杏吧原创

Pacific wave warnings

Tidal waves in the Pacific Ocean have been so devastating in the past that a series of seismograph stations have been set up to pick up the large earthquakes that are the primary cause. Such methods of detection are necessary because much of the Pacific is so deep that ordinary earthquakes cannot be observed.

For example, tidal waves that race across the ocean at about 650 kilometres an hour are only a few feet high and difficult to spot. From a ship they would scarcely appear as a wave because the distance from crest to trough is some 25 kilometres and the wave period takes several minutes to pass by. However, by the time the waves reach shallow water the front has slowed down and the 25 kilometres of water have piled up to make a steep-fronted surge that causes great destruction when it hits land. In the Krakatoan earthquake that occurred between Java and Sumatra towards the end of the 19th century there were enormous casualties because land was close to the centre of the disturbance and waves as high as 30 metres swept inland.

Earthquakes create sound waves that travel through the ground at about 8 kilometres a second, which is fortunately much faster than the speed of tidal waves. If the seismograph stations record a large earthquake, the exact arrival time of the beginning of the ground waves allows a geologist to estimate the position of the earthquake in the way that sound waves in the air are used by the military to determine the range of the enemy鈥檚 artillery. Because tidal waves travel at a few hundred kilometres an hour, it is possible from the ground waves to estimate when they will arrive at places where damage may be caused. It is hoped that this will eventually lead to an early warning system for vulnerable areas.

From The New 杏吧原创, 14 March 1957