Seismology and Earthquake Waves

Topics: Earthquake, Plate tectonics, Seismology Pages: 10 (2231 words) Published: October 8, 1999
I chose to research earthquakes and the prediction of earthquakes because I was curious as to how they work. In this paper, I

will discus the history of earthquakes, the kinds and locations of earthquakes, earthquake effects, intensity scales, prediction,

and my own predictions.

An earthquake can be defined as vibrations produced in the earth's crust. Tectonic plates have friction between them which

builds up as it tries to push away and suddenly ruptures and then rebounds. The vibrations can range from barely noticeable to

a disastrous, and destructive act of nature. Six kinds of shock waves are generated in the process. Two are classified as body

waves, that is, they travel through the inside of the earth and the other four are surface waves. The waves are further classified

by the kinds of motions they incur to rock particles. Primary or compressional waves, known as P waves, send particles

moving back and forth in the same direction as the waves are traveling, as secondary or transverse shear waves, known as S

waves, create vibrations perpendicular to their direction of travel. P waves always travel at faster speeds than S waves, so

whenever an earthquake occurs, P waves are the first to arrive and to be recorded at geophysical research stations worldwide.

During ancient times very little was know about. Some of the ancient Greek philosophers connected earthquakes to

underground winds, where others blamed them on fires in the depths of the earth. Around AD 130 the Chinese scholar Chang

Heng, believing that waves must ripple through the earth from the source of an earthquake, created a bronze object to record

the directions of such waves. Eight balls were carefully balanced in the mouths of eight dragons placed around the outside of

the object. When a passing earthquake occurred the wave would cause one or more of the balls to drop.

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 recreated and advanced in 1859 by an Irish engineer, Robert


Perhaps recalling on his knowledge of the strength and behavior of construction materials, Robert 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, an English geologist, John Milne created a device similar to

one of today's earthquake-recording device, a seismograph, which in Greek seismos means earthquake. A simple pendulum

and needle suspended above a smoked-glass plate, it was the first instrument to allow visual difference of primary and

secondary earthquake waves. The modern seismograph was invented in the early 20th century by a Russian seismologist,

Prince Boris Golitzyn. His device used a magnetic pendulum suspended between the poles of an electromagnet, created the

modern era of earthquake research.

There are three general classes of earthquakes that are now recognized: tectonic, volcanic, and artificially produced. The

tectonic kind is by far the most devastating, and these earthquakes create many difficulties for scientists trying to develop ways

to predict them. The main cause of tectonic earthquakes is stress set up by movements of the dozen major and minor plates

that make up the earth's crust. Most tectonic quakes occur at the boundaries of these plates, in zones where one plate slides

past another, such as at the San Andreas Fault in California, North America's most quake-prone area, or where one plate

slides beneath the other plate, subduction. Subduction-zone earthquakes count for nearly half of the world's destructive seismic

events and 75 percent of the earth's seismic energy. They are concentrated along the "Ring of Fire", a narrow band about

38,600 km long,...
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