Loading...
HomeMy WebLinkAbout977700051 Geologic Hazard Assessment 1 Stratum Group File: 9.27.22 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 October 28, 2022 Jonas Rubenson Jonas@psu.edu Re: Geologic Hazard Assessment Jefferson County Parcel 977700051 Dear Jonas Rubenson: 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 an area on the western half of the subject property is a potential seismic hazard area. It is our understanding that there are plans to construct a new home and septic system 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 Geologic Map of Northeastern Jefferson County, Washington (Gayer 1976) indicates that the subject property is underlain by Vashon till (Figure 1). Vashon till consists of as a very poorly sorted mix of clay, silt, sand, and gravel with scattered cobbles and boulders. The unit is highly compacted to the point of resembling concrete and commonly forms steep, nearly vertical coastal bluffs in the Salish Sea region. Vashon till was deposited directly by overlying glacial ice between 18,000 and 14,000 years ago. The maximum ice thickness in the area has been estimated to be approximately 4,000 feet thick during the last major regional glaciation (Schasse and Slaughter, 2005; Thorsen, 1981). 2 May 19, 2022 Jefferson County Parcel 977700051 Geologic Hazard Assessment Stratum Group File: 9.27.22 Figure 1. Clipped and annotated geologic map (Gayer, 1976) showing the subject property location. Vlt= Vashon till. Geologic observations made on the site are not entirely consistent with the above-described mapping. Medium dense, unstratified to weakly stratified medium to coarse sand and pebble to cobble gravel with lesser silt consistent with Vashon ice-contact deposits was observed in hand and machine dug test pits on the subject property (Figures 2 and 3). Vashon ice-contact deposits include sediments that were deposited directly by thin wasting glacial ice as well as sediments deposited by meltwater that was in close proximity to glacial ice. This unit can be highly variable in composition, even within a relatively small area. Contreras and others describe the unit as loose to compact diamicton, pebble and cobble gravel, sand, lacustrine mud, and isolated boulders. Diamicton is a poorly sorted mix of clay, silt, sand, and gravel with scattered cobbles and boulders. The discrepancy between our observations and the above-described mapping is likely due to mapping scale issues in which all areas are not able to mapped precisely. The Gayer (2016) map shows almost the entirety of southern Marrowstone Island as underlain by Vashon till despite there being significant variability in this area. 3 May 19, 2022 Jefferson County Parcel 977700051 Geologic Hazard Assessment Stratum Group File: 9.27.22 Figure 2. Vashon ice contact material underlying the subject property. 4 May 19, 2022 Jefferson County Parcel 977700051 Geologic Hazard Assessment Stratum Group File: 9.27.22 Figure 3. Vashon ice-contact deposits underlying the subject property. SITE SPECIFIC OBSERVATIONS The 12.59-acre subject property is located in an upland area on southern Marrowstone Island approximately 1500 feet northwest of Lip Lip Point. Access to the property is via Moen Road. The property is currently undeveloped. The subject property consists of gentle slopes between 0 and 10 degrees (Figure 4). Areas of wet muddy ground were observed in the central portion of the property. Vegetation on the site consists of mixed forest stands of Douglas fir, western red cedar, red alder, and big leaf maple with an understory of sword fern, Oregon grape, Himalayan blackberry, stinging nettle, other brush, and grass. 5 May 19, 2022 Jefferson County Parcel 977700051 Geologic Hazard Assessment Stratum Group File: 9.27.22 Figure 4. The gently sloping subject property. GEOLOGIC HAZARD ASSESSMENT 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 6 May 19, 2022 Jefferson County Parcel 977700051 Geologic Hazard Assessment Stratum Group File: 9.27.22 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 5). The map indicates that C soils are medium to relatively soft soils potentially subject to ground shaking amplification. Figure 5. 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 7 May 19, 2022 Jefferson County Parcel 977700051 Geologic Hazard Assessment Stratum Group File: 9.27.22 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 (Figure 6). Figure 6. 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, medium to coarse sand (SP) with lesser gravel and very minor silt. These soils have a low liquefaction potential because they are well drained, coarse grained, and moderately well sorted. Furthermore, these soils are glacial 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 8 May 19, 2022 Jefferson County Parcel 977700051 Geologic Hazard Assessment Stratum Group File: 9.27.22 The subject property is gently sloping and not at risk of seismically induced landslides. Risk 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 home 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