Earthquakes
Suddenly the ground beneath your feet begins to move, trees and poles begin to sway, another thud moves under your feet and the buildings near you start to shake and move. Earthquake! Anyone who has experienced even a small tremor knows that the above description cannot hope to describe the feeling when the solid dependable Earth suddenly starts to move and fall apart. Fortunately, large Earthquakes are rare in Australia, but as was discovered in Newcastle, New South Wales, on 28th December 1989, they do not have to be large to be devastating. Characteristics of earthquakes Most earthquakes occur at depths of less than 30km where the rock is brittle, and tends to snap when placed under stress. This break in the rock creates the vibrations that we know as an earthquake. Deeper than 30km, the crust is warmer and tends to slowly bend and fold when it is placed under stress. This produces folded and warped rocks, but doesn't result in an earthquake. Earthquakes can occur anywhere, but are most common near the edges of the tectonic plates along deep cracks in the Earth called "Fault Lines". Earthquakes are also associated with volcanic activity. During earthquakes, the land surface can move up or down in relation to surrounding ground forming raised blocks, move sideways or even split to form steep sided rift valleys. The Cataract Gorge in Launceston, Tasmania is an example of a fault that split to leave a deep gorge. Earthquakes in Australia Both maps are extracted from the pamphlet "Earthquake Awareness for Australians" published by Emergency Management Australia. Measuring
earthquakes Not all earthquakes with the same magnitude will do the same amount
of damage, this depends on the depth of the earthquake's focus, local
geology and population. Roughly the Richter Scale can be rated against effect as follows:
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