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Home My WebLink About995400104 Geotech Report1 Stratum Group File: 4.14.22 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 May 19, 2022 Tristen Byers byerstristen@gmail.com Re: Geologic Hazard Assessment 137 Mountain View Drive, Quilcene, WA Jefferson County Parcel 995400104 Dear Tristen Byers: This geologic hazard assessment was conducted to assess the risk of seismic hazards on the subject property to inform future development of the property. The Jefferson County Public Land Records seismic hazard map indicates that the subject property is a potential seismic hazard area. It is our understanding that there are plans to construct a new home on the site. Based on our geologic hazard assessment, the risk of seismic shaking to the proposed new home is low to moderate but at an acceptable level for residential use. The risk of liquefaction on the subject property is low. There is no risk of seismically induced landslides on the subject property. Development of the site does not warrant additional seismic design mitigation. This assessment included a visual inspection of the property and vicinity, review of available geologic mapping in the area, review of lidar (light distance and ranging) imagery of the vicinity and our own notes and observations we have made in the vicinity of the site and at locations with similar geologic conditions. GENERAL GEOLOGY The Lidar-revised Geologic Map of the Uncas 7.5 Quadrangle Clallam, and Jefferson Counties, Washington (Tabor and others, 2011) indicates that the subject property is underlain by Holocene and Pleistocene older alluvial fan deposits. The alluvial fan deposits are described by Tabor and others (2011) as Holocene and Pleistocene aged, moderately sorted, unconsolidated deposits of cobble gravel, pebbly sand, and sandy silt along the floodplain of Snow and Salmon Creeks south of Port Discovery and along Snow Creek. Geologic observations made on the site are consistent with the above-described mapping. Coarse sand and pebble gravel with minor medium sand consistent with old alluvial fan deposits were observed in hand-dug test pits on the subject property (Figures 2 and 3). RECEIVED BY DCD 7/11/2022 2 May 19, 2022 Jefferson County Parcel 995400104 Geologic Hazard Assessment Stratum Group File: 4.14.22 Figure 1. Clipped and annotated geologic map (Tabor and others, 2011) showing the subject property location. Qf4= older alluvial fan deposits. Figure 2. Old Alluvial fan deposits (coarse sand and gravel) observed in a hand dug test pits on the subject property. RECEIVED BY DCD 7/11/2022 3 May 19, 2022 Jefferson County Parcel 995400104 Geologic Hazard Assessment Stratum Group File: 4.14.22 Figure 3. Old alluvial fan deposits (coarse sand and gravel) excavated from a hand dug test pit on the subject property. SITE SPECIFIC OBSERVATIONS The subject property is located in the Snow Creek Ranch community in Quilcene. Access to the property is via Mountain View Drive. The property is currently developed with an unoccupied mobile home (Figure 4). A gravel drive and parking area are located in from of the mobile home adjacent to Mountain View Drive. The subject property consists of gentle slopes with slope angles between 0 and 5 degrees (Figures 4 and 5). Vegetation on the site consists primarily of planted grass with scattered Douglas fir and deciduous trees in the backyard. RECEIVED BY DCD 7/11/2022 4 May 19, 2022 Jefferson County Parcel 995400104 Geologic Hazard Assessment Stratum Group File: 4.14.22 Figure 4. The front yard area of the subject property. Figure 5. The backyard area of the subject property. RECEIVED BY DCD 7/11/2022 5 May 19, 2022 Jefferson County Parcel 995400104 Geologic Hazard Assessment Stratum Group File: 4.14.22 Seismic Hazard Assessment Seismic Shaking Western Washington is seismically active and seismic shaking may occur on the site due to earthquakes along the Cascadia Subduction Zone (CSZ) or other faults present in the area. The subject property is located approximately 150 miles from the CSZ. The Washington Department of Natural Resources Cascadia Seismic Scenario Shaking intensity map indicates that the subject property along with all of eastern Jefferson County will have very strong shaking intensity with a modified Mercalli intensity level of 7. Expected ground conditions for this scale include negligible damage to well-built structures of good design and construction and slight to moderate damage in well-built ordinary structures. Damage may be considerable in poorly built structures. The subject property is not located in close proximity to any known or mapped seismogenic features including active faults, active folds, paleo-seismic trench locations, earthquake epicenters, or earthquake induced mass movements. However, there are several faults in the Puget and Juan de Fuca lowlands that are poorly constrained that could extend beyond their known locations towards the subject property. This includes the Lofall Fault, the Southern Whidbey Island Fault zone, and possibly other previously unrecognized faults or fault zones. An earthquake along a previously unrecognized fault near the subject property could produce moderate to strong ground shaking. National Earthquake Hazard Reduction Program (NEHRP) seismic site class mapping has been completed by the Washington state Department of Natural Resources (WA DNR). This mapping provides guidance as to the potential ground-motion amplifying effects of soft soils during an earthquake. The NEHRP mapping indicates that the subject property is located within site class C (Figure 6). The map indicates that C soils are medium to relatively soft soils potentially subject to ground shaking amplification. RECEIVED BY DCD 7/11/2022 6 May 19, 2022 Jefferson County Parcel 995400104 Geologic Hazard Assessment Stratum Group File: 4.14.22 Figure 6. Clipped and annotated NEHRP Site Class Map of Jefferson County (Palmer and others, 2004) showing the subject property location. Based on our assessment, it is our opinion that the seismic shaking hazard at the site is low to moderate due to the potential for damaging shaking to occur during a Cascadia Subduction Zone or other faults in the area. However, the site is no worse than any other location with similar soils in eastern Jefferson County. It is our opinion that subject property is not prone to severe ground- motion amplification due to the soils underlying the site. Liquefaction Liquefaction is a process whereby saturated, unconsolidated sandy soils temporarily become liquefied and loose strength due to strong ground shaking. Liquefaction can result in differential settlement and/or lateral displacement. The soils found to be most susceptible to liquefaction are loose, fine sands containing very little silt or clay. Potential liquefaction decreases with coarser grained sand or higher silt content or if the fine sand is denser. Higher ground shaking accelerations or long periods of shaking can increase the potential for soil to liquefy. Hence, a close proximity quake may cause high accelerations and liquefaction and a distant quake with lower site acceleration but long time periods of shaking can also cause soils to liquefy. The Liquefaction Susceptibility Map of Jefferson County, Washington (Palmer and others, 2004) indicates that the liquefaction susceptibility at the subject property is very low. RECEIVED BY DCD 7/11/2022 7 May 19, 2022 Jefferson County Parcel 995400104 Geologic Hazard Assessment Stratum Group File: 4.14.22 Figure 7. Clipped and annotated Liquefaction Susceptibility Map of Jefferson County (Palmer and others, 2004) showing the subject property location. The soils underlying the site are composed of moderately dense, coarse sand (SP) with lesser gravel and very minor medium sand. These soils have a low liquefaction potential because they are very well drained, coarse grained, and moderately well sorted. Furthermore, these soils are old alluvial sediments and that would have undergone multiple previous seismic events with resulting compaction and densification. Therefore, they would not be subject to dramatic compression from seismic loads during an earthquake. Based on our assessment, it is our opinion that the liquefaction hazard at the subject property is low. Seismically Induced Landslides Our landslide hazard assessment considered the potential for seismically induce landslides. The subject property and vicinity are very gentle and are not at risk of seismically induced landslides. Risk RECEIVED BY DCD 7/11/2022 8 May 19, 2022 Jefferson County Parcel 995400104 Geologic Hazard Assessment Stratum Group File: 4.14.22 Risk is a measure of potential harm determined by combining the degree of hazard and the degree of exposure. Based on our geologic hazard assessment, the seismic shaking hazard at the site is low to moderate and the liquefaction hazard at the site is low. Human exposure to potential seismic hazards (shaking and liquefaction) within home is low to moderate due to the fact that people will occupy the building for extended periods of time. Therefore, it is our opinion that the seismic hazard risk to people within the proposed home is low to moderate but at an acceptable level for residential use. Development of the site does not warrant additional seismic design mitigation. CONCLUSIONS Based on our geologic hazard assessment, the risk of seismic shaking to the proposed new home is low to moderate but at an acceptable level for residential use. The risk of liquefaction on the subject property is low. There is no risk of seismically induced landslides on the subject property. Development of the site does not warrant additional seismic design mitigation. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding this assessment please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Geoff Malick, L.G., M.Sc. Licensed Geologist RECEIVED BY DCD 7/11/2022