Barbara J. Tewksbury, the William R. Kenan Professor of Geology, gave a geological perspective on the Dec. 26 Indian Ocean tsunami in a talk called "When The Seas Rise Up" on Jan. 24. Tewksbury explained the geological phenomena that cause tsunami, the waves' behavior, and the historical context of the Indian Ocean event. Tewksbury will give a repeat of this lecture on Tuesday, Feb. 1 at 7 p.m. in the Chemistry Auditorium.
To understand tsunami, Tewksbury said, you have to understand earthquakes. Tsunami are caused by any phenomena that disturbs the entire water column, but the most common cause is submarine earthquakes. Earthquakes occur when the energy that has been stored up by the pressure of two tectonic plates moving against each other is suddenly released by the plates sliding against each other. This causes vibrations to emanate out from the center of the quake.
However, as Tewksbury explained, it is not the vibrations from the earthquake that cause the tsunami, but rather the displacement of the ocean floor and the disturbance of the water column. When the water column is moved up or down by the movement of the seafloor, it creates an oscillation that radiates out in all directions. Tewksbury likened this to bouncing your hand gently on the top of a still pool of water and seeing ripples moving out from the center. The Indian Ocean tsunami was caused by a 9.0 magnitude earthquake that occurred when the Eurasian plate was subducted at Sumatra, slipping 20 meters along the fault and causing the seafloor to rise only a couple of meters.
Tewksbury went on to outline the differences in the behavior of tsunami waves from the wind-produced waves we are used to. First of all, while wind waves move only along the top of the water, tsunami waves move the entire water column at every point. They have incredibly long wavelengths in the open ocean however, up to hundreds of kilometers, and the height of the wave is less than a meter in the open water. As Tewksbury said, this makes the events in movies such as "The Poseidon Adventure," where a cruise ship is overturned by a tsunami in the open water of the Mediterranean Sea, impossible.
While tsunamis have long wavelengths and low waveheights out at sea, as they come closer into the shore the water column is pushed up, the wavelength is shortened, and the waveheight gets larger. The wavelength is still relatively large, however, which is why the crests of tsunami waves appear on shore as surges of water rather than breaking waves. Either the crest of the trough of a tsunami can appear on land first, meaning that sometimes the first change in the ocean is a large recession of the water on the coastline. Tewksbury showed pictures from Sri Lanka showing how the waterline originally receded, then seven minutes later inundated the land when the crest arrived, and then receded again 11 minutes later when the next trough arrived.
Tsunami can also be caused by other geological phenomena like volcanoes and submarine landslides, Tewksbury said. Tsunami cause geological change as well, depositing sediment in some places and causing erosion in others. Looking at tsunamigenic patterns in the soil can help scientists reconstruct historical tsunami events. About six major, damaging tsunami occur in the Pacific Ocean every century. While the danger is larger in the Pacific, no ocean or sea that could have earthquakes or landslides is immune. In fact, a tsunami is likely to occur from an earthquake in the Carribean sometime in the next 50 years. However, Tewksbury reminded the audience, both hurricane storm surge and terrestrial earthquakes can be just as deadly and destructive as a tsunami.
--- By Caroline O'Shea '07
