The majority of earthquakes occur along the boundaries of Earth's tectonic plates. At these boundaries, slabs of oceanic or continental crust 5 to 30 kilometers (3 to 19 mi) or more thick are pulled apart, collide, or slip past one another. These movements put stress on the rock that makes up the plates. When pulling or pushing forces exceed the strength of the rock, it fractures and a fault forms. Movement along these faults causes most earthquakes. Usually the movements are relatively small and go unnoticed by people living nearby. Occasionally, however, large, violent fractures send seismic waves emanating dozens of kilometers through the surrounding crust.
Seismic waves are vibrations that travel in all directions away from the point where a fracture occurs. This point, called the focus, is usually several kilometers below the surface. Scientists call the location on the surface directly above this location the earthquake's epicenter.
Earthquakes generate three prominent types of seismic waves. The first two, P and S waves, travel through Earth's crust. P-waves cause rock particles to be compressed and decompressed in the same direction the wave is traveling. S-waves cause rock particles to move at right angles, either up and down or side to side, relative to the direction that the wave is traveling. The third type, called surface waves, cause Earth's surface -- rocks, water, buildings -- to move both up and down and side to side, relative to the direction that the wave is traveling.
Scientists use instruments called seismometers to measure seismic waves. These tools provide several kinds of information about earthquakes. For example, they record both the intensity of the vibration, as measured by the height of the line on the seismogram, and the vibration's frequency, which is measured by the distance between the peaks or valleys on the graph. Depending on its configuration, a seismometer can record either vertical or horizontal motion. The design of the seismometer in this video segment is intended to record horizontal motion.
To learn about how scientists use seismometers to predict volcanic eruptions, check out Seismic Signals.
To learn more about the instruments scientists use to detect earthquakes, check out Earthquakes: The Seismograph.
To learn more about the effects of earthquakes, check out Earthquake Prediction, Earthquakes: San Francisco, and Earthquakes: Los Angeles.