tsunamis, giant waves caused by submarine earthquakes, landslips or volcanic activity, are known to wreck havoc in Japan. Now, the country's civil defence officials should be able to predict tsunamis through a new altering system that uses Global Positioning System ( gps) navigation satellites to monitor the vertical motion of a buoy moored out in the ocean. In Japan, tsunamis are considered almost as big a threat as earthquakes on land. In 1960, a powerful earthquake 160 km off the coast of Chile caused a tsunami. It inundated a Japanese coastline killing 119 people.

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.