The surface of the Earth is constantly being shaken by violent disturbances that come from below. These are earthquakes. They are caused by movements of the Earth's crust usually when tectonic plates slide under or over each other. An earthquake can release far more energy than any nuclear weapon ever tested and can cause widespread damage on the surface. Earthquakes are measured on a scale called the Richter scale (This was developed in 1935 by Charles Richter). Earthquake severities are measured on a scale of 1.0 to 10 - the higher numbers being the more severe earthquakes. A severe earthquake (say 9.0 on the Richter scale) would release about a million times as much energy as the atomic bomb dropped on Hiroshima!
Large earthquakes are very rare in Britain but I was on holiday in Vanuatu when the house trembled due to an earthquake that measured 2.8 on the Richter scale. In April 2007 part of Kent in Southern England was shaken by a 4.2 scale earthquake that occured some ten miles out under the sea in the English Channel.
When an earthquake
occurs beneath the sea a tsunami is formed. This is a wave that travels across the surface of
the sea. Tsunamis far out at sea are very fast moving (over 600 km/hour) with a wave height
of less than a metre. However when they reach shallow water near land the wave slows
down to a few tens of metres per second but its height increases. Devastating waves of tens
of metres high have been the result when a tsunami moves into shallow water.
The
massive earthquake off the island of Sumatra on 26th December 2004 read 9.3 on the Richter
scale and caused a 25m high tsunami that reached 25m high on neighbouring islands. It was
due to a complex slipping of the fault line off the coast where the Pacific plate joins the Indo
Australian plate. The energy released was equivalent to exploding 30 Gigatons (30 000
million tons) of TNT.
Predicting when and where an earthquake or a tsunami will occur is
very difficult. Buoys are used to detect tiny tremors at sea that might be the beginning of an
earthquake. Seismographs are used on land that will give warning of the movement of the
Earth's crust.
Earthquakes and tsunamis are much more likely in certain regions of
the world where the tectonic plates are moving under, over or alongside each other and so
scientists concentrate their detection devices in those areas. Even so the oceans are so vast
that it is unlikely that they will ever be able to detect every tiny quake.
Magnitude | Earthquake description | Estimated global number per year |
Less then 2.5 | Usually not felt but can be recorded on a seismograph | 900000 |
2.5 to 3.9 | Often felt – causes minor damage | 30000 |
4.0 to 4.9 | Some damage – shaking of items within a house | 1000 |
5.0 to 6.0 | Slight damage to buildings and other structures particularly those that are badly built. People woken up. Pendulum clocks stopped. | 500 |
6.1 to 6.9 | Considerable damage in populated areas up to 50 miles from the epicentre | 100 |
7.0 to 7.9 | Serious damage - a major earthquake | 20 |
Above 8.9 | Communities near epicentre totally destroyed – widespread damage – a great earthquake | One every 5 to 10 years |