Estimating site amplifications of the longmenshan region through inversions of the aftershock data

Considering 1173 recordings of 35 stations from 67 aftershocks of the 12 May 2008 Wenchuan M_w (moment magnitude) 7.9 earthquake, we investigate site amplifications and their variations in the Longmenshan region. Site responses of 35 stations are analyzed using the coda-wave and S-wave methods. For these methods, the site amplifications are computed using a generalized inversion method. Generally, the calculated site amplifications from coda-wave and S-wave inversions are not very large, which is probably resulted from the special geology conditions in the Longmenshan region and most peak ground acceleration values of recordings less than 0.1 g. Because coda-wave amplitudes attenuate slowly along the propagation path, the site amplifications computed through the coda-wave inversion are relatively larger than those through the S-wave inversion. The comparison of the intraevent and interevent residuals of the coda-wave inversion with those of S-wave inversion demonstrates that the coda-wave inversion is more reasonable to calculate the site amplifications of the Longmenshan region. Moreover, for the Longmenshan region, the averaged site amplifications of the young geology sediments are not considerably different from those of the old geology rocks in some frequencies. If the sites of stations are classified by the National Earthquake Hazard Risk Reduction Program (NEHRP) site classifications, the averaged site coefficients of the Longmenshan region are usually smaller than the corresponding NEHRP site coefficients.