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# Earthquakes

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Earthquake basics

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### Earthquakes

1. 1. Earthquakes.An earthquake is the shaking and vibration of the Earths crust due to movement of the Earthsplates . Earthquakes are caused by the release of built-up pressure inside the Earths crust. An earthquakes power is measured on the Richter scale using an instrument called a seismometer. The point inside the crust where the pressure is released is called the focus. The point on the Earths surface directly above the focus is called the epicentre.
2. 2. TYPES OF EARTHQUAKE WAVES• 1. PRIMARY OR P WAVES: these are body waves which propagate within the body of a rock. As it spreads out, it alternately pushes (compresses) and pulls (dilates) the rock.It is a very fast wave. These waves are relatively weak and cause the surface to move in a back and forth motion horizontally.• 2. SECONDARY OR S-WAVES: This is the slower of the body waves. As an S wave propagates, it shears the rock sideways at right angles to the direction of travel. these waves cause the crust to move from side to side at right angles to the outward motion of the main wave. They are also called TRANSVERSE waves and are known to cause the most damage.• 3. SURFACE WAVES: This wave is called a surface wave because its motion is restricted to near the ground surface. Such waves correspond to ripples of water that travel across a lake. Surface waves in earthquakes can be divided into two types. The first is called a Love wave. Its motion is essentially that of S waves that have no vertical displacement; it moves the ground from side to side in a horizontal plane but at right angles to the direction of propagation. The horizontal shaking of Love waves is particuly damaging to the foundations of structures. The second type of surface wave is known as a Rayleigh wave. Like rolling ocean waves, Rayleigh waves wave move both vertically and horizontally in a vertical plane pointed in the direction in which the waves are travelling.•
3. 3. Measuring Earthquakes• Earthquake magnitude can be measured using the Ritcher Scale while the Mercali Scale measures the intensity of an earthquake.. The Richter Scale measures the amount of seismic energy released in an earthquake, While the Mercali scale measures the intensity of shaking and the amount of damage done by an earthquake.
4. 4. Effects of an Earthquake.• Effects are often classified as primary and secondary impacts. Primary effects occur as a direct result of the ground shaking. Examples of primary effects include collapse of buildings, Death, injuries and disruption of transport and communication routes. Secondary effects occur as a result of the primary effects, e.g. tsunamis or fires due to ruptured gas mains and homelessness.• Earthquake Hazards• I. Ground Shaking: The first main earthquake hazard is the effect of ground shaking. Buildings can be damaged by the shaking itself or by the ground beneath them settling to a different level than it was before the earthquake (subsidence).• II. Liquefaction: This is the mixing of sand or soil and groundwater during the shaking of a moderate or strong earthquake. When the water and soil are mixed, the ground becomes very soft and acts similar to quicksand. If liquefaction occurs under a building, it may start to lean, tip over, or sink several feet. The ground firms up again after the earthquake has past and the water has settled back down to its usual place deeper in the ground. Liquefaction is a hazard in areas that have groundwater near the surface and sandy soil.
5. 5. • III. Tsunamis: A tsunami is formed in a body of water, such as an ocean or sea, by an event that causes enough of a disturbance so that a vertical displacement of the water column occurs. Tsunamis can be caused by undersea earthquakes, volcanic eruptions or landslides. An earthquake-induced tsunami is caused by the shifting of the sea floor. A tectonic earthquake; that is, one that is caused by changes in the earth’s crust, but happening under the sea, causes the water above the area where the movement occurred to be displaced. As it attempts to get back to normal, in accordance with gravitational pull, the sea floor either raises or lowers, which can cause the tsunami.
6. 6. • IV. Ground displacement(ground movement) This usually occurs along a fault. If a structure (a building, road, etc.) is built across a fault, the ground displacement during an earthquake could seriously damage or rip apart that structure
7. 7. Factors affecting the impact of an earthquake.• Distance from the epicentre – the effects of an earthquake are more severe at its centre.• The higher on the Richter scale, the more severe the earthquake is.• Level of development (MEDC or LEDC) – MEDCs are more likely to have the resources and technology for monitoring, prediction and response.• Population density (rural or urban area). The more densely populated an area, the more likely there are to be deaths and casualties.• Communication - accessibility for rescue teams.• Time of day influences whether people are in their homes, at work or travelling. A severe earthquake at rush hour in a densely populated urban area could have devastating effects.
8. 8. Predicting and monitoring earthquakes• 1. Animal Behaviour: some animals are very sensitive to sound, temperature, touch, light intensity and even magnetic fields, so it is perfectly possible that they can indeed detect the seismic activity which precedes an earthquake.• 2. Foreshocks. These are small earthquakes before a major one that can give warnings.• 3. Seismic History: Seismologists can study the seismic history of earthquakes and try and make predictions of when future earthquakes are likely to happen.• 4. Radon: The release of radon has been studied as a precursor to a major earthquake. However, this is not exactly conclusive.
9. 9. • LEDC’s often suffer more from the effects of volcanoes and earthquakes than MEDC’s.This is because of a number of reasons such as:• Construction standards tend to be poor in LEDCs. Homes and other buildings may suffer serious damage when a disaster occurs.• Buildings collapsing can cause high death tolls.• Evacuation and other emergency plans can be difficult to put into action due to limited funds and resources.• Clearing up can be difficult. There may not be enough money to rebuild homes quickly and safely. Many people could be forced to live in emergency housing or refugee camps.