Earthquakes are the most deadly natural hazards in the world and around 100 earthquakes could cause serious damage in a year. Earthquakes normally strike without any warning and many of the earthquake regions coincide with areas of high population density. When large earthquakes occur in such areas the results can be catastrophic, with very bad loss of human lives and unexpected economic cost.
3.1 How earthquakes happen
The Earth's rocky outer layers are divided into large sections known as tectonic plates. These are separated by fractures known as faults. An earthquake is caused when there is a movement of masses along the fault line and its build up of stress along a fault and rocks crack or slip past each other. Nevertheless, they are normally created from pressure generated by the movement of continental plates on the mantle of the earth. These plates were first theorized by a German meteorologist and astronomer named Alfred Lothar Wegener (1880-1930).At the time he was ridiculed for his ideas, but later they were substantiated by further research. Even the theory of plate tectonics only became widely accepted within the last half of the 1900s.
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There is a constant pressure on continental plates to move..Yet the friction of plate masses being pushed together only allows movement to occur in fits and starts. So, when movement ultimately comes as a result of all this continental pushing, we frequently feel it, and we call it an earthquake. Some of the geological activities induced by plate tectonics include volcanoes, mountain creation, island creation and a phenomenon known as .subduction.
Subduction occurs when one plate gets pushed beneath another plate (usually an ocean plate gets pushed beneath land mass). Imagine one huge mass of rock, being pushed beneath another huge mass of rock. As might be imagined, it is within subduction zones that the most numerous and most severe earthquakes happen. One of the most prominent subduction zones runs along the Pacific Coast of the United States. This is why California is infamous for its earthquakes.
Fig 3.1.1 The world plate tectonic map
At the Earth's surface, earthquakes manifest themselves by a shaking and sometimes displacement of the ground level. When a large earthquake occurs, the epicenter of the earthquake is located offshore; the seabed sometimes suffers sufficient displacement to cause a Tsunami. The shaking during earthquakes can also trigger landslides and occasionally, volcanic activity.
The risk of seismic activity World Map as shown in the figure
Figure3.1.2 The risk of seismic activity World Map
3.2 Type of faults
A fault is a fracture or zone of fractures between two blocks of rock which there have been a relative movement take place. If this movement rapidly occur then it's called earthquake, if it's slowly then its called creep. Length of the faults may be start from millimetres to thousands of kilometres. Most faults produce repeated displacements over geologic time. Fault relative movement can be divided in to three common types. This divided procedure depending on their motion.
3.2.1 Normal fault
In this normal fault the block above the fault has moved downward relative to the block below. This happened because of the tensional forces and it will be response to extension.
3.2.2 Reverse fault
Here the upper block, above the fault plane, move up and over the lower block. This happened because of the compression force. Reverse fault is often described as a thrust fault.
3.2.3 Strike-slip fault
It is a fault on which the two blocks slide past one another. These faults are identified as either right lateral or left lateral depending on whether the displacement of the far block is to. the right or the left when viewed from either side
3.3 Size and Frequency of Occurrence
Earthquakes use to record by a seismometer, also known as a seismograph. The magnitude of an earthquake is measured by the Richter scale. Earthquakes with a Richter magnitude of 3 or lower are mostly imperceptible and those with a magnitude of 7 cause serious damage over large areas. The Modified Mercalli Intensity Scale is commonly used by seismologists seeking information on the severity of earthquake effects. The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more significant measure of severity to the non-scientist than the magnitude, because intensity refers to the effects actually experienced at the place. The maximum intensity generally occurs near the epicentre. Intensity ratings are expressed as Roman numerals between I. the low end and XII. at the high end (http://www.seismo.unr.edu)
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Not felt except by a very few under exceptionally favourable circumstances.
Felt by persons at rest, on upper floor, or favourably placed. Delicately suspended objects may swing.
Felt indoors; hanging objects swing; vibration similar to passing of light trucks; duration may be estimated; may not be recognized as an earthquake.
Hanging objects swing; vibration similar to passing of heavy truck, or sensation of a jolt similar to a heavy ball striking the walls; standing motor cars rock; Dishes, windows, doors rattle; glasses clink and crockery clashes; in the upper range of IV wooden walls and frames creak.
V. Rather Strong
Felt outdoors; direction may be estimated; sleepers wakened, liquids disturbed, some spilled; small unstable objects displaced or upset; doors swing, close or open; shutters and pictures move; pendulum clock stop, start, or change rate.
Felt by all; many frightened and run outdoors; walking unsteady; Windows, dishes, glassware broken; knick-knacks, books, etc. fall from shelves and pictures from walls; some heavy furniture moved or overturned; a few instances of fallen plaster. small bells ring(church or school).trees bushes shaken.
VII. Very Strong
Difficult to stand; noticed by drivers of motor cars; hanging objects quiver; Furniture broken; damage to masonry, including cracks; weak chimneys broken at roof line. Fall of plaster, loose bricks, stones , tiles, cornices; waves on ponds; water turbid with mud; small slides and caving in along sand or gravel banks; large bells ring; concrete irrigation ditches damaged.
Steering of motor cars affected; partial collapse in masonry. Fall of stucco and some masonry walls; twisting and fall of chimneys, factory stacks, monuments, tower and elevated tanks; frame houses moved on foundations if not bolted down; loose panel walls thrown out; decayed piling broken from trees; changes in flow or temperature of springs and wells; cracks in wet ground and on steep slopes.
General panic; general damage to foundations; frame structures if not bolted shifted off foundations; frames racked; serious damage to reservoirs; underground pipes broken; conspicuous cracks in ground; in alleviated areas sand and mud ejected, earthquake fountains and sand craters appear.
most masonry and frame structures destroyed with foundation. Some well-built wooden structures and bridges destroyed; serious damage to dams, dikes and embankments; large landslides; water thrown on banks of canals, rivers, lakes, etc.; sand and mud shifted horizontally on beaches and flat land; Rails bent slightly.
XI. Very Disastrous
Bridges destroyed; Rails bent greatly; underground pipelines completely out of service.
Damage nearly total; Almost everything is destroyed; large rock masses displaced; lines of sight and level distorted; objects thrown in to the air.
Table Various Modified Mercalli Intensity ratings
Dr. Charles F. Richter's most valuable contributions was to recognize that the seismic waves radiated by all earthquakes can provide good estimates of their magnitudes. He collected the recordings of seismic waves from a large number of earthquakes, and developed a calibrated. system of measuring them for magnitude.
Richter showed that, the larger the intrinsic energy of the earthquake, the larger the amplitude of ground motion at a certain distance. He calibrated his scale of magnitudes using measured maximum amplitudes of shear waves on seismometers particularly sensitive to shear waves with periods of about one second. The recorded details had to be obtained from a specific kind of instrument, called a Wood-Anderson seismograph
Here are the typical effects in various magnitudes of earthquakes ranges adapted from U.S. Geological Survey documents (http://www.seismo.unr.edu).
Frequency of Occurrence
Less than 2.0
Micro-earthquakes, not felt.
About 8,000 per day
Generally not felt, but recorded.
About 1,000 per day
Often felt, but rarely causes damage.
49,000 per year (est.)
Noticeable shaking of indoor items, rattling noises. Significant damage unlikely.
6,200 per year (est.)
Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings.
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800 per year
Can be destructive in areas up to about 100 miles across in populated areas.
120 per year
Can cause serious damage over larger areas.
18 per year
Can cause serious damage in areas several hundred miles across.
1 per year
Devastating in areas several thousand miles across.
1 per 20 years
Never recorded; see below for equivalent seismic energy yield.
Extremely rare (Unknown)
Table Magnitude of Earthquake on the Richter scale
Based on US Geological Survey
Small earthquakes occur constantly in most places like California, Alaska in US and Chile, Peru, Indonesia, Iran, Portugal, New Zealand, Greece. In United Kingdom large Earthquake occur less frequently. It has been calculated that the average reoccurrences are:
an earthquake of 3.7 - 4.6 .every year
an earthquake of 4.7 - 5.5 every 10 years
an earthquake of 5.6 or larger every 100 years
In fact; in the recent years, the number of major earthquakes per year has actually decreased,. although this is probably a statistical fluctuation.
3.4 Seismic Waves
The deformations, dynamic motions, are basically sound waves radiated from the earthquake as it ruptures. While most of the tectonic plate energy driving fault ruptures is taken up by static deformation, up to 10% may dissipate .immediately in the form of seismic waves. Waves in water, rock, and air, are transfer energy long distances without moving the constituent particles of these substances very far.Â For example, a sound wave in air can travel tens and hundreds of kilometres but the air molecules themselves only shift a fraction of a millimetre. Similarly an ocean wave can go across an ocean but each individual water molecule only moves a few meters back and forward. Equivalent types of wave motion happen in the solid rock as well.
Table 3.2 .Main types of seismic waves.
P waves normally called the name of longitudinal waves. This ways travel through solid rocks and fluids like liquid layers of the earth or water.Â They are compression waves and rely on the compression strength and elasticity of the materials to propagate.Â They are known as body waves because they travel though the body of a material in all directions and not just at the surface, as water waves do. .For P waves, the motion of the material particles that transmit the energy move parallel to the direction of propagation. It's just like sound waves. The fastest kind of seismic wave is the P-wave and always arrives firstly at your house. P-waves typically travel 1.68 times faster than S-waves and 2 to 3 times faster than the Surface-waves, which typically travel at about 3.7 km/s
Fig P waves motion
Shear waves (or secondary waves) are very slower and much more caustic than P-Waves. They can travel in solid but not in liquid. They are transverse waves. This means that they make the earth vibrate .perpendicularly to the direction of the wave travel. This waves cause damage due to its configuration. It causes buildings to be thrust upwards from the ground, and then the ground drops out from under it as the wave movements on. Below the figure shows the passageway of S-waves through the earth.
Fig passageway of the S-waves
3.4.3 Rayleigh Waves
This is a combination of a P and S-Wave. It is moving in the both longitudinal and transverse direction. The surface, it moves up and down and side-to-side to the ground and the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the R-wave, which can be much massive than the other waves.
Fig Rayleigh waves
3.4.4 Love Wave
Love waves are the fastest surface waves and move the ground from side-to-side. These tell the surface to move forwards and backwards and left and right at the same time.
3.5 Effect or Impacts of Earthquakes
There are many effects of earthquakes such as landslides, shaking, ground rupture, avalanches, fires, soil liquefaction, tsunami and human impacts.
3.5.1 Avalanches and Landslides
An avalanche is a rapid flow of snow down a slope; landslides is a geological phenomenon which includes the wide range of ground movements such as rock falls ,deep failure of slopes. Both are happened due to the earthquake which may damage in hilly and mountainous areas.
3.5.2 Shaking and Ground Ruptures
Shaking and ground ruptures are the major effect of the earthquake and it make severe damage to buildings and other rigid structures. This severe damage mainly depends on the magnitude of earthquake, the distance from the epicenter and the local geological and geomorphologic conditions, which may amplify or reduce the wave propagation. The ground-shaking is measured by ground accelerations.
Specific local geological, geomorphologic, and .geostructural features can induce high levels of shaking on the ground surface even from low - intensity earthquakes. This effect is called site or local amplification. It is principally. due to the transfer of the seismic motion from hard, deep soils to soft, superficial soils. and to the effects of seismic energy focalization owing to the typical geometrical setting. of the deposits.
Earthquake may initiate fires when the electric cables or gas line breaks. This depends on the location of the earthquake. In the event of water mains rupture and. a loss of pressure, it may also become difficult to stop the spread of fire once it begins.
3.5.4 Soil Liquefaction
Soil liquefaction, it's an impact which happened because of the shaking, water-saturated granular material temporarily loses .its strength and transforms from a solid into a liquid. Soil liquefaction may cause rigid structures, such as buildings or bridges, to tilt or sink into the. liquefied deposits.
Undersea earthquakes and earthquake-.triggered landslides into the sea can cause Tsunamis, such as the Indian Ocean earthquake2004.
3.5.6 Human Impacts
Earthquakes may result in cost of death and injury; disease; the lack of basic necessities, higher insurance claims, general property damage, road and bridge damage, and the collapse of buildings or the destabilization of the base of buildings which may lead to collapse during future earthquakes.
3.6 Earthquake Prediction
An earthquake prediction is a guessing that an earthquake within a specific magnitude range will occur in a specific region and time window. Predictions are considered as such, to the extent that they are reliable for practical, as well as .scientific, purposes. Although there is evidence that at least some earthquakes in some tectonic regions are predictable.with useful accuracy of time and space, the reliability and reproducibility of the prediction techniques have not been established and are therefore generally not accepted by seismologists. For practical purposes,.seismologists produce seismic hazard assessment programmes by estimating the probability that a given earthquake or suite of earthquakes will occur.
It is claimed that the animals (eg. dog) can detect the p-wave or ultrasonic wave generated by a big underground explosion or rupture of an earthquake, even if the waves are too small for humans to sense. These waves travel faster than the Love and Rayleigh earthquake waves, which are most strongly wave, shake the ground and cause the most damage. When this happens, animals can detect the incoming earthquake wave, and start to behave in an agitated or nervous manner.