Geography Earthquakes Notes Zimbabwe

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Earthquakes

Definition
Earthquakes are vibrations in the earth's crust. They occur along all plate boundaries but are more destructive usually at destructive plate boundaries

Structure of Earthquakes

How Earthquakes occur
Earthquakes occur from the slow build up of pressure within crustal rocks. If this pressure is suddenly released, then parts of the crustal rocks may experience vibrations. The point at which this pressure is released within the crust is known as the focus. The point directly above the focus on the surface experiences the worst shocks and is known as the epicenter. From the focus, vibrations move towards the surface as body waves and they move from the epicenter outwards as surface waves.

How are they measured
The magnitude or size of an earthquake can be measured by an instrument called seismometer and are shown on a seismograph. The earthquakes are measured on the Richter scale from a value of 1 to 10 . Each level of magnitude is 10 times more powerful than the previous. Therefore a level 7 is 100 time more powerful than a level 5.

The Distribution of Earthquakes
Earthquakes are mostly concentrated along plate tectonic boundaries. They are concentrated along the western coasts of north and South America where the north and South American plates are colliding with the pacific plate. They also stretch along the southern coastal lands of Asia and North Africa where the Eurasian plate is colliding with the African plate and the Australian plate. This region stretches into the coastal areas of eastern Asia affecting the island areas of Philippines and Japan where the Eurasian plate is colliding with the plate. Earthquakes are also concentrated along the mid oceanic ridges in the Atlantic and Indian Ocean where the plates are moving apart. In Africa they occur along the east African rift valley where crustal rocks are tension because of the movement of plates underneath. They are more destructive along destructive boundaries where the plates are moving towards each other. Earthquakes

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