An earthquake is the rapid relative displacement of the rock on
either side of a fracture, or fault, in the interior of the
solid earth. Elastic strain energy that had been stored in the
rocks is suddenly released. Some of the energy released
dissipates as friction along the fault. The rest is transferred
to seismic waves that radiate outward in all directions from the
initial point of rupture and cause ground motion at the earth's
The severity of an earthquake can be measured in a variety of
ways and places. These include measurements of what occurs on
the surface, where ground motion can damage structures and
infrastructure. Other measurements enable seismologists to infer
what happened at the point within the earth where the rupture
initiated, known as the hypocenter. The current practice among
earth scientists and engineers is to use the word
"magnitude" to characterize the energy released at an
earthquake's hypocenter, and the word "intensity" to
refer to the observed effects of an earthquake at the surface.
While the magnitude of an earthquake is independent of the
location at which measurements are made, intensity is dependent
upon the distance from the rupture, the intervening geology, and
local site conditions.
Most of the earth's seismic energy is released at the
boundaries of tectonic plates. There are three types of plate
boundaries. The first is known as the convergent type, where
plates move toward one another. A dramatic example of a
convergent plate boundary is the continent-continent collision
zone between the Indian Subcontinent and Asia, which has
resulted in the formation of the Himalayan Mountains. The most
common manifestation of this type of boundary, however, is the
subduction zone, where oceanic and continental crusts collide
and the oceanic plate is thrust under the continental plate due
to the oceanic plate's higher density. Most of the earth's
seismic energy is released along these zones.
The second type of plate boundary is known as the transform,
or strike-slip fault, where plates slide past one another.
Perhaps the most well-known example of this type of plate
boundary is the San Andreas fault in California.
The third type is known as the divergent plate boundary,
where plates move away from one another. Examples are the
Mid-Atlantic Ridge and the East-Pacific Rise. These plate
boundaries occur almost exclusively deep in the ocean, and
therefore present a negligible seismic hazard.
While the vast majority of earthquakes occur at plate
boundaries, they can also occur in the interior of plates.
Geologists believe that such areas are characterized by traces
of ancient geological deformations or by variations in
temperature and strength of the lithosphere. Earthquakes that
occur in such areas are referred to as "intraplate"
earthquakes. The New Madrid Seismic Zone in the Central United
States is an example of region for which there is significant
hazard from intraplate earthquakes.