Source parameters, path attenuation and site effects from strong-motion recordings of the Wenchuan aftershocks (2008-2013) using a non-parametric generalized inversion technique

Abstract

Secondary (S) wave amplitude spectra from 928 strong-motion recordings were collected to determine the source spectra, path attenuation and site responses using a non-parametric generalized inversion technique. The data sets were recorded at 43 permanent and temporary strong-motion stations in 132 earthquakes of Ms 3.2-6.5 from 2008 May 12 to 2013 December 31 occurring on or near the fault plane of the 2008 Wenchuan earthquake. Some source parameters were determined using the grid-searching method based on the omega-square model. The seismic moment and corner frequency vary from 2.0 × 1014 to 1.7 × 1018 N·m and from 0.1 to 3.1 Hz, respectively. The S-wave energy-to-moment ratio is approximately 1.32×10-5. It shows that the moment magnitude is systematically lower than the surface wave magnitude or local magnitude measured by the China Earthquake Network Center. The seismic moment is approximately inversely proportional to the cube of the corner frequency. The stress drop values mainly range from 0.1 to 1.0MPa, and are lognormal distributed with a logarithmic mean of 0.52MPa, significantly lower than the average level over global earthquake catalogues. The stress drop does not show significant dependence on the earthquake size and hypocentre depth, which implies self-similarity for earthquakes in this study. The ε indicator was used to determine the stress drop mechanism. The low stress drop characteristic of the Wenchuan aftershocks may be interpreted by the partial stress drop mechanism, which may result from remaining locked sections on the fault plane of the main shock. Furthermore, we compared the stress drop distribution of aftershocks and slip distribution on the fault plane of the main shock. We found that aftershocks with higher stress drop occurred at areas with smaller slip in the main shock. The inverted path attenuation shows that the geometrical spreading around the seismogenic region ofWenchuan earthquake sequence is weak and significantly dependent on frequency for hypocentre distances ranging from 30 to 150 km. The frequency-dependent S-wave quality factor was regressed to 151.2f 1.06 at frequencies ranging from 0.1 to 20 Hz. The inverted site responses provide reliable results for most stations. The site responses are obviously different at stations in a terrain array, higher at the hilltop and lower at the hillfoot, indicating that ground motion is significantly affected by local topography.