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Vantage to Pomona Heights Chapter 3 230 kV Transmission Line Project FEIS Affected Environment PAGE 3-292 The majority of faulting in the area is associated with creation of this fold belt during the late Miocene Epoch; therefore, they are not considered active for transmission line design purposes. Faults that are considered active are shown on the Geohazards Map in Appendix A and are discussed in more detail in Section 3.15.4. The Project study area was subject to as many as 40 catastrophic floods during the Pleistocene Epoch (18,000-10,000 years ago), as a result of glaciers damming and releasing the Clark Fork River in northern Idaho and Montana. At Wallula Gap, south of the Tri-Cities, the constricted topography trapped the flooding water, allowing it to back up into the Project area where sediments settled onto hillsides, terraces, and valleys (Lind and Vachon n.d.). Evidence of these events on Joint Base Lewis-McChord Yakima Training Center (JBLM YTC) consists of backwater deposits of granite erratics, silts, sands, and gravel. More recently, during the late Pleistocene (10,000 years ago), the continental and alpine glaciers melted back, releasing large amounts of water and trapped sediment. Windblown glacial silt, called loess, was deposited in a thick layer across eastern Washington. Loess comprises the primary component of the rich, silt-loam soils of the area (Lind and Vachon n.d.). 3.15.2.2 Geologic Hazards Topography in the Project study area consists of gently rolling to moderate hilly plateaus and steep slopes from Umtanum Ridge, Manastash Ridge (MR), and the Saddle Mountain Ridge to the Columbia River and Lower Crab Creek. Elevations in the Project study area range from 400 to 3,400 feet above sea level. Geologic hazards in the Project area generally consist of Quaternary faults and their associated seismogenic events such as earthquakes, liquefaction, and landslide susceptibility. Earthquakes are the expression of large energy releases that result from sudden movement along faults. Quaternary faults are considered active and therefore are likely to have earthquakes occur along their length in the future. The U.S. Geological Survey (USGS) measures seismicity as the probability an area would be affected by a damaging earthquake. It is measured as the probability of a certain degree of ground shaking in terms of the percentage of acceleration due to gravity. In accordance with the National Electric Safety Code, Pacific Power is required to consider the potential for seismic activity in the design of transmission line structures and facilities and must construct the transmission line structures and substation facilities to withstand seismic forces. The Project study area is located in a moderately active seismic region of Washington designated by the Uniform Building Code as Seismic Zone 2B, which is the middle of the scale. The USGS Quaternary Fault and Fold Database (part of the Earthquake Hazards Program) and data obtained from WDGER were reviewed to identify potentially hazardous faults near the Project study area. The Saddle Mountains Fault is an active fault located along the bottom of the northern slope of the Saddle Mountains. Additional active faults identified in close proximity to the Action Alternative route segments included Umtanum Ridge-Gable Mountain-Associated Structure Faults (crossing at Route Segment NNR- 3) and Saddle Mountains-Associated Structure Faults (Route Segments NNR-4, NNR-6, NNR-7, NNR-8, and MR-1). All of the structure faults within two miles of the Action Alternatives are considered to be of indeterminate age at this time and are therefore classified as Class B structures. This classification indicates further study would need to be conducted to determine whether the faults are Quaternary in age and, therefore, considered active. Both of these fault systems are thrust faults associated with anticlines and both show evidence of quaternary-age movement along some portion of the fault systems, but not along the structure faults that are in close proximity to the Project’s Action Alternatives. The nearest fault with evidence of quaternary movement is a concealed trace of the Saddle Mountains Fault, lying just east of the Columbia River approximately three miles from Route Segment NNR-8. Two additional faults lie