An earthquake, one of the most destructive natural phenomena, consists of rapid vibrations of rock near the surface of the earth. It is the most terrifying of all natural phenomena and has brought 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 a 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.
The Greek word for “shaking,” and (when it applies to the earth) “earthquakes,” is seismos. The science of earthquakes, therefore, is called seismology The scientific study of earthquakes is comparatively new. Until around the 18th century, few accurate descriptions of earthquakes were 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 or the large blocks of the earth’s crust near the surface along lines called faults. When the stress that develops in the crust becomes too big for the rock to support, one side of vibrations that transmit waves which travel for long distances through the interior of the earth 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 vibrate, and are what we call earthquakes. The two main types of earthquake waves are P waves and S waves. P waves or primary waves are the fastest and can travel through solid and liquid materials. In other words, P wave is a fast body wave that travels through the interior 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. Due to the slow speed that S waves can travel, they can not pass through the liquid materials. 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. About 10 extremely 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 losses, which collapsed as a result of the quake.
The Catastrophic effects of earthquakes are generated in four main principal kinds: Ground motion, which damages or sometimes completely destroys building. 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. Most damages are caused by structural failures or by fire. Another major cause of destruction is landslides this is when large amounts of earth slide off a hillside. Because of theses, 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 have struck Hawaii and destroyed everything in their path. The destructiveness of quake is determined by such factors as velocity, acceleration period and duration or the earthquake waves. The size, shape, material and quality of construction of building are the most important factors to determine the range of destruction and consequently the number of deaths. Most fatalities attributed to earthquakes are caused by the collapse of buildings. At the beginning of this century, less than one out of three earthquakes with large magnitude caused at least one death. Unfortunately today, two out three earthquakes have fatal results.
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, the constant motion of earth’s surface causes earthquakes. 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 the twentieth century. The approach to protection is necessary 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.