Earthquakes Essay, Research Paper

Earthquakes: Causes, Mapping, and Predicting

Throughout history, man has made many advancements.

These advancements have been made to make life easier. The one

thing man can’t do is to control Mother Nature. Mother Nature can

cause many things such as earthquakes. The causes of earthquakes

have been theorized in many ways. According to the book

Predicting Earthquakes by Gregory Vogt, the Greeks, “blamed the

earthquakes on Poseidon, god of the sea”(25). The Hindu believed

that “the earth was a platform that rested on the back of eight

great elephants. When one of the elephants grew weary, it

lowered and shook its head causing the ground above to

tremble”(Vogt 25). Margaret Poynter writes “many primitive people

thought that the earth rested upon the back of some sort of animal.

When that animal became restless, great cracks appeared in the

ground, and tall trees swayed and fell. In South America, the animal

was a whale. In Japan, it was a great black spider or giant catfish.

One ancient tribe thought that four bulls supported the earth on

their horns. To amuse themselves, they sometimes tossed it from one

to another”(6). In the same book, Poynter says “The Chinese believed

that monsters lived in the caves inside the earth. When the

creatures fought, the surface of the earth trembled (6).” “In Greece,

it was not an animal, but a titan named Atlas who was condemned

to support the world upon his shoulders. Later, about the third

century B.C., a Greek philosopher, Aristotle, had a more scientific

explanation. He thought that earthquakes occurred only when hot

air masses tried to escape from the center of the earth. Two

centuries later, Lucretius, a Roman, wrote that underground

landslides caused the earth’s surface to move”(Poynter 7).2 Last

Name

Today, scientists have found a more logical reason to

earthquakes. Scientists say almost 600 million years ago, all the

continents were connected to form a huge super continent called

Pangaea. At about 220 million years ago, Pangaea began to break

up into sub-blocks. According to the book Volcanoes, Earthquakes,

and the Formation of Continents, these sub-blocks were called

“Gondwana (which corresponds approximately to the continents of

the present southern hemisphere) and Laurasia (the northern

hemisphere)” (Kohler 15). According to Pierre Kohler, “The earth’s

surface is divided into 13 plates: seven large ones (the largest

corresponding to the Pacific) and six small ones” (18-19). The book

Earthquakes by Margaret Poynter states that a person named

Robert Mallet studied earthquakes. He made tests, drew a world

map, and recreated earthquakes only to find that rocks are being

overstressed at the faults. “A fault is the place where two plates

meet and are rubbed against each other” (Groiler Electronic

Publishing, Inc.). The book, Predicting Earthquakes, the author points

out “There are generally three kinds of faults: normal, reverse, and

strike-slip. By careful observation and measurement, geologists,

acting like detectives, can tell how much a fault moved, which part

went up, which part went down, and which way the fault moved”

(Vogt 26). “When one of the plates slip under the great amount of

stress at the fault, an earthquake occurs. The shaking we feel are

the passing of long waves” (Putnam 443). “The L-waves (long

waves) travel at slower velocities that the primary and secondary

waves. These waves make the largest squiggles on a seismograph

but their effect diminishes rapidly with distance. The L-waves are

limited to the crust” (Putnam 443). One of the two kinds of waves

are “Primary waves are a kin to sound waves, and thus produce

alternate compression and rarefaction in the medium through which

they travel much like the waves that spread out through the air in

all directions from a tuning fork” (Putnam 444). The second of the

two kinds of waves are “Secondary waves, the particles in the rock

through which the wave is traveling vibrate at right angles, or

transversely, to the direction of propagation. The velocity of

P-waves are almost twice as fast as S-waves” (Putnam 444). “A

seismologist cannot locate the epicenter (where the earthquake

took place) of an earthquake that has shown up on his

seismometer from the seismogram, or written record, alone. All the

seismogram tells him are the times when the P and S waves reach

his station, and how violent they are” (Marcus 62). Rebecca Marcus,

in her work The First Book of Volcanoes & Earthquakes, explain how

scientists locate an earthquake’s epicenter. “To locate a quake, the

seismologist first finds the difference between the time of arrival of

the P wave and that of the S wave. Let us suppose that an S wave

reaches a station in New York on a certain date at 10:30 P.M., 4

minutes and 42 seconds after a P wave. the seismologist then refers

to a table, which tells him that the epicenter is 2,000 miles away.

Although he has found its distance, he does not know its direction

from his station. “Now he needs the cooperation of at least two

other stations. Messages are sent, let us say, to a station in San

Francisco and to another in Rio de Janeiro, asking for their distance

from an earthquake that occurred on that date at eighteen

seconds past 10:25 P.M., New York time. When the seismologist

receives his answers, he learns that the epicenter was 3,800 miles

from San Francisco and 3,500 miles away from Rio de Janeiro. Using

a radius representing 2,000 miles, he draws a circle around New

York. Next , using the same scale, he draws a circle with a radius

representing 3,800 miles around Riode Janeiro. The three circles cross

at a point near the Dominican Republic. And here is the epicenter of

the earthquake” (63). To measure and identify these waves, an

invention called the seismometer was used. Rebecca Marcus tells

about the first seismograph. “The very first attempt to detect a

distant earthquake was make about A.D. 136 in China by an

inventor whose name was Chang Heng. Chang Heng’s invention

consisted of a large hollow ball standing on its base which was set

on level ground. A heavy weight was suspended inside the shell.

Around the large ball, at equal distances, were eight

open-mouthed bronze dragon heads, and on the tongue of each

was a small copper ball. A bronze open-mouthed toad stood

beneath each dragon head. The whole instrument was so arranged

that, at the slightest jarring by an earthquake wave, the suspending

weight would cause one of the balls to shoot out of the dragon’s

mouth into the toad’s mouth. Which ball shot out depended on the

nearest path of the wave” (Marcus 57). In today’s modern world,

technology would make Chang Heng’s seismometer look crude and

untrust worthy. Today’s modern seismometer looks similar but more

complicated because it involves photographic paper, a beam of

light, a mirror, and many other things. In 1935, Charles Richter

created a scale that measured earthquakes. In February 1977,

Richter’s scale was slightly revised because of the fact that there

are now more sophisticated ways of measurement and the scale

didn’t cover the higher magnitude rating. In the book Geological

Disasters, by Thomas G. Aylesworth, the author states: “There is

another method of measuring earthquakes, the modified Mercali

scale. This is a twelve-point scale of intensity that grades the quake

by describing the kinds of damage and other effects caused by it.

You might say that the Richter scale measures the actual power of

the earthquake, while the modified Mercali scale measures the

earthquake’s effect on humans. In any case, the two scales do not

give us quite the same information. Here are the twelve points that

make up the modified Mercali scale: I- Just detectable by

experienced observers when prone. Microseisms (tiny vibrations).

2-Felt by few. Delicately poised objects may sway. 3- Vibration, but

still unrecognized by many. Feeble. 4- Felt by many indoors but by

few outdoors. Moderate 5- Felt by almost all. Many awakened.

Unstable objects moved. 6-I Felt by all. Heavy objects moved. Alarm.

Strong. 7- General alarm. Weak buildings considerably damaged.

Very strong. 8- Damage general except in proofed buildings. Heavy

objects overturned. 8- Buildings shifted from foundations, collapse,

ground cracks. Highly destructive. 10- Masonry buildings destroyed,

rails bent, serious ground fissures. Devastating. 11 Few if any

structures left standing. Bridges down. Rails twisted. Catastrophic. 12

Damage total. Vibrations distort vision. Objects thrown in air. Major

catastrophe” (14-15). In the book Earthquake by Bryce Walker there

is an article that explains how to predict earthquakes. Most of the

theories involve the other planets lining up and putting great stress

on the earth which causes earthquakes. “I think that all of the oil

and soft medals that we are taking is creating friction and causing

the earthquakes in California” . In, When Nature Runs Wild, the

author states that “At the present time, man cannot predict

earthquakes. Several nations, however, have research programs

that are investigating these problems. Leaders in this field are the

United States, Japan, and the Soviet Union” (Johnson 12). The World

Book Encyclopedia, writes that “Scientists can make fairly accurate

long-term predictions of where earthquakes will occur. They know,

for example, that about 80 percent of the world’s major

earthquakes happen along a belt encircling the Pacific Ocean. This

belt is sometimes called the Ring of Fire because it has many

volcanoes, earthquakes, and other geologic activity” (Earthquakes;

Karen C. McNally 38).