Seismic and Geotechnical Site Characterizations at Four Earthquake Strong Motion Sites in Washington State

Abstract

As part of on going program for generating maps addressing geologic site effects in Washington, the Washington State Department of Natural Resources (DNR), Division of Geology and Earth Resources (DGER) drilled 30‐meter‐deep geotechnical boreholes at four strong‐motion sites operated by the Pacific Northwest Seismic Network (PNSN). Invasive (soil sampling, Standard Penetration Test (SPT) and downhole seismic) and noninvasive (Multichannel Analysis of Surface Waves, (MASW)) methods were used to determine shear‐wave velocity profiles, specifically for the top 100 feet of soil column, at each of these strong motion sites. The boreholes were logged for visual soil classification and SPTs, and selected soil samples were tested in the laboratory to obtain plasticity and gradation values. In addition, S‐and P‐waves generated at the surface were received by a 3‐component geophone placed at 1‐m intervals in each borehole. Then the borehole sites were later surveyed by using MASW method to estimate the Vs profiles at each borehole site. The MASW survey shows a very good correlation with the downhole S‐velocity profiles. SPT blow counts are consistent with soil conditions in the Puget Sound area. Site classes E, D‐E, D, C‐D and C, as described in the International Building Codes (IBC 2006), were determined by using average shear‐wave velocities and SPT blow count values to 30 meters (˜100 feet) of soil column of the four boreholes. An updateable geospatial database incorporating shear‐wave data and velocities, borehole geotechnical information (soil samples and their laboratory tests, SPT blow counts, etc.) will be generated and periodically updated. This database will directly be available through the DGER's interactive mapping service for end users such as federal and local government agencies, urban planning and emergency response groups and seismic networks, such as PNSN.