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Chapter 7:Hazard Analysis—Earthquakes <br /> CHAPTER 7 - EARTHQUAKES <br /> 7.1 Description of Hazard <br /> An earthquake is the sudden release of strain energy in the Earth's crust,resulting in waves of <br /> shaking that radiate outward from the earthquake source.22 The Earth's crust, which can be <br /> oceanic or continental, is the uppermost layer of the lithosphere23. Figure 7-1 illustrates the <br /> layers that compose the Earth including the crust and the lithosphere. The oceanic crust is <br /> approximately 3 to 6 miles thick while the continental crust is approximately 20 to 30 miles <br /> thick.24 When stresses in the crust exceed the strength of the rock, it breaks along lines of <br /> weakness (either a pre-existing or new fault plane) and results in earthquakes. <br /> The point where an earthquake starts is termed the focus or hypocenter and may be many <br /> kilometers deep within the earth. The point at the surface of the crust directly above the focus <br /> is called the earthquake epicenter. The distance between the hypocenter and the epicenter is <br /> termed the focal depth. In the case of underwater earthquakes, the focal depth is measured <br /> from the hypocenter to the surface of the oceanic crust. The severity of earthquakes is <br /> dependent on the energy released from the fault or epicenter. Other factors influencing the <br /> severity of an earthquake include: magnitude, proximity to the epicenter, depth of the <br /> epicenter, duration, soil characteristics, and type of ground motion. The effects of an <br /> earthquake can be felt far from the epicenter. <br /> C_ <br /> PZ <br /> Mantle Ma,ale <br /> con[iWYS doom <br /> to ouref Care <br /> V <br /> Qa[er <br /> O core of <br /> �ICa meta <br /> Solid rn:td <br /> :i <br /> Figure 7-1. Cutaway Showing Earth's Layers25 <br /> 22 Pacific Disaster Center Website,Retrieved October 6,2009 from <br /> http://w-ww.pdc.org/iweb/earthquakes.]sp?subg=l <br /> zs The lithosphere constitutes the rigid outer layer of the planet.it includes the crust and the upper mantle. <br /> 24 Wikipedia Online Encyclopedia Website,Retrieved October 7,2009 from <br /> http://en.wikipedia.org/wiki/Crust_%28 geology%29#cite_note-amonline-0 <br /> 2'image from Solar Computer House Website,Retrieved October 7,2009 from <br /> tILtp://www.solcomhouse.com/geothennal.htm <br /> 7-1 Hawaii County Multi-Hazard Mitigation Plan <br />