USING TWO-STATION MICROTREMOR ARRAY METHOD TO ESTIMATE SHEAR-WAVE VELOCITY PROFILES IN SEATTLE AND OLYMPIA, WASHINGTON

Abstract

We performed two-station microtremor array measurements (2ST-MAM) at several sites in Seattle and Olympia, Washington. We used three-component broad-band sensors (accelerometers) for microtremor data acquisition with variable station separations ranging from 10 to 3000 meters for each site. We recorded the microtremor data ranging from 10 minutes (for closer separations) to 1 hour (for large separations), with a total of several hours of measurements per site. We used the spatial autocorrelation (SPAC) method to calculate phase velocities from the 2ST-MAM data, and obtained clear dispersion curves between 0.2 to 30 Hz. Maximum phase velocity is about 1500m/s at the frequency of 0.2Hz at downtown of Seattle. A Genetic Algorithm and a non-linear least squares method were used for the inversion, and S-wave velocity profiles down to a depth of several kilometers were estimated in the downtown Seattle area. A low velocity layer with S-wave velocity lower than 600m/s was determined down to a depth of 10m. A velocity layer with S-wave velocity of 700m/s was determined down to a depth of 700m. Bedrock with S-wave velocity higher than 1500m/s was determined at a depth of greater than 2500m. At the Olympia downtown site, a low velocity layer with S-wave velocity less than 400m/s was determined to a depth of 90m and there is a clear velocity boundary at a depth of 90m. Bedrock with S-wave velocity higher than 1000m/s was determined at a depth of about 250m. These preliminary results have shown that using the 2ST-MAM method is applicable to deep and shallow depth-to-bedrock investigations, and provides fast, cost-effective and accurate S-wave velocity estimates.