SIGHTINGS


 
New System Could Speed
Warnings Of Giant
Quake-Generated Tsunamis
10-15-98
 
 
WASHINGTON (AP) -- A new system developed by Mexican scientists could give coastal residents a crucial few extra minutes warning when earthquakes generate giant killer waves known as tsunamis.
 
Tsunamis, sometimes mistakenly referred to as tidal waves, have claimed untold lives over the years, including more than 2,000 victims last summer when one hit Papua New Guinea in the Pacific.
 
Hawaii, Mexico, South America and Indonesia are among areas that have been hit hard in the past.
 
The new system "may provide a reliable tsunami warning very rapidly," said Nikolai Shapiro of the Universidad National Autonoma in Mexico City.
 
Indeed, warnings could occur as quickly as five minutes after a quake, compared with the 10 minutes to 20 minutes now necessary, according to Shapiro's report in today's edition of Geophysical Research Letters.
 
That could be a life saving difference in cases when an earthquake generates a tsunami near a populated coastal area.
 
The university's scientists devised a seismic ratio that can help determine whether a quake will produce devastating waves. The system uses data generated by relatively simple and affordable seismograph stations.
 
A tsunami results from earthquakes, landslides or volcanoes that occur beneath the ocean. They generate deep waves spreading out in all directions.
 
The waves may be little noticed at sea, but when they near shore the rising sea floor causes them to lift up, sometimes to massive heights that can sweep away entire coastal villages.
 
When the waves must travel long distances, the Pacific Tsunami Warning Centre in Hawaii can provide alerts. But when they originate near shore there is often little chance to warn people. Even a brief warning can give people time to try to move inland or uphill to safer ground.
 
Shapiro said the warning can be produced "with the data available from a single broadband (seismograph) station" and is thus economically and technically viable in many countries.
 
Eddie Bernard, a tsunami expert who heads the U.S. National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, called the study "a nice piece of work."
 
But Bernard cautioned it is based on only about 20 earthquakes over a decade, too small a sample "to get too carried away."
 
In studying earthquakes that occur off Mexico's west coast, Shapiro and colleagues found that those happening near a subduction trench -- where the plates that make up the Earth's surface come together -- are more likely to cause tsunami than those that occur closer to the coast.
 
They also found that the more hazardous events near trenches produced fewer high frequency vibrations than the near-shore quakes. The researchers worked out a ratio of seismic waves that allowed them to tell quickly which quakes were near subduction trenches and which were near the coast.
 
The main requirement is a broadband seismograph located within a few hundred kilometres of the quake.
 
In many regions of the world, broadband seismographs have not been installed yet or were put in only recently, Shapiro said. Those regions don't have enough data yet to sort out the dangerous quakes.
 
However, he added, "we believe that the phenomenon reported in our paper is a fundamental property of (tsunami-causing) subduction-zone earthquakes. Therefore, we expect that similar observations can be done in other regions which are located close to subduction zones."
 
Asked if the system could have helped the residents of New Guinea, Shapiro noted that while there is a broadband seismic station at Port Moresby, it is too far from the northern coast where the wave struck.
 
To get a fast tsunami alert, it would be better to have a broadband station in the island's central part, he said.





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