Ewarethquake Essay
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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...
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