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Geotechnical Engineering Evaluation Miller Property Slope Stabilization Everett, Washington Table 1. 2018 IBC Seismic Design Parameters. NGA File No. 1333322 July 1, 2022 Page 7 Site Class Spectral Acceleration Spectral Acceleration Site Coefficients Design Spectral at 0.2 sec. (g) at 1.0 sec. (g) Response SS Si Parameters Fa Fv Sys Sri D 1.449 0.555 1.000 1.500 0.966 0.555 The spectral response accelerations were obtained from the USGS Earthquake Hazards Program Interpolated Probabilistic Ground Motion website (2008 data) for the project latitude and longitude. The peak ground acceleration for the site is given a value of 0.617g under these data. Although no known faults intersect the property, the site is located within proximity to the South Whidbey Island Fault Zone (SWIFZ): an active, shallow region of seismicity within central Puget Sound stretching from the Strait of Juan de Fuca to North Bend. Information published in 2013 by the Washington State Department of Natural Resources suggests the SWIFZ last ruptured less than 2,700 years ago, and that the fault zone can produce a M7.5 earthquake. The nearest mapped strand of the SWIFZ is located just under 3 miles to the southwest of the site. In our opinion, the possibility of faulting ground rupture caused by this fault zone is considered low. Hazards associated with seismic activity include liquefaction potential and amplification of ground motion. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. It is our opinion that the dense glacial deposits interpreted to underlie the site at depth and form the core of the slope have a low potential for liquefaction or amplification of ground motion. Also, stability concerns during seismic activity have been addressed through a subsurface pile wall as described in the remainder of this report. Erosion Hazard The criteria used for determination of the erosion hazard for affected areas include soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative cover and the specific surface soil types, which are related to the underlying geologic soil units. The Soil Survey of Snohomish County Area, Washington by the Natural Resources Conservation Service (NRCS), classifies the sloping portions of the site which are the subject of this study as Alderwood-Everett gravelly sandy loams, 25 to 70 percent slopes, for which a moderate to severe soil erosion rating has been mapped. Based on our experience in the area and our observations in the field, it is our opinion that the site has a moderate erosion hazard for areas where the soils are exposed. Recommendations are included in this report to reduce the potential for erosion of the native soils during and after landslide repair. NELSON GEOTECHNICAL ASSOCIATES, INC.