Earthquakes

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motions they impart to rock particles. Primary or
compressional waves (P waves) send particles oscillating back and forth in the
same direction as the waves are traveling, whereas secondary or transverse shear
waves (S waves) impart vibrations perpendicular to their direction of travel. P
waves always travel at higher velocities than S waves, so whenever an earthquake
occurs, P waves are the first to arrive and to be recorded at geophysical
research stations worldwide.(Associated Press 1993)
Earthquake waves were observed in this and other ways for centuries, but
more scientific theories as to the causes of quakes were not proposed until
modern times. One such concept was advanced in 1859 by the Irish engineer Robert
Mallet. Perhaps drawing on his knowledge of the strength and behavior of
construction materials subjected to strain, Mallet proposed that earthquakes
occurred �either by sudden flexure and constraint of the elastic materials
forming a portion of the earth's crust or by ...

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the margins of
the Pacific Ocean. The points at which crustal rupture occurs in such quakes
tend to be far below the earth's surface, at depths of up to 645 km (400 mi).�
(Monastersky Dec, 95) Alaska's disastrous Good Friday earthquake of 1964 is an
example of such an event.
Seismologists have devised two scales of measurement to enable them to
describe earthquakes quantitatively. "One is the Richter scale named after the
American seismologist Charles Francis Richter�which measures the energy released
at the focus of a quake. It is a logarithmic scale that runs from 1 to 9; a
magnitude 7 quake is 10 times more powerful than a magnitude 6 quake, 100 times
more powerful than a ...

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