Describe And Explain The Global Patterns Of Earthquakes.

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“Earthquakes are probably the most dramatic of Man’s natural enemies.”
(Hallam 1977)
“Earthquakes are vibrations of the earth caused by the rupture and sudden movement of rocks that have been strained beyond their elastic limits.” (Hamblin and Christiansen 2001).
Shockwaves are generated when an earthquake occurs. There are three types of seismic waves: Primary waves, secondary waves and surface waves. Earthquakes tend not to be deeper than 700km into the earth. Shallow-focus earthquakes occur from the surface to a depth of 70km. Intermediate-focus earthquakes occur between 70km-300km below the surface and finally deep-focus earthquakes between 300km-700km. The magnitude of an earthquake is measured on a Richter Scale.
If the locations of earthquakes are plotted on a map it becomes clearly evident that there is a pattern. Earthquakes are generally concentrated in areas where plates meet. If we plot the areas where shallow, intermediate, and deep focus earthquakes occur it becomes apparent that virtually all deep earthquakes are associated with ocean trenches and island arc structures bordering the Pacific. The reasoning behind this is plate tectonics.
The earth can be internally divided into three major areas: The outer crust, the mantle and the core. The rigid outermost layer of the crust comprises of both continental and oceanic crust. Underlying the crust is the mantle, it comprises the largest portion of the earth and finally the innermost portion is the core.
Plate tectonics tells us that the earth’s outermost layer (the lithosphere) is broken into both oceanic and continental plates which slide over the uppermost layer of the mantle (the athenosphere). These plates are in constant motion and interact with their boundaries. It is these interactions which are the cause of earthquakes.
We now know there are seven major crustal plates about 80km thick and all in constant motion relative to each other. It is along t...