Earthquakes


Earthquakes have plagued our lives for as long as people have inhabited
the earth. These dangerous acts of the earth have been the cause of many deaths
in the past century. So what can be done about these violent eruptions that take
place nearly with out warning? Predicting an earthquake until now has almost
been technologically impossible. With improvements in technology, lives have
been saved and many more will. All that remains is to research what takes place
before, during, and after an earthquake. This has been done for years to the
point now that a successful earthquake prediction was made and was accurate.
This paper will discuss a little about earthquakes in general and then about how
predictions are made.
Earthquake, "vibrations produced in the earth\'s crust when rocks in
which elastic strain has been building up suddenly rupture, and then
rebound."(Associated Press 1993) The vibrations can range from barely noticeable
to catastrophically destructive. Six kinds of shock waves are generated in the
process. Two are classified as body waves—that is, they travel through the
earth\'s interior—and the other four are surface waves. The waves are further
differentiated by the kinds of 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 their giving way and becoming
fractured.”(Butler 1995)
Later, in the 1870s, the English geologist John Milne devised a
forerunner of today\'s earthquake-recording device, or seismograph. A simple
pendulum and needle suspended above a smoked-glass plate, it was the first
instrument to allow discrimination of primary and secondary earthquake waves.
The modern seismograph was invented in the early 20th century by the Russian
seismologist Prince Boris Golitzyn. “His device”, using a magnetic pendulum
suspended between the poles of an electromagnet, "ushered in the modern era of
earthquake research." (Nagorka 1989)
"The ultimate cause of tectonic quakes is stresses set up by movements
of the dozen or so major and minor plates that make up the earth\'s
crust."(Monastersky Oct, 95) Most tectonic quakes occur at the boundaries of
these plates, in zones where one plate slides past another—as at the San Andreas
Fault in California, North America\'s most quake-prone area—or is subducted
(slides beneath the other plate). “Subduction-zone quakes account for nearly
half of the world\'s destructive seismic events and 75 percent of the earth\'s
seismic energy. They are concentrated along the so-called Ring of Fire, a narrow
band about 38,600 km (about 24,000 mi) long, that coincides with 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 magnitude 5 quake, 1000 times more powerful than a
magnitude 4 quake, and so on."(Associated Press 1992)
The other scale, introduced at the turn of the 20th century by the
Italian seismologist Giuseppe Mercalli, “measures the intensity of shaking with
gradations from I to XII.” (Associated Press 1992) Because seismic surface
effects diminish with distance from the focus of the quake, the Mercalli rating
assigned to the quake depends on the site of the measurement. “Intensity I on
this scale is defined as an event felt by very few people, whereas intensity XII
is assigned to a catastrophic event that causes total destruction. Events of
intensities II to III