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What Are Earthquakes?

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What Are Earthquakes? Introduction: Living in England as a public we know next to nothing about earthquakes, compared to what people know about them in America, the reason they know so much about them is because they have to deal with them on a very regular basis. This article is put together to inform the public of Great Britain about what earthquakes do and how they are caused. So What Are Earthquakes: Earthquakes are, vibrations produced in the earth's crust when rocks in which elastic strain has been building up suddenly rupture, and then rebound. What Are Seismic Wave That Are Produced From Earthquakes? Seismic waves are waves of energy that are produced from the rebound of the earths crust after the rocks reach there elastic limit and rupture. There are two types of seismic waves these are P waves (primary Waves) and S Waves (Shear Waves). P Waves: P waves are longitudinal Waves. This means they cause the particles to vibrate in the direction of the shockwave. P waves are the faster of the two waves and therefore reach the epicenter of the earthquake faster than S waves. ...read more.


At present, China, Japan, Russia, and the United States are the countries most actively supporting such research. In 1975 the Chinese predicted the magnitude 7.3 quake at Haicheng, evacuating 90,000 residents only two days before the quake destroyed or damaged 90 per cent of the city's buildings. One of the clues that led to this prediction was a chain of low-magnitude tremors, called foreshocks, that had begun about five years earlier in the area. Other potential clues being investigated are tilting or bulging of the land surface and changes in the earth's magnetic field, in the water levels of wells, and even in animal behavior. A new method under study in the United States involves measuring the build-up of stress in the crust of the earth. On the basis of such measurements the US Geological Survey, in April 1985, predicted that an earthquake of magnitude 5.5 to 6 would occur on the San Andreas Fault, near Parkfield, California, sometime before 1993. Many unofficial predictions of earthquakes have also been made. In 1990 a zoologist, Dr. Iben Browning, warned that a major quake would occur along the New Madrid fault before the end of the year. ...read more.


In the earths crust the mean velocity of the P Waves is 6.1 km/s, whilst for S Waves it is 4.1 km/s. It would take the P waves about 16 seconds to travel 100 km, whereas S Waves would take about 24 seconds to travel the same distance. There the P Waves would arrive at the seismometer 8 seconds before the S Waves if the Epicentre were 100 km away. This means that the Lag-Time is 8 seconds, and also that for every second of Lag-Time the epicentre is 12.5 km away (i.e., 100/8 = 12.5). If you have the lag times from three seismometers, then you can work out the position of the epicentre. For Example, if you look at the three seismograms that you are given, then you can see that the lag times are as follows; A:24 seconds. B:48 seconds. C:28 seconds. By Multiplying these by 12.5, you can workout the distance of each of the seismometers from the epicentre; A:24x12.5 = 300 km B:48x12.5 = 600 km C:28x12.5 = 350 km On a globe or a scale map you can then draw a circle of radius 300 km for A, 600km for B and 350 km for C. The point where the three circles cross each other is the epicentre. The principle is shown on the map below. ...read more.

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