An earthquake is the shaking of the ground caused by an abrupt shift of rock along a fracture in the Earth, called a fault (Figure 1). Within seconds, an earthquake releases stress that has slowly accumulated within the rock, sometimes over hundreds of years.

It is also possible for the accumulated stress to be released more gradually, by continuous slippage along a fault; this movement may amount to only a few millimeters a year. Such faults are said to undergo aseismic fault creep because the stress release occurs without earthquakes.

Faults are a record of past earth movements, just as fossils are a record of plants and animals that once inhabited the Earth. However, like volcanoes, faults may be extinct or active. Some faults are continuously active, while others may have occasional earthquakes and long periods of quiescence. Thousands of "extinct faults" have been mapped in Washington. A few active faults have also been mapped; these active faults are said to be active because they have experienced surface movement in the last 10,000 years. However, in the last 100 years earthquakes in Washington have not been associated with known active faults.

The earthquake process can be compared to the bending of a stick until it snaps. Stress accumulated during bending is suddenly released when the stick breaks. Vibrations are produced as the stick springs back to its pre-stressed position. In the Earth, seismic waves (Figure 2) are the vibrations caused by the sudden release of stress built up in rocks on either side of a fault. The rupturing of a fault may release all or only some of the stress. Any residual stress is often released by later minor readjustments along the fault causing smaller earthquakes called aftershocks.

Earthquakes generate several kinds of seismic waves that vibrate the ground (Figure 2). These seismic waves travel through the Earth at speeds of several kilometers per second, and they cause ground motions that can be detected by seismographs (or by accelerographs) far from the epicenter of the earthquake. In 1987, the University of Washington was operating more than 100 seismograph stations in Washington and northern Oregon (Figure 3). Several thousand seismographs are operated throughout the world by other groups of seismologists.

Typical components of a modern seismograph station are shown in Figure 4 The signals produced by the seismographs in response to ground vibrations from an earthquake are commonly recorded on paper and magnetic tape. The display of ground motion versus time on paper record is called a seismogram (Figure 5).Seismographs can detect ground motions caused by sources other than earthquakes, such as explosions, volcanic eruptions, sonic booms, helicopters, and cars. Each of these sources can generally be identified from their characteristic signals recorded on seismograms.