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<br />10-1 <br />CHAPTER 10. <br />EARTHQUAKE <br /> <br />10.1. GENERAL BACKGROUND <br />10.1.1 How Earthquakes Happen <br />An earthquake is the vibration of the earth’s surface <br />following a release of energy in the earth’s crust. This <br />energy can be generated by a sudden dislocation of the <br />crust or by a volcanic eruption. Most destructive quakes <br />are caused by dislocations of the crust. The crust may first <br />bend and then, when the stress exceeds the strength of the <br />rocks, break and snap to a new position. In the process of <br />breaking, vibrations called “seismic waves” are generated. <br />These waves travel outward from the source of the <br />earthquake at varying speeds. <br />Earthquakes tend to reoccur along faults, which are zones <br />of weakness in the crust. Even if a fault zone has recently <br />experienced an earthquake, there is no guarantee that all <br />the stress has been relieved. Another earthquake could still <br />occur. <br />Earthquakes in the Pacific Northwest have been studied <br />extensively. It is generally agreed that three source zones <br />exist for Pacific Northwest quakes: a shallow (crustal) <br />zone; the Cascadia Subduction Zone; and a deep, <br />intraplate “Benioff” zone. These are shown in Figure 10-1. <br />More than 90 percent of Pacific Northwest earthquakes <br />occur along the boundary between the Juan de Fuca plate <br />and the North American plate. <br />Geologists classify faults by their relative hazards. Active <br />faults, which represent the highest hazard, are those that <br />have ruptured to the ground surface during the Holocene <br />period (about the last 11,000 years). Potentially active <br />faults are those that displaced layers of rock from the <br />Quaternary period (the last 1,800,000 years). Determining <br />if a fault is “active” or “potentially active” depends on <br />geologic evidence, which may not be available for every <br />fault. Although there are probably still some unrecognized active faults, nearly all t he movement between <br />the two plates, and therefore the majority of the seismic hazards, are on the well-known active faults. <br />•DEFINITIONS <br />•Earthquake—The shaking of the <br />ground caused by an abrupt shift of <br />rock along a fracture in the earth or a <br />contact zone between tectonic plates. <br />Epicenter—The point on the earth’s <br />surface directly above the hypocenter of <br />an earthquake. The location of an <br />earthquake is commonly described by <br />the geographic position of its epicenter <br />and by its focal depth. <br />•Fault—A fracture in the earth’s crust <br />along which two blocks of the crust <br />have slipped with respect to each other. <br />•Focal Depth—The depth from the <br />earth’s surface to the hypocenter. <br />•Hypocenter—The region underground <br />where an earthquake’s energy <br />originates <br />•Liquefaction—Loosely packed, water- <br />logged sediments losing their strength <br />in response to strong shaking, causing <br />major damage during earthquakes. <br />Seiche—A standing wave in an <br />enclosed or partly enclosed body of <br />water and normally caused by <br />earthquake activity and can affect <br />harbors, bays, lakes, rivers and canals. <br />These events usually don’t occur in <br />proximity to the epicenter of a quake, <br />but possibly hundreds of miles away <br />due to the fact that the shock waves a <br />distance away is of a lower frequency.