When forces deep in the Earth push the rigid crust
beyond endurance, the rocks can no longer take the
strain, rupturing and shearing in a massive burst of energy.
The result is an earthquake that can lay whole cities flat.
(Natural Disasters, Readers Digest, 1996)
The aftermath of an earthquake
(World Disaters - Earthquakes)
Earthquakes are caused when tension is released from the rocks in the Earth's crust and upper mantle. This tension is due to friction between what scientists believe are large 'plates' floating on magma on the Earth's surface. Sometimes earthquakes happen when the rocks in the earth's crust bend and break. This causes shock waves to travel on the earth's surface, resulting in widespread destruction.
|Scientists believe that the earth is made up of four main layers. The outermost
layer being the crust, then the mantle, the outer core and at the centre of the
earth the inner core.|
The crust is made up of hard rock, mainly granite. The mantle is mainly molten
lava on which the crust is floating. The core is mostly iron, with the outer core
being liquid and the inner being solid. The mantle is continually moving; this is
Earthquakes usually occur in places where two plates meet, called faults. Earthquakes are mostly generated deep within the earth's crust, when the pressure between two plates is too great for them to be held in place. The underground rocks then snap, sending shock waves out in all directions. These are called seismic waves. The underground origin of an earthquake is called the focus. The point at which an earthquake originates on the surface is called the epicentre.
|I||Only felt by instruments.||VII|
|II||Felt by people at rest, especially on upper floors. Suspended objects may swing.||VIII||Considerable damage to most buildings. Heavy furniture overturned. Some sand fluidised.|
|III||Felt indoors. Vibrations like passing traffic.||IX||Even well-designed and sturdy buildings badly damaged, moved from their foundations. Ground cracks. Pipes break.|
|IV||Many people feel it indoors, a few outdoors. Crockery and windows rattle. Standing cars rock. Some sleepers awake.||X||Most masonry destroyed. Landslides occur. Water slops from reservoirs and lakes. Railway lines bend.|
|V||Felt by nearly everyone. Tall objects rock. Plaster cracks.||XI||Few structures remain upright. Bridges fall. Extensive fissures in the ground. Underground pipes totally out of action.|
|VI||Most people run outdoors. Damage to weakly constructed buildings. Felt by people in moving vehicles.||XII||Total destruction. Ground thrown into waves. Objects flung into the air. You would be lucky to survive this one.|
|Scale about 3.0||Scale about 6.0||Scale about 9.0|
Earthquakes cannot be predicted although areas most at risk can be identified. The buildings in these areas then can be modified to withstand earthquake shocks. For example, buildings constructed near the San Andreas fault in San Fransisco, have to meet extremely strict building regulations.