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Seismology and Earthquake Waves

Oct 08, 1999 2231 Words
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

Mallet.

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, that meet with the border of the Pacific Ocean. The points at which rupture occurs in these earthquakes tend

to be far below the earth's surface, at depths of up to 645 km. Alaska's disastrous Good Friday earthquake of 1964 is an

example of one such event. Tectonic earthquakes beyond the "Ring of Fire" occur in a variety of geological settings. Mid-ocean

ridges, which are the seafloor-spreading centers of tectonic plates, are the sites of many events of moderate intensity that take

place at relatively shallow depths. These quakes are seldom felt by anyone and account for only 5 percent of the earth's seismic

energy, but they are recorded daily on the instruments of the worldwide network of seismological stations. Another setting for

tectonic earthquakes is an area stretching across the Mediterranean and Caspian seas, and the Himalaya, ending in the Bay of

Bengal. In this zone, which releases about 15 percent of the earth's seismic energy, continental landmasses riding on the

Eurasian, African, and Australian plates are being forced together to produce high, and new mountain chains. The resulting

earthquakes, which occur at shallow to intermediate depths, have often devastated areas of Portugal, Algeria, Morocco, Italy,

Greece, the Former Yugoslav Republic of Macedonia, Turkey, and other countries partly or completely on the Balkan

Peninsula, Iran, and India.

One other category of tectonic earthquakes includes the not often happening but large and destructive earthquakes that occur

in areas far from other forms of tectonic activity. Main examples of these "midplate earthquakes" are the three large tremors

that shook the region around New Madrid, Missouri, in 1811 and 1812. Which was powerful enough to be felt 1000 miles

away, these tremors created movements that rerouted the Mississippi River. Another example is the quake that struck

Charleston, South Carolina, in 1886. Geologists believe that the New Madrid quakes are "a symptom of forces tearing apart

the earth's crust, forces such as those that created Africa's Rift Valley." Of the two classes of nontectonic earthquakes, those of

volcanic origin are rarely very large or destructive. They are of interest mainly because they usually create threatening volcanic

eruptions, as they did in the weeks before the eruption of Mount St. Helens in Washington, in May 1980. Such earthquakes

originate as magma which works its way upward, filling the spaces below a volcano. As the top and bottom of the volcano

swell and are tilted, explosion of the held back magma may create many small earthquakes. On the island of Hawaii,

seismographs may register as many as 1000 small earthquakes a day before an eruption occurs. Humans can create

earthquakes through a variety of ways, such as the filling of new reservoirs, the underground detonation of explosives, or the

pumping of fluids deep into the earth through wells. For example, in 1962 the city of Denver, Colorado, began to experience

earthquakes for the first time in its history. The discovery was made that the tremors where created at the same time as the

pumping of waste fluids into deep wells occurred. When pumping was discontinued, the earthquakes continued for a little while

and then stopped.

Earthquakes produce many effects that concern the populations of seismically active regions. They can create many deaths by

destroying structures such as buildings, bridges, and dams, and they can also start devastating landslides. For example, an

earthquake near Hebgen, Montana, in 1959 caused a landslide that killed several people and temporarily blocked the Madison

River, which created a lake and threatened the town of Ennis with a major flood. Another destructive effect of earthquakes is

the creation of tidal waves. Because these waves are not related to the tides, they are called seismic sea waves or, their

Japanese name tsunamis. These huge waves of water have hit populated coastlines with such violent actions that entire towns

have been destroyed. In 1896 in Sanriku, Japan, with a population of 20,000, suffered from such and event. Where buildings

have been constructed on filled ground, the liquefaction of soils is another seismic hazard. When subjected to the shock waves

of an earthquake, soil used in landfills may lose all its bearing strength and create a substance like quicksand. Buildings resting

on these materials have literally sank into the ground.

Seismologists have created two scales of measurement to enable them to describe an earthquake's power. One is the Richter

scale, named after the American seismologist Charles Francis Richter, which measures the energy released at the focus of an

earthquake. It is a scale that runs from 1 to 9. A magnitude 7 earthquake is 10 times more powerful than a magnitude 6

earthquake, 100 times more powerful than a magnitude 5 earthquake, 1000 times more powerful than a magnitude 4

earthquake, and so on. An estimated 800 earthquakes of magnitudes 5 to 6 occur annually worldwide, in comparison with

about 50,000 quakes of magnitudes 3 to 4, and only about one earthquake of magnitudes 8 to 9. Theoretically, the Richter

scale is an open-ended one, meaning that the earth can create an earthquake more powerful than 9, but until 1979 an

earthquake of magnitude 8.5 was thought to be the most powerful possible. Since then, improvements in seismic measuring

techniques have enabled seismologists to redefine the scale, and 9.5 is now considered to be the practical limit. On the basis of

the new scale, the magnitude of the 1906 San Francisco earthquake has been revised from 8.3 to 7.9, while the Alaskan

earthquake of 1964 has been upgraded from 8.4 to 9.2. The other scale, introduced at the turn of the 20th century by an

Italian seismologist, Giuseppe Mercalli, measures the intensity of shaking with gradations from I to XII. Because seismic

surface effects decrease with further distances from the focus of the earthquake, the Mercalli rating assigned to the earthquake

depends on the site of the measurement. Intensity I on this scale is defined as an event felt by very few people, where intensity

XII is assigned to a catastrophic event that causes total destruction. Events of intensities II to III are roughly equivalent to

quakes of magnitude 3 to 4 on the Richter scale, and XI to XII on the Mercalli scale can be correlated with magnitudes 8 to 9

on the Richter scale.

Attempts at predicting when and where earthquakes will occur have had some success in recent years. At this time China,

Japan, Russia, and the U.S. are the countries most actively supporting this research. In 1975 the Chinese predicted the

magnitude 7.3 earthquake at Haicheng, evacuating 90,000 people only two days before the earthquake destroyed or damaged

90 percent of the city's buildings. One of the clues that led to this prediction was a chain of low-magnitude tremors, called

foreshocks, that had started about five years earlier in the area. Other potential clues being investigated are tilting or bulging of

the land surface and changes in the earth's magnetic field, in the water levels of wells, and even in animal behavior. A new

method under study in the U.S. involves measuring the buildup of stress in the crust of the earth. On this idea of such

measurements the U.S. Geological Survey, in April 1985, predicted that an earthquake of magnitude 5.5 to 6 would occur on

the San Andreas fault, near Parkfield, California, sometime before 1993. Many unofficial predictions of earthquakes have also

been made. In 1990 a zoologist, Dr. Iben Browning, warned that a major earthquake would occur along the New Madrid fault

before the end of the year. Like most predictions of this type, it was proved wrong.

While I was trying to predict earthquakes, my first prediction wasn't too far off of my ‘target', I predicted one about 100 miles

south of the California-Mexico border. There were a couple of earthquakes that occurred in California, near L.A, they were of

course very minor and couldn't be felt, they were only detectable by seismographs. The next day I predicted that there was

going to be an earthquake in the same spot that it occurred the day before. I was correct, in fact there were three. The next

day I picked a spot about 50 miles north of the earthquake that occurred the day before, this time I was wrong, there were

two that occurred near the San Francisco bay area and none within a 50 mile radius of my approximation. The next couple

days I predicted earthquakes that were within a 100 mile radius than were they actually occurred. From my experiments I

concluded that predicting earthquakes was easy, you just have to pick a spot on the fault. The only thing that troubled me and

probably most scientists, is magnitude, there is no possible way of predicting an earthquakes magnitude. Which is what we are

really trying to predict. Earthquakes happen all the time, but what we are really trying to figure out how to predict is when a

major earthquake is going to occur.

I learned that earthquakes are almost unpredictable, and devastating acts of nature. I also learned how earthquakes occur and

almost all of the "earthquake dictionary". There is still a lot more to be known about earthquakes that we still do not know

about today. Prediction of large earthquakes is still under development, where prediction of small, unnoticeable earthquakes

can be easy to predict because they happen mainly around fault lines.

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