FREE ESSAY ON EARTHQUAKES

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. 
Earthquake, vibrations produced in the earth's crust when rocks in which elastic
strain has been building up suddenly rupture, and then rebound.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.
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.
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 magnet. 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 long, that
coincides with the margins of the Pacific Ocean. 
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.
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. 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 are roughly equivalent to quakes of magnitude 3 to 4 on the Richter scale, and XI
to
XII on the Mercalli scale can be correlated with magnitudes 8 to 9 on the Richter scale.
Attempts at predicting when and where earthquakes will occur have met with some
success in recent years. At present, China, Japan, Russia, and the U.S. are the
countries
most actively supporting such research. In 1975 the Chinese predicted the magnitude 7.3
quake at Haicheng, evacuating 90,000 residents only two days before the quake destroyed
or damaged 90 percent of the city's buildings. One of the clues that led to this
prediction
was a chain of low-magnitude tremors, called foreshocks, that had begun about five years
earlier in the area. Other potential clues being investigated are tilting or bulging of
the
land surface and changes in the earth's magnetic field, in the water levels of wells,
and
even in animal behavior. A new method under study in the U.S. involves measuring the
buildup of stress in the crust of the earth. On the basis of such measurements the U.S.
Geological Survey, in April 1985, predicted that an earthquake of magnitude 5.5 to 6
would occur on the San Andreas fault, near Parkfield, California, sometime before 1993.
Many unofficial predictions of earthquakes have also been made. In 1990 a zoologist, Dr.
Iben Browning, warned that a major quake would occur along the New Madrid fault
before the end of the year. Like most predictions of this type, it proved to be wrong.
Groundwater has also played an important part in earthquake predictions. A peak in radon
in the groundwater at Kobe, Japan 9 days before the 7.2 earthquake cause quite a stir.
Radon levels peaked 9 days before the quake, then fell below the normal levels 5 days
before it hit.
The whole idea behind earthquake predicting is to save lives. With the
improvement in technology, lives have been saved. New ideas and equipment is starting to
prove to be very helpful in predicting were and when an earthquake will strike. The time
and research put into earthquake prediction.