Subject: Undersea earthquakes and tsunamis

How do undersea earthquakes cause tsunamis?

   By changing the shape of the seafloor. If an earthquake ruptures all the way to (or even just close to) the seafloor, then part of the seafloor will be raised or lowered. Imagine that the seafloor is raised. Seawater itself is almost incompressible, so by raising the seafloor the earthquake will also be raising the sea surface. The sea surface, however, likes to be horizontal. An extra lump of water at the surface will collapse under gravity to produce a series of waves, the tsunami. This is directly analogous to dropping a pebble into a pond - there you create a hole in the water surface; the hole in the surface is gravitationally unstable, so it collapses to give you a series of ripples. What makes tsunamis so alien from our everyday experience is the fact that they have exceptionally long wavelength (the distance from one crest to the next). Because the area affected by the earthquake may be huge, the wavelength of the waves may be huge too - as much as 200 miles. What that means is that individual wave crests may be as much as half-an-hour apart. In other words, when a tsunami hits a coastline, the edge of the water is dangerous for hours.
   Eleven people at Crescent City, California died in 1964 and 59 people at Hilo, Hawaii, died in 1960 because they did not understand: the people evacuated, but returned to shoreline houses or businesses before it was safe. The fist waves of those tsunamis were small; the later waves killed people.

Gerard Fryer
Hawaii Institute of Geophysics and Planetology
University of Hawaii, Honolulu HI 96822

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