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Earthquakes are by far the most destructive short-term natural force on earth and have plagued civilisations for millennia. An Earthquake is the set of vibrations produced in the Earth's crust when rocks in which elastic strain has been building up suddenly rupture, and then rebound. The vibrations can range from barely noticeable to catastrophically destructive. Earthquakes can release energy thousands of times greater than the world's first atomic bomb.
Earthquakes are almost always classified according to their cause. Therefore they are most often referred to as tectonic hazards as they are caused by the movements of the plates of the earth's crust known as tectonic activity. Mini earthquakes can be caused by other ways, often due to volcanic activity or by human activity. If volcanic activity is the cause then the earthquake is still classified as a tectonic hazard as volcanoes are also tectonic phenomena. Humans can induce earthquakes through a variety of activities, such as the filling of new reservoirs, the underground detonation of atomic explosives, or the pumping of fluids deep into the Earth through wells. In England, for example, there are sporadic small earthquakes caused by the collapse underground of old mine workings. If human activity has caused the earthquake then it can be classified as a quasi-natural hazard as it has been initiated by humans but has still occurred within the natural environment.
As I have said earthquakes can have three general causes, due to tectonic, volcanic or human activity, but it is the tectonic variety that causes most risk and on the largest scale so it is these that I shall be talking about. The ultimate cause of plate tectonic quakes is stresses set up by movements of the dozen or so major and minor plates that make up the Earth's crust.
Most tectonic quakes occur at the boundaries of these plates, known as faults, in zones where one plate slides past another-as at the Pacific Rim and at the San Andreas Fault in California-or is subducted (slides beneath the other plate). A fault is most accurately defined as a fracture in the crust of the earth along which rocks on one side have moved relative to those on the other side. Most faults are the result of repeated displacements over a long period of time.
An earthquake is caused by a sudden slip on a fault. Stresses in the earth's outer layer push the sides of the fault against each other. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake. However earthquakes do not occur regularly and not at all conservative or destructive faults. This is due to 'creeping' of the plates.
There are three general types of fault at which earthquakes can occur. They are :
Compressional and faults can cause considerably more damage than transform faults with equal magnitude as their nature causes vertical movement rather than horizontal. Transform faults will cause buildings to sway back and forth allowing flexible frames to absorb most of the force. However, compressional faults suddenly raise and drop buildings inches or even feet at a time, creating forces that will topple even the most well designed buildings.
Shock waves propagate like ripples from the focus and epicentre, decreasing in intensity as they travel outwards. Six kinds of shock waves are generated in the process. Two are classified as body waves-that is, they travel through the Earth's interior-and the other four are surface waves. The waves are further differentiated by the kinds of motions they impart to rock particles. The main types of seismic waves are primary waves (P waves) and shear waves (S waves). P waves cause particles to vibrate in the same direction as the shock wave (left). P waves are the first to be recorded in an earthquake because they move faster than S waves (right), which cause vibrations perpendicular to the direction of travel. P waves always travel at higher velocities than S waves, so whenever an earthquake occurs, P waves are the first to arrive and be recorded at geophysical research stations throughout the world.
Quakes tend to occur in clusters that strike the same area within a limited time period. The largest quake in a cluster is called the mainshock, those before it are called foreshocks, and those after it are called aftershocks.
In any cluster, most quakes are aftershocks. Most aftershocks are too small to cause damage, but following a large mainshock one or more may be powerful. These occur as the stress that has built up is released over a longer period of time in more, smaller, shocks. Foreshocks can be a good prediction tool, while aftershocks can further contribute to damage caused by the main shock and hamper aid efforts to help those affected by the initial quake.
Another destructive effect of earthquakes is the generation, usually by undersea tremors, of so-called tidal waves. Because such waves are unrelated to the tides, they are more properly called seismic sea waves or-their Japanese name-tsunamis. These towering walls of water have struck populated coastlines with such violent fury that entire towns have been destroyed. In 1896 Sanriku, Japan, with a population of 20,000, suffered such a devastating fate.
Where buildings have been constructed on filled ground, the liquefaction of soils is another seismic hazard. Liquefaction is a physical process that takes place during some earthquakes that may lead to ground failure. As a consequence of liquefaction, soft, water-saturated, fine grain sands and silts behave as viscous fluids rather than solids. Liquefaction takes place when seismic shear waves pass through a saturated granular soil layer, distort its granular structure, and cause some of its pore spaces to collapse. The collapse of the granular structure increases pore space water pressure, and decreases the soil's shear strength. If pore space water pressure increases to the point where the soil's shear strength can no longer support the weight of the overlying soil, buildings, roads, houses, etc., then the soil will flow like a liquid and cause extensive surface damage. Buildings resting on material that has become liquefied have literally been swallowed up.
Most earthquakes originate at plate boundaries where the plates of the earth's crust, lithosphere plates, are converging, diverging or sliding past each other. The most powerful earthquakes are associated with plate subduction , where one plate thrusts under another in deep subduction zones. Subduction-zone quakes account for nearly half of the world's destructive seismic events and 75 per cent of the Earth's seismic energy. They are concentrated along the so-called Ring of Fire, a narrow band about 38,600 km (24,000 mi) long, that coincides with the margins of the Pacific Ocean. The points at which crustal rupture occurs in such quakes tend to be far below the Earth's surface, at depths of up to 680 km (422 mi).
Tectonic earthquakes beyond the Ring of Fire occur in a variety of geological settings. Mid-ocean ridges-the seafloor-spreading centres of plate tectonics-are the sites of numerous such events of moderate intensity that take place at relatively shallow depths. These quakes are seldom felt by anyone and account for only about 5 per cent of the Earth's seismic energy, but they are recorded daily on the instruments of the world-wide network of seismological stations.
Another setting for tectonic earthquakes is a zone stretching across the Mediterranean and Caspian seas and the Himalayas, terminating in the Bay of Bengal. Within this zone, which releases about 15 per cent of the Earth's seismic energy, continental land masses riding on the Eurasian, African, and Australian plates are being forced together to produce high, young mountain chains. The resulting quakes, which occur at shallow to intermediate depths, have often devastated areas of Portugal, Algeria, Morocco, Italy, Greece, the Former Yugoslav Republic of Macedonia, Turkey and other countries partly or completely on the Balkan Peninsula; Iran; and India.
One other category of tectonic earthquake includes the infrequent but large and destructive quakes that occur in areas far removed from other forms of tectonic activity. Prime examples of these so-called midplate quakes are the three massive tremors that shook the region around New Madrid, Missouri, in 1811 and 1812. Powerful enough to be felt 1,600 km (1,000 mi) away, these shocks produced movements that re-routed the Mississippi River. Geologists believe that the New Madrid quakes are a symptom of forces tearing apart the Earth's crust, forces such as those that created Africa's Rift Valley.
As I have previously mentioned earthquakes can be caused by volcanoes or human activity. Where this is the case the quakes are limited to the area immediate to the cause.
Thousands of earthquakes occur yearly. Fortunately, only a few are powerful enough to be destructive. During this century, the world average was about 18 major earthquakes of magnitude 7 on the Richter scale per year. As earthquakes are generally caused by the build up of stress between two plates, which is subsequently released during the quake, it follows that an earthquake will occur in a region on a vaguely regular period. The stress will accumulate at a constant rate and the two plates will release at a certain point regularly. However it is very dangerous to assume this as earthquakes are quite random and could occur at any time, with the phenomenon of fault creep adding to the unpredictability.
It has recently been said throughout the world that earthquakes are on the increase, and it may seem that more earthquakes are occurring. However earthquakes of magnitude 7.0 or greater have remained fairly constant throughout this century and according to records have actually seemed to decrease in recent years.
The explanation for this is that in the last 20 years we have been able to locate more earthquakes yearly because of the increase in the number of seismograph stations in the world and improved global communications. [e.g., 1931 there were 350 stations operating in the world, today more than 4,000 stations and the data comes in rapidly from these stations by telex, computer and satellite. This increase has help us and other seismological centres to locate many small earthquakes which were undetected in earlier years.
Earthquakes can cause massive damage and destruction if they occur in populated areas and it is for prevention and warning reasons that humans use scales to measure the magnitude and intensity of an earthquake are measured. When warnings are given people need to know approximately how large the quake will be in order to take action and a measurement scale is the easiest way of communicating this information. Measurements are also made so that comparisons of earthquakes an be made.
The effect of an earthquake on the Earth's surface is called the intensity. The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally - total destruction. Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli (MM) Intensity Scale. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects. The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the non-scientist than the magnitude because intensity refers to the effects actually experienced at that place. The maximum observed intensity generally occurs near the epicentre.
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. Structural engineers usually contribute information for assigning intensity values of VIII or above.
Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs. Seismographs record a zig-zag trace that shows the varying amplitude of ground oscillations beneath the instrument. The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs.
Adjustments are included for the variation in the distance between the various seismographs and the epicentre of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value. Events with magnitudes of about 4.5 or greater - there are several thousand such shocks annually - are strong enough to be recorded by sensitive seismographs all over the world.
Great earthquakes, such as the 1964 Good Friday earthquake in Alaska, have magnitudes of 8.0 or higher.
When they occur in heavily populated areas earthquakes can be disastrous. Many thousands of people can be killed and injured. This is especially true in ELDC countries where proper earthquake warnings, building precautions and response plans are not available. Many of the most disastrous earthquakes have occurred in ELDC countries and also in the early stages of development in EMDC countries, for example in California and Tokyo in the 19th early 20th centuries. On April the 18 1906 an earthquake occurred along the San Andreas Fault causing considerable damage and loss of life to the city of San Francisco as 3,000 people died. The town of Santa Rosa to the north of the city was totally destroyed in the quake that measured 7.9 on the Richter scale. 200,000 people died in Tokyo in 1802 and 100,000 lives were lost in 1857 as two large earthquakes hit. In 1923 a magnitude 8,3 quake struck near Tokyo and Yokohama causing both cities to be engulfed in massive firestorms that burnt the cities to the ground.
ELDC countries that have been devastated by earthquakes include India where, in 1737, around 300,000 people were killed by an earthquake. The largest earthquake ever to be recorded occurred on May 22nd 1960 in Chile with a magnitude of 9.5 on the Richter scale. Shock waves caused destruction that spread over 90,000 square miles, destroying 50,000 homes and killing 6,000 people. This death toll is small compared to the two in Japan and in India as the area was affected was predominantly countryside with a low population density, so giving a reduced death toll compared to the earthquakes that occurred in the cities.
As I have shown earthquakes can be a devastating phenomenon when they hit a densely populated area, causing death and disruption, but have their effect dramatically reduced in more sparsely populated areas.
I have just mentioned earthquakes cause tremendous disruption when they hit cities, but what does this disruption entail. The level and type of disruption of a city depends on a number of factors including the levels of warning system it has, the design and structure of buildings and the response plan that is in place to prevent deaths after the initial shock has finished. In cities without these precautions devastation can be complete. Buildings will collapse, bridges will fall, dams may break, fires will start and emergency services will be unprepared and slow to reach areas of devastation.
The type of construction determines how well a structure survives a quake. Lightweight, steel-framed buildings with strength and flexibility, and reinforced concrete buildings with few structure weakening window and door openings generally suffer little damage. However, even if building are able to withstand an earthquake, they are still vulnerable to foundation failure, which cause buildings to topple over when the ground gives way beneath them or due to soil liquefaction.
The human response to an earthquake depends both on the magnitude of the earthquake and the position that the person is in at the time. For small quakes the action taken will be less severe as the danger is less. Geographical location is another factor as people are less likely to take action if they are in the countryside than if they were in a city as they is less danger in outside of urban areas.
In cities the response will depend upon what you are doing when the earthquake strikes. If you are sat in a car or on a bus there is little that you can do except stop the vehicle or move it to an area away from tall structures. If you are in a building the advice is to get under a table or stand under a door frame or other strong point in the buildings structure.
It is possible to prevent an earthquake from occurring as it is a part of nature and involves the workings and structure of the planet itself. The only real way of preventing the hazard that an earthquake presents is to move away from the area at risk. This would reduce risk to 0. However this does not often occur for obvious reasons; whole cities have been built in earthquake prone areas and to move would be impractical. Instead humans find ways of adapting to the risks presented. I shall outline some of those ways.
In EMDC cities that are under threat from earthquakes this sort of advice is widely spread. I found this on the internet. Find table and put it up.
The need for education is one of the best ways to reduce the effects of the an earthquake. People are also told to prepare themselves 'quake kits' which include :
As I have already mentioned the design of structures is very important to reduce the risk of collapse and bridges must be built to withstand shaking and 'rippling' along their length. All human underground activity must also be designed to withstand an earthquake
In planning for earthquakes response plans must be drawn up with plans for emergency food, water and shelter provision, emergency rescue teams organised and emergency power supplies for important buildings such as hospitals.
Finally for the best protection from an earthquake an adequate warning system is required so that residents and emergency services can prepare themselves and take safety action.
Earth scientists have begun to forecast damaging earthquakes in California. Although quake forecasting is still maturing, it is now reliable enough to make official earthquake warnings possible. These warnings help government, industry, and private citizens prepare for large earthquakes and conduct rescue and recovery efforts in the aftermath of destructive shocks. Earthquake forecasts declare that a tremor has a certain probability of occurring within a given time, not that one will definitely strike. In this way they are similar to weather forecasts. Scientists are able to make earthquake forecasts because quakes tend to occur in clusters that strike the same area within a limited time period. The U.S. Geological Survey (USGS) first began forecasting aftershocks following the 1989 magnitude 7.1 Loma Prieta, California, earthquake. By studying previous earthquakes, scientists had detected patterns in the way aftershocks decrease in number and magnitude with time.
With such knowledge, scientists can estimate the daily odds for the occurrence of damaging aftershocks following large California tremors. Aftershock forecasts assist government, industry, and emergency response teams in deciding when it is safe to demolish, repair, or allow people to use damaged structures. In the aftermath of the 1989 Loma Prieta earthquake, the "probability of aftershocks given by the USGS was one of the factors ... used in deciding how many fire-fighters to keep on duty"
Some large earthquakes are preceded by foreshocks. Knowledge of past earthquake patterns allows scientists to estimate the odds that an earthquake striking today is a foreshock and will soon be followed by a larger mainshock in the same area. These odds depend on the earthquake's magnitude and the seismic history of the fault on which it occurred. When a moderate earthquake hits California, scientists immediately estimate the probability that a damaging mainshock will follow. If the threat is significant, a warning is issued.
Other ways of prediction include using data from previous earthquakes and their magnitudes and intensities. By predicting the intensity of shaking due to an earthquake before the earthquake occurs plans to prevent damage can be made. Doing this rapidly after an earthquake can help manage the emergency response efforts.
Intensity is a measure of the effects of earthquakes. The variations in the seismic threat across an area are depicted on maps such as this as zones of different risk levels. These maps predict where the greatest intensities will occur in an earthquake and so the buildings and structures here can be made with greater earthquake survival specifications. This will mean that the structure is less likely to fail and so will reduce risk. These maps are produced by the government so that building regulations can be enforced.
The perception of earthquakes varies widely depending on where a person lives. If a person lives in an area that is at risk from earthquakes their perception will be much greater, an they will have more understanding of the devastation that they can cause, than a person who lives in area where earthquakes are unlikely to occur. This is not as bad as it can be for some other hazards as earthquakes are quite limited to where they occur and so if you live in an area where they do not occur it will not cause you too much risk. However if you live in an area where earthquakes are common it is imperative that you understand what to do if an earthquake strikes and how best you can act to maintain your personal safety.
I have shown through my presentation how earthquakes occur due to movement of the earth's crust and the characteristics of an earthquake such as shock waves and Tsunamis. I have shown where earthquakes occur, mainly around the pacific rim and on plate boundaries, how often they occur and how large they can be. I have used several case studies to do this and have illustrated the after effect that can occur in an urban area due to failure of structures, liquefaction and rupture of utilities. Finally I have shown the types of action that should be taken in an earthquake and the necessary measures that should be taken to reduce earthquake hazard and the measures that are being developed to improve earthquake prediction.
Earthquakes are a high risk hazard for those living in urban areas in areas prone to seismic activity, but risk is reduced in rural areas. It should be remembered that the only way to take yourself out of the risk of an earthquake hazard is to move out of the area in which they can occur.