How Earthquakes Happen:
The primary cause of earthquakes is the movement of masses of earth along fault lines. Fault lines can occur anywhere. Nevertheless, they are normally created from pressure generated by the movement of continental plates on the mantle of the earth. These plates were first theorized by a German meteorologist and astronomer named Alfred Lothar Wegener (1880-1930). (Photograph below courtesy of the Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany.) At the time he was ridiculed for his ideas, but later they were substantiated by further research. Even so, the theory of plate tectonics only became widely accepted within the last half of the 1900s.
There is constant pressure on continental plates to move. Yet the friction of land masses being pushed together only allows movement to occur in fits and starts. So when movement finally comes as a result of all this continental pushing, we often feel it, and we call it an earthquake. Some of the geological activities caused by plate tectonics include volcanoes, mountain creation, island creation and a phenomena known as subduction.
Subduction occurs when one plate (usually an ocean plate) gets pushed beneath another plate (usually a land mass). Imagine one huge mass of rock, being pushed beneath another huge mass of rock. As might be imagined, it is within subduction zones that the most numerous and most severe earthquakes happen. One of the most prominent subduction zones runs along the Pacific Coast of the United States. This is why California is notorious for its earthquakes.
Intra-plate earthquakes can also occur far away from plate boundaries. They may be caused by the shifting of old faults and plate adjustments from crust movements occurring slowly over very long periods. Though these quakes may be as severe as quakes in subduction zones, they tend to be far less frequent.
Faults that occur close the surface of a plate usually generate the most significant quakes. Earthquakes seldom cause the earth to actually crack open along a fault line. Nevertheless, earthquakes can cause landslides, avalanches and the demise of land formations that are already prone to collapse.
Earthquakes may also be felt a long distance from the fault plane where they occur. This is due to a wave action that ripples out from the epicenter of a quake. There are two types of waves. Body waves are generated and move rapidly underground, yet it can break out onto the surface where it becomes a surface wave. The surface wave, not unnaturally, is what causes most of the damage in an earthquake. The body wave travels faster than the surface wave and will often be felt a second or more before the arrival of the destructive surface wave.
As waves radiate outward from their source, they weaken. Yet the character of the wave can be changed by the types of soil and rock that it travels through, sandy soils increase the damage potential of a quake.
After the main quake, aftershocks are common. They are a result of a realignment of the crust around a major fault movement. Aftershocks usually occur within a few days of the main quake, becoming less and less serious over time. Generally, there will be at least one earthquake of one degree of magnitude smaller than the main shock and up to 10 aftershocks that are two degrees smaller. There will likely also be a large number of shocks even smaller. These aftershocks can pose a great danger to rescue and cleanup crews.