Civil defence officials in Japan should soon get advance warning of tsunamis, the giant waves that can wreak havoc when they hit the coast. A new alerting system uses Global Positioning System navigation satellites to monitor the vertical motion of a buoy moored out in the ocean.
Tsunamis are caused by submarine earth-quakes, landslips or volcanic activity. In Japan, tsunamis are considered almost as big a threat as earthquakes on land. A powerful earthquake 100 miles off the coast of Chile in 1960 caused a tsunami that inundated the Japanese coastline, killing 119.
Until now, the only way to predict these waves has been to monitor distant earthquakes and guess the likely speed, size and direction of any tsunami they might produce. But this is fraught with errors caused by unknown variables such as the topography of the seabed. "The idea of using a buoy may seem obvious, but it has only been made possible because of recent advances in GPS technology," says Teruyuki Kato of Tokyo University's Earthquake Research Institute (ERI).
The 24 satellites that make up the GPS network are arranged so that at least three of them are always in view from a receiver anywhere on the Earth's surface. The satellites transmit precise time signals, and receivers calculate their position by decoding the differences in the signals from three or more satellites. In this way the buoy's GPS receiver can measure its vertical movement to an accuracy of just a few centimetres.
Kato says it is not hard to distinguish tsunamis from normal waves as the former have much longer wavelengths. The ERI is now testing the system off the coast of the Miura peninsular, southwest of Tokyo.
Author: Peter Hadfield, Tokyo
New Scientist issue 8th May 1999
PLEASE MENTION NEW SCIENTIST AS THE SOURCE OF THIS ARTICLE - THANK YOU