Saturday, April 11, 2009

Tsunamis and Subduction Zones

Subduction zones occur where one tectonic plate is forced downward beneath another plate. Most of these zones are found along the edges of the Pacific Ocean but there are others scattered across the globe, including the eastern border of the Caribbean and the western edge of Indonesia.

The downward warping of the two plates creates an oceanic trench in these regions. While the subducting plate continues to move downward, eventually melted by the underlying mantle, the edge of the overlying plate is also bent downward, a result of friction between the two plates. Eventually (and intermittently), this potential energy is released and the plate edge rebounds upward, pushing the overlying ocean upward and triggering a tsunami.

Like the effect of a pebble dropped in a pool, the tsunami wave rushes off in all directions, reaching speeds of over 500 mph. Relatively modest in height in the open ocean, the wave builds as it enters shallower water along continental shelves; this is due to increasing friction on the leading edge of the wave and persistent force from behind. Finally, the wave moves on shore with tremendous power, sweeping away buildings, trees and coastal residents. The only protection from these recurrent geologic events is an advance warning system, triggered by offshore buoys, that allows coastal residents to move to higher ground before the tsunami strikes.