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Chapter 4.1 Earthquake Hazard Hazard Type EARTHQUAKE Probability of Occurrence HIGH Vulnerability Chapter 4.1 HIGH Risk Earthquake Hazard Profile HIGH Introduction and Oregon residents about how such a destructive event would forever change the Of all hazards that impact the Pacific Northwest, Pacific Northwest. The article successfully earthquakes cause the most widespread increased public awareness about the region’s damage to transportation, communications, seismic hazards. However, much work remains utilities, buildings, and business, and to prepare people and communities for disrupt services across all sectors of society. what most earth scientists consider a certain Earthquakes are among the most feared natural catastrophic event. hazard because they strike without warning and most of the population has little to no personal At least 20 damaging earthquakes have rattled experience with them. Washington State in the last 125 years — most in Western Washington. Since 1970, over The July 20, 2015 Pulitzer Prize winning New 5,300 earthquakes with epicenters within a Yorker article titled, “The Really Big One” 40-mile radius from central Thurston County described in detail the effects of the entire have been detected.1 Most of these events are Cascadia Subduction Zone rupturing with a simply captured as data points by seismographs magnitude 9.0 earthquake and the ensuing and pass without notice. Ninety-three of these tsunami. The article generated significant seismic events had epicenters in Thurston conversation and concern among Washington County, most less than magnitude 2. 4.1-1 Hazards Mitigation Plan March 2017 Chapter 4.1 Earthquake Hazard Figure 4.1.1 Earthquake Epicenters in Thurston County The 1949, 1965, and 2001 Nisqually This earthquake hazard profile presents an earthquakes that shook Thurston County are overview of the source, effects, risks, and a clear indication that seismic events similar a summary of historical incidents. Three to the Nisqually quake’s magnitude or greater earthquake scenarios were modeled using are likely to recur within a 25-year horizon – a a Geographical Information System (GIS) high probability of occurrence. Each of these software tool, HAZUS, to evaluate potential historic events caused significant widespread losses within Thurston County. In addition, damage to the region. The Nisqually quake is GIS hazard exposure data is shown for the a reminder of the region’s vulnerability and as incorporated and unincorporated portions of such, the Thurston Region has a high risk rating Thurston County, including local government for earthquake hazards. essential facilities that are potentially at risk to the effects of liquefaction. March 2017 Hazards Mitigation Plan 4.1-2 Chapter 4.1 Earthquake Hazard Hazard Identification An earthquake is the result of elastic energy bound within a fault releasing, due to a sudden fracture and movement of rocks inside the Earth. A fault is a fracture in the Earth where the two sides have been displaced relative to each other. Most faults in Washington, such as the Seattle fault, are a combination of strike-slip fault and a thrust or reverse fault. When a fault ruptures, the seismic energy is dispersed in waves that move through the earth in all directions, and with sufficient magnitude will cause the ground to shake violently. This shaking motion and the subsequent behavior of the earth’s surface – liquefaction, landslides, ruptures, or ground failure – causes the destruction of buildings and other infrastructure. Large earthquake can also produce secondary destructive effects including tsunamis, flooding, and fires. Figure 4.1.2 Known and Suspected Faults in Washington State Numerous known and suspected faults or fault zones exist throughout the greater Puget Sound Basin. The Olympia fault runs from northwest to southeast across Thurston County. 4.1-3 Hazards Mitigation Plan March 2017 Chapter 4.1 Earthquake Hazard Severity – Measuring the Size of an Earthquake Magnitude Several common measures are used to articulate earthquake strength. Magnitude (M) is a measurement of the total quantified energy released by an earthquake. “Moment magnitude” is calculated from the amount of movement on the fault causing the earthquake and the area of the fault surface that ruptures during the earthquake. It is a base-10 logarithmic scale, where each whole number increase in magnitude represents a ten-fold increase in measured amplitude, and about 32 times more ‘elastic’ energy released in the form of seismic waves than the magnitude that precedes it. For example, an M7 earthquake releases about 32 times more energy than a M6, while an M8 releases about 30 times more energy than an M7. A M9 earthquake thereby releases nearly 1,000 times more energy than a large M7 earthquake and nearly 33,000 times more energy than an M6 event. Figure 4.1.4 illustrates the scale of the magnitude of historic earthquakes. Figure 4.1.4 Comparison of Recent and Historic Earthquakes by Energy Release (Magnitude)2 March 2017 Hazards Mitigation Plan 4.1-4 Chapter 4.1 Earthquake Hazard Peak Ground Acceleration observations provide qualitative statements about the likely extent of damages for Peak ground acceleration (PGA) is equal earthquakes with various levels of ground to the maximum ground acceleration that shaking (PGA) at a given site: occurred during earthquake shaking at a location. PGA is equal to the amplitude of • Ground motions of only 1% g or 2% g the largest absolute acceleration recorded on are widely felt by people; hanging plants an accelerogram at a site during a particular and lamps swing strongly, but damage earthquake. Below is an excerpt from the levels, if any, are usually very low. Washington State Enhanced Hazards Mitigation • Ground motions below about 10% g Plan describing Peak Ground Acceleration.3 usually cause only slight damage. PGA is a measure of the intensity of shaking, • Ground motions between about 10% relative to the acceleration of gravity (g). For g and 30% g may cause minor to example, an acceleration of 1.0 g PGA is moderate damage in well-designed an extremely strong ground motion, which buildings, with higher levels of damage does occur near the epicenter of large in more vulnerable buildings. At this level earthquakes. With a vertical acceleration of ground shaking, some poorly built of 1.0 g, objects are thrown into the air. buildings may be subject to collapse. With a horizontal acceleration of 1.0 g, • Ground motions above about 30% g objects accelerate sideways at the same may cause significant damage in well- rate as if they had been dropped from the designed buildings and very high levels ceiling. 10% g PGA means that the ground of damage (including collapse) in poorly acceleration is 10% that of gravity, and so designed buildings. on. • Ground motions above about 50% g Damage levels experienced in an may cause significant damage in most earthquake vary with the intensity of ground buildings, even those designed to resist shaking and with the seismic capacity seismic forces. of structures. The following generalized 4.1-5 Hazards Mitigation Plan March 2017 Chapter 4.1 Earthquake Hazard The United States Geological Survey’s National Seismic Hazard Maps program produces data and maps derived from seismic hazard curves calculated on a grid of sites across the United States that describe the annual frequency of exceeding a set of ground motions. The figure below depicts probabilistic ground motions with a two percent probability of exceedance in 50 years for Washington State. Figure 4.1.5 Two Percent Probability of Exceedance in 50 Years Map of Peak Ground Acceleration4 March 2017 Hazards Mitigation Plan 4.1-6 Chapter 4.1 Earthquake Hazard Modified Mercalli Intensity The Modified Mercalli Intensity (MMI) Scale measures the earthquake intensity by the damage it causes. Peak ground acceleration (PGA) is a measure of the strength of ground movements. It expresses an earthquake’s severity by comparing its acceleration to the normal acceleration due to gravity. The MMI value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects actually experienced at that place. The lower numbers of the intensity scale generally deal with how people feel the earthquake. The higher numbers of the scale are based on observed structural damage. Structural engineers usually contribute information for assigning intensity values of VIII or above. The intensity of an earthquake is also dependent upon the magnitude, the epicenter, the depth, and the soil or rock conditions at the site. The intensity of ground shaking increases with the amount of energy released and decreases with distance from the causative fault or epicenter. The following is an abbreviated description of the levels of Modified Mercalli intensity. Intensity Shaking Description/Damage I Not felt Not felt except by a very few under especially favorable conditions. II Weak Felt only by a few persons at rest, especially on upper floors of buildings. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many III Weak people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated. Felt indoors by many, outdoors by few during the day. At night, some awakened. IV Light Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable V Moderate objects overturned. Pendulum clocks may stop. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen VI Strong plaster. Damage slight. Damage negligible in buildings of good design and construction; slight to moderate in VII Very strong well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
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