Earthquakes and tsunamis

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.
Photo credit: NOAA / NOAA Center for Tsunami Research
(See: 14- 16/General/Text/Continental drift)

Predicting earthquakes and tsunamis

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.

Methods of earthquake prediction:

(a) keep a record of where earthquakes have occurred in the past – are they likely to occur again in the same region?
(b) watch for changes in stresses in the Earth's crust that might be the build up to something big! This can be done by shing laser beams across a fault line and measuring any tiny movements using interference techniques (See: 14-16/Light/Text/Interference)

The San Andreas fault in California is probably the most watched fault line in the world since the massive earthquake in San Francisco in 1906.







Charles F. Richter and the Richter scale
For each increase in the scale of magnitude on the Richter scale the movement of the ground increases by ten times. So in a magnitude 5.0 earthquake the ground would move ten times as much as during a 4.0 earthquake and the energy released would be 32 times greater.
A magnitude 8 earthquake releases as much energy as detonating nearly 1000 million tons of TNT.