Estimating Deep S‐Wave Velocity Structure Using Microtremor Array Measurements and Three‐Component Microtremor Measurements in San Francisco Bay Area
- 1 January 2012
- conference paper
- conference paper
- Published by Society of Exploration Geophysicists in Symposium on the Application of Geophysics to Engineering and Environmental Problems 2012
Abstract
Microtremor array measurements and three‐component microtremor measurements have been performed at several sites in South Bay of San Francisco Bay Area. A couple of seismographs including three‐component accelerometers were used for data acquisition. Separations of two accelerometers vary from 5 to 4125m and several different separations were used in the microtremor array measurements at one site. Length of microtremor data for one separation is about 10 to 60 minutes and measurements of one site took several hours. A spatial autocorrelation was used for calculating phase‐velocities and clear dispersion curves were obtained between 0.2 to 10 Hz. Maximum phase velocity is about 2300m/s at the frequency of 0.2Hz. Same microtremor data was used for the calculation of horizontal and vertical ratio (H/V) of spectra. There are two peak frequencies of H/V spectra. Higher peaks vary from 1 to 2Hz and lower peaks vary from 0.2 to 1Hz. A joint inversion of an H/V spectrum and a dispersion curve is applied to observed data and S‐wave velocity models down to a depth of about two kilometers were obtained. A low velocity layer with S‐wave velocity lower than 500m/s existed down to a depth of 50 to 100m at all sites. Intermediate bedrock with S‐wave velocity higher than 1000m/s existed at a depth of 500 to 1000m. Deepest bedrock with S‐wave velocity higher than 3000m/s seems to be existed at a depth of at least 1500m. It seems that the lower peak frequency of 0.2 to 1Hz in the H/V spectra is mainly due to the deepest bedrock.Keywords
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