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.
Many of these earthquake effects are depicted in the slides included in this set.
