Chapter 2

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University of Toronto Scarborough
Environmental Science
Jovan Stefanovic

Chapter 2 Eatherquakes the most disastrous earthquakes in moderm times occurred in China in July 1976 when an entire city was destroyed and more than 250 000 people killed in less than six minutesThe consequences of an earthquake depend on its magnitude depth distance from populated areas the naturae of the local earth materials and engineering and construction practicesWhat can explain the large differences in the number of caulaties and adamge from these four earthquakes o They occurred at different depths within Earth o In deep earthquakes some of the energy dissipates by the time the seismic waves reach the surface resulting in less damage o Building and zning regulations were important in determinig the damage these earthquakes caused o Finally In El Salvador as in many cities it was not just how structures were builth that affect the number of casulties buy also where they were builtAlthough we cannot control the geologic environment or depth of an earthquake we can do much to reduce the damage and loss of life 21 Introduction to EarthquakesEarthquakes result from rupture of rocks along a fault a fracture in Earths crustRocks on opposite sides of a fault move suddenly and energy is released in the form of seismic wavesThe epicentre of an earthquake is the point on the surface of Earth directly above the fault ruptureThe focus or hypocentre is the location of the initial rupture along the fault directly below the epicentre At the instant of the rupture seismic waves radiate outwared in all directions from the focusSeismologist scientists who study earthquakeMagnitude express the amount of energy an earthquake releasesIntensity provides a measure of the effects of a quake on people and structuresEarthquake ground motions are recorded by an intrusment called a siemogrpahEarthquake Magnitude Richter Scale quanties the magnitude of earthquakes as the logarithm to the base 10 of the maximum signal wave amplitude recorded on a thenstandard seismogram at a distance of 100 km from the epicenterOther magnitude scales have been developed since Richters time
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