RECENT CONVENTIONAL SHALLOW AND DEEP SEISMIC SURVEY METHODS TO ESTIMATE SHEAR-WAVE VELOCITIES USED IN SEISMIC HAZARD ASSESSMENT AND GEOLOGIC MAPPING IN WASHINGTON

Abstract

The Washington State Department of Natural Resources, Division of Geology and Earth Resources has been conducting various shallow and deep active and passive seismic surveys to estimate shear-wave velocity (Vs) profiles in Washington and Oregon. We have used cost-effective, practical, and accurate shallow active and passive seismic survey methods such as SH-wave refraction, Multi-Channel Analysis of Surface Waves (MASW), and Microtremor Array Measurements (MAM) to estimate shallow Vs profiles for calculating time-averaged shear-wave velocity of the top 30 meters of the soil (Vs30m). Recently, we have tested new methodologies such as Horizontal-to-Vertical Spectral Ratios (H/V), joint inversion modeling of the H/V and MASW, and a Two-Station MAM to further estimate the Vs profiles with depths of 100 to 3000 meters. Each method used for estimating the shallow and deep Vs profiles has shown its strengths and weaknesses depending on the field conditions, subsurface geology and instrumental capacity. We will present various examples addressing these strengths and weaknesses, and present our online subsurface database (geotechnical, well logs, geologic and deep oil/gas or geothermal boreholes) and its contribution to processing and interpretation of seismic data obtained from the pronounced methods. Finally, we will present examples showing how to use these methods in geologic mapping, and earthquake hazard assessment applications.