Earthquake

Rapid slip along a fault generates waves in much the same way as does a pebble falling into a pool of water, and waves moving outward from an earthquake focus are reflected and refracted each time they encounter a different rock type. There are four different kinds of seismic waves, two of which are known as body waves and two of which are known as surface waves. Body waves travel deep through the Earth, whereas surface waves travel along the Earth's surface and generally cause the most damage.

The two types of body waves are P-waves and S-waves. P-waves, also known as primary waves, travel the fastest of the four types. They move by alternately compressing and stretching the rock through which they pass. P-wave velocity depends on the rock type and density, but it is generally about 6 km/s (4 mi/s). S-waves, also known as secondary waves, move by shearing or moving from side to side the rock through which they pass. S-waves move more slowly than P-waves and, depending on the type of rock, have a velocity of about 2 mi/s (3 km/s).

The two types of surface waves are known as Rayleigh and Love waves. They travel more slowly than either P- or S-waves, but often cause more damage than body waves because they travel along the Earth's surface and have a greater effect on buildings.

Seismologists can determine the epicenter of an earthquake by noting the times that seismic waves arrive at three or more different seismometers. Multiplication of the wave velocity by the travel time gives the distance to the epicenter, which is the radius of a circle with its center at the seismometer. The radii from at least three circles will intersect at a point that is the earthquake epicenter. In practice, seismologists first make a rough estimate of the epicenter and then refine their estimate as additional data become available, for example by using velocities corresponding to specific rock types rather than a general estimate.