Deadly tsunami measured by satellite

The tsunami that killed more than 150,000 around the Indian Ocean was caught by a series of radar satellites, allowing National Oceanic and Atmospheric Administration scientists to develop measurements of the wave in mid-ocean.

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 two feet .

An hour and 15 minutes later it was down to about 16 inches. After eight hours the main wave was down to about two to four inches, though a portion in the Bay of Bengal was still at about 10 inches, the NOAA scientists.

An massive 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 hit 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 analyse, came from the TOPEX/Poseidon and Jason satellites operated by Nasa and the French space agency, CNES; the European Space Agency’s Envisat and the US Navy’s Geosat Follow-On.

“These observations are unique and of tremendous value for testing and improving tsunami computer models and developing future tsunami early warning systems,” said Lee-Lueng Fu of Nasa’s Jet Propulsion Laboratory in Pasadena, California.

Walter Smith, a geophysicist at NOAA, emphasised that the office does not get satellite data until several hours after a tsunami has developed – too late to be used as a real-time forecast.

“Right now, this technique is not a first line of defence in tsunami hazard monitoring and warnings, but it gives scientists a window to tsunami activity in the deep and in remote parts of an ocean basin, too far away from coastal tide gauges and other instruments that could detect it,” Smith said.