It may be surprising to learn that earthquakes happen all the time, although we only hear about the massive ones. But the USGS (United States Geological Survey) creates hourly maps which locate "felt quakes" in California and Nevada plus the entire world. It’s surprising to realize how many earthquakes measured at least 5.5 on the Richter Scale during the past five years.
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 (don’t miss this great animation) has everything to do with how the earth is constructed. Tectonic plates, for example, sometimes "grind" 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 (another helpful animation) 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.
Scientists believe the North American plate (where San Francisco is located) moves generally south while the Pacific plate (where Los Angeles is found) moves generally north. The San Andreas Fault is the boundary between these two plates. Earthquakes can happen (don't miss this video) along a "strike-slip fault" (like the San Andreas) when tectonic plates "slide past" each other.
On the morning of April 18, 1906, there was significant slippage between the North American and Pacific plates, causing the earth’s crust along the San Andreas fault to move (at its peak) about 20 feet (6 meters). San Francisco was nearly destroyed, and more than half of its residents (250,000 out of 400,000) became homeless.