MODULE 1 EARTH AND ITS INTERIOR Earth is formed by the coalescence of a large collection of material masses. Formation of earth which was a fusion process generated large amount of heat. When earth cooled down, different layers were formed. Heavier and denser materials sank to the centre and the lighter ones rose to the top. The differentiated earth consists of different layers (geospheres) like 1. Core 2. Mantle 3. Crust Core It is the densest central part of the earth. It is also called barysphere. It is composed of inner and outer cores. The inner core has a radius of about 1221 km. It is solid and consists of heavy metals such as nickel and iron. It has a density of about 16000 kg/m3. The outer core surrounding the inner core is 2259 km thick. It is composed of nickel and iron alloyed with silica. The outer core is liquid in form and has a density of 12000 kg/m3. The temperature at the core is about 25000C and the pressure is about 4million atm. Mantle Mantle is the layer surrounding the core. It is also called asthenosphere which is 2685 km thick. It is composed of hot, dense ultrabasic igneous rock in a plastic state. Mantle with a density of 5000 – 6000 kg/m3 has the ability to flow. The mantle consists of 1) Upper Mantle made of olivine and pyroxene and 2) Lower Mantle made of more homogeneous mass of magnesium and iron oxide and quartz. No earthquakes are recorded in the lower mantle. The specific gravity of mantle is about 5. The mantle has an average temperature of about 2200degree Celsius. Crust Crust also known as lithosphere is the thinnest outer solid shell. It is 200 km thick with a density of 1500 kg/m3. The temperature of the crust is about 250C and the pressure within it is 1 atm. It is the part of the earth where life exist. The average thickness of crust beneath continents is about 40km where as it decreases to as much as 5km beneath oceans. The oceanic crust is constituted by basaltic rocks and continental part by granitic rocks overlying the basaltic rocks. Compared to the layers below, this layer has high rigidity and anisotropy.
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Interior of earth CIRCULATIONS The entire lifetime of earth is a continous sequence of underground movements. This movement is produced by the Convection currents developed in the viscous Mantle, because of prevailing high temperature and pressure gradients between the 2
Crust and the Core, like the convective flow of water when heated in a beaker. The energy for the above circulations is derived from the heat produced from the incessant decay of radioactive elements in the rocks throughout the Earth's interior. These convection currents result in a circulation of the earth's mass; hot molten lava comes out and the cold rock mass goes into the Earth. The mass absorbed eventually melts under high temperature and pressure and becomes a part of the Mantle, only to come out again from another location, someday. Many such local circulations are taking place at different regions underneath the Earth's surface, leading to different portions of the Earth undergoing different directions of movements along the surface producing sliding of crust and some portions of mantle on the hot molten outer core. These movements of earth masses produce a division of crust in some portions called tectonic plates moving in different directions and with different velocities.
Effect of convection currents PLATE TECTONICS The movement of tectonic plates in different directions with different speeds is called plate tectonics. The movement of tectonic plates are at a rate of 5 to 10 cm per year on the plastic mantle. The plates are the part of rigid outer shell of the earth called the lithosphere. It includes both the crust and some of the upper mantle. There are 12 major tectonic plates, 20 smaller ones and many filler plates. The major tectonic plates are the African, the Eurasian, the Indian, the Australian,
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the Arabian, the Philippines, the North American, the South American, the Pacific, the Nasca, the Cocus and the Antartic plates. The various Causes of plate motion are - Convection currents - Slab pull- the subducting oceanic plate becomes colder and denser than the surrounding mantle and pulls the rest of slab along - Ridge push – gravitational sliding of the lithosphere slab away from the oceanic ridge raised by rising material in the asthenosphere. Plate tectonics is responsible for features such as - Continental drift- two plates move away from each other - Mountain formation- front plate is slower than rear plate due to which rear plate collides with front plate - Volcanic eruptions - Earthquakes The theory of plate tectonics, presented in early 1960s, explains that the lithosphere is broken into seven large (and several smaller) segments called plates. The upper most part of the earth is considered to be divided into two layers with different deformation properties. The upper rigid layer, called the lithosphere, is about 100 km thick below the continents, and about 50 km under the oceans, and consists of Crust and rigid upper-mantle rocks. The lower layer, called the asthenosphere, extends down to about 700 km depth. The rigid lithospheric shell is broken into several irregularly shaped major plates and a large number of minor or secondary plates. The lithospheric plates are not stationary, on the contrary, they float in a complex pattern on the soft rocks of the underlying asthenosphere like rafts on a lake. These plates bear the loads of land masses, water bodies or both and are in constant motion over the viscous mantle. These plates move in different directions and at different speeds from those of the neighbouring ones. Sometimes, the plate in the front is slower; then, the plate behind it comes and collides (and mountains are formed). On the other hand, sometimes two plates move away from one another (and rifts are created). In another case, two plates move side-by-side, along the same direction or in opposite directions. Some segments of adjacent plates remain immovable and produce seismic vibrations along boundaries causing destruction. This theory requires a source that can generate tremendous force acting on the plates. The widely accepted explanation is based on the force offered by convection currents created by thermo-mechanical behavior of the earth’s 4
subsurface. The variation of mantle density with temperature produces an unstable equilibrium. The colder and denser upper layer sinks under the action of gravity to the warmer bottom layer which is less dense. The lesser dense material rises upwards and the colder material as it sinks gets heated up and becomes less dense. These convection currents create shear stresses at the bottom of the plates which drags them along the surface of earth. The relative motion of crustal plates gives rise to three kinds of plate boundaries or marginal zones which is the dividing zone between two plates. These are 1. Divergent plate boundaries 2. Convergent plate boundaries 3. Transform plate boundaries 1. Divergent plate boundaries A divergent plate boundary is a boundary between plates that are moving apart. A new crust is formed when two plates move apart. This is the location where the less dense molten rock from the mantle rises upwards and becomes part of crust after cooling. Highest rate of spreading or expansion between plates is found to occur near Pacific Ocean ridges and the lowest rate of spreading occurs along midAtlantic ridges. Generally, spreading ridges or divergent boundaries are located beneath the oceans. A few areas where the spreading occurs along the continental mass are East African rift valley and Iceland.
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2. Convergent plate boundaries A convergent plate boundary is a boundary between plates that move towards each other. Here crust gets destroyed as one one plate dives under another. Here plates may also collide.
a. Subduction boundaries These boundaries are created when either oceanic lithosphere subducts beneath oceanic lithosphere (ocean-ocean convergence), or when oceanic lithosphere subducts beneath continental lithosphere (ocean-continent convergence), at the junction where the two plates meet, a trench known as oceanic trench is formed. When two plates of oceanic lithosphere run into one another, the subducting plate is pushed to depths where it causes melting to occur. When a plate made of oceanic lithosphere runs into a plate with continental lithosphere, the plate with oceanic lithosphere subducts because it has a higher density than continental lithosphere. The subducted plate melts as it encounters higher temperature regime inside earth melts and produces magma. This magma rises to the surface to produce chains of volcanos and islands known as island arcs. One of the areas around Indian peninsula where subduction process is in progress is near Andaman-Sumatra 6
region, where the Indo-Australian plate is subducting below the Andaman and Sunda plates,
b. Collision boundaries When two plates with continental lithosphere collide, subduction ceases and a mountain range is formed by squeezing together and uplifting the continental crust on both plates, The Himalayan Mountains between India and China were formed in this way.
3. Transform plate boundaries It is the boundary between plates that move horizontally past each other. Here crust is neither produced nor destroyed as the plates slide horizontally past each other.
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TYPES OF EARTHQUAKES Earthquakes are the Earth's natural means of releasing stress. When the Earth's plates move against each other, stress is put on the upper mantle (lithosphere). When this stress is great enough, the lithosphere breaks or shifts. As the Earth’s plates move they put forces on themselves and each other. When the force is large enough, the crust is forced to break. When the break occurs, the stress is released as energy which moves through the Earth in the form of waves, which we feel and call an earthquake. Rock breakage is called faulting and causes a release of energy when stored stress is suddenly converted to movement. Vibrations known as seismic waves are produced - they travel outwards in all directions at up to 14 kilometers per second. At these speeds, it would take the fastest waves only 20 minutes to reach the other side of the Earth by going straight through its centre - that's a distance of almost 13,000 kilometers. The waves distort the rock they pass through, but the rock returns to its original shape afterwards. The epicenter is the point on the Earth’s surface directly above the source of the earthquake. The source, also known as the focus, can be as deep as 700 kilometers. Earthquakes do not occur deeper than this because rocks are no longer rigid at very high pressures and temperatures - they can't store stress because they behave plastically. Smaller events occur more frequently - in fact, most earthquakes cause little or no damage. A very large earthquake can be followed by a series of smaller aftershocks while minor faulting occurs during an adjustment period that may last for several months.
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Earthquakes can be classified according to location, epicentral distance, focal depth, magnitude and geological make up of region. 1. Based on location (i)
Interplate The earthquake that occurs at a plate boundary is known as inter-plate earthquake. Not all earthquakes occur at plate boundaries.
(ii)
Intraplate Though, interior portion of a plate isusually tectonically quiet, earthquakes also occur far from plate boundaries. These earthquakes are known as intra-plate earthquakes. The recurrence time for an intraplate earthquake is much longer than that of inter-plate earthquakes.
2. Based on epicentral distance (i) (ii) (iii)
Local earthquake < 1 degrees Regional earthquake 1–10 degrees Teleseismic earthquake >10 degrees
3. Based on focal depth (i) (ii) (iii)
Shallow depth 0–70 km Intermediate depth 71–300 km Deep earthquake > 300 km
4. Based on magnitude (i) (ii) (iv) (iii) (v) (iv)
Micro earthquake M < 3 Intermediate earthquake 3–4 Moderate earthquake 5–5.9 Strong earthquake 6–6.9 Major earthquake 7–7.9 Great earthquake >8 9
5. Based on geological make up of region (i)
tectonic earthquakes These occur when rocks in the Earth's crust break due to geological forces created by movement of tectonic plates
(ii)
volcanic earthquakes These occur in conjunction with volcanic activity.
(iii)
collapse earthquakes These are small earthquakes in underground caverns and mines
(iv)
explosion earthquakes These result from the explosion of nuclear and chemical devices.
SEISMIC WAVES Earthquake vibrations originate from the point of initiation of rupture and propagates in all directions. These vibrations travel through the rocks in the form of elastic waves. Mainly there are three types of waves associated with propagation of an elastic stress wave generated by an earthquake. These are primary (P) waves, secondary (S) waves and surface waves. Both P and S waves are called bodywaves because they move within the Earth's interior. Their speeds vary depending on the density and the elastic properties of the material they pass through, and they are amplified as they reach the surface. In addition, there are sub varieties among them. The important characteristics of these three kinds of waves are as follows: Primary (P) Waves These are known as primary waves, push-pull waves, longitudinal waves, compressional waves, etc. These waves propagate by longitudinal or compressive action, which mean that the ground is alternately compressed and dilated in the direction of propagation. P waves are the fastest among the seismic waves and travel as fast as 8 to 13 km per second. Therefore, when an earthquake occurs, these are the first waves to reach any seismic station and hence the first to be recorded. The P waves resemble sound waves because these too are compressional or longitudinal waves in nature, which mean that they compress and expand matter as they move through it. Hence, the particles vibrate to and fro in the direction of propagation (i.e. longitudinal particle motion). These waves are capable of traveling through solids, liquids and gases. 10
The P-waves propagates radial to the source of the energy release and the velocity is expressed by
where E is the Young’s modulus; n is the Poisson’s ratio (0.25); and r is the density.
Primary wave motion Secondary (S) Waves These are also called shear waves, secondary waves, transverse waves, etc. These are the waves directly following the P waves Compared to P waves, these are relatively slow. These are transverse or shear waves, which mean that the ground is displaced perpendicularly to the direction of propagation. In nature, these are like light waves, i.e., the waves move perpendicular to the direction of propagation. Hence, transverse particle motion is characteristic of these waves. These waves are capable of traveling only through solids. . S waves cannot travel through liquid because, while liquid can be compressed, it can't shear. S waves are the more dangerous type of waves because they are larger than P waves and produce vertical and horizontal motion in the ground surface. If the particle motion is parallel to prominent planes in the medium they are called SH waves. On the other hand, if the particle motion is vertical, they are called SV waves.
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The shear wave velocity is given by
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They travel at the rate of 5 to 7 km per second. For this reason these waves are always recorded after P waves in a seismic station.
Secondary wave motion Surface Waves When the vibratory wave energy is propogating near the surface of the earth rather than deep in the interior, two other types of waves known a Rayleigh and Love waves can be identified. These are called surface waves because their journey is confined to the surface layers of the earth only. Surface waves travel through the earth crust and does not propagate into the interior of earth unlike P or S waves. Surface waves are the slowest among the seismic waves. Therefore, these are the last to be recorded in the seismic station at the time of occurrence of the earthquake. They travel at the rate of 4 to 5 km per second. Complex and elliptical particle
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motion is characteristic of these waves. These waves are capable of travelling through solids and liquids. They are complex in nature and are said to be of two kinds, namely, Raleigh waves and Love waves. The Rayleigh surface waves are tension-compression waves similar to the P-waves expect that their amplitude diminishes with distance below the surface of the ground. Similarly, the Love waves are the counterpart of the “S” body waves; they are shear waves that diminishes rapidly with distance below surface, The damage and destruction associated with earthquakes can be mainly attributed to surface waves. This damage potential and the strength of the surface waves reduce with increase in depth of earthquakes.
Love waves
Rayleigh waves 13
MEASURING INSTRUMENTS The vibratory motion produced during an earthquake could be measured in terms of displacement, velocity or acceleration. People who record and interpret seismic waves are called seismologists. Seismologists study the interior of our planet by observing the way seismic waves travel through Earth. This process is similar to using X rays to create a CAT scan of the interior of a human body. A seismologist is interested in even small amplitude ground motions (in terms of displacement) that provides insight into the wave propagation characteristics and enables him to estimate the associated earthquake parameters. As accelerations are the causative phenomena for forces that damage structures (Force = mass x acceleration), engineers are more concerned with the earthquake causing structural damage, hence are interested in acceleration measurement. The instruments that measure the ground displacements are called seismographs. Seismographs show the kinds of waves that occur, their strength, and the time that they arrive at the instrument. Seismographs are located all around the world at seismic stations on land, and in special locations in the oceans. The record obtained from a seismograph is called a seismogram. The seismograph has three components – the sensor, the recorder and the timer. The principle on which it works is simple and is explicitly reflected in the early seismograph – a pen attached at the tip of an oscillating simple pendulum (a mass hung by a string from a support) marks on a chart paper that is held on a drum rotating at a constant speed. A magnet around the string provides required damping to control the amplitude of oscillations. The pendulum mass, string, magnet and support together constitute the sensor; the drum, pen and chart paper constitutes the recorder; and the motor that rotates the drum at constant speed forms the timer. One such instrument is required in each of the two orthogonal direction. Some instruments do not have a timer device (i.e., the drum holding the chart paper does not rotate). Such instruments provide only the maximum extent (or scope) of motion during the earthquake; for this reason they are called seismoscopes. The analog instruments have evolved over time, but today, digital instruments using modern computer technology are more commonly used. The digital instrument records the ground motion on the memory of the microprocessor that is in-built in the instrument. By varying the characteristics of equipment one could record displacement, velocity or acceleration during an earthquake The devices that measure the ground accelerations are called accelerometer. The accelerometers register the accelerations of the soil and the record obtained is called an accelerogram. .
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Seismograph
LOCATION OF FOCUS Seismologists use the elapsed time between the arrival of a P-waves and S-waves at a given site to assist them in estimating the distance from the site to the focus. The distance from hypocenter to observation point is given by
where, T=difference in time of arrival of P and S waves at an observation point; S= distance from hypocenter to observation point; and Vp and Vs are the velocity of P and S waves, respectively.The time T can be taken as the time of duration of the initial tremor to it built-up while Vp and Vs are geological properties for a given locations. Thus, the distance from the hypocenter to the observation point is approximately proportional to the time of duration of the initial tremor; the coefficient of proportionality is about 8 km/sec. When S has been determined for each of three observation points the hypocenter is located as the point of intersection of these spheres.
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CHARACTERISTICS OF STRONG GROUND MOTION Vibration of the earth’s surface is a net consequence of motions, vertical as well as horizontal, caused by seismic waves that are generated by energy release. These waves arrive at various instants of time, have different amplitudes , and carry different levels of energy. Thus the motion at any site on the ground is random in nature, its amplitude and direction varying randomly with time. Large earthquakes at great distances can produce weak motions that may not damage structures or even be felt by humans. However earthquakes in the vicinity or with high intensity can produce strong ground motions that can possibly damage structures. The motion of the ground can be described in terms of displacement, velocity or acceleration.The ground motion is usually recorded with strong motion accelerographs placed at various locations. The acceleration record of a strong earthquake usually consists of two horizontal components and one vertical component. Generally, the two horizontal components are of equal magnitude and the vertical component is somewhat smaller. The accelerograph record frequently includes instrumentation errors, owing to frequency characteristics of the accelerograph and other inherent features that must be corrected by filtering and other procedures. The corrected accelerogram is then integrated to obtain the velocity and displacement histories of ground motion. On firm ground, accelerogram is irregular and complex. On the other hand, on the surface of the soft strata the earthquake ground motion assumes an almost harmonic nature, resulting from filtering of the seismic waves as they travel through soft strata. Earthquake accelerograms are thus complex and can vary considerably from one another. They are significantly affected by local site conditions, distance from the causative fault, and the transmission path of the seismic waves. Newmark and Rossenblueth classified earthquake ground motion into four groups in accordance with their surface ground motion characteristics: 1. Single shock type. This occurs only at close proximity with epicentre on firm strata and for shallow earthquakes. Port Hueneme earthquake is an example for this. 2. A moderately long, extremely irregular motion. This is associated with an intermediate focal depth and occurs only on firm ground. It is typical of earthquakes originating in the circum-Pacific belt. The NS component of 1940 E1 Centro earthquake is indicative of this type. 16
3. A long ground motion exhibiting pronounced prevailing periods of vibration. Motions of the type are recorded at layers of soft strata, through which seismic waves have been filtered and subjected to multiple reflections at the layer boundaries. The 1964 Mexico City earthquake exemplifies this behaviour. 4. A ground motion involving large-scale permanent deformation of the ground. These types of earthquake may entail landslides or soil liquefaction. The Alaska and Niigata earthquakes of 1964 characterize this type of earthquake. Three characteristics of ground motions are important: (1) peak of maximum ground motion; (2) duration of ground motion; and (3) the frequency content. The structural response is affected by each of these factors. Peak ground motion, primarily peak ground acceleration (PGA), influences the vibration amplitude, and has been employed to scale earthquake design spectra and acceleration time forces. The severity of ground shaking is significantly influenced by the duration of ground motion. For example, an earthquake with high peak acceleration poses a high hazard potential, but if it is sustained for only a short period of time it is unlikely to inflict significant damage to many types of structures. Conversely an earthquake with moderate peak acceleration and a long duration can build up damaging motion in certain types of structure. Finally, ground motion amplification to a structure is more likely to occur and the frequency content of ground motion is in close proximity to the natural frequency of the structure. A correlation equation for peak ground acceleration can be given in terms of Richter magnitude M as Log10 PGA = –2.1 + 0.81M–0.027M2 Although PGA decreases with distance from the causative fault, the rate of decrease is relatively small, over a distance comparable to the vertical dimensions of the shipped fault. INTENSITY The intensity of an earthquake refers to the degree of destruction caused by it. In other words, intensity of an earthquake is a measure of severity of the shaking of ground and its attendant damage. This, of course, is empirical to some extent because the extent of destruction or damage that takes place to a construction at a given place depends on many factors. Some of these factors are: (i) distance from the epicenter, (ii) compactness of the underlying ground, (iii) type of construction (iv) magnitude of the earthquake (v) duration of the earthquake and (vi) depth of 17
the focus (vii) degree of consolidation. Intensity is the oldest measure of earthquake. It is also calles destructive power. It is an evaluation of severity of ground motion at a given location represented by a numerical index.it is measured in relation to the effect of earthquake on human life. Generally destruction is defeined in terms of damage caused to buildings, dams, bridges etc as reported by witnesses. It is not a unique precisely defined characteristic of an earthquake. It is a qualitative measure. It is based on direct observation by individuals rather than on instrumental measurements.it is represented by roman capital numerals. The seismic intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally - total destruction. The intensity scales are based on three features of shaking: • perception by people and animals • performance of buildings • changes to natural surroundings. Numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the most popular is the Modified Mercalli Intensity (MMI) Scale. This scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects. The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage. Another intensity scale is Mendvedev-Spoonheuer-Karnik scale (MSK 64). This scale is more comprehensive and describes the intensity of earthquake more precisely. Indian seismic zones were categorized on the basis of MSK 64 scale. Some of the other intensity scales used are Rossi-Forel (RF) scale, Japanese Meteorological Agency (JMA) intensity scale, etc. An imaginary line joining the points of same intensity of the earthquake is called an 'iso-seismal'. In plan, the different iso-seismals will appear more or less as concentric circles over a plain, homogeneous ground if the focus of the earthquake is a point. On the other hand, if the focus happens to be a linear tract, the isoseismals will occur elongated. Naturally, the areas or zones enclosed by any two successive isoseismals would have suffered the same extent of destruction. Over the years, researchers have tried to develop more quantitative ways for estimating earthquake intensity. 18
MAGNITUDE The magnitude of an earthquake is related to the amount of energy released by the geological rupture causing it, and is therefore a measure of the absolute size of the earthquake, without reference to distance from the epicenter. While earthquake intensity is depicted in Roman numerals and is always a whole number, magnitude is depicted in Arabic numerals and need not be a whole number. It is a more precise measure than intensity. Earthquake magnitudes are based on direct measurements of the size of seismic waves, made with recording instruments, rather than on subjective observations of the destruction caused. Similar to intensity scales, over the years, a number of approaches for measurement of magnitude of an earthquake have come into existence. Richter Magnitude, ML A workable definition of magnitude was first proposed by C.F. Richter. He based on the data from Californian earthquakes, defined the earthquake magnitude as the logarithm to the base 10 of the largest displacement of a standard seismograph (called Wood-Anderson Seismograph with properties T=0.8 sec; m=2800; and damping nearly critical ≈ 0.8) situated 100 km from the focus. M=log10 A where A denotes the amplitude in micron (10-6m) recorded by the instrument located at an epicentral distance of 100 km; and M is the magnitude of the earthquake. Because of the logarithmic nature of the definition a difference of 1.0 in the magnitude represents a difference of 10 in the seismograph amplitude. Magnitude observations by different recording stations usually differ quite widely, often by as much as one magnitude, which is later corrected taking into account the recordings from a large number of instruments. Moment magnitude Over the years, scientists observed that different magnitude scales had saturation points and the magnitudes estimated by different approaches did not point to a unique value of earthquake size The Richter magnitude saturates at about 6.8, and the surface wave magnitude at about 7.8. In addition, these magnitude estimates did not have a linear relation with the energy released due to earthquake rupture. To address these short falls, Hanks and Kanamori, in 1979 proposed a magnitude 19
scale, termed as ‘moment magnitude’, based on the seismic moment due to earthquake rupture. In addition to the magnitude scales as discussed, Surface wave magnitude, Ms, based on the amplitude of Rayleigh waves having a period of about 20 seconds, body wave magnitude, Mb based on the amplitude of first few P wave cycles are also being used. ENERGY RELEASE An approximate relationship between surface wave magnitude, Ms, and the energy released by an earthquake, E, is given by log10E = 4.8 +1.5Ms where E is measured in joules. Thus the ratio of energies released by twoearthquakes differing by 1 is magnitude is equal to 31.6. The ratio is 1000 for earthquakes differing by 2 in magnitude, Comparisons have been made between natural forces and nuclear weapons. The energy released by a 1 megaton hydrogen bomb is roughly equivalent to a magnitude 7.4 earthquake.
DIRECT AND INDIRECT EFFECTS OF EARTHQUAKE Earthquakes are major hazards and can cause catastrophic damage. They have two types of effects- direct and indirect.Direct effects cause damages directly and include ground motion and faulting, whereas indirect effects cause damages indirectly as a result of the processes set in motion by an earthquake. Direct effects - Seismic waves especially surface waves result in ground motion which can damage and destroy buildings. If a structure , such as a building or a road, straddles a fault, then the ground displacement that occurs during an earthquake will seriously damage or rip apart that structure. - earthquake vibration causes landslides in regions of steep slopes which can damage buildings and lead to loss of life - Soil vibration causes foundation failure or detachment of building off its foundation - liquefaction of soil - lateral spreading 20
Indirect effects -
Tsunamis Seiches Causes fire by damaging gas lines and electric wires Can rupture dams and levees Causes floods
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