An earthquake, one of the most destructive natural phenomena, consist of rapid vibrations of rock near the earth s surface. It is the most terrifying of all natural phenomena and brings fear since ancient times because of its sudden unpredictable occurrence and enormous capacity of destruction. A single shock usually last no more than a few seconds , although several quakes may last for as much as couple of minutes. The quake as felt on the surface is always the result never the cause of some geologic process although the damage done may be immense. Stamatopoulos 4 Causes The Greek word for "shaking", and (when it applies to the earth) "earthquake", is seismos. The science of earthquakes, therefore, is called seismology (Oxford Advanced learner s Dictionary). The scientific study of earthquakes is comparatively new. Until the 18th century, few accurate descriptions of earthquakes was recorded, and the natural cause of earthquakes was little understood. Although earthquake can be the result of sea-floor spreading, volcano eruptions and plate tectonics the great majority of earthquakes are caused by sudden movement of the large blocks of the Earth s crust less than 70 Km from the surface along lines called faults. (Earthquake," Microsoft Encarta 96 Encyclopedia. ) When the stress that develop in the crust become too big for the rock to support, one side of the fault slips on the other side. This movement causes vibrations that transmit waves which travel for long distances through Earth interior or along surface. The energy released by an earthquake moves in the form of waves through the rock which act as an elastic body. These Waves make the ground to vibrate, and are what we call earthquake. (Keller A. Edward. Environmental Geology , MacMillan, 6th edition, p151) The two main types of earthquake waves are P waves and S waves (photo 4). P waves or primary waves are the fastest and can travel through solid and liquid matirials. "The rate of propagation through granite mountains for P waves is approx. 5.5km/sec; The rate is much reduced through liquid materials. (water: 1,5 km/sec)" (Keller A. Edward. Environmental Geology , MacMillan, 6th edition 145-146). In other words, P wave is a fast body wave that travel through the intrerior of Earth and arrives first at the seismograph. On the other hand, S waves (secondary or shear waves) are slower body waves than P waves and have approximately velocity 3km/sec. Due to slow speed that S waves can travel, they can not pass through the liquid materials. Stamatopoulos 5 Earthquake magnitudes are expressed on the Richter scale, and measure the amount of energy released. Seismograph, an important geological instrument, record earthquake waves. According to this, scientists can predict what kind of waves will come up to surface and for how long. Of the million or so earthquakes per year, strong enough to be noticed, only few release enough energy to do serious damage.( Keller A. Edward. Environmental Geology , MacMillan, 6th .p146 ) About 10 really violent earthquakes occur each year on the average but only one of them happens to be involved in a populated urban area. " The most destructive earthquake or record occurred in 1556 in Shensi Province in China, where an estimated 830,000 people died. Those people lived in Cave dwellings excavated in loess which collapsed as a result of the quake. (Flint R. Foster and Skinner J. Brian. Physical Geology, 2nd edition, p395) Stamatopoulos 6 Effects The catastrophic effects of earthquakes are generated in four main principal kinds: Ground motion which damages or sometimes completely destroys buildings. Although, today, modern Engineering in order to prevent such damage has given many solutions to the problem, in very strong earthquakes there is no possible structure of safety. The ground motion, causes fires not only because it breaks the gas lines but also because it displaces the electrical wires and stoves. More than 90% of San Francisco building damage was caused by fire. "In regions of hills and steep slopes, earthquakes vibrations may cause regolith to slip and start rapid mass wasting movements."( Flint F. Richard and Skinner J. Brian. Physical Geology 2nd Edition, p394). As a result of these, almost all structures are destroyed by rapidly moving regolith. (layer of Earth) Finally the seismic sea waves causes movement of the sea floor. These waves are called tsunami. These huge waves struck Hawaii and destroyed 1500 houses and killed more than 150 people. (Flint F. Foster and Skinner J. Brian. Physical Geology, p394) The Destructiveness of quake is determined by such factors as velocity, accelaration period and duration of the earthquake waves. The size, shape, metirial and quality of construction of building is the most important factors to determine the range of destruction and consequently the number of deaths. " About 75% of fatalities attributed to earthquake are caused by the collapse of buildings." (Coburn Andrew and Spence Robin. Earthquake Protection, ,p6-7 ) At the beginning of this century less than one out of three earthquakes with large magnitude causes at least one death. Unfortunately today, two out of three earthquakes have fatal results. The San Francisco Earthquake of 1906 was one of the most destructive in the history of America. The Earthquake and the fire that Stamatopoulos 7 followed killed nearly 700 people and left the city with smashed ruins. It was one of the most significant earthquakes of the early years. That great earthquake that struck San Francisco a little after 5:00am on the morning of the 19, April, 1906 is in special interest because its study gave us the opportunity to formulate the elastic rebound theory of earthquake source. "This theory remains today as the principal model of the earthquake cycle" (http://quake.wr.usgs.gov/more/1906, "San Francisco Earthquake 1906", Internet Source ). Many buildings collapsed especially those on marshy or filled ground. Others buildings with solid rock was little damage by the shock. (photo 2). On the other hand big fire burned an area of 4.7 sq. miles, and destroyed many of building that left standing(photo 1). As a result of this more than 225,000 people out of 400,000 became homeless (photo 3). Although the loss of life could only be estimate, it probable exceed 700 direct deaths and more than 3000 indirect deaths. On the other hand, material loss was more that $400 million. The strong shaking with magnitude 7.7 7.9 R last 45 to 60 seconds. " The frequently quoted value of 700 deaths caused by the earthquake and fire is now believed to underestimate the total loss of life by a factor of 3 or 4" (http://quake.wr.usgs.gov/more/1906, "San Francisco Earthquake 1906," Internet Source.) Stamatopoulos 8 Conclusion An earthquake is a natural phenomenon like rain. Earthquakes have occurred for billions of years. Descriptions as old as recorded history show the significant effects they have had on people's lives. Long before there were scientific theories for the cause of earthquakes, people around the world created folklore to explain them. In simple terms, earthquakes are caused by the constant motion of Earth's surface. This motion creates buildup and release of energy stored in rocks at and near the Earth's surface. Earthquakes are the sudden, rapid shaking of the Earth as this energy is released. "However, the science and practice of how to protect ourselves, our buildings and our cities from earthquakes has also been developing during the second half of the twentieth century. The approach to protection is necessarily and requires a wide range of measures including public education programs, better building design and increasing quality of construction in the areas most likely to suffer an earthquake." (Coburn Andrew and Spence Robin. Earthquake Protection, p2)

Stamatopoulos 8 Work-Cited Coburn Andrew and Spence Robin. Earthquake Protection, New York: Wiley, 1992 Flint F. Richard and Skinner J. Brian. Physical Geology, 2nd edition Edward A. Keller, Environmental Geology, Macmillan: 2nd edition Oxford Advanced Learner s Dictionary , Oxford Univ. Press: 1992 Earthquake," Microsoft Encarta 96 Encyclopedia. 1993- 1995 Microsoft Corporation. All rights reserved. Funk & Wagnalls Corporation. All rights Internet site: "San Francisco Earthquake." http://Quake.wr.usgs.gov/ /gifts/ (excerpted from Ellsworth, 1990)

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