Earthquakes Essay

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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...

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