INTRODUCTION An earthquake is a sudden natural movement of a portion of the earth's crust that produces trembling or shaking of the ground. During earthquakes, rocks suddenly release energy. An earthquake originates from a particular region and spreads out in all directions. Among the different natural hazards that our earth is subjected to, an earthquake has the most devastating natural impact, resulting in huge loss of human life and material. Those of us who have experienced an earthquake know that there is a rumbling sound of doors windows rattle and beds and tables in the room shake. While standing, we feel the floor move away from under our feet. DEFINITION OF EARTHQUAKE The ground vibrations, both feeble and strong, produced on the surface of the earth due to any reason what so ever are described as earthquakes. Whenever these vibrations traverse, an earthquake is said to have taken place. It may also be defined as "Violent shaking of earth's surface due to abrupt release of large amount of energy that has accumulated over a long time inside the earth". It is important to mention here that about 90 percent of all earthquakes are caused naturally whereas 10 percent earthquakes are related to man made sources such as nuclear explosions, reservoir induced tremors, etc. EARTHQUAKE ENGINEERING OR SEISMOLOGY The branch of science that deals with the study of earthquake and the structure of the earth is termed as earthquake engineering or seismology. Seismology includes study of generation, propagation and recording of seismic waves that occur inside the earth along with the sources that produce them. The research and development carried out in the area of earthquakeresistant constructions is also a part of earthquake engineering. IMPORTANT TERMS ASSOCIATED WITH ENGINEERING SEISMOLOGY. Some of the important terms related to earthquake engineering are described below. (a) Focus or Seismic centre: The exact point or place inside the surface of the earth at which an earthquake originates is termed as focus or seismic centre. It is also termed as Hypocentre. It is actually a point on the fault plane where slip or movement of tectonic plate occurs. Most of the damaging earthquakes have shallow focus with focal depths less than 70 kilometers. Its position is determined with the help of seismographic records. (b) Epicentre: The point or line on the surface of the earth lying vertically above the focus is termed as 'epicentre' or 'epicentral line'. As earthquake cannot possibly originate at a mere point alone, therefore whole of the disturbed area inside the earth is 'focus' and as a consequence, the epicentre is a tract and not an isolated point located vertically above the focus. (c) Focal depth: The depth of focus from the epicentre is termed as focal depth or simply the vertical distance between epicentre and hypocentre is called focal depth. (d) Epicentral depth: The horizontal distance from the epicentre to any place of interest or discussion is termed as epicentral distance. Intensity of vibrations are more intense near the epicentre and as the epicentral distance increases, these become feeble and weak and ultimately die out. (e) Anticentre: The point on the surface of the earth diametrically opposite to the epicentre is called anticentre. In seismology, anticentre is of little importance, as focus is rarely determined accurately. Therefore, most frequently the term epicentre or epicentral area is indicated in earthquake engineering. (f) Seismic waves : The waves transmitted in all directions due to large strain energy released from the focus during an earthquake are termed as seismic waves. These are of two types, body waves and surface waves, which obey the laws of reflection and refraction as they pass through the earth materials of different densities. (g) Seismograph: An instrument which used to record ground vibrations or surface displacements is termed as a 'Seismograph'. These seismographs are used only to record weak motions and are unsuitable to record vibrations of very high intensity. For recording higher intensities, special strong motion instruments called accelerographs are employed. (i) Isoseismal: An imaginary line on the surface of the earth along which the intensity of a measured seismic shock is equal at every point is termed as 'isoseismal'. It is just like a contour line which joins points of same elevation. In an earthquake hit area, places of similar intensity can be easily marked and a line joining these points is an 'isoseismal line'. (j) Isoseismal map: A map showing different isoseismals for a particular earthquake is termed as an isoseismal map. Such an isoseismal map is quiet helpful to scientists and engineers in the development of seismic zones and is used to carry out long term planning on earthquake preparedness in the absence of earthquake prediction. (k) Seismic zones (Seismic belt): The regions of earth's crust where earthquakes occur frequently and repeatedly are termed as seismic zones or seismic belt. Depending on the intensity of seismic activity, these zone are further divided into highly seismic, moderately seismic and poorly seismic zones. The Himalayan ranges and the Indo-gangetic plains are the areas which are considered as highly seismic zones. (l) Fault: A fracture or a crack along which blocks of earth's mass move relatively on either side parallel to the fracture is termed as a fault. This sliding of earth's mass takes place in pieces called "Tectonic plates'. This sudden slip of rock masses along the fault plane releases large strain energy stored in the interface rocks generating seismic shocks. It is important to note that the energy released during 2001 Bhuj earthquake (Gujrat, India) was 200 times of that of energy released by Atom bomb dropped on Hiroshima (Japan) in the year 1945. (m) Active fault: A fault is termed as an active, if it is likely to have another earthquake or seismic shock in the near future. Remember that all faults are considered as active if they have caused movement of blocks of crust once or more times in the past. The term 'aseismic refers to a fault where no earthquakes or seismic shocks have been observed. (n) Foreshocks and Aftershocks: Foreshocks are defined as relatively smaller earthquakes or minor seismic shocks that precede the major tectonic earthquake known as "main shock. Foreshocks are generally caused by small ruptures or plastic deformations. "Aftershocks are defined as earthquakes of smaller intensity that follow the mainshock. Aftershocks generally occur in sequence and continue for a long period due to fresh ruptures or readjustments of the fractured mass. (0) Tsunamis : Tsunamis refer to giant sea waves or harbour waves. It is a Japanese term composed of two words "Tsu' meaning harbour and 'nami' meaning wave. Tsunami are a series of large sea waves caused by earthquake or seismic activity near the coastal regions or at the ocean floor. (p) Earthquake hazard: Any phenomenon associated with an earthquake such as Tsunamis, landslides, groundshaking, surface faultings, and ground deformation etc. that may affect the normal life of people is termed as earthquake hazard. NECESSITY OF EARTHQUAKE ENGINEERING (SIGNIFICANCE OF SEISMOLOGY) The current seismic zone map indicates that more than 60 percent of the land area in India is considered prone to earthquakes. Many recent earthquakes of high magnitude which occurred in India during the last decade sound an alarming bell that the seismic risk in the country has been increasing day by day. Most deaths during an earthquake are caused by collapse of man made structures. The basic knowledge of civil engineering and structural engineering without any exposure to earthquake engineering is not sufficient to build earthquake resistant structures. Hence the most important step towards an earthquake-resistant India is to train the practicing professional engineers in the subject of earthquake engineering. The knowledge of earthquake engineering is essential for a civil engineer to enable him to build earthquake resistant constructions that can appropriately withstand earthquakes. The vast devastation in Gujrat during the Bhuj earthquake of 2001, clearly shows that seismic design, detailing and related earthquake resistant practices are not being followed in the construction of buildings and other structures. The inadequate preparedness of the country to face damaging earthquakes is due to poor knowledge of science of seismology. Thus a basic knowledge of earthquake engineering' is needed to make a significant impact towards earthquake safety in our country. The knowledge of earthquake engineering or seismology is thus essential: (i) To design and build earthquake resistant structures for earthquake safety. (ii) To carry out advanced research and development for effective earthquake management. (iii) For earthquake preparedness f.e. to learn how to prepare facing earthquakes. (iv) For seismic evaluation and retrofitting. The term retrofitting refers to upgrading the strength and structural capacity of a seismically deficient building to enable it to safely withstand the effect of seismic shocks in future. CAUSES OF EARTHQUAKES We all know that earthquakes originate due to a sudden impact on the body of the earth. When the impact is of lighter intensity, milder or feeble vibrations are set up, whereas a heavier impact generates strong tremors having disastrous effects. The causes of earthquakes may be broadly classified into following three categories. 1. Superficial or surface causes 2. Volcanic causes 3. Tectonic causes 1. Superficial causes (surface causes): Earthquakes of mild intensity which occur over the ground surface caused by the dynamic agencies operating upon the surface of the earth are termed as Superficial ones. The various surface causes producing seismic tremors of low intensity are enlisted below: (a) A huge landslide or a rock fall along hill slopes. (b) Giant sea waves and crashing breakers along sea shores. (c) Running water, descending falls and cascades upon valley floor. (d) Heavy vibrating machinery in Industrial area. (e) Movement of locomotives and other heavy vehicles on earth surface. (f) Man made explosions and other nuclear tests. (g) Mining blasts in mining areas. heavy excavations causing land subsidence, which sets minor tremors in (h) Large scale heavy excavations causing land subsidence, which sets minor tremors in the vicinity. Some of the other microseismic causes producing feeble seismic shocks are due to cultural noise or disturbed vibrations generated by Industry and the traffic. From the causes enlisted above, it is quiet clear that the superficial agencies initiate earthquakes no doubt, but very few of them are strong enough to be perceptible to our senses and none of them can possible have any destructive effect on human life. 2. Volcanic causes: Volcanic activities taking place in different parts of the world, oftenly produce volcanic outbursts or explosions during which the surface of the earth trembles. The impact thus generated is sometimes so strong that it causes earthquakes or seismic shocks in the nearby areas. But however it is not necessary to be concluded that all sorts of volcanic eruptions on earth produce earthquakes. There are number of volcanoes which erupt frequently and yet initiate no earthquakes on the earth's surface. People living in countries like Japan, Italy, Newzealand, Iceland, Java, Sumatra etc. have enough experience of such volcanic activities. Volcanic earthquakes, caused by high pressures exerted by movement of hot molten lava known as magma with in the earth's crust are generally shallow earthquakes of mild intensity. In India, there are no active volcanoes. However, the peninsular and extra-peninsular India has observed volcanic activity on a large scale in the past, some millions of years back. There are even traces of dormant and extinct volcanoes in the islands of Bay of Bengal and Andaman regions. (An active volcano is one which erupts frequently whereas extinct volcano shows no volcanic activity one with rare eruption is called dorment volcano) 3. Tectonic causes: Tectonic causes are those which originate within the earth's crust and are necessarily associated with the relative movements of rock masses forming the crust of the earth. The seismic shocks which occur due to sudden release of enormous strain owing to crustal movements are termed as tectonic earthquakes. It is an established fact that all major earthquakes causing large scale devastation on the surface of the earth are of tectonic origin. Tectonic earthquakes occur due to : (i) (ii) Displacement or rupturing of rock masses along pre existing cracks or faults. Development of new fault planes. The term 'tectonic' relates to structural distortions occurring in the earth's crust caused by upheavals and other movements within it, whereas 'plate tectonics' refers to interrelating movements of the rigid plates or sections that make up the earth's crust, riding on the semi-molten rock mass of the interior. The rocks forming the earth's crust when subjected to stress, yield only upto a certain limit. However, when the accumulating stress increases beyond the said limit, the rocks are subjected to fracturing and the fractured blocks suffer relative displacement accompanied by enormous strain released by rupturing of blocks along confined regions. This produces vibrations giving rise of earthquakes whose magnitude depends upon the amount of strain energy released. The disastrous Bhuj earthquake which occurred in Gujrat (India), in the year 2001 was also of tectonic origin.