Nathan Moore, Josh Schuster, Dan Krause, Sean Hignite

 RESEARCH PAGE

TABLE OF CONTENTS

1. INTRODUCTION

2. PART I. HOW EARTHQUAKES WORK

3. PART II. LIMITING EARTHQUAKE DAMAGE

4. CONCLUSION

5. WORKS CITED

Introduction     

Everyday over 8,000 earthquakes occur around the world. Although most of these cannot even be felt, every year earthquakes cost billions of dollars and take many lives. For centuries, people have wondered about and feared these tremendous forces. Different theories abounded, but until about 200 years ago, few concrete facts were known about earthquakes. Scientific findings solved the problem as to why earthquakes occurred, but did little to limit their deadly force. Even today, scientists continue to study earthquakes hoping to gain a better understanding of these natural occurrences, while at the same time, developing technology to prevent complete destruction during an earthquake.         

Part I.   How Earthquakes Work

The earth’s surface is made up of plates of hard rock that float on a layer of molten rock. These plates are the cause of most earthquakes. As the plates float on the hot, soft rock, they bump, grind, and dive above and below each other. At places where plates collide, mountain ranges can form, while where plates diverge, large valleys called rifts are created. The causes of this movement are gravity and the cooling of hot molten rock. As the molten rock cools at the edge of a plate and becomes hard, the plate’s edge becomes very dense and can sink down. Plates can also be pushed in certain directions by gravity, especially at ocean ridges, where the plates slide downhill away from the ridge. Large plates are also able to move smaller plates. 

At the edges of these plates are faults. Faults are displacements of rock layers in the Earth’s crust in response to stress, accompanied by a break in the continuity of the rocks on each side of the fault line. In other words, a fault is a crack in the Earth’s crust where two other plates of the Earth’s surface meet. Four kinds of faults exist: normal faults, reverse faults, thrust faults, and transcurrent faults. Firstly, normal faults’ movement usually follows the gravitational pull on the fault blocks involved in an earthquake. The gravitational pull of the fault blocks of a normal fault tends to be outward. This gravitational pull in an earthquake causes one fault block to move upward and the other downward, with respect to the fault plane. The fault block that moves upward forms a fault scarp, which is a cliff like feature. Fault scarps can range in height from a few to hundreds of meters and their length can continue for three hundred kilometers or more. Secondly, reverse faults are like normal faults except the general movement of the fault blocks is toward each other, not away from each other as in a normal fault. The gravitational inward movement of the fault blocks of reverse faults forms a thrust fault type expression (material over material) on the Earth’s surface after an earthquake. The third type of faults are thrust faults, which are similar to reverse faults. During earthquake activity on a thrust fault one fault block moves into the other fault block causing one of the blocks to move upward. When thrust faults are exposed on the surface overburden material lies over the main block. Thrust faults are usually associated with mountainous regions and areas with folded surfaces. Lastly, the most well known and studied faults are transcurrent faults, also known as strike-slip faults. Movement on these faults is usually horizontal. Anything that crosses this fault line in time is either slowly torn apart or offset. Rivers that exist on transcurrent faults are called offset streams because of the abnormal shape they have due to activity on the fault line. These fault lines can be very long; the San Andreas Fault line is six hundred miles long just to give an example. To conclude, the four types of faults are normal faults, reverse faults, transcurrent faults, and thrust faults.  

At the fault line, rocks are stressed and store energy like a spring as the plates on each side of the fault slowly move. At times, the rocks are no longer able to withstand the stress, and an earthquake occurs. Rocks grind together or break apart, causing a great release of energy. This release of energy is expressed through two major types of waves: body waves and surface waves. Body waves are emitted from the actual site of the quake inside of the earth, or the hypocenter. The first body waves are called P waves, or pressure waves. These waves, which can travel at almost 4.2 miles per second, stretch the earth in some areas, while compressing it in others. Following the P waves are the secondary, or sheer, waves known as S waves, which travel at about half the speed of P waves. These waves move the earth in directions perpendicular to the wave’s movement. Once these body waves reach the earth’s surface, much more damaging waves spread out. The first type of surface waves are Rayleigh waves. These waves act like ocean waves and move the earth up and down. Love waves work in the opposite direction and shake the earth from side to side. Both types of surface waves travel at about 2.5 miles per second and are the reason for the collapse of most buildings and bridges during an earthquake.  

As waves emanate from the origin of the earthquake, they can be measured, and the magnitude, or strength, of the earthquake can be determined. The higher the energy released in the earthquake, the higher the magnitude. Waves of all types are picked up on the seismogram, an instrument that records the amount of ground movement during the earthquake either digitally or with a needle or pen that moves over paper. The size of the waves recorded (see right) is use to determine the magnitude, which can be described on a number of different scales. The two most widely used are the Richter scale and the moment magnitude scale, used to measure large earthquakes. Both scales rate the earthquake with a number system in which a jump of one number (say from three to four) indicates an increase by ten times of the size of waves recorded on the seismogram. This increase also means that 30 times the amount of energy released in a 3.0 magnitude quake is released in a 4.0 quake. The largest earthquake every recorded occurred near Chile and measured 9.5 on the moment magnitude scale.   

Part II.  Limiting Earthquake Damage

            The power of earthquakes is able to damage and destroy buildings, bridges, and almost any other man-made structures. As these structures collapse, people can be killed by debris or be trapped. In the aftermath of the quake, relief supplies can be hard to find and obtain, and many more people can die because of starvation or disease.

            The safest way to prevent this widespread disaster is to predict the occurrence of an earthquake. Seismologists try to estimate the time, location, and intensity of an earthquake. They analyze faults to determine which ones may be more active than others. Predicting when the quake will occur is much more difficult, however. As a result, people in earthquake prone areas may know it will occur at sometime but cannot prepare for or evacuate the area when a major quake is about to strike.

            Because of earthquakes’ unpredictability, engineers have turned to redesigning man-made structures to make them more quake resistant. The simplest way to do this is to reinforce the structures. Engineers add trusses, girders, and cross-braces to structures, hoping to prevent the buildings’ complete collapse. These reinforcements must be flexible, however, to withstand the sheer power of an earthquake. Engineers can also build buildings on base isolators. Base isolators separate a building from the ground, instead of rigidly anchoring it in the ground. If the building was anchoring firmly in the ground, all the force of the earthquake would be directly applied to the building. When the frequency of the earthquake matches the building’s natural frequency, or the number of times the building sways back and forth each second, the building will collapse. As the ground moves during an earthquake, the base isolators absorb some of the movement, while at the same time, lowering the frequency of the building, and the building does not sway dramatically. A third way in which engineers “earthquake proof” structures is by adding dampers to beams and braces inside of buildings. The dampers are able to counteract the earthquake’s movements and absorb the vibrations, reducing the damage to a structure. At the same time, the dampers work to keep the building’s center of gravity over its base. Even after the earthquake is over, energy is still moving through the building. The dampers act as an outlet for this energy. In addition to these three major ways of stabilizing structures during a quake, many more are being used and developed.

            Even with these added safety features, earthquakes still cause damage and devastation. Because of this, emergency preparation is of the utmost importance. Emergency personnel must be able to respond at all times to an earthquake rattled area. Emergency supplies must also be strategically placed to provide the best, most efficient relief response. Because engineers will never be able to completely tame an earthquake’s tremendous power, preventing further loss of life in the aftermath of an earthquake is extremely important. 

Conclusion

            Earthquakes are one of nature’s most powerful forces. They are caused by the moving and colliding of plates, which make up the earth’s crust. At places were the plates meet, which

are called faults, earthquakes occur regularly. These earthquakes emit devastating waves which rock the earth’s surface, destroying buildings, bridges, and landforms. Because of these disastrous effects, scientists and engineers study the forces of earthquakes and learn how to limit them. They do this with specially designed buildings, as well as predicting and preparing for the next big earthquake. Although earthquakes still cause billions of dollars of damage each year, some of these techniques have proved to be successful and may be even more widely used in the future. 

Works Cited

Bonser, Kevin. “How Smart Structure Will Work.” howstuffworks. 2006. HowStuffWorks, Inc. 20 Dec. 2006. <http://science.howstuffworks.com/smart-structure.htm>

“Fault line: Seismic Science at the Epicenter.” Exploritorium.edu. 1999. The Exploritorium. 26 Nov. 2006. <http://www.exploritorium.edu/faultline/index.html> 

Jacobs, Andrew. “Base Isolation.” Illumin. 2003. USC Viterbi School of Engineering. 21 Dec. 2006. <http://illumin.usc.edu/article.php?articleID=127>

McNally, Karen C. “Earthquake.” The World Book Encyclopedia. 2000 ed.

Mustoe, M. “The Four BASIC Types of Faults.” Tinynet.com. 1997. tinynet. 15 Dec. 2006. <http://www.tinynet.com/faults.html>