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The two main answers to ‘how earthquakes happen’ is: as a result of tectonic plates colliding and volcanic eruption. The shock waves associated with nuclear weapons testing and other man-made explosions. To be considered an earthquake a shock wave has to be of natural origin.
Earthquakes Caused By Tectonic Plates
The theory of plate tectonics explains how the crust of the Earth is made of several plates, large areas of crust which float on the Mantle. Since these plates are free to slowly move, they can either drift towards each other, away from each other or slide past each other. Many earthquakes happen in areas where plates collide or slide past each other. The Elastic Rebound Theory applies to these quakes.
Major earthquakes are sometimes preceded by a period of changed activity. This might take the form of more frequent minor shocks as the rocks begin to move,called foreshocks , or a period of less frequent shocks as the two rock masses temporarily ‘stick’ and become locked together. Following the main shock, there may be further movements, called aftershocks, which occur as the rock masses settle into their new positions. Aftershocks cause problems for rescue services because they can bring down buildings that were weakened by the main quake.
Earthquakes Caused By Volcanoes
Volcanic earthquakes are far less common than tectonic plate related ones. They are triggered by the explosive eruption of a volcano. When a volcano explodes the associated earthquake effects are usually confined to an area 16 to 32 km around its base.
The volcanoes which are most likely to explode violently are those which produce acidic lava. Acidic lava cools and sets very quickly when it contacts air. This chokes the volcano’s vent and blocks the escape of pressure. The only way a blockage can be removed is by the pressure building up until it literally explodes the blockage outward. The volcano will explode in the direction of its weakest point, so it is not always upward. Extraordinary levels of pressure can produce an earthquake of considerable magnitude. The shock waves have been known to produce a series of tsunami in some instances.
There you have the answer to ‘how earthquakes happen’. Keep in mind that there have been man-made shock waves following large explosions, but they are not considered earthquakes because of their artificial origin.
We’ve also recorded related episodes of Astronomy Cast about Plate Tectonics. Listen here, Episode 142: Plate Tectonics.