The earth is never free from earthquakes fro long and more than 50,000 of them are recorded annually. Minor earth tremors caused by gentle waves of vibration within the earth's crust occur every few minutes. Major earthquakes, usually caused by movement along faults, can be very disastrous particularly in densely populated areas. Earthquakes themselves may cause only restricted damage in the regions of occurrence, but their after-effects can be very catastrophic. They produce gigantic tidal waves, called tsunamis by the japanese, which flood towns and drown thousands of people. Fires break out beyond control as gas mains are shattered and buildings collapse. In severe earthquakes, fissures gape open, and the ground writhes and undulates in the passage of the 'surface waves'. A wave height of a quarter of an inch in the upheaval is sufficient to bring down most ordinary buildings. Roads, railways and bridges are buckled and twisted; telecommunications are cut when the cables are snapped. Hills are so shaken that landslides are widespread. As the vibration thins out at the edges, like the series of waves set up by a stone thrown into the water, damage is greatly reduced. Only the highly sensitive seismograph can record the movements of earthquake waves.