Seismic polarization measurements using multi-taper spectral analysis

The authors characterize the site response at a high frequency seismic array and 2 boreholes by examining the frequency-dependent polarization, between 1 and 45 Hz, of the initial longitudinal arrival (P-wave) from local earthquakes. They use a multi-taper spectral analysis to determine the polarizations. The data display a strong frequency-dependence in the apparent incidence angle. the authors show this variation is mainly due to an irregular weathered granite surface layer found in the region. Although scattering becomes prominent at high frequencies they are able to model many of the gross features of the polarization with a simple shallow 1D velocity model. The borehole polarizations also display strong frequency dependent incidence angles due to the reflected energy from the free surface. Unlike the surface data the borehole results are insensitive to the top-most surface layer, instead they help constrain the average velocity and attenuation above the sensors