When the crust suddenly fractures, elastic energy is released and transferred by high amplitude L waves travelling through the surface layers of the Earth. The most destructive effects of this energy are felt at the epicentre.
Ground MovementEarthquake vibrations travel through the ground, causing little permanent damage is the intensity is low. Stronger, larger amplitude earthquakes cause shear or lateral movement of the ground, which is more damaging. Earthquakes of intensity 12 on the Mercalli scale make 'the ground surface move as a series of waves, with cracks opening at the crests and closing at the troughs'. This causes objects to be thrown upwards and is highly destructive.
Damage to StructuresMovement of the ground separates one part of the structure from another. For example: bricks and stonework separate along the mortar, causing walls to collapse floors separate from supporting walls, causing them to 'pancake' on top of each other bridges builts in sections separate from their supporting piers sections of gas, water and drainage pipes separate from each other and leak Buildings sway when their foundations move sideways but the top storeys lag behind.
LiquefactionVibration in the bedrock is transmitted into superficial deposits at the surface. In wet sand, and other unconsolidated deposits, the water separates from the solid particles and rises to the surface. Houses built on alluvial deposits can suddenly find themselves standing in water and large amounts of damage are caused.
LandslipsOn steep slopes made unstable by high rainfall, the vibration may trigger landslides and mudflows, partly assisted by liquefaction. This causes loss of life by burial, and can prevent successful rescue attempts.
AftershocksThe main movement along the fault releases most of the energy, but subsequent movements, minutes, hours or even days afterwards cause aftershocks. Structures weakened by the main shock may collapse during this period, causing further damage and added difficultis for rescue workers.
TsunamisA tsunami is a water wave which is set up at the same time as an earthquake, but is not a seismic wave. A tsunami is caused by bodily displacement of a large volume of water by the movement of a large section of the crust on the sea floor. In open water, the tsunami has a amplitude of a meter, a wavelength of several hundred kilometres and a speed up to 700km/hr. When the tsunami approaches the shallow waters of the coast, the height of the wave increases dramatically, surging across low-lying coastal areas with devastating effects. A more imediate warning is given as the sea drains away from the shore-line immediately before the tsunami strikes. Tsunamis also result from the displacement of water by large landslides e.g. when the flanks of a volcano collapse into the sea.
Early warning of tsunamisThe catastrophic effects of tsunamis can be mitiagted by a good early warning system., by maintaining a barrier of trees and other vegetatio along the coastal strip to absorb the waves energy, and by building houses back from the shore-line, preferably raised above ground level.The earthquake arrives well before the tsunami, giving several hours warning for distant epicentres. However, not all strong eathquakes are followed by tsunamis. Mid-ocean detectors needed, connected to effective communications on land. Countries which can afford this are well protected. The Indian Ocean Tsunami Warning System has been set up by UNESCO after 230,000 people died following the Sumatran earthquake of 2004.