Geography can change a wave's energy


In some ill-fated stretches of Indonesia's Aceh province, the ocean roared inland as far as 10 miles. Elsewhere around the Indian Ocean basin, the line of damage extended only a few hundred yards from the beach.

Why the difference?

Indonesia's proximity to last weekend's 9.0 magnitude earthquake clearly contributed to the roiling "wall of water" estimated at more than 30 feet high that swept through the provincial capital of Banda Aceh on the island of Sumatra.

But scientists say a number of geographical features also can help to either dissipate or focus a tsunami's enormous energy and determine how far and how fast it surges ashore.

Normally created after a powerful earthquake or volcanic eruption, a tsunami begins as a massive upwelling of ocean water, with its energy extending from the ocean surface to its floor. By some estimates, last Sunday's rupture miles beneath the Indian Ocean floor likely uplifted the sea above it by several yards.

In the open ocean, the wave rippling outward from this disturbance could keep pace with a jetliner and reach a total length of 150 miles or more.

As a tsunami nears land, the shallower ocean depth decreases its velocity and the distance between wave crests, forcing the wave's considerable energy into a much more compressed space, causing it to grow in height.

Shallow beaches and harbors are especially susceptible because they offer little to slow the wave front and accompanying surge.

A steep, straight coastline that plunges toward the deep ocean, however, would likely receive far less damage because the wave "would just bounce off the wall like a tennis ball," said Nano Seeber, a seismologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y.

Underwater topography also influences a tsunami. A wide continental shelf with a gradual slope can lead to a more measured slowdown of a wave and increase in its height.

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