What causes an earthquake to occur?
Beneath the earth are solid rock plates that are in constant motion; when two plates are pushing up against each other, but neither one "gives", potential energy is stored; this is similar to two people deadlocked in an arm wrestling contest, where the interlocked arms are perpendicular to the surface because neither person can move the other's arm. The quake of the plates is similar to the end of the wrestling match where one person finally overcomes the deadlock, and pushes the other's arm to the surface; the plates have pushed and pushed against each other until one gives and the resulting energy pushes upward in the same way that the losing arm wrestler's forearm has been crushed downward. This is a natural process that can cause grave consequences, of course, when it occurs underneath a heavily populated area such as the recent tragedies in Japan and Haiti.
Most earthquakes are causally related to compressional or tensional stresses built up at the margins of the huge moving lithospheric plates that make up the earth's surface. The immediate cause of most shallow earthquakes is the sudden release of stress along a fault, or fracture in the earth's crust, resulting in movement of the opposing blocks of rock past one another. These movements cause vibrations to pass through and around the earth in wave form, just as ripples are generated when a pebble is dropped into water. Volcanic eruptions, rockfalls, landslides, and explosions can also cause a quake, but most of these are of only local extent. Shock waves from a powerful earthquake can trigger smaller earthquakes in a distant location hundreds of miles away if the geologic conditions are favorable.