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2025 Hazard Mitigation Plan Kittitas Gounty, Washington 4,6,4, Earthquake 4.6,4.1. Description Earthquakes are rapid shaking of the earth caused by the release of energy stored in rocks. The United States Geological Survey (USGS) defines an earthquake as the sudden slip on a fault and the resulting ground shaking and radiated seismic energy caused by the slip, or by volcanic or magmatic activity, or other sudden stress changes in the earth. Ground shaking is a term utilized to describe the vibration of the ground during an earthquake. Other than the initial ground shaking, additional earthquake hazards include building collapses, displacement or cracking of the earth's surface, landslides, liquefaction, tsunamis, seiches, flooding as a result of damage to dams or levees, and fires from ruptured gas lines, downed power lines and other sources..125 Landslides and flooding will be addressed separately in this Plan. Earthquakes cause both vertical and horizontal ground shaking which varies both in amplitude (the amount of displacement of the seismic waves) and frequency (the number of seismic waves per unit time), usually lasting less than 30 seconds.126 Buildings with foundations resting on unconsolidated landfill and other unstable soil, and trailers or homes not tied to their foundations are at risk because they can be shaken off their mountings during an earthquake. When an earthquake occurs in a populated area, it may cause deaths, injuries, and extensive property damage. The Pacific Northwest's geography (Washington, Oregon, and northern California) is shaped by the Cascadia Subduction Zone (CSZ), the largest active fault. The CSZ "megathrust" fault is approximately 621 miles long and it stretches from Northern Vancouver lsland to Cape Mendocino, California and it separates the Juan de Fuca and North American plates. 127 This fault produces some of the largest and most damaging earthquakes in the world. Most of the Pacific Northwest deep earthquakes are a result of the rupture faults within the subducting Juan de Fuca plate (beneath Puget Sound at depths from 19 to 44 miles)..128 In the Pacific Northwest, depending on where the earthquake occurs, seismologists characterize the Cascadia region earthquakes into four (4) different types - Cascadia megathrust, deep intraplate, crustal faulting, and volcanic earthquakes. Volcanic earthquakes are generally too small and remote to cause direct damage, but they provide evidence about potential volcanic eruptions..l2e Geologists classify faults by their relative hazards and determining if a fault is "active" or "potentially active" depends on geologic evidence, which may not be available for every fault. The Washington Geological Survey identifies a fault active when there is evidence for movement within the Holocene time period (since about 12,000 years ago). lt usually also means that there are earthquakes (even small 125 U.S Geological Survey. (n.d.). Earthquake Hazards Program Glossary. Retrieved from httos ://www.usqs.qov/olossarv/earthquake-hazards-proqram. 126 U.S Geological Survey. (n.d.). What are the Effects of Earthquakes. Retrieved from https://www.usqs.qov/proqrams/earthquake-hazards/what-are-effects-earthquakes. 127 Pacific Northwest Seismic Network. (n.d.). Cascadia Subduction Zone. Retrieved from httos://pnsn.orq/outreach/earthquakesources/csz. 128 Pacific Northwest Seismic Network. (n.d.). Deep Earthquakes. Retrieved from https://pnsn.org/outreach/earthquakesources/deepearthquakes. 12e Pacific Northwest Seismic Network. (n.d.). PNW Earthquake Sources Overview. Retrieved from https://pnsn.orqioutre uakesources Chapter 4: Hazard ldentification and Risk Assessment 107