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<br /> <br />Earthquake <br /> <br />10.1.4 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 /> <br />Magnitude <br />An earthquake’s magnitude is a measure of the energy released at the source of the earthquake. Magnitude is <br />commonly expressed by ratings on the moment magnitude scale (Mw), the most common scale used today <br />(USGS, 2017). This scale is based on the total moment release of the earthquake (the product of the distance a <br />fault moved and the force required to move it). The scale is as follows: <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 <br /> Micro—Mw < 3 <br /> <br />Intensity <br />The most commonly used intensity scale is the modified Mercalli intensity scale. Ratings of the scale as well as <br />the perceived shaking and damage potential for structures are shown in Table 10-1. The modified Mercalli <br />intensity scale is generally represented visually using shake maps, which show the expected ground shaking at <br />any given location produced by an earthquake with a specified magnitude and epicenter. An earthquake has only <br />one magnitude and one epicenter, but it produces a range of ground shaking at sites throughout the region, <br />depending on the distance from the earthquake, the rock and soil conditions at sites, and variations in the <br />propagation of seismic waves from the earthquake due to complexities in the structure of the earth’s crust. A <br />shake map shows the variation of ground shaking in a region immediately following significant earthquakes. <br /> <br />Table 10-1. Mercalli Scale and Peak Ground Acceleration Comparison <br />Modified <br />Mercalli Scale <br /> <br />Perceived Shaking <br />Potential Structure Damage Estimated PGAa <br />(%g) Resistant Buildings Vulnerable Buildings <br />I Not Felt None None <0.17% <br />II-III Weak None None 0.17% - 1.4% <br />IV Light None None 1.4% - 3.9% <br />V Moderate Very Light Light 3.9% - 9.2% <br />VI Strong Light Moderate 9.2% - 18% <br />VII Very Strong Moderate Moderate/Heavy 18% - 34% <br />VIII Severe Moderate/Heavy Heavy 34% - 65% <br />IX Violent Heavy Very Heavy 65% - 124% <br />X – XII Extreme Very Heavy Very Heavy >124% <br />a. PGA = peak ground acceleration. Measured in percent of g, where g is the acceleration of gravity <br /> <br /> <br /> <br /> <br />10-3