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
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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
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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