Earthquake prediction

One way to predict earthquakes is to study past quakes. If there has been no earthquake in an earthquake zone for a while, there will be one soon. The longer it has been since the last quake, the bigger the next one will be.

Seismic gaps are places in active earthquake zones where there has been no earthquake activity. this is where a big earthquake will probably occur.

Seismologists make very accurate surveys with ground instruments and laser beams bounced off satellites ( see earthquake measurement ). They can spot tiny deformations of rock that show strain building up.

Alinked network of four laser-satellite stations called Keystone is set to track ground movements in Tokyo Bay, Japan, so that earthquakes can be predicted better.

The level of water in the ground may indicate stress as the rock squeezes groundwater towards the surface. Chinese seismologists check water levels in wells. Rising surface levels of the underground gas radon may also show that the rock is being squeezed.

Other signs of strain in the rock may be changes in the ground's electrical resistance or its magnetism. Before an earthquake dogs are said to howl, chickens flee their roosts, rats and mice scamper from their holes and fish trash about in ponds.