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.
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Kidd, J.S. & Kidd, R. A. (1997). On shifting ground ?the story of continental drift?. New
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Erickson, J. (1992). Plate Tectonics. New York: Facts on File, Inc.
Glen, W. (1982). The Road to Jaramillo. Stanford, California: The Stanford University
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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
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