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CHAPTER 10. EARTHQUAKE <br />10-2 <br /> <br /> <br />Figure 10-1. Earthquake Types in the Pacific Northwest <br />Faults are more likely to have earthquakes on them if they have more rapid rates of movement, have had <br />recent earthquakes along them, experience greater total displacements, and are aligned so that movement <br />can relieve accumulating tectonic stresses. A direct relationship exists between a fault’s length and location <br />and its ability to generate damaging ground motion at a given site. In some areas, smaller, local faults <br />produce lower magnitude quakes, but ground shaking can be strong, and damage can be significant as a <br />result of the fault’s proximity to the area. In contrast, large regional faults can generate great magnitudes <br />but, because of their distance and depth, may result in only moderate shaking in the area. <br />10.1.2 Earthquake Classifications <br />Earthquakes are typically classified in one of two ways: by the amount of energy released, measured as <br />magnitude, or by the impact on people and structures, measured as intensity. <br />Magnitude <br />Currently the most commonly used magnitude scale is the moment magnitude (Mw) scale, with the follow <br />classifications of magnitude: <br />• Great—Mw > 8 <br />• Major—Mw = 7.0—7.9 <br />• Strong—Mw = 6.0—6.9 <br />• Moderate—Mw = 5.0—5.9 <br />• Light—Mw = 4.0—4.9 <br />• Minor—Mw = 3.0—3.9