Footnote: Section 1
It has been stated that the answer to the question of, "What causes the forces that create earthquakes?", can, in large part, be found in the theory of plate tectonics. This is true; an overwhelming percentage of earthquakes are the direct result of tectonic activity. A notable fraction of world-wide seismic activity, however, is either not directly related to tectonics, or related only in a secondary manner.
The earthquakes least connected to plate tectonics involve the rapid removal or deposition of a large amount of material, either at the surface or in the rocks below. This can be natural, as when the glaciers receeded at the end of the last Ice Age, or caused by man, as is the case in certain mining or drilling operations, and even in the construction of certain large dams. Because the weight of the overlying material on the rocks below is rapidly altered, the Earth's crust may "rebound", resulting in uplift, or sag, resulting in the downward shift of material. Either way, this movement along fractures in the Earth's crust will cause earthquakes.
Earthquakes may also be triggered by volcanism. The movement of magma in the Earth's crust can cause volumes of rock to shift. Volcanoes are always accompanied by earthquakes, even though these quakes may be very minor. However, all volcanism is essentially a product of plate tectonics as well (if you consider hot spot activity to be an integral part of the theory), so even these quakes have their basis in plate tectonics.
It should also be noted that some areas of significant non-volcanic earthquake activity are far removed from plate boundaries. The most notable example in the United States (also one of the most striking in the world) is the New Madrid seismic zone, located along the Mississippi River in southern Missouri, and extending into several nearby states. This is a weakened section of the continental crust known as the Reelfoot Rift, was probably formed millions of years ago when the North American Plate broke away from surrounding plates. The break was likely not clean, and several failed rifts probably formed before the plate finally separated "cleanly". These rifts persist even after the rifting episode is over, and because of their weakened nature, they remain seismically active. Of course, since these failed rifts are a product of tectonic forces, these earthquakes also have their basis in plate tectonics.
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