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Much of our understanding of earthquakes is based upon a revolutionary theory of earth dynamics, called PLATE TECTONICS.
4. Earthquake Damage
The damage caused is both directly and indirectly related to the shockwaves produced by the quake.
San Francisco burning after the earthquake of 1906
The vibrations of the ground cause a great deal of damage. The violence of the shaking depends on:
1. the magnitude of the quake,
2. the depth of the focus,
3. the structure of the rocks and soil and,
4. the distance to the epicentre.
The waves of energy travel through the earth in three forms, each arriving at different times:
1. Primary (P) waves. P waves arrive first and are associated with stretching and compressing soil/rock volumes as they move.
2. Secondary (S) waves. S waves are the next to arrive and these cause the most damage as they cause the ground to move back and forth causing buildings to sway.
3. Surface waves. These are the last to arrive and are usually felt close to the epicentre only and behave like ripples in water.
By recording the speed of arrival of the different waves, scientists can locate their origin and find the epicentre and determine the magnitude. If these movements are sufficiently violent structures (e.g. buildings and dams) will collapse.
Ground movement can cause damage indirectly. If the earthquake occurs on the ocean floor, the energy generated creates enormous waves (called Tsunami or tidal waves) that crash onshore. On land quakes can generate landslides and can cause loose soil to liquefy (behave like quicksand). In San Francisco much of the city is built on soils that quickly turn to 'jelly' when shaken.
