The USGS encourages people to report earthquake tremors (or "tremblors" as they are typically called). Such events can predict volcanic eruptions - just like tremors in 79 AD (had they been understood) would have predicted the eruption of Mt. Vesuvius which destroyed Pompeii and Herculaneum.

Why do earthquakes happen? The process has everything to do with how the earth is constructed. Tectonic plates, for example, sometimes “grind” (don't miss this video) against each other. Those movements can dislocate segments of the Earth’s crust and is one cause of earthquakes:

An earthquake is the vibration, sometimes violent, of the Earth's surface that follows a release of energy in the Earth's crust. This energy can be generated by a sudden dislocation of segments of the crust, by a volcanic eruption, or event by manmade explosions. Most destructive quakes, however, are caused by dislocations of the crust. The crust may first bend and then, when the stress exceeds the strength of the rocks, break and "snap" to a new position. In the process of breaking, vibrations called "seismic waves" are generated. These waves travel outward from the source of the earthquake along the surface and through the Earth at varying speeds depending on the material through which they move. Some of the vibrations are of high enough frequency to be audible, while others are of very low frequency. These vibrations cause the entire planet to quiver or ring like a bell or tuning fork.

Earthquakes typically occur along a “fault line” which is, in essence, a fracture of the Earth’s crust. In the United States, the San Andreas Fault (located in Central California’s Carrizo Plain) is the most famous. It has been the source (this USGS map was created by Darrell Herd) of California’s worst earthquakes including the Loma Prieta quake which disrupted the World Series in 1989.