An earthquake is the motion or trembling of the ground produced by sudden displacement of rock in the Earth's crust. Earthquakes result from crustal strain, volcanism, landslides, and collapse of caverns. Stress accumulates in response to tectonic forces until it exceeds the strength of the rock. The rock then breaks along a preexisting or new fracture called a fault. The rupture extends outward in all directions along the fault plane from its point of origin (focus). The rupture travels in an irregular manner until the stress is relatively equalized. If the rupture disturbs the surface, it produces a visible fault.

Earthquakes can affect hundreds of thousands of square kilometers; cause damage to property measured in the tens of billions of dollars; result in loss of life and injury to hundreds of thousands of persons; and disrupt the social and economic functioning of the affected area. Although earthquakes in the United States occur most frequently in states west of the Rocky Mountains, devastating earthquakes have also occurred in the Midwest and East. All 50 states have some degree of risk from earthquakes.

Earthquakes can be measured in terms of either the amount of energy they release (magnitude) or the degree of ground shaking they cause at a particular locality (intensity). Magnitude is calculated from the record (wave amplitude) made by an earthquake on a calibrated seismograph. The magnitude scale is logarithmic. An increase of one in magnitude represents a tenfold increase in the recorded wave amplitude. However, the energy release associated with an increase of one in magnitude is not tenfold, but about thirty fold. For example, approximately 900 times more energy is released in an earthquake of magnitude 7 than in an earthquake of magnitude 5.

Intensity is determined from observations of the earthquake's effect on people, structures, and the earth's surface at a given locality. When a fault ruptures, seismic waves propagate outward in all directions and ground shaking results. Generally the severity of ground shaking increases as magnitude increases and decreases as distance from the fault rupture increases. The severity of the ground shaking can be enhanced by certain soil and subsoil types. The intensity of the earthquake is affected by the severity of the ground shaking, the duration of the shaking, the response of structures in the affected area, etc.

Hazards associated with earthquakes include ground shaking, surface faulting, earthquake-induced ground failures, tectonic uplift and subsidence, and tsunamis. Surface faulting, the offset of the earth's surface by differential movement across a fault, shears and tears structures built on the fault. Surface faulting is generally accompanied by horizontal or vertical distortion of the earth's surface that can distort or tilt structures constructed near the fault. Regional uplift and subsidence may accompany earthquakes caused by large displacements on shallow faults. Such changes can damage harbor facilities, canals, roads, railroads, and other structures. A tsunami is a water wave or a series of waves generated by an impulsive vertical displacement of the surface of the ocean or other body of water by an earthquake or other cause. These waves can extend the damaging effects of an earthquake event thousands of kilometers from the earthquake focus.

Ground failures accompanying earthquakes include landslides, liquefaction, lateral spreads, differential settlements, and ground cracks. Earthquake shaking often dislodges rock and debris on steep slopes, triggering rock falls, avalanches, and landslides. These slides have been known to bury entire towns and may be the most damaging aspect of the earthquake event.

Liquefaction occurs where ground water is near the surface in soils composed of sands and silts. The soil temporarily loses strength and behaves as a viscous liquid. Structures can settle or tip in the liquefied soil or be ripped apart as the ground spreads laterally or flows. Flow failures can move over kilometers at speeds of tens of kilometers per hour. They usually develop in loose, saturated sand on slopes greater than five percent. When subsurface sand layers lose strength because of liquefaction, lateral spreading can occur in overlying sediments allowing them to move down even the gentlest slopes. Soils may lose shear strength allowing heavy structures to settle or tip and lightweight, buried structures to rise buoyantly. Cracking may result from movement along faults, differential compaction of the soil, or slides. Strong ground shaking has compacted loose cohesionless materials and caused differential ground settlements ranging from 5 cm to more than a meter. Many of these earthquake effects are depicted in the slides included in this set.