_by Kaye M. Shedlock and
Louis C. Pakiser_
For sale by the U.S. Government Printing Office
Superintendent of Documents
Mail Stop: SSOP, Washington, DC 20402-9328
One of the most frightening and destructive phenomena of nature is a
severe earthquake and its terrible aftereffects. An earthquake is a
sudden movement of the Earth, caused by the abrupt release of strain
that has accumulated over a long time. For hundreds of millions of
years, the forces of plate tectonics have shaped the Earth as the huge
plates that form the Earth’s surface slowly move over, under, and past
each other. Sometimes the movement is gradual. At other times, the
plates are locked together, unable to release the accumulating energy.
When the accumulated energy grows strong enough, the plates break free.
If the earthquake occurs in a populated area, it may cause many deaths
and injuries and extensive property damage.
Today we are challenging the assumption that earthquakes must present an
uncontrollable and unpredictable hazard to life and property. Scientists
have begun to estimate the locations and likelihoods of future damaging
earthquakes. Sites of greatest hazard are being identified, and definite
progress is being made in designing structures that will withstand the
effects of earthquakes.
[Illustration: _USGS scientist uses portable seismic recording equipment
near Mount St. Helens, Washington._]