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EARTHQUAKE 1 WHAT IS AN EARTHQUAKE An earthquake is a vibration or oscillation of the surface of the earth, ranging from a faint tremor to a wild motion caused by a transient disturbance of the elastic or gravitational equilibrium of the rocks at or beneath the earth the surface. It forms energy of wave motion transmitted through the surface layer of the earth in widening circles from a point of sudden energy release. 2 INTERIOR OF THE EARTH Thus, on the basis of seismic investigation, the earth can be divided into three major zones, these are Crust Mantle Core 3 CAUSES OF THE EARTHQUAKE Earthquakes are caused mainly due to disequilibrium in any part of the crust of the earth. A number of causes have been assigned to cause disequilibrium or isostatic imbalance in the earth’s crust such as: Vulcanicity Faulting & elastic rebound theory Hydrostatic pressure and anthropogenic causes Plate tectonic theory 4 NATURE OF EARTHQUAKE When a stone is thrown into the pool, waves spread through the water in all directions. Similarly, waves spread out through the earth when there is a sudden localized disturbance of the rock. An earthquake is one such type of disturbance. When an earthquake occurs, the fracturing rocks emit SEISMIC WAVES, which radiates in all directions from the focus. Waves: These are of two types: Body waves & Surface Waves 5 Body Waves: The body waves are two types: P-Waves: Primary waves. These are fastest, compressional and capable of propogating through both, the solid and liquid media S-Waves: Secondary waves. These are transverse, comparatively slower and pass only through solid media 6 Surface Waves: The body waves are two types: L-waves: Shear motion in horizontal plane. Rayleigh waves: Motion in vertical plane. Surface Waves are more disastrous than the Body Waves. 7 Shocks The earthquake vibrations shake the ground even hundreds of kilometers away. The shaking is strongest at its source. When the vibrations are felt in the bedrock and ground, they are called shocks. Some earthquakes are preceded by smaller foreshocks, and larger earthquakes are always followed by aftershocks. 8 Focus & Epicenter EPICENTRE Earth’s surface FOCUS MANTLE The point where the earthquakes are generated is called focus. The point on the earth’s surface, directly above the focus, is called the epicenter . 9 CLASSIFICATION OF THE EARTHQUAKE Each earthquake differs from the other and thus it becomes difficult to classify all the earthquakes in certain categories. Inspite of these limitations earthquakes are classified on the basis of common characteristics as given below: 1. On the basis of causative factors: a. Natural earthquakes: Natural earthquakes are those which are caused by natural processes i.e. due to endogenetic forces. These are further divided into four subcategories 10 Volcanic earthquakes Tectonic earthquakes Isostatic earthquakes Plutonic earthquakes b. Artificial or man-induced earthquake or anthropogenic earthquakes: Artificial or man-induced earthquake or anthropogenic earthquakes are caused by human activities such as pumping of water and mineral oil from underground aquifers and oil reserves respectively, deep underground mining, blasting of rocks by dynamites for constructional purposes e.g. for the construction of dams and reservoirs, roads etc., nuclear explosion, storage of huge volume of water in big reservoirs etc. 11 Examples, 1931 earthquakes of Greece due to Marathon Dam, 1936 earthquakes of Hoover Dam (USA) due to Lake Mead, Koyna earthquake (Maharastra, India) of 1967 due to Koyna reservoir etc. 2. On the basis of focus: Guttenberg has divided the world seismic centres on the basis of the depths of their foci into 3 types viz. Shallow focus earthquakes – foci are located at the depths from the ground surface to 70 km, Intermediate focus earthquakes – foci at the depths between 70 km to 300 km, and Deep focus earthquakes – foci at the depths between 300 km to 700 km. 12 3. On the basis of human causalities: Earthquakes are grouped into 3 categories on the basis of their hazardous impacts in terms of human casualities. Moderately hazardous earthquakes – when human deaths caused by severe seismic tremors are below 50,000 mark, Highly hazardous earthquakes causing human deaths ranging between 51,000 to 1,00,000 mark, and Most hazardous earthquakes causing human casualities above 1,00,000 mark. 13 INTENSITY OF THE EARTHQUAKE It is based on the degree of damage occurred at the Earth’s surface and on the people’s reaction to the shock. Italian seismologists Prof. Guiseppe Mercalli proposed the following scale to measure the intensity of an earthquake. Mercalli Intensity Effects I Instrumental Detected only by seismograph II Feeble Noticed only by sensitive people Richter Magnitude 3.5 to 4.2 14 III Slight Like the vibrations due to 3.5 to 4.2 a passing lorry; felt by people at rest, especially an upper floors IV Moderate Felt by people while walking, rocking of loose objects, including standing vehicles V Rather strong Felt generally: most sleepers are awakened and bells ring VI Strong Trees away and all suspended objects swing, damage by overturning and falling of loose objects 4.3 to 4.8 4.9 to 5.4 15 VII Very strong General alarm, wall cracks, plaster falls VIII Destructive Car drivers seriously disturbed, masonry fissured, chimneys fall, poorly constructed buildings damaged IX Ruinous Some houses collapse where ground begins to crack, and pipes break open X Disastrous Ground cracks badly, many buildings destroyed and railways lines bent, landslides on steep slopes 5.5 to 6.1 6.2 to 6.9 7 to 7.3 16 XI Very disastrous Few buildings remain standing, bridges destroyed, all services (railways, pipes and cables out of action), great landslides and floods 7.4 to 8.1 XII Catastrophic Total destruction, objects thrown into air, ground rises and fall in waves >8.1 (maximum known 8.9) 17 MAGNITUDE OF THE EARTHQUAKE It is a numerical scale of magnitudes from 0 to 9, proposed by Prof. Charles Richter of California Institute of Technology in 1935. The idea of magnitude was based on observed surface movements. The magnitude of an earthquake is a measure of the amount of energy released. Earthquake magnitudes are based on direct measurements of the size (amplitude) of seismic waves, made with recording instruments (seismograph), rather than on subjective observations of destruction. The total energy released by an earthquake can be calculated from the amplitude of the waves and distance from the epicenter. 18 EFFECTS OF THE EARTHQUAKE Loss of life and property Landslides and Avalanches Development of fissures and cracks Disturbances of groundwater circulation and drainage pattern Fires Upwarp and downwarp of the crust TSUNAMIS 19 WORLD DISTRIBUTION Earthquakes are not randomly distributed over the globe but they tend to occur in narrow continuous belt. The most notable aspects of the distribution of earthquakes in the world is their locating along a narrow belt. There are three well-defined belts or zones of seismic activity in the world, where most earthquakes originate are identified. These earthquake zones are: The Circum-Pacific Belt The Mediterranean and Trans-Asiatic Belt The Mid-oceanic Ridges and the African rift system Belt 20 INDIA AND EARTHQUAKE Seismic zonation map of country is guide to the earthquake vulnerability of the region. India has been divided into four zones with respect to severity of earthquakes. Of this zone V is seismically the most active and zone II is the least active. Delhi comes under zone IV of seismic hazard map. Asiatic and Indian plates where the process of mountain building is still in progress due to the Asiatic plate is moving southward whereas the Indian plate is moving northward and hence the northern margin of the Indian plate is being subducted below the Asian plate. The collision of Asiatic and Indian plates and resultant subduction of Indian plate and consequent folding and faulting and gradual rise of the Himalayas at the rate of 50 mm per year cause earthquakes of northern India, Tibet and Nepal 21 Seismic Zoning Map of India Resource Developemnt Centre, New Delhi 22 PREDICTION OF AN EARTHQUAKE The Astrologers and Mystics are the people who predict an Earthquake, which has no scientific basis. There is no scientific way to predict an earthquake. However, continuous recording of small - scale earthquakes is sometimes useful in prediction of a major earthquake. 23 HOW CAN I PREPARE FOR AN EARTHQUAKE Learn first aid procedures. Keep emergency items on hand, including first aid kit, supply of important medicines, flashlight, battery-powered radio, batteries, some drinking water and nonperishable food, tools to shut off utilities, and a fire extinguisher. Plan to be self sufficient for a minimum of 72 hours after an earthquake or other major disaster. Conduct an earthquake safety check of your house periodically Develop a family earthquake plan and communication plan for those first hours after an earthquake. 24 Learn how to shut off all utilities in your home. Avoid placing heavy objects on shelves or heavy pictures on walls where they could fall during a quake. Use bolts or straps to secure heavy items that might topple over, such as a bookcase, china cabinet, or water heater. Avoid placing heavy objects on shelves or heavy pictures on walls where they could fall onto a bed. Keep flammable or hazardous liquids such as paints, pest sprays, or cleaning products in cabinets or secured on lower shelves. 25 WHAT SHOULD I DO DURING AN EARTHQUAKE If you’re Indoors, duck under a desk or sturdy table or stand in a corner. If not near a desk or table, move against an interior wall, and project your head with your arms. Stay away from windows, bookcases, file cabinets, heavy mirrors, hanging plants, and other heavy objects that could fall. Watch out for falling plaster or ceiling tiles. Stay under cover until the shaking stops. In a high-rise building, do not use the elevators. Do not be surprised if the fire alarm or sprinkler systems activate. Outdoors, move to a clear area, away from trees, signs, buildings, or downed electrical wires and poles. 26 On a sidewalk near buildings, and there is no safe haven from overhead wires or other hazards, duck into a doorway to protect yourself from falling bricks, glass, plaster, and other debris. Driving, pull over to the side of the road and stop. Avoid overpasses, power lines, and other hazards. Stay inside the vehicle until the shaking is over. In a crowded store or other public place, do not rush for exits. Once the shaking stops, crowds trying to rush to the exits may be more dangerous than the earthquake itself. In a wheelchair, stay in it. Move to cover, if possible, lock your wheels, and protect your head with your arms. 27 In the kitchen, move away from the refrigerator, stove, and overhead cupboards. In a stadium or theater, stay in your seat and protect your head with your arms. Do not try to leave until the shaking is over. Then leave in a calm, orderly manner. In bed, hold on and stay there, protecting your head with a pillow. In a coastal area, move to higher ground. In a mountainous area or near unstable slopes or cliffs, be alert for falling rocks and other debris that could be loosened by the earthquake. After an earthquake, be prepared for aftershocks 28 WHAT TO DO AFTER AN EARTHQUAKE Check yourself for injuries and provide help injured or trapped persons. Report any fires, chemical hazards, gas leaks, or broken water lines to your emergency personnel. Stay out of damaged buildings and away from damaged areas. Wear shoes in all areas near debris and broken glass. Stay away from beaches. Tsunamis and seiches sometimes hit after the ground has stopped shaking. If you're at school or work, follow the emergency plan or the instructions of the person in charge. 29 Clean up spilled medicines, bleaches, gasoline, or other flammable liquids immediately and carefully. Open closet and cabinet doors cautiously. Listen to radio (or television) for updated emergency information and instructions. Use battery-powered lanterns or flashlights to inspect your home. Do not use flame producing devices/open flame appliances if gas leak is suspected. Do not touch downed power lines or objects touched by them. Use the telephone only for emergencies Expect aftershocks. 30