Deep S-wave velocity structure at Hawaii Islands obtained by microtremor array measurements

Abstract:

Microtremor array measurements and three-component microtremor measurements have been performed at the west coast of Hawaii Island (Figure.1). Two seismographs with three-component accelerometers were used for data acquisition. At each site, one seismograph was fixed in one place and data was acquired at that location for the entire survey. Data was acquired by a second seismograph at larger separations ranging from 5 to 3403m from the fixed seismograph. Data acquisition was repeated at each new separation. In each measurement, 10 to 60 minutes of ambient noise was recorded. As the separations of seismographs increased, the record length of ambient noise was increased. The sampling interval used was 10msec. An entire measurement took several hours. Data acquisition was performed in the day-time and the seismographs were placed in relatively quiet places such as in parks or residential areas. A spatial autocorrelation was used for calculating phase velocity and a clear dispersion curve (Figure 2a) was obtained in frequency range from 0.2 to 30 Hz. A joint inversion was applied to the observed dispersion curve, and H/V spectrum, and S-wave velocity model was analyzed for the site. In the inversion, phase velocities of the dispersion curve and the absolute value and peak frequencies of the H/V spectra were used as observation data. The unknown parameters were layer thickness and S-wave velocity. A Genetic Algorithm was used for optimization. Theoretical H/V spectra and phase velocities are generated by calculating the weighted average of the fundamental mode and higher modes (up to the 5th mode) based on medium response.

Figure 2b shows an S-wave velocity model obtained by the inversion. We can see that a low velocity layer with S-wave velocity from 250 to 700 m/s exists to a depth of 90 m. A velocity layer with S-wave velocity from 900 to 1500 m/s exists at a depth range of 90 to 1300 m. Bedrock with S-wave velocity about 3000 m/s exists at a depth of 2200 m.