Abstract
The September 21, 1999, Chi-Chi earthquake induced strong shaking, resulting in severe damage in the Puli area. According to Huang and Tarng (2005), the collapse of many structures during the earthquake was very closely related to site effects. Shallow shear-wave velocities are widely used for earthquake ground-motion site characterization. Thus, we investigate S-wave velocity structures for the Puli area by performing microtremor array measurements at 16 sites. Dispersion curves at these sites are calculated using the F-K method (Capon, 1969) for the vertical component; S-wave velocity structures for the Puli area are then estimated by surface wave inversion (Herrmann, 1991). If the S-wave velocity of the bedrock is assumed to be 2000 m/s, the depths of the Quaternary sediments in the Puli area are between 300 m (FAL, PIP) and 870 m (DAH). Moreover, there are 3∼6 distinct interfaces in the shallow velocity structure (0∼1000 m). The depth of the bedrock gradually increases from the edge (SIN, PIP) to the center (PUL, DAH) of the basin and the thickest Quaternary sediments appear near Heng-Chih-Cheng (DAH).
Highlights
The S-wave velocity is a very important factor in theoretical simulations and ground motion predictions
After removing the time segment contaminated by instrumental or arti cial noise, we estimate phase velocities by f -k spectral analysis based on the Maximum Likelihood Method (MLM) (Capon, 1969)
It indicates that sites AIL, deepest area is near Heng-Chih-Cheng (DAH) and YUY have higher S-wave velocities at the near-surface shallow structures. At these three sites (AIL, DAH and YUY), the observed phase velocities become rather at between frequencies 1.0 and 2.7 Hz with phase velocities of about 1.3–1.5 km/s. These results suggest that a thicker layer with an S-wave velocity of about 1.4–1.6 km/s was deposited at these sites. 4.2 Inversion of the S-wave velocity structures
Summary
The S-wave velocity is a very important factor in theoretical simulations and ground motion predictions. It is largely obtained using well-logging which is a timeconsuming and costly method. A number of studies have inverted surface-wave phase velocities, obtained from microtremor recordings, to derive near-surface material properties (Horike, 1985; Matsushima and Okada, 1990; Satoh et al, 2001a, b). One of its main functions was to clarify the composition and characteristics of microtremor behavior, and to investigate the analytical capacities of different microtremorbased techniques for obtaining a quantitative description of site features, by analyzing and comparing observational data with simulations.
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