2019_KittitasCountyHMP_Volume 1_For Adoption

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CHAPTER 10. EARTHQUAKE 10-3 • Micro—Mw < 3 Estimates of moment magnitude roughly match the local magnitude scale (ML) commonly called the Richter scale. One advantage of the moment magnitude scale is that, unlike other magnitude scales, it does not saturate at the upper end. That is, there is no value beyond which all large earthquakes have about the same magnitude. For this reason, moment magnitude is now the most often used estimate of large earthquake magnitudes. Intensity Currently the most commonly used intensity scale is the modified Mercalli intensity scale, with ratings defined as follows (USGS, 1989): • I. Not felt except by a very few under especially favorable conditions • II. Felt only by a few persons at rest, especially on upper floors of buildings. • III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it is an earthquake. Standing cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated. • IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like a heavy t ruck striking building. Standing cars rocked noticeably. • V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop. • VI. Felt by all; many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight. • VII. Damage negligible in buildings of good design and construction; slight in well -built ordinary structures; considerable in poorly built or badly designed structures. Some chimneys broken. • VIII. Damage slight in specially designed structures; considerable damage in ordinary buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. • IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. • X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent. • XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly. • XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air. 10.1.3 Ground Motion Earthquake hazard assessment is also based on expected ground motion. This involves determining the annual probability that certain ground motion accelerations will be exceeded, then summing the annual probabilities over the time period of interest. The most commonly mapped ground motion parameters are the horizontal and vertical peak ground accelerations (PGA) for a given soil or rock type. Instruments called accelerographs record levels of ground motion due to earthquakes at stations throughout a region. These readings are recorded by state and federal agencies that monitor and predict seismic activity.