a local tsunami that destroyed all trees and vegetation up to a height of 524 meters (1720 feet) above the bay. Unusually high tsunamis are referred to as megatsunamis. This category includes landslide-generated tsunamis as well as tsunamis generated by a meteor impact, such as the meteor that struck 66 million years ago, causing the extinction of the dinosaurs. The height of the wave caused by that impact is estimated to have been as much as 5 kilometers (3 miles) high!

Figure 10.7: Changes to a tsunami wave as it approaches the shore.

Figure 10.8: The difference between normal water waves and tsunami waves as they approach the shore.

Often the trough of a wave will arrive before the crest, making it appear as if the sea is retreating. This strange sight might give residents time to escape before the crest arrives, if they recognize that the drawdown is a warning sign and not just the tide going out very quickly. The shape of a bay or harbor can accentuate the effects of a tsunami, funneling the wave’s energy inland—such was the case in Crescent City, California, after the Great Alaskan Earthquake of 1964. The earthquake created a tsunami that arrived at Crescent City four hours later. Although the first three waves did only minor damage, the fourth wave, approximately 6 meters (20 feet) tall, inundated the town, killing 12 residents and injuring over 100, and destroying hundreds of buildings. Forty-seven years later, Crescent City was once again struck by a tsunami, this time from the 2011 M9.0 Tohoku-Oki Japan earthquake. Fortunately, there was only one death because at-risk areas of the city were evacuated before the arrival of the first wave.

Figure 10.9: A typical tsunami evacuation route sign.

The West Coast of North America is prone to tsunamis because of the seismically active “Ring of Fire” around the Pacific Ocean. The National Oceanic and Atmospheric Association’s (NOAA’s) Pacific Tsunami Warning Center has created a tsunami warning system based upon earthquake data and real-time tsunami detectors in the deep ocean. If a tsunami is generated at a distance, residents of coastal communities can be warned so that they can follow established tsunami evacuation routes to high ground (Figure 10.9). If, however, a tsunami is generated just offshore, there is not enough time for a warning, as was the case for coastal Japan with the 2011 M9.0 earthquake and associated tsunami.

The Japan 2011 and Alaskan 1964 earthquakes are good examples of megathrust earthquakes that are likely to create tsunamis. As one tectonic plate tries to subduct below another, the two plates can become stuck together. The pressure from the subducting plate builds up and deforms the overlying plate by causing it to buckle (Figure 10.10). When the subducting plate finally moves, the overlying plate snaps back, uplifting the ocean and creating a tsunami. Not only could this happen along the Aleutian Island Arc’s trench, it could happen again along the Cascadia subduction zone. The location of the trench, just offshore of northern California, Oregon, and Washington, is likely to give coastal residents little time for evacuation.