Satellite data reveal new details of tsunami
The shaded region shows the height of sea surface measured by a radar altimeter onboard the Jason satellite two hours after the earthquake while traversing above the Indian Ocean. The maximum height of the leading wave crest was about 50 cm followed by a trough of sea surface depression of 40 cm. The arrows show the directions of wave propagation along the satellite track. — NASA/JPL/CNES/National Institute of Advanced Industrial Science and Technology, Japan
FOR THE first time, orbiting satellites have observed and measured a major tsunami event in open ocean. U.S. and French teams working in parallel with altimetry data from the joint NASA/French Space Agency Jason and Topex/Poseidon oceanography satellites have independently confirmed the satellites' measurements of the height of the tsunami waves as they radiated from the earthquake's epicentre located southeast of Sumatra on December 26, 2004.
Tremendous value
The Indian Ocean tsunami resulted from the magnitude 9 earthquake southwest of Sumatra on December 26.
The measurements will aid our understanding of these events as they are of tremendous value to researchers worldwide. Approximately two hours after the quake, the satellites flew over the Bay of Bengal about 150 kilometres apart. "These two satellites make only about 13 earth revolutions daily, with each orbit passing over the earth approximately 3,000 kilometres away from its last," said Dr. Philip Callahan of NASA's Jet Propulsion Laboratory, Pasadena, California.
Very low probability
Callahan has been searching for tsunami signals in satellite radar altimeter data since Topex/Poseidon's launch in 1992. "There is a very low probability of capturing observations in any given location within two hours of an event like this. The fact that Jason captured the tsunami's signals is serendipitous, but is nevertheless a major boon for oceanographers," he said.
"The observations made by Jason and Topex/Poseidon are unique and of tremendous value for testing and improving tsunami computer models and developing future tsunami early warning systems," said JPL's Dr. Lee-Lueng Fu, Jason and Topex/Poseidon project scientist.
"The satellite altimeter data currently take a minimum of five hours to process, so they cannot provide early warning of such events," said Fu. Dr. Callahan received the Jason data the morning of December 27. Showing the signals of the tsunami waves, the figure displays changes in sea surface height from previous observations made along the same ground track 20 to 30 days before the earthquake. The satellites recorded a maximum sea surface elevation gain (deviation from normal) of 50 centimetres on the open ocean about 1,200 kilometres south of Sri Lanka at the leading crest of a tsunami wave raging out of the Bay of Bengal.
A trough of sea surface depression of 40 centimetres below normal followed it. The distance from one wave crest to the next was about 800 kilometres.
A second wave with a crest height of 40 centimetres above normal followed the first. Approaching the coasts of Myanmar near the northern end of the Bay, were two waves with crest heights of 40 centimetres and 20 centimetres above normal.
These tsunami waves eventually reached shallow waters along the coasts of Sumatra, Sri Lanka, Thailand and Southern India, spreading across the Bay of Bengal from the earthquake zone offshore from Western Sumatra.
Their open ocean speeds reduced from that of a jet plane, 800 kilometres per hour, to about 32 kilometres per hour, building the open ocean wave heights of 0.5 metres or less to walls of water up to 10 metres high with great destructive power.
Jason and Topex/Poseidon are collaborative satellite missions of NASA and the French Space Agency, Centre National d'Estudes Spatiales.
"The detection of these tsunami waves provides yet another demonstration of the important value of satellite radar altimeter observations," said Dr. Yves Menard, Jason and Topex/Poseidon project scientist at the Centre National d'Estudes Spatiales. — Our Bureau
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