What is the relationship between faults, earthquakes, and plate boundaries?

Think of the Earth as an onion, composed of layers made of rocks of various temperatures, getting hotter as you go deeper. The outer layer, the crust, is pretty rigid and floats along on top of the mantle, which can flow (slowly). The mantle can flow because radioactive elements (uranium and thorium) left over from the formation of the earth provide the heat. The top layer, which we live on, is 7-35 km thick, out of a total of 6368 km, so the skin is 0.2% of the radius, really very thin. Since the heated rocks making up the interior are in constant slow motion, the skin floating on top of it is fractured by the motion. We see this effect as continental drift, a well known historical phenomenon.

The skin is divided into about a dozen tectonic plates. Plate boundaries are always faults, but not all faults are plate boundaries. The movement of the plates relative to each other distorts the crust in the region of the boundaries creating systems of earthquake faults.

There are also major faults and systems of faults in the interiors of plates. One of these is in the region of Missouri in the central United States, called the New Madrid fault system. A series of major earthquakes occurred there in 1811-1812, causing the Mississippi River to change its course.

As the plates move, mechanical energy is stored near the faults in the same way that energy is stored by a stretched spring. The energy is associated with a buildup of force or stress around the fault. Meanwhile, the fault is held together by the force of friction. When the elastic forces get to be large enough, the friction force is overcome, and the fault slips, producing the earthquake. The slipping fault also produces elastic waves that travel outward from the fault and cause the ground to shake. When this wave reaches an observer, the rapid motion of the earth is interpreted as an earthquake.

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