S-wave velocity profiles near the Hayward fault obtained by active and passive surface wave methods
Abstract
We measured S-wave velocity (Vs) profiles at five sites the Hayward (Fig.1) using active and passive surface wave methods. The methods are multichannel analysis of surface waves using an active source (MASW), a passive surface wave method using geophones in a linear array (MAM), and a two-station spatial autocorrelation method (2ST-SPAC) using long-period accelerometers. Maximum array size varied from 345 to 565 m depending on the site. Minimum frequency and corresponding maximum wavelength ranged from 0.6 to 2 Hz and 500 to 1500 m, depending on the site. Phase velocities obtained from the three methods were combined to produce a single dispersion curve for each site. A nonlinear inversion consisting of a least squares method and a genetic algorithm was used to estimate Vs profiles from the dispersion curves to a depth of 400 to 700 m depending on the site. Vs profiles (Fig.2) show significant differences across the Hayward fault. On the west side of the fault (Southgate Park to Cemetery), there is a low velocity layer at the surface, with Vs less than 400 m/s to a depth of 40 to 90 m. A thick intermediate velocity layer with Vs ranging from 400 to 1200 m/s lies beneath the low velocity layer. Bedrock with Vs greater than 1200 m/s was measured at depths greater than 450 m. On the east side of the Fault (CSU East Bay), thickness of the low velocity layer (Vs < 400 m/s) is less than 5 m, and depth to bedrock (Vs > 1200 m/s) is less than 170 m. The results of this investigation are generally consistent with existing borehole Vs logs and other geophysical investigations, and indicate that bedrock depth changes approximately 300 m across the fault. To evaluate the effect of a significant change of bedrock depth on surface ground motion due to earthquakes, a representative Vs cross section perpendicular to the fault was constructed and theoretical amplification including two-dimensional structure was calculated using a viscoelastic finite-difference method. Calculation results show that the low frequency (1 to 3 Hz) component of ground motion is locally amplified on the west side of the fault because of the effect of two-dimensional structure.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMNS32A..03H
- Keywords:
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- 7212 SEISMOLOGY Earthquake ground motions and engineering seismology;
- 7255 SEISMOLOGY Surface waves and free oscillations;
- 0935 EXPLORATION GEOPHYSICS Seismic methods;
- 0545 COMPUTATIONAL GEOPHYSICS Modeling