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# Ch.7 hw. - down rolling motion and the side-to-side...

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Crystal Cozart ESC1000 September 4, 2009 Ch.7 Homework 2.) Earthquakes occur either when the elastic energy accumulated in rock exceeds the frictions that holds rock along a fault, or when the elastic energy exceeds the strength of the rock and the rock breaks. 4.) The focus is an initial rupture point that lies below the surface where abrupt movement creates an earthquake. The point on Earth’s surface directly above the focus is the e picenter. 5.) A P wave is a compressional elastic, body wave that causes alternate compression and expansion of the rock. They are called primary waves because they are so fast that they are the first seismic waves to reach an observer. An S wave is another type body wave called a shear wave. It arrives after P waves and is the secondary wave to reach an observer. Surface waves travel more slowly than body waves. The two types are distinguished by motion: the up-and-
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Unformatted text preview: down rolling motion and the side-to-side vibration. 8.) The point on the Earth’s surface above the focus is the epicenter. Geologists used a time-travel curve to calculate the distance between an earthquake epicenter and a seismograph. To make this curve, a number of seismic stations at different locations record the time of arrival of seismic waves from an earthquake with a known epicenter and occurrence time. Then a graph is drawn using this data. This graph can then be used to measure the distance between a recording station and an earthquake whose epicenter is unknown. 11.) Only shallow earthquakes occur along the Mid-Oceanic Ridge because here the asthenosphere rises to levels as shallow as 10 to 15 kilometers below Earth’s surface and is too hot and plastic to fracture....
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