4 EARTHQUAKES

Sushmitha Baskar

Module 04: EARTHQUAKES

Structure

4.0 Objectives

4.1 Introduction

4.2 Definitions

4.3 Continental drift theory

4.4 Plate tectonics

4.5 Causes of earthquakes

4.6 Seismic waves

4.7 Distribution of earthquakes

4.8 Types of earthquakes

4.9 Hazards due to earthquakes

4.10 Prediction of earthquakes

4.11 Precautions to be taken – Handling earthquakes

4.12 Tsunami

4.13 Hazards associated with tsunamis

14 Prediction of Tsunami and some early warning systems  

    4.15 Summary

4.16 Key Words

4.17 References and Suggested Further Readings

4.0 OBJECTIVE

• After reading this unit, you should be able to:
• define the term earthquakes;
• describe the types of earthquakes;
• explain the hazards associated with earthquakes; and describe tsunami and its effects.

4.1 Introduction

This chapter introduces you to earthquakes, their causes and occurrence in different parts of the world. We have all experienced the ground shaking or vibrating at some time or the other. Sometimes we don’t even feel the earthquake but see them reported in the newspaper. So earthquakes can be mild or severe. Earthquakes indicate that our planet is dynamic and changing system. They provide us knowledge of earth’s interior by studying the seismic waves generated by them. The seismic waves can be compared to ripples generated on water surfaces when an object is thrown. Similar events take place in an earthquake. When the energy stored in elastically strained rocks is released suddenly an earthquake occurs. This sends seismic waves. They occur along faults. Volcanoes are another reason for earthquakes.

Let us now learn about some definitions relating to earthquake.

4.2 Definitions

Before going further let us discuss the terminology associated with the study of earthquakes (Figure 2.1a).

Earthquake: It occurs due to the sudden movement of a part of the earth’s crust.

Seismology: It is the study of the generation, propagation and measurement of seismic waves through earth and the sources that generate them. The word seismology originates from the Greek words, ‘seismos’ meaning earthquake and ‘logos’ meaning science. The study of seismic wave propagation through earth provides the maximum input to the understanding of internal structure of earth.

Seismometer: It is the instrument used to record the seismic vibrations.

Seismogram: The graph that shows the seismic vibrations at a particular area is called a seismogram.

Focus or Hypocentre or Centrum: It is the point or place of origin of earthquakes. Thus, focus is the initial position of rupture within the Earth which generates seismic waves.

Epicentre or Epifocal: Epicenter is a point directly above the focus of an earthquake on the Earth’s surface.

Isoseismal Lines: These lines join points of equal intensity on the surface where the earthquake occurs. These are closed circles around epicentre.

Homoseismal or coseismals lines: These are the lines drawn through the points at which the earthquake is recorded at the same time.

Pleistoseistic region: Located around the epicentre is the region which suffers the maximum destruction as a result of an earthquake.

Figure 4.1a General terminology used in mode of propagation of the earthquakes (a) cross sectional view and (b) view from top.

4.3 Theory of Continental drift theory

This is a very important theory related to plate tectonics. The German scientist Alfred Wegener, in 1915, proposed that the continents present today were once a single landmass called Pangaea. Wegener’s observations were based on the similarity of coastlines and the geology between South America, Africa and Indian peninsula, Australia and Antarctica. He further proposed that Pangaea  existed approximately 200 million years ago and it was surrounded by the ocean known as Panthalassa. It was this super continent that broke up and formed the present continents. He was of the view that the continents had drifted due to centrifugal pseudo force of the Earth’s rotation or by a small component of astronomical precession. But findings by researchers showed that these forces could not cause continental drift.

4.4 Plate tectonics

This theory explains that the lithosphere of the earth is broken into seven major segments known as plates which slide past each other over the hot molten mantle known as asthenosphere. The plates act like a hard and rigid shell. The lithosphere consists of the crust and the upper part of mantle. Just below the lithosphere is the asthenosphere. This is a partially molten layer which allows the hard and rigid lithosphere to float or glide. Lithosphere is approximately 100 km thick below the continents, and 50 km under the oceans. The asthenosphere extends to about 700 km depth. The lithospheric plates move with a velocity of 2-10 cm/year on the underlying asthenosphere. The convection currents upwelling in the mantle are the driving force the movement of lithospheric plates. The major plates are African, American, Antarctic, Indo-Australian, Eurasian and Pacific plates. In addition there exist smaller plates like Andaman, Philippine plate and so on. The movement of the plates with respect to each other is the main cause for the occurrence of earthquakes and volcanic eruptions. You will read in detail about plate tectonics theory in the module ‘Plate Tectonics’.

• (i) Inter-plate earthquake: Earthquakes that occur at plate boundaries are referred to as inter-plate All earthquakes do not occur at the plate boundaries.

• (ii) Intra-plate earthquakes: The interior part of a plate is usually tectonically quiet. But earthquakes can also occur far from plate boundaries. Such earthquakes are called intra-plate earthquakes. The recurrence time for these earthquakes is longer than that of inter-plate earthquakes.

4.5 Causes of earthquakes

Let us discuss the causes of earthquake. The causes can be placed into the following three groups:

1. Surface Causes: They produce earthquakes of minor intensity which can be caused due to:

Collapse of Caves: The collapse of caves and their impact in the surrounding area may result in their collapse causing feeble earthquake.

Blasting of rocks: This can generate tremors in surrounding area which may induce landslides.

Landslides: Massive landslide often causes minor earthquake in the surrounding area.

2. Volcanic causes: The earthquakes are also associated with volcanoes.

3. Tectonic Causes: They include important causes for major earthquakes. These are:

• (i) Plate Tectonics: We have read that the Earth’s crust has been divided into number of plates which may be continental, oceanic or transistional. Movement of these plates produces earthquakes. In most of the cases the earthquakes are disastrous.

• (ii) Movement along Fault planes: Crustal displacements or structural disturbances cause sudden slipping of the Earth’s crust along the faults. As a result of movement of the adjacent blocks of fault, major earthquakes are produced.

Elastic Rebound Theory was propounded by Harry Fielding Reid after studying faultline of 1906 San Francisco earthquake. The theory describes the mechanism by which earthquakes are generated. According to Elastic Rebound theory the materials of the Earth, are elastic and they can withstand a certain amount of stress without being deformed permanently. But if stress is prolonged or if it is increased in magnitude, the rocks will undergo permanent deformation and eventually rupture. Rupture occurs in the rocks on either side of the fault tend to return to their original shape-position. Because of their elasticity and an elastic rebound occurs (Figure 2.1b). Elastic-rebound theory is a concept that accounts for the earthquakes generated by the sudden slippage of rocks on either side of a fault plane.

4.6 Seismic waves

Study of earthquakes is called ‘Seismology’. These seismic waves travel in the form of vibrations and seismograph is used to record these vibrations. The seismogram is the resulting graph which shows the vibrations and is used in the monitoring stations for prediction and forecast. The source of an earthquake is called the focus. It is usually the exact point where seismic waves are generated. The point on the earth’s surface directly above the focus is the epicentre (Figure 2.2). The earthquake has the highest intensity at the epicentre. Maximum damage to life and property occurs at the epicentre. The intensity of earthquake decreases as we moves away from the epicentre.

The study of seismic waves provides a complete picture of the Earth’s interior. Seismic waves from large earthquakes pass through the Earth. These seismic waves differ from each other in respect of their propagation velocity, wavelength and path of travel and nature of vibration

Seismic waves are basically of two types –

Body waves Surface waves

• (1) Body waves: They are generated due to the release of energy at the focus of the earthquake and move in different directions in the Earth. Denser the material more is the velocity. Their direction also changes as they reflect or refract when cut across materials with different densities known as seismic reflection and seismic refraction, respectively. Body waves are of two types:

• (a) Primary waves (P): The P waves are faster than S waves and are also known as called compressional waves. They can travel through solid, liquid and gaseous material. P-waves have a tendency to vibrate parallel to the direction of wave propagation. P-waves are compression waves because they travel through solid, liquid or gaseous materials as a succession of compressions and expansions and therefore, also referred as longitudinal or compressional waves. This causes density differences in the material through which they travel. They are the first to come hence known as primary waves.
• (b) Secondary waves (S): These waves are also known as shear waves as they displace material at right angles to their path of travel.. They can travel only through solid materials. The direction of vibration of these waves is perpendicular to the direction of wave propagation. Thus they are transverse or transitional waves Therefore, they create crests and troughs in the materials through which they pass. They arrive later hence known as secondary waves.

Surface waves:

Surface waves are those travelling on surface of the earth. They travel slower than Body waves. The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane confined to the Earth’s surface, like waves on the ocean. Their velocity is slightly less than that of S-waves. Surface waves are usually the most destructive waves in a large shallow focus earthquake especially in sedimentary basins. They have low velocity, low frequency and long wavelength. Surface waves include Love waves which have a complex horizontal ground movement and Rayleigh waves have rolling motion.

Magnitude and Intensity of an Earthquake: The size of an earthquake is referred to as the Richter Magnitude, named after the geophysicist Professor Charles Richter. It measures the amplitude of the largest recorded seismic wave at a specific distance from the earthquake. The Richter magnitude scale assigns earthquakes a number between 1 and 10 in order of increasing intensity. The Mercalli intensity scale is another seismic scale. It is also used to measure the intensity of earthquakes. . It is the measure of earthquake strength as interpreted from the maximum wave amplitude recorded by a seismograph. It was developed by the Italian volcanologist Giuseppe Mercalli in 1884. It was later expanded to include 12 degrees of intensity in 1902 by Adolfo Cancani. Then it was modified by Harry O. Wood and Frank Neumann in 1931. Intensity is used to define the extent of destruction an earthquake has caused and is dependent not only on the magnitude of the quake, but also soil type, building materials, population, and other human settlements. Mercalli intensity scale is the 12-point scale that measures earthquake severity in terms of the damage inflicted. Presently it is known as the Modified Mercalli Intensity Scale. This scale describes the intensity of an earthquake based on its observed effects. It labels an earthquake from I to XII depending on the effects of the earthquake. Both the scales are useful and have different applications and measurement techniques. The Mercalli scale is linear and the Richter scale is logarithmic. It is the measure of earthquake strength as interpreted from the maximum wave amplitude recorded by a seismograph.

Duration: They can last from a few seconds to several minutes. The greater the intensity of the shocks, the longer the earthquakes last. The average duration from 1 – 2 minutes of shocks of sufficient intensity can cause sufficient damage.

Wave Velocity: These waves travel approximately 5 – 8 km per second. They travel through the outer part of the crust.

Isoseismic Line: It is the line that connects all points on the earth’s surface where the intensity of shaking due to earthquake waves is the same.

4.7 Distribution of earthquakes

Millions of earthquakes of varying intensity occur every day. But the major and catastrophic ones capture our attention. Earthquakes usually occur on mainland and coastlines and tectonic plate boundaries. They are found to occur along the Pacific Ocean, the Indo-Australian plate boundary, Eastern side of Eurasian plate and the Western side of North American plate. The distribution of earthquakes is referred as seismicity of that particular. It is relatively more along the narrow belts that coincide with plate boundaries (Figure 2.3). Earthquakes along these zones are two types a) shallow focus earthquakes (that have focal depths less than about 70 km) or b) deep focus earthquakes (that have focal depths between 75 and 700 km).

• (a) Earthquakes at diverging plate boundaries: These plate boundaries are the zones where two plates move divergent or away from each other in opposite directions. Examples of this type are the mid oceanic ridges. Earthquakes along such boundaries exhibit normal fault motion. They are shallow focus earthquakes and have focal depths less than 20 km (Figure 2.4)

• (b) Earthquakes at transform plate boundaries: These occur at where lithospheric plates slide past each (Figure 2.5). Earthquakes along these boundaries show strike-slip motion on the faults. They are usually shallow focus earthquakes and have depths usually less than about 50 km. San Andreas Fault of California and the South Island of New Zealand are examples of this type. They record moderate tremors on the Richter’s scale of magnitude.
• (c) Earthquakes at converging plate boundaries: These occur at boundaries where two plates move towards each other. In this type compressional stress is active and reverse faults take place. There are two types of converging plate boundaries. They are: (1) subduction boundaries, where the oceanic lithosphere is pushed underneath either oceanic or continental lithosphere; and (2) collision boundaries in which continental lithospheric plates collide (Figure 2.6).

plate and nearby plates and corresponds to the Pacific ‘Ring of Fire’. Eight out of ten largest earthquake since 1900 have occurred along this seismic belt.

• (ii) The Mid-Atlantic Belt: This belt runs from north of Iceland to South Atlantic along a divergent plate margin. The depth of foci here is intermediate.
• (iii) The Mid-Continental Belt: This belt runs through the Alps-Himalayan young mountain The belt is found along the convergent margins between the Eurasian plate with the African and Indian plates.

4.8 Types of earthquakes

Earthquakes are usually classified based on their origin. Since they originate due to the movements of the tectonic plates they are also known as tectonic hazards. Human activities and volcanic eruptions can also trigger earthquakes.

• (a) Tectonic Earthquakes: This type of earthquake occurs when the crust is subjected to strain. The tectonic plates can converge, diverge or slide past each other (Figure 2.7). Benioff zone is a deep active seismic area in a subduction It is located beneath volcanic island arcs and continental margins above active subduction zones. The differential motion results in earthquakes.

Figure 2.7 Relationship between plate tectonic and earthquakes

• (b) Volcanic earthquakes: Volcanoes can also cause earthquakes. When a volcano explodes, the possible earthquake effects are affected in an area 10 to 20 miles around its base. This is unlike a tectonic earthquake effect which is felt around larger areas. The volcanoes that produce acidic lava explode violently.

• (c) Volcano – tectonic earthquake: They occur as a result of injection or withdrawal of magma within the rocks. It can result in faulting and subsidence. These earthquakes are recorded over a long period. These are monitored and used for the prediction of volcanic eruptions. The prolonged tremors caused by movement of magma are referred to as harmonic tremors. The seismicity records of Mount St. Helens during the period 1980 to 1992 establish the relationship between volcanic events and the above type of earthquakes.

• (d) Human activities and earthquakes: Some human activities that can cause earthquakes include: filling of new reservoirs / dams, injecting fluid into deep wells for waste disposal, the detonation of atomic explosives under the ground, or the injection of fluids deep inside the Earth through wells. Further, incidents, like landslides, breaking up of glacier and such disturbances can also trigger earthquakes.

Based on depth of their origin, earthquakes may be broadly classified into three types:

• (i) Normal or shallow depth earthquakes originate from within a depth of 50 km.
• (ii) Earthquakes of intermediate depth originate at a depth of 50 to 240 km.
• (e) (iii)Deep-focus earthquakes originate at a depth of several hundred kilometres (240 to725 km).

4.9 Hazards due to earthquakes

Earthquakes are hazardous processes and can damage life and property. Hazards and risk due to earthquakes is related to the population density, construction standards mainly building codes used and emergency preparedness. Most damages due to earthquakes are caused by seismic waves especially the surface waves. It is more severe near the epicentre and is lesser as it goes away from the epicentre. The seismic waves cause more damage in loose sediments as they are

subject to more intense shaking than solid bedrock. The size of the earthquake is an important factor. Usually, high magnitude earthquakes and higher duration causes more intense shaking. The damage to structures or buildings due to earthquakes depends on the type of construction materials used to a certain extent. For example, concrete is brittle and susceptible to damage. Some materials such as wood and steel are flexible and are less susceptible to damages. Some hazards where earthquakes damage roads, buildings, gas line and so on are discussed in the following paragraphs.

• (i) Damage to Buildings: Buildings can be damaged due to earthquakes. When the earth moves, the entire building rocks and it can result in cracks and complete breakdown also. This depends on the magnitude of the earthquake. It depends on the earthquake size, distance from the event, frequency of waves and duration of shaking.

• (ii) Surface Geology: The composition of the soil/rock type, water content and thickness of the soil/ rock affects the shaking. Further, liquefaction is another hazard, when water-saturated soils cause the material flows like a fluid. Buildings can sink into the ground due to soil liquefaction.

• (iii) Ground Displacement: This is a hazard due to ground movement along a fault. When a structure, for example a road is built across a fault, then ground displacement during an earthquake can damage the structure.

• (iv) Fault zone: Ground rupture generally occurs only along the fault zone. Thus, buildings constructed over fault zones may be affected and damaged. But those that are built adjacent to the faults may not be affected.

• (v) Aftershocks: They occur after a main earthquake and are usually smaller earthquakes. They are generally very dangerous because they cause further damage to buildings/structures already damaged by the main earthquake. They occur due to differences in the stress pattern in the areas near the epicenter. The earth’s crust needs to adjust to these changes that have taken place.

• (vi) Other Secondary Hazards due to earthquakes:

Mass movements: Hilly terrains are subjected to earthquakes. Therefore ground shaking in these regions can trigger landslides, rock and debris falls, slumps, and debris avalanches.

Flooding: This occurs due to the damages in dams, due to tsunamis. This can also occur due  to ground subsidence after an earthquake.

Fire: When electric cable lines and natural gas lines get disrupted due to an earthquake, fires can result.

Contamination of water: During earthquakes ground water contamination can occur and cause related health hazards.

Changes in Ground Level: Earthquakes may cause changes in ground level. This includes both uplift and subsidence of the land surfaces.

Tsunamis: Tsunamis and seiches can cause a lot of damage. Earthquakes that occur below sea level and along coastal areas cause tsunamis. They can result in devastation in several areas on the other side of the ocean too. Seiches can be compared to smaller tsunamis. They occur on lakes due to earthquakes and are a few feet high.

4.10 Prediction of earthquakes

The geological knowledge helps us to understand that most earthquakes will occur along the plate boundaries. The best preparation we need to follow is adopting appropriate building constructions. Geoscientists map the surface geology to study the potential size and location of earthquakes.

• (a) Long-Term Forecasting: It is based on information about the instances where earthquakes have occurred in the past. For understanding this knowledge of paleoseismology and seismic gaps are important. The zone along a tectonically active area where no earthquakes have occurred in the recent past is referred to as seismic gap. If seismic gaps are identified, then it possible the area can expect a large earthquake in the near future.

• (b) Short-Term Prediction: This process involves monitoring of processes that occur in an area near an earthquake prone fault. Precursor events are the anomalous events that precede an earthquake. Some precursor events that may be important include: (i) Uplift of ground and their tilting: Around active faults prior to an earthquake, the ground is uplifted. This is due to the increase in size of rocks caused by strain building on the fault. It can lead to the formation of cracks resulting in small earthquakes called foreshocks. (ii) Shift in water level in wells: When rocks become strained near a fault, some changes take place in pressure of the groundwater. This forces  groundwater to move to higher or lower elevations, causing changes in the water levels in wells. (iv)Emission of Radon gas: Radon is an inert gas produced by the radioactive decay of uranium and other elements in rocks. It is inert and remains in a crystal structure until some event forces the gas out. Deformation resulting from strain can form micro cracks. Through these cracks radon can enter the groundwater. Such increases in the amount of radon emissions in groundwater have been reported prior to some earthquakes. (v) Drop in the Electrical Resistivity of Rocks: In general, rocks are poor conductors of electricity. If micro cracks develop and groundwater is forced through the cracks, the electrical conductivity increases. In some cases a 5-10% drop in electrical resistivity has been observed prior to an earthquake. (vi)Unusual Radio Waves: Just prior to the Loma Prieta Earthquake of 1989, some researchers reported observing unusual radio waves. The reasons are not yet known, but research is under process. (vii) Strange animal behavior: Prior to earthquakes some strange animal behavior has been observed. Before the magnitude 7.4 earthquake in Tanjin, China, zookeepers reported unusual animal behaviour. They include: unusual behavior in snakes, swans refusing to go near water and pandas screaming. Foreshocks: They are useful for earthquake prediction. Before the 7.3 magnitude earthquake in China, in the year 1975; the observation of foreshocks helped to evacuate the city of Haicheng.

We now know that earthquakes are caused by strong forces. So we cannot stop earthquakes from occurring. However, predictions and emergency preparedness can help to a certain extent.

4.11 Precautions to be taken – Handling earthquakes

There a number of precautions that can be taken prior to an earthquake, during and also after an earthquake.

(a) Before an earthquake: We should avoid keeping heavy objects on shelves as they are likely to fall during earthquakes. Heavy furniture should be anchored to the walls. Fire extinguishers and first aid kits come handy and should be kept at home and offices.

During an earthquake: During an earthquake we should not panic. We need to stay calm and if caught indoors stay under a table. Stay away from windows and doors as they can fall. It is always better to stay outdoors in the open away from power lines.

• (c) After an earthquake: Damages due to earthquakes are many. Injuries are many so first aid requirements should be kept handy. Further gas, water and power lines should be checked for damage. We should stay away from damaged buildings. Aftershocks can also strike the area so we need to be connected with media updates. The media is very helpful for broadcasting the news updates.

SAQ 1

• What is an earthquake and explain seismic waves?
• Discuss the distribution of earthquakes.
• What are the hazards associated with earthquakes?

4.12 Tsunami

Tsunami refers to a series of gigantic waves that occur after some kind of disturbances under the sea or ocean. It can be due to an earthquake or a volcanic eruption. The name ‘Tsunami’ in Japanese means harbour wave. The tsunami waves travel in all directions from the point of the disturbance. This can be compared to the ripples on the surface of water bodies. The waves can travel in the open sea. They can travel at about approximately 450 miles per hour. When the big waves approach shallow waters along the coast they grow to great heights and hit the shore. The waves can be as high as 100 feet and can result in lot of destruction on the shore. Tsunamis can also travel across oceans. Therefore, a large earthquake along the coast of South America can produce a tsunami that can destroy coastal regions of Hawaii and Japan. These waves are highly devastating to the coastal regions. In 1992-93 three large tsunamis occurred: Japan, Indonesia, and Nicaragua that were highly damaging.

4.13 Hazards associated with tsunamis

Tsumanis are primarily destructive. The secondary effects include the debris that act as projectiles when they run into other objects. They also include erosion that can destroy the structures built along coastal areas. Fires can result due to disruption of gas supply and power lines. The tertiary effects include loss of crops and water for survival and life.

4.14 Prediction of Tsunami and some early warning systems

It is important to predict tsunamis so that lives n the coastal regions can be saved. In case of those areas that are situated at greater distances from the earthquake epicentre there is time for warnings to be given to the coastal areas. One good example is of Hawaii. It is located away from most of the sources of tsunami. The installation of early warning systems has been useful. For earthquakes occurring anywhere on the subduction margins of the Pacific Ocean, there is a approximately four hours of warning before a tsunami would occur on the Hawaiian Islands. The Pacific Tsunami Warning Centre has been set up by the National Oceanic and Atmospheric Administration for Pacific warning system for areas in the Pacific Ocean. The centre has an international network of seismographic stations, and  tidal stations around the Pacific basin that transmits information to the Centre located in Hawaii. So when an earthquake occurs anywhere in the region, the centre analyses the data and observes for tsunami signals. The tidal stations are also monitored. In case a tsunami is round the corner, an early warning is given to areas on the Pacific coast. So the hazards can be controlled by installing warning systems, satellites engineering solutions that reduce their effects and save lives. In the event of a tsunami, we should be connected to the radio, television, internet for tsunami warnings. When a tsunami occurs we should move inland and to a higher ground to safeguard ourselves. We should also stay away from the beach and coastal areas until official advice.

SAQ 2

1. What is a tsunami?
2. How can tsunami be predicted?

4.15 Summary

In this unit we have studied about earthquake and tsunami. We understood the causes, distribution and types of earthquakes. We have also learnt the various earthquake hazards and forecasting of the events. In the final part of the unit details have been included on tsunami, its causes and effects and the forecasting of tsunamis. The thorough understanding of natural hazards is the first step towards disaster preparedness and response. For this we need to have good knowledge on the natural processes occurring on the earth in order to predict hazards and prepare ourselves. We have no control over geological processes but we can predict and attempt to reduce the hazards to life and property. More rigorous scientific research is required to forecast future events.

4.16 Key words

• Aftershocks: They occur after a main earthquake and are usually smaller earthquakes.
• Isoseismic Line: It is the line that connects all points on the earth’s surface where the intensity of shaking due to earthquake waves is the same.
• Tsunami: It refers to a series of gigantic waves that occur after some kind of disturbances under the sea or ocean. It can be due to an earthquake or a volcanic eruption.
• Low velocity zone: This zone occurs close to the boundary between the lithosphere and the asthenosphere in the upper mantle.

4.17 References and Suggested further readings

• Baskar, S., Baskar, R. 2009. Natural Disasters. Unicorn books, Pustak Mahal, India. 159p.
• Bryant, E. 2005. Natural Hazards. 2nd Edition, Cambridge university press, 330p.
• Bullen, K.E., Bruce, B.A. 1985. An Introduction to the Theory of Seismology: Fourth Edition, Cambridge University Press.
• Burton, I., Kates, R.W. 1964. The perception of natural hazards in resource management, Natural Resources Journal 3, 412 – 41.
• Burton, I., Kates, R.W., White, G.F. 1978. The Environment as Hazard New York: Oxford University Press.
• Keller, E.A. 2010. Environmental Geology, 9th Edition, Pearson publication, 624 p.
• Stein, S., Wysession, M. 2002. An Introduction to Seismology, Earth and Earth Structure: Blackwell Publishing.

Key to SAQ

Answers to SAQ 1

1. Your answer should include the following points:

Earthquakes occur when energy stored in elastically strained rocks is released all of a sudden. This release of energy sends waves of elastic energy, called seismic waves and causes the ground to shake. This ground shaking can be mild to severe. They occur along faults, planar  breaks in rocks along which there is displacement of rocks on one side relative to the other.

Focus

Epicentre

Body waves: (a) Primary waves (P): (b) Secondary waves (S) Surface waves:

2. Your answer should include the following points:

Earthquakes usually occur on mainland and coastlines and tectonic plate boundaries. They are found to occur along the Pacific Ocean, the Indo-Australian plate boundary, Eastern side of Eurasian plate and the Western side of North American plate. Earthquake distribution is called seismicity and is highest along relatively narrow belts that coincide with plate boundaries.

Earthquakes at diverging plate boundaries Earthquakes at transform plate boundaries Earthquakes at converging plate boundaries

Earthquakes are centered to certain regions in the world. They include three important belts. They are: The Circum-Pacific Belt, The Mid-Atlantic Belt and The Mid-Continental Belt.

3. Your answer should include the following points:

Earthquakes are hazardous processes and can damage life and property. Hazards and risk due to earthquakes is related to the population density, construction standards and emergency preparedness. Most damages due to earthquakes are caused by seismic waves especially the surface waves. It is more severe near the epicentre and is lesser as it goes away from the epicentre.

Damage to Buildings Surface Geology

Ground Displacement Fault zone

Aftershocks

Other Secondary Hazards due to earthquake

Answers to SAQ 2

1. Your answer should include the following points:

Tsunami refers to a series of huge waves that occur after an undersea disturbance, such as an earthquake or a volcanic eruption. The name ‘Tsunami’ in Japanese means harbour wave. The waves travel in all directions from the area of disturbance. This can be compared to the ripples on the surface of water bodies. The waves can travel in the open sea and are approximately 450 miles per hour. When the big waves approach shallow waters along the coast they grow to great heights and hit the shore. The waves can be as high as 100 feet and can result in lot of destruction on the shore.

2. Your answer should include the following points:

The Pacific Tsunami Warning Centre has been set up by the National Oceanic and Atmospheric Administration for Pacific warning system for areas in the Pacific Ocean. The centre has an international network of seismographic stations, and tidal stations around the Pacific basin that transmits information to the Centre located in Hawaii.

Installing of warning systems Engineering solutions.