The earth is made of layers, divided into the crust, mantle, and core. The crust is the Earth’s surface, a thin, hard layer of rock, broken into many pieces. Each of these pieces is known as a crustal plate. Some form continents, others the ocean floor, but they are always moving. According to the theory of plate tectonics, the earth’s crust is made of six major plates and nine smaller ones that lie on the mantle, a thicker, denser layer of hot, soft, molten rock. These plates float around within the mantle, in a hot, soft zone known as the asthenosphere.
The core is made up of even hotter rocks below the mantle, and currents of burning rock rise up through the mantle. These currents spread out once they hit the bottom surface of the crust. This behavior tends to tear the crust, pulling the apart, grinding some plates against others, colliding them into one another. Continental drift (when major plates are slowly but steadily moved apart) also contribute, carrying plates until they collide. It is through these collisions that mountain ranges are also formed. This movement of our dynamic planet produces earthquakes and volcanoes.
Earthquakes occur most frequently (about 95% of the time) at the point where two plates scrape against one another. When these two plates move against each other, the crack is known as a fault. A famous example is the 700-mile-long San Andreas Fault running up the length of California (United States). When plates jam against one another, stress builds up in between. When the pressure becomes too great, the bends and snaps free with a jerky motion. This sudden motion is an earthquake.
There are also many different types of faults (great fractures between masses of rock at the earth's surface):
Forces of Nature: ThinkQuest 2000 (Team #C003603)
http://library.thinkquest.org/C003603/english/earthquakes/thecauses.shtml
Everyone knows that Earth has seven continents - North America, South America, Europe, Asia, Africa, Australia, and Antarctica. German meteorologist Alfred Wegener noticed in 1912 that the continents fit together much like the pieces of a jigsaw puzzle. He hypothesized that the continents were all connected 200 million years ago. His theory was later supported by findings of similar fossils on every continent. Because prehistoric creatures could not possible have crossed the oceans to live on each continent, Earth must once have had a single land mass.
However, earthquakes can also occur where they are least expected. In 1812 and 1886, a series of major earthquakes occurred in the Midwestern United States, often so great in magnitude that they were felt over a distance of 1.5 million square miles. The epicenters of these earthquakes were located in the interior of continental plates. Researchers discovered that friction between the fluid mantle and a previously unknown fault caused the 1812 tremors. The 1886 earthquake in Charleston, South Carolina, is believed to have been caused by pluton, a dense mass of solidified molten rock that flowed into the crust, where it created dangerously violent stresses in the earth.