EARTHQUAKE REFERENCE FILES Earthquake, shaking of the earth?s surface caused by rapid movement of the earth?s rocky outer layer. Earthquakes occur when energy stored within the earth, usually in the form of strain in rocks, suddenly releases. This energy is transmitted to the surface of the earth by earthquake waves. The study of earthquakes and the waves they create is called seismology. Scientists who study earthquakes are called seismologists. (Webster?s p.423) The destruction an earthquake causes, depends on its magnitude or the amount of shaking that occurs. The size varies from small imperceptible shaking, to large shocks felt miles around. Earthquakes can tear up the ground, make buildings and other structures collapse, and create tsunamis (large sea waves).
Many Lives can be lost because of this destruction. (The Road to Jaramillo p.211) Several hundred earthquakes, or seismic tremors, occur per day around the world. A worldwide network of seismographs detect about one million small earthquakes per year. Very large earthquakes, such as the 1964 Alaskan earthquake, which measured 8.6 on the Richter scale and caused millions of dollars in damage, occur worldwide once every few years. Moderate earthquakes, such as the 1989 tremor in Loma Prieta, California (magnitude 7.0), and the 1995 tremor in K?be, Japan (magnitude 6.8), occur about 20 times a year. Moderate earthquakes also cause millions of dollars in damage and can harm many people. (The Road to Jaramillo p.213-215) In the last 500 years, several million people have been killed by earthquakes around the world, including over 240,000 in the 1976 T?ang-Shan, China, earthquake. Worldwide, earthquakes have also caused severe property and structural damage. Good precautions, such as education, emergency planning, and constructing stronger, more flexible structures, can limit the loss of life and decrease the damage caused by earthquakes. (The Road to Jaramillo p.213-215,263) AN EARTHQUAKES ANATOMY Seismologists examine the parts of an earthquake, like what happens to the earth?s surface during an earthquake, how the energy of an earthquake moves from inside the earth to the surface, and how this energy causes damage.
The Term Paper on Tropical Cyclone Earthquakes Hurricane Damage
... surface material, such as poorly compacted fill or river deposits; bedrock areas receive fewer effects. The worst damage occurs ... year. Great earthquakes (intensity 8. 0 or higher) occur once a year, major earthquakes (intensity 7. 0-7. 9) occur 18 times a year, strong earthquakes ... volcanoes are known to have erupted on the earth's surface since historic times, and many more have erupted ...
By studying the different parts and actions of earthquakes, seismologists learn more about their effects and how to predict ground shaking in order to reduce damage. (On Shifting Ground p.109-110) Focus and Epicenter The point within the earth along the rupturing geological fault where an earthquake originates is called the focus, or hypocenter. The point on the earth?s surface directly above the focus is called the epicenter. Earthquake waves begin to radiate out from the focus and follow along the fault rupture. If the focus is near the surface between 0 and 70 km (0 and 40 mi.) deep shallow focus earthquakes are produced. If it is deep below the crust between 70 and 700 km (40 and 400 mi.) deep a deep focus earthquake will occur. Shallow-focus earthquakes tend to be larger, and therefore more damaging, earthquakes. This is because they are closer to the surface where the rocks are stronger and build up more strain. (The Ocean of Truth p.76 & The road to Jaramillo p.94-97) Seismologists know from observations that most earthquakes originate as shallow-focus earthquakes and most of them occur near plate boundaries areas where the earth?s crustal plates move against each other.
Other earthquakes, including deep-focus earthquakes, can originate in subduction zones, where one tectonic plate subducts, or moves under another plate. (The Ocean of Truth p.54-56) I Faults Stress in the earth?s crust creates faults places where rocks have moved and can slip, resulting in earthquakes. The properties of an earthquake depend strongly on the type of fault slip, or movement along the fault, that causes the earthquake. Geologists categorize faults according to the direction of the fault slip. The surface between the two sides of a fault lies in a plane, and the direction of the plane is usually not vertical; rather it dips at an angle into the earth. When the rock hanging over the dipping fault plane slips downward into the ground, the fault is called a normal fault. When the hanging wall slips upward in relation to the bottom wall, the fault is called a reverse fault or a thrust fault. Both normal and reverse faults produce vertical displacements, or the upward movement of one side of the fault above the other side, that appear at the surface as fault scarps. Strike slip faults are another type of fault that produce horizontal displacements, or the side by side sliding movement of the fault, such as seen along the San Andreas fault in California.
The Essay on Water Ice Mars Surface Earth
Mars (Greek: Ares) is the god of War. The planet probably got this name due to its red color; Mars is sometimes referred to as the Red Planet. (An interesting side note: the Roman god Mars was a god of agriculture before becoming associated with the Greek Ares; those in favor of colonizing and terra forming Mars may prefer this symbolism. ) The name of the month March derives from Mars. Mars has ...
Strike-slip faults are usually found along boundaries between two plates that are sliding past each other. (Plate Tectonics p.49-53) II Waves The sudden movement of rocks along a fault causes vibrations that transmit energy through the earth in the form of waves. Waves that travel in the rocks below the surface of the earth are called body waves, and there are two types of body waves: primary, or P, waves, and secondary, or S, waves. The S waves, also known as shearing waves, cause the most damage during earthquake shaking, as they move the ground back and forth. (Plate tectonics p.133) Earthquakes also contain surface waves that travel out from the epicenter along the surface of the earth. Two types of these surface waves occur: Rayleigh waves, named after British physicist Lord Rayleigh, and Love waves, named after British geophysicist A. E. H. Love. Surface waves also cause damage to structures, as they shake the ground underneath the foundations of buildings and other structures. Body waves, or P and S waves, radiate out from the rupturing fault starting at the focus of the earthquake. P waves are compression waves because the rocky material in their path moves back and forth in the same direction as the wave travels alternately compressing and expanding the rock.
P waves are the fastest seismic waves; they travel in strong rock at about 6 to 7 km (4 mi.) per second. P waves are followed by S waves, which shear, or twist, rather than compress the rock they travel through. S waves travel at about 3.5 km (2 mi.) per second. S waves cause rocky material to move either side to side or up and down perpendicular to the direction the waves are traveling, thus shearing the rocks. Both P and S waves help seismologists to locate the focus and epicenter of an earthquake. As P and S waves move through the interior of the earth, they are reflected and refracted, or bent, just as light waves are reflected and bent by glass. Seismologists examine this bending to determine where the earthquake originated. (Encarta 98) On the surface of the earth, Rayleigh waves cause rock particles to move forward, up, backward, and down in a path that contains the direction of the wave travel. This circular movement is somewhat like a piece of seaweed caught in an ocean wave, rolling in a circular path onto a beach. The second type of surface wave, the Love wave, causes rock to move horizontally, or side to side at right angles to the direction of the traveling wave, with no vertical displacements.
The Essay on When There Is A Sudden Breaking Of Rocks Within
... waves. A crack or fracture in the earth's surface is called a fault; motion along a fault normally occurs and this movement can cause earthquakes ... volcano.A fault marks the boundary between two plates. There are three main types of faults. A divergent fault occurs when two plates are ... Dalrymple made a successful method of determining ages of rocks collected from all over the world, and eventually ...
Rayleigh and Love waves always travel slower than P waves and usually travel slower than S waves. (The Floor of the Sea p.76-78, 112-115) III CAUSES Most earthquakes are caused by the sudden slip along geologic faults. The faults slip because of movement of the earth?s tectonic plates. This concept is called the elastic rebound theory. The rocky tectonic plates move very slowly, floating on top of a weaker rocky layer. As the plates collide with each other or slide past each other, pressure builds up within the rocky crust. Earthquakes occur when pressure within the crust increases slowly over hundreds of years and finally exceeds the strength of the rocks. Earthquakes also occur when human activities, such as the filling of reservoirs, increase stress in the earth?s crust. (Encarta 98) ELASTIC REBOUND THEORY In 1911 American seismologist Harry Fielding Reid studied the effects of the April 1906 California earthquake. He proposed the elastic rebound theory to explain the generation of earthquakes that occur in tectonic areas, usually near plate boundaries. This theory states that during an earthquake, the rocks under strain suddenly break, creating a fracture along a fault. When a fault slips, movement in the crustal rock causes vibrations.
The slip changes the local strain out into the surrounding rock. The change in strain leads to aftershocks, which are produced by further slips of the main fault or adjacent faults in the strained region. The slip begins at the focus and travels along the plane of the fault, radiating waves out along the rupture surface. On each side of the fault, the rock shifts in opposite directions. The fault rupture travels in irregular steps along the fault; these sudden stops and starts of the moving rupture give rise to the vibrations that propagate as seismic waves. After the earthquake, strain begins to build again until it is greater than the forces holding the rocks together, then the fault snaps again and causes another earthquake. (Plate tectonics p.56-59) DISTRIBUTION Seismologists have been monitoring the frequency and locations of earthquakes for most of the 20th century. They have found that the majority of earthquakes occur along plate tectonic boundaries, while there are relatively few intraplate earthquakes, that occur within a tectonic plate. The categorization of earthquakes is related to where they occur, as seismologists generally classify naturally occurring earthquakes into one of two categories: interplate and intraplate.
The Dissertation on Earthquake Performance of Reinfoced Concrete Buildings
... estimated at 100 kg/cm2; nevertheless, when earthquakes occur and buildings fall apart, this value can plummet ... allowed to be built on active faults. Many buildings were not engineered, but built according ... of the 1997 California Code for both rock and firm soil sites. Recognizing that the ... iron-plated immunity “. (Gulkan, 2001). Experience is one of the important need for earthquake engineering. ...
Interplate earthquakes are the most common; they occur primarily along plate boundaries. Intraplate earthquakes occur within the plates at places where the crust is fracturing internally. Both interplate and intraplate earthquakes may be caused by tectonic or volcanic forces. (Naked Earth p.134-135) I Tectonic Earthquakes Tectonic earthquakes are caused by the sudden release of energy stored within the rocks along a fault. The released energy is produced by the strain on the rocks due to movement within the earth, called tectonic deformation. The effect is like the sudden breaking and snapping back of a stretched elastic band. (The Ocean of truth p.122) II Volcanic Earthquakes Volcanic earthquakes occur near active volcanoes but have the same fault slip mechanism as tectonic earthquakes. Volcanic earthquakes are caused by the upward movement of magma under the volcano, which strains the rock locally, and leads to an earthquake. As the fluid magma rises to the surface of the volcano, it moves and fractures rock masses and causes continuous tremors that can last up to several hours or days. Volcanic earthquakes occur in areas that are associated with volcanic eruptions, such as in the Cascade Mountain Range of the Pacific Northwest, Japan, Iceland, and at isolated hot spots such as Hawaii.
The Term Paper on Caused By The Earthquake People Afghanistan Buildings
Contents Page. Page 1 - Introduction to report and Afghanistan Earthquake. Page 2 - Afghanistan Earthquake Page 3 - Afghanistan and beginning of Colombian Earthquake Page 4 - Colombian Earthquake Page 5 - Colombian Earthquake and Start of comparison Page 6 - comparison and Preparation Page 7 - Preparation and explanation of Richter Scale. 15/5/02 Think We " re Badly Off? Think again, recently hit ...
(Plate tectonics p.74) LOCATIONS Seismologists use global networks of seismographic stations to accurately map the focuses of earthquakes around the world. After studying the worldwide distribution of earthquakes, the pattern of earthquake types, and the movement of the earth?s rocky crust, scientists proposed that plate tectonics, or the shifting of the plates as they move over another weaker rocky layer, was the main underlying cause of earthquakes. The theory of plate tectonics arose from several previous geologic theories and discoveries. Scientists now use the plate tectonics theory to describe the movement of the earth’s plates and how this movement causes earthquakes. They also use the knowledge of plate tectonics to explain the locations of earthquakes, mountain formation, deep ocean trenches, and predict which areas will be damaged the most by earthquakes. It is clear that major earthquakes occur most frequently in areas with features that are found at plate boundaries: high mountain ranges and deep ocean trenches. Earthquakes within plates, or intraplate tremors, are rare compared with the thousands of earthquakes that occur at plate boundaries each year, but they can be very large and damaging.
(On shifting ground p.17-19) Earthquakes that occur in the area surrounding the Pacific Ocean, at the edges of the Pacific plate, are responsible for an average of 80 percent of the energy released in earthquakes worldwide. Japan is shaken by more than 1000 tremors greater than 3.5 in magnitude each year. The western coasts of North and South America are very also active earthquake zones, with several thousand small to moderate earthquakes each year. (U.S.G.S.) Intraplate earthquakes are less frequent than plate boundary earthquakes, but they are still caused by the internal fracturing of rock masses. The New Madrid, Missouri, earthquakes of 1811 and 1812 were extreme examples of intraplate seismic events. Scientists estimate that the three main earthquakes of this series were about magnitude 8.0 and that there were at least 1500 aftershocks. (The ocean of truth p.67-69) EFFECTS Ground shaking leads to landslides and other soil movement. These are the main damage causing events that occur during an earthquake. Primary effects that can accompany an earthquake include property damage, loss of lives, fire, and tsunami waves. Secondary effects, such as economic loss, disease, and lack of food and clean water, also occur after a large earthquake.
The Term Paper on Haiti Earthquake
... the human history. For the regions with high seismic activity, 7. 0-8. 0-magnitude earthquakes are a norm of life which, despite ... cause much additional damage but became the source of serious emotional effects. The Haiti earthquake was another good lesson to seismologists, and once ... feet: he saw a 400sq m house collapsing on the ground, with people trying to pull an elderly woman out of ...
(On shifting ground p.47) Ground Shaking and Landslides Earthquake waves make the ground move, shaking buildings and structures and causing poorly designed or weak structures partially or totally collapse. The ground shaking weakens soils and foundation materials under structures and causes dramatic changes in fine-grained soils. During an earthquake, water-saturated sandy soil becomes like liquid mud, an effect called liquefaction. Liquefaction causes damage as the foundation soil beneath structures and buildings weakens. Shaking may also dislodge large earth and rock masses, producing dangerous landslides, mudslides, and rock avalanches that may lead to loss of lives or further property damage. (The road to Jaramillo p.43-45) REDUCING DAMAGE Earthquakes cannot be prevented, but the damage they cause can be greatly reduced with communication strategies, proper structural design, emergency preparedness planning, education, and safer building standards. In response to the tragic loss of life and great cost of rebuilding after past earthquakes, many countries have established earthquake safety and regulatory agencies. These agencies require codes for engineers to use in order to regulate development and construction.
Buildings built according to these codes survive earthquakes better and ensure that earthquake risk is reduced. (On shifting ground p.56) Tsunami early-warning systems can prevent some damage because tsunami waves travel at a very slow speed. Seismologists immediately send out a warning when evidence of a large undersea earthquake appears on seismographs. Tsunami waves travel slower than seismic P and S waves in the open ocean, they move about ten times slower than the speed of seismic waves in the rocks below. This gives seismologists time to issue tsunami alerts so that people at risk can evacuate the coastal area as a preventative measure to reduce related injuries or deaths. Scientists radio or telephone the information to the Tsunami Warning Center in Honolulu and other stations.(The floor of the sea p.59) Engineers minimize earthquake damage to buildings by using flexible, reinforced materials that can withstand shaking in buildings. Since the 1960s, scientists and engineers have greatly improved earthquake resistant designs for buildings that are compatible with modern architecture and building materials. They use computer models to predict the response of the building to ground shaking patterns and compare these patterns to actual seismic events, such as in the 1994 Northridge, California, earthquake and the 1995 K?be, Japan, earthquake.
They also analyze computer models of the motions of buildings in the most hazardous earthquake zones to predict possible damage and to suggest what reinforcement is needed. (Martin Alfred p.62) Structural Design Geologists and engineers use risk assessment maps, such as geologic hazard and seismic hazard zoning maps, to understand where faults are located and how to build near them safely. Engineers use geologic hazard maps to predict the average ground motions in a particular area and apply these predicted motions during engineering design phases of major construction projects. Engineers also use risk assessment maps to avoid building on major faults or to make sure that proper earthquake bracing is added to buildings constructed in zones that are prone to strong tremors. They can also use risk assessment maps to aid in the retrofit, or reinforcement, of older structures. (The ocean of truth p.23) In urban areas of the world, the seismic risk is greater in non-reinforced buildings made of brick, stone, or concrete blocks because they cannot resist the horizontal forces produced by large seismic waves. Fortunately, single-family timber-frame homes built under modern construction codes resist strong earthquake shaking very well.
Such houses have laterally braced frames bolted to their foundations to prevent separation. Although they may suffer some damage, they are unlikely to collapse because the strength of the strongly jointed timber-frame can easily support the light loads of the roof and the upper stories even in the event of strong vertical and horizontal ground motions.(On shifting groung p.73) Emergency Preparedness Plans Earthquake education and preparedness plans can help significantly reduce death and injury caused by earthquakes. People can take several preventative measures within their homes and at the office to reduce risk. Supports and bracing for shelves reduce the likelihood of items falling and potentially causing harm. Maintaining an earthquake survival kit in the home and at the office is also an important part of being prepared. (On shifting ground p.97) In the home, earthquake preparedness includes maintaining an earthquake kit and making sure that the house is structurally stable. The local chapter of the American Red Cross is a good source of information for how to assemble an earthquake kit. During an earthquake, people indoors should protect themselves from falling objects and flying glass by taking refuge under a heavy table.
After an earthquake, people should move outside of buildings, assemble in open spaces, and prepare themselves for aftershocks. They should also listen for emergency bulletins on the radio, stay out of severely damaged buildings, and avoid coastal areas in the event of a tsunami. (The floor of the sea p.46) In many countries, government emergency agencies have developed extensive earthquake response plans. In some earthquake hazardous regions, such as California, Japan, and Mexico City, modern strong motion seismographs in urban areas are now linked to a central office. Within a few minutes of an earthquake, the magnitude can be determined, the epicenter mapped, and intensity of shaking information can be distributed via radio to aid in response efforts.(The floor of the sea p.18) STUDYING EARTHQUAKES Seismologists measure earthquakes to learn more about them and to use them for geological discovery. They measure the pattern of an earthquake with a machine called a seismograph. Using multiple seismographs around the world, they can accurately locate the epicenter of the earthquake, as well as determine its magnitude, or size, and fault slip properties. (Alfred Wegener & encarta 98) I Measuring Earthquakes An analog seismograph consists of a base that is anchored into the ground so that it moves with the ground during an earthquake, and a spring or wire that suspends a weight, which remains stationary during an earthquake.
In older models, the base includes a rotating roll of paper, and the stationary weight is attached to a stylus, or writing utensil, that rests on the roll of paper. During the passage of a seismic wave, the stationary weight and stylus record the motion of the jostling base and attached roll of paper. The stylus records the information of the shaking seismograph onto the paper as a seismogram. Scientists also use digital seismographs, computerized seismic monitoring systems that record seismic events. Digital seismographs use re-writeable, or multiple-use, disks to record data. They usually incorporate a clock to accurately record seismic arrival times, a printer to print out digital seismograms of the information recorded, and a power supply. Some digital seismographs are portable; seismologists can transport these devices with them to study aftershocks of a catastrophic earthquake when the networks upon which seismic monitoring stations depend have been damaged. (Plate Tectonics p.56-58, 64) There are more than 1000 seismograph stations in the world. One way that seismologists measure the size of an earthquake is by measuring the earthquake?s seismic magnitude, or the amplitude of ground shaking that occurs.
Seismologists compare the measurements taken at various stations to identify the earthquake?s epicenter and determine the magnitude of the earthquake. This information is important in order to determine whether the earthquake occurred on land or in the ocean. It also helps people prepare for resulting damage or hazards such as tsunamis. When readings from a number of observatories around the world are available, the integrated system allows for rapid location of the epicenter. At least three stations are required in order to triangulate, or calculate, the epicenter. Seismologists find the epicenter by comparing the arrival times of seismic waves at the stations, thus determining the distance the waves have traveled. Seismologists then apply travel-time charts to determine the epicenter. With the present number of worldwide seismographic stations, many now providing digital signals by satellite, distant earthquakes can be located within about 10 km (6 mi.) of the epicenter and about 10 to 20 km (6 to 12 mi.) in focal depth. Special regional networks of seismographs can locate the local epicenters within a few kilometers. (the Ocean of truth) . All magnitude scales give relative numbers that have no physical units.
The first widely used seismic magnitude scale was developed by the American seismologist Charles Richter in 1935. The Richter scale measures the amplitude, or height, of seismic surface waves. The scale is logarithmic, so that each successive unit of magnitude measure represents a tenfold increase in amplitude of the seismogram patterns. This is because ground displacement of earthquake waves can range from less than a millimeter to many meters. Richter adjusted for this huge range in measurements by taking the logarithm of the recorded wave heights. So, a magnitude 5 Richter measurement is ten times greater than a magnitude 4; while it is 10 x 10, or 100 times greater than a magnitude 3 measurement. (The floor of the sea p.89-91) Today, seismologists prefer to use a different kind of magnitude scale, called the moment magnitude scale, to measure earthquakes. Seismologists calculate moment magnitude by measuring the seismic moment of an earthquake, or the earthquake?s strength based on a calculation of the area and the amount of displacement in the slip. The moment magnitude is obtained by multiplying these two measurements. It is more reliable for earthquakes that measure above magnitude 7 on other scales that refer only to part of the seismic waves, whereas the moment magnitude scale measures the total size.
The moment magnitude of the 1906 San Francisco earthquake was 7.6; the Alaskan earthquake of 1964, about 9.0; and the 1995 K?be, Japan, earthquake was a 7.0 moment magnitude; in comparison, the Richter magnitudes were 8.3, 8.6, and 6.8, respectively for these tremors. (U.S.G.S.) Earthquake size can be measured by seismic intensity as well, a measure of the effects of an earthquake. Before the advent of seismographs, people could only judge the size of an earthquake by its effects on humans or on geological or human-made structures. Such observations are the basis of earthquake intensity scales first set up in 1873 by Italian seismologist M. S. Rossi and Swiss scientist F. A. Forel. These scales were later superseded by the Mercalli scale, created in 1902 by Italian seismologist Guiseppe Mercalli. In 1931 American seismologists H. O. Wood and Frank Neumann adapted the standards set up by Guiseppe Mercalli to California conditions and created the Modified Mercalli scale. Many seismologists around the world still use the Modified Mercalli scale to measure the size of an earthquake based on its effects. The Modified Mercalli scale rates the ground shaking by a general description of human reactions to the shaking and of structural damage that occur during a tremor.
This information is gathered from local reports, damage to specific structures, landslides, and peoples? descriptions of the damage. (The road to Jaramillo p.122) II Predicting Earthquakes Seismologists try to predict how likely it is that an earthquake will occur, with a specified time, place, and size. Earthquake prediction also includes calculating how a strong ground motion will affect a certain area if an earthquake does occur. Scientists can use the growing catalogue of recorded earthquakes to estimate when and where strong seismic motions may occur. They map past earthquakes to help determine expected rates of repetition. Seismologists can also measure movement along major faults using global positioning satellites (GPS) to track the relative movement of the rocky crust of a few centimeters each year along faults. This information may help predict earthquakes. Even with precise instrumental measurement of past earthquakes, however, conclusions about future tremors always involve uncertainty. This means that any useful earthquake prediction must estimate the likelihood of the earthquake occurring in a particular area in a specific time interval compared with its occurrence as a chance event.
(The ocean of truth p.29) The elastic rebound theory gives a generalized way of predicting earthquakes because it states that a large earthquake cannot occur until the strain along a fault exceeds the strength holding the rock masses together. Seismologists can calculate an estimated time when the strain along the fault would be great enough to cause an earthquake. As an example, after the 1906 San Francisco earthquake, the measurements showed that in the 50 years prior to 1906, the San Andreas fault accumulated about 3.2 meters (10 feet) of displacement, or movement, at points across the fault. The maximum 1906 fault slip was 6.5 meters (21 feet), so it was suggested that 50 years x 6.5 meters/3.2 meters, about 100 years, would elapse before enough energy would again accumulate to produce a comparable earthquake. (Plate Tectonics) Scientists have measured other changes along active faults to try and predict future activity. These measurements have included changes in the ability of rocks to conduct electricity, changes in ground water levels, and changes in variations in the speed at which seismic waves pass through the region of interest. None of these methods, however, has been successful in predicting earthquakes to date.
(U.S.G.S) Seismologists have also developed field methods to date the years in which past earthquakes occurred. In addition to information from recorded earthquakes, scientists look into geologic history for information about earthquakes that occurred before people had instruments to measure them. This research field is called paleoseismology. Seismologists can determine when ancient earthquakes occurred. (The floor of the sea p.118) Seismology, basically, the science of earthquakes, involving observations of natural ground vibrations and artificially generated seismic signals, with many theoretical and practical ramifications. A branch of geophysics, seismology has made vital contributions to understanding the structure of the earth?s interior. (Webster?s) SEISMIC PHENOMENA Different kinds of seismic waves are produced by the deformation of rock materials. A sudden slip along a fault, for example, produces both longitudinal push-pull (P) and transverse shear (S) waves. Compressional trains of P waves, set up by an quick push or pull in the direction of wave propagation, cause surface formations to shake back and forth. Sudden shear displacements move through materials with slower S-wave velocity as vertical planes shake up and down.
When P and S waves encounter a boundary such as Mohorovi?i? discontinuity (Moho), which lies between the crust and the mantle, they are partly reflected, refracted, and transmitted, breaking up into several other types of waves as they pass through the earth. Travel times depend on compressional and S-wave velocity changes as they pass through materials with different elastic properties. Crustal granitic rocks typically show P-wave velocities of 6 km/sec, where as underlying mafic and ultramafic rocks show velocities of 7 and 8 km/sec. In addition to P and S waves?body-wave types?two surface seismic waves are the Love waves, named for the British geophysicist Augustus E. H. Love, and Rayleigh waves, named after the British physicist John Rayleigh. These waves travel fast and are guided in their propagation by the earth?s surface. (Plate Tectonics p.142) INTRAMENTS OF STUDY Longitudinal, transverse, and surface seismic waves cause vibrations at points where they reach the earth?s surface. Seismic instruments have been designed to detect these movements through electromagnetic or optical methods. The main instruments, called seismographs, were perfected following the development by the German scientist Emil Wiechert of a horizontal seismograph about the turn of the century.
(Naked Earth p.36-42) Some instruments, such as the electromagnetic pendulum seismometer, employ electromagnetic recording; that is, induced tension passes through an electric amplifier to a galvanometer. A photographic recorder scans a rapidly moving film, making sensitive time-movement registrations. Refraction and reflection waves are usually recorded on magnetic tapes, which are readily adapted to computer analysis. Strain seismographs, employing electronic measurement of the change in distance between two concrete pylons about 30 m (100 ft.) apart, can detect compressional and extensional responses in the ground during seismic vibrations. The Benioff linear strain seismograph detects strains related to tectonic processes, those associated with propagating seismic waves, and tidal yielding of the solid earth. Still more recent inventions used in seismology include rotation seismographs; tiltmeters; wide frequency band, long-period seismographs; and ocean bottom seismographs. (Alferd Wegener p.118-120) Similar seismographs are deployed at stations around the world to record signals from earthquakes and underground nuclear explosions. The World Wide Standard Seismograph Network (WWSSN) incorporates some 125 stations.
(U.S.G.S.) Richter Who? Richter, Charles (1900-1985), American seismologist who wrote fundamental seismology texts, and who established an earthquake magnitude scale with German-American seismologist BenoGutenberg. (Encarta 98) Richter was born in Ohio but moved to Los Angeles as a child. He attended Stanford University and received his undergraduate degree in 1920. In 1928 he began work on his Ph.D. in theoretical physics from the California Institute of Technology (Caltech), but before he finished it, he was offered a position at the Carnegie Institute of Washington. At this point, he became fascinated with seismology. After he worked at the new Seismological Laboratory in Pasadena, under the direction of Beno Gutenberg. In 1932 Richter and Gutenberg developed a standard scale to measure the relative sizes of earthquake sources, called the Richter scale. In 1937 he returned to Caltech, where he spent the rest of his career, eventually becoming professor of seismology in 1952. Richter and Gutenberg also worked to locate and catalog major earthquakes and used them to study the deep interior of the earth. Together they wrote a very influential textbook, published in 1954, called Seismicity of the Earth.
In 1958 Richter published the textbook, Elementary Seismology, which many consider his greatest contribution to the field. Richter visited Japan on a Fulbright Fellowship in 1959-1960. (Encarta 98) Richter was also involved in public awareness and safety issues surrounding earthquakes, taking a sensible stance rather than using scare tactics. He was devoted to his work in science and learned several languages in order to read the global earthquake literature. Richter was so interested in earthquakes, he even installed a seismograph in his living room of his Los Angeles home. He influenced Los Angeles building codes that city officials credited with saving many lives in the 1971 earthquake in San Fernando, California. After retirement he continued to work on earthquake safety design. (Encarta 98) (PUT MONTH) EARTHQUAKE FINDINGS During the month of march we charted all of the bigger earthquakes that occurred . We charted the earthquakes measuring from 4 to 7 on the Richter scale. We plotted this data to see where most of the earthquakes would occur. Also to see how high most of the quakes would be on the scale. According to our analyses most of the earthquakes occurred around the plate boundaries.
Especially in South America along the South American plate and Mexico along the North American plate. Yet, to our surprise there weren?t many earthquakes whatsoever, along the boundary between the Eurasian plate and the African plate. We also found Seismic activity in some unusual areas like the arctic region above Europe and the Antarctic region. Most of the quakes we recorded were not generally large either. Most of them were recorded at 4 on the Richter scale. There were not many large earthquakes in the month of March. The largest quake we recorded was 6.8 in Xizang-India border region. We also found that there were an unusually high number of earthquakes in the month of March. From the data that we collected we noticed that earthquakes can also occur in the middle of the ocean. In conclusion from the data we have constructed we came to find out that large earthquakes are rare and far in between. We have come to realize how devastating earthquakes can really be to people and their surroundings. REFERENCES Kidd, J.S. & Kidd, R. A. (1997).
On shifting ground ?the story of continental drift?. New York: Facts on File, Inc. Erickson, J. (1992).
Plate Tectonics. New York: Facts on File, Inc. Glen, W. (1982).
The Road to Jaramillo. Stanford, California: The Stanford University Press. Menarld, H.W. (1986).
The Ocean of Truth. Princeton, N.J.: The Princeton University Press Suhwartzbach, M. (1986).
Alfred Wegener. Madison, Wisconsin.: Science Teck Inc. Vogel, S. (1992).
Naked Earth. New York: Dutton Books. Wertenbacher, W. (1974).
The Floor of the Sea. Boston Massachusetts.: Little Brown and Co. Internet. (1999).
wwwneic.cr.usgs.gov/neis/bulletin.html. Computer source.: Internet explorer. Apsell, P. S. (Producer).
(1990).
Nova Earthquake. [Video Tape]. Western Video
