Causes of Tsunami Still Cause Controversy

Since November 2005, passengers leaning on ship’s rails before the Indonesian coastline can observe a yellow object bobbing up and down: it’s the first buoy of the Tsunami Early Warning System that the Germans and the Indonesians have begun to install.

TEWS doomed to failure?

At the Third Conference on Early Warning (EWC3), held in Bonn from March 27 to March 29, 2006, Tony Song from the NASA Jet Propulsion Laboratory, California Institute of Technology, demonstrated that the causes of the tsunami had not been thoroughly comprehended. He concluded: “Without knowing how tsunamis arise from earthquakes, it would be impossible to predict a tsunami precisely.”

Does that mean that all efforts at early warning are doomed to failure?

Standard approach

The standard approach is based on the assumption that tsunamis occur during a submarine earthquake when the seabed rises and pushes the water in a vertical direction. Hence earthquakes that involve a significant vertical motion will be more effective in generating tsunamis. Song, on the other hand, could demonstrate that the Tsunami of December 26, 2004 was triggered by horizontal displacement. According to Song, movements of continental margins, rather than the local vertical uplift should be the focus of tsunami observation and prediction. Horizontal motions of continental slopes confer five times the energy of a vertical displacement. Song’s findings are raising questions that even newer studies cannot answer. Richard Briggs, seismologist from the NASA, tried to explain in Science why the earthquake that occurred near Sumatra on March 28, 2005 did not cause tsunamis despite its magnitude. His explanation was that the seabed did not rise except near the islands.

Parameters

According to Dr. Jörn Lauterjung, physicist and head of the staff of the scientific executive board from the “Geo-Forschungszentrum Potsdam” (GFZ), Song’s new theory will not change the planning “because all propositions that Song has made about the parameters that should be measured and about the methods that should be applied are covered by the early warning system anyway.”

Lauterjung explains that the TEWS (Tsunami Early Warning System) assesses the tectonic activity, apart from measuring the wave in the ocean or before the coast. “Seismometers observe the seismic activity. Deformations and motion of the surface are registered via GPS. Both measurements taken together can give evidence about the nature and quantity of tectonic movements which then enter the early warning system.”

As such, Lauterjung does not question the value of the TEWS and says: “We do not even have to rethink the system, since we also want to register the possible causes of a tsunami which do not lie in an earthquake. For example, submarine slope failures can be triggered by much smaller earthquakes which could not cause a tsunami by themselves. We believe that with our approach all data necessary for a tsunami early warning system are indeed measured.”