Geología, Topografía y Minas
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
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
II Felt by few. Delicately poised objects may sway.
III Vibration, but still unrecognized by many. Feeble.
IV Felt by many indoors but by few outdoors. Moderate
V Felt by almost all. Many awakened. Unstable objects moved.
VI Felt by all. Heavy objects moved. Alarm. Strong.
VII General alarm. Weak buildings considerably damaged. Very strong.
VIII Damage general except in proofed buildings. Heavy objects overturned.
IX Buildings shifted from foundations, collapse, ground cracks. Highly destructive.
X Masonry buildings destroyed, rails bent, serious ground fissures. Devastating.
XI Few if any structures left standing. Bridges down. Rails twisted. Catastrophic.
XII Damage total. Vibrations distort vision. Objects thrown in air. Major
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" (Last Name).
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).
Aylesworth, Thomas G. Geological Disasters "Franklin Watts" 1979: 14-15.
The New Grolier Multimedia Encyclopedia "Grolier Electronic Publishing Inc." 1993: No Page
Johnson, Thomas P. When Nature Runs Wild "Creative Education Press" 1986: 12-14.
Kohler, Pierre Volcanoes Earthquakes and the Formation of Continents "Barron's Educational
Series, Inc." 1987: 14-22.
Marcus, Rebecca B. The First Book of Volcanoes & Earthquakes "Franklin Watts, Inc." 1972:
Poynter, Margaret Earthquakes "Enslow Publishers, Inc." 1990: 6-7.
Putnam, William C. Geology "Oxford University Press" 1964: 439-450.
Vogt, Gregory Predicting Earthquakes "Franklin Watts" 1989: 24-25.
Walker, Bryce Earthquakes "Time Life Books" 1982: 94-100.
Karen C. McNally Earthquakes World Book Encyclopedia "World Book Inc." 1994: 38-39.
|Enviado por:||Pedro Ramirez|