5001 23RD AVE W Geotech Report 2022-10-25

applicable or valid under normal circumstances. All private property owners should make a note of this. Earthquake Related Hazards: The following paragraphs describe potential earthquake related hazards that are known to exist within most of the northwestern United States. Earthquakes in the Pacific Northwest occur due to tectonic activity associated with the subduction of the Juan de Fuca Oceanic plate beneath the North American Continental plate. The Juan de Fuca plate is converging on and thrusting beneath the North American Continental plate along the Cascadia Subduction Zone (CSZ), which is situated offshore along Washington. This convergence along the CSZ is the source of three types of earthquakes in western Washington. These are (1) deep intraplate earthquakes originating in the Juan de Fuca plate, (2) large subduction zone-interplate earthquakes that may occur along the interface between the Juan de Fuca and the North American Plates, and (3)shallow crustal earthquakes generated along faults. Most of the intraplate earthquakes have occurred within the Puget Sound region. The estimated maximum magnitudes of CSZ intraplate earthquakes are in the range of M7.0 to M7.5. Available research indicates that there is a potential for a large subduction zone earthquake near the Washington coast. To interpret earthquake potential of the CSZ plate interface, geologic lines of evidence such as coastal subsidence, stratigraphic evidence for flooding associated with earthquakes and turbidity in the ocean have been used. Based on the available geologic evidence, there is a sufficient scientific consensus to consider the CSZ plate interface as a potential earthquake source. The estimated maximum magnitudes of CSZ interplate earthquakes are in the range of M8.0 to M9.0+. The estimated recurrence interval is 350 to 500 years. Crustal earthquakes are generally concentrated above a depth of approximately 10 to 20 km. Based on our literature review, the estimated maximum magnitudes of these crustal earthquakes are in the range of M6.0 to M6.5. Based on site geology, topography, and our preliminary evaluation, in our opinion, the site may be susceptible to severe ground shaking and landsliding during a major earthquake. Ground acceleration more than 0.3g may occur at the site. As mentioned above, it should be noted that most of the northwestern United States is susceptible to similar earthquake-related hazards. A detailed site-specific seismic evaluation of any kind was beyond the scope of this report. Based on the results of our subsurface explorations and review of available literature (2009 International Building Code), in our opinion, a site classification "C" and a design PGA of 0.35 may be used for this project. However, this site classification may be revised by performing a Project No: 0314-WA19 August 07,2019 Page No:-6ref-1.2