Monday, January 10, 2005

Killer waves were sometimes only inches high

A color-coded map, based on data from four Earth-observing satellites, shows how the wave generated by the Sumatra quake spread out to varying heights three and a half hours after the seismic event.

While a tsunami can rise to great heights when it arrives at the shore, such waves are often barely noticeable in the ocean.

In this case, scientists found that two hours after the undersea quake that launched the tsunami, the wave was about 2 feet (60 centimeters).

An hour and 15 minutes later it was down to about 16 inches (40 centimeters).

After eight hours the main wave was down to about 2 to 4 inches (5 to 10 centimeters), though a portion in the Bay of Bengal was still at about 10 inches (25 centimeters), the N0AA scientists said Monday.

An earthquake deep beneath the ocean off Indonesia caused the tsunami by shifting the sea floor, resulting in displacement of the water overhead and causing a wave to spread out from that location.

Unlike surface waves that affect only a shallow amount of water, a tsunami stretches all the way to the sea floor and, as that rises to the land, so does the wave. Arriving at shore, such waves can grow suddenly by dozens of feet.

The satellite imaging did not provide a depth for the waves that came ashore.

The new measurements were based on data from four Earth-orbiting satellites. Researchers hope the work will help them develop models to improve tsunami forecasts.

The data, which took several days to analyze, came from the TOPEX/Poseidon and Jason satellites operated NASA and the French space agency, CNES; the European Space Agency’s Envisat; and the U.S. Navy’s Geosat Follow-On.