Site and propagation effects on the spectra of an S‐to‐S reflected phase recorded from a set of microearthquakes in the Northern Apennines (Italy)

Abstract

We use recordings at a single station of a shallow crustal S‐to‐S reflected phase generated by microearthquakes (ML, 2.5–3.5) in the Northern Apennines (Italy) to analyze source, path attenuation and site effects. A site resonance phenomenon in a narrow frequency band (6–8 Hz) has been clearly detected in the shear wave acceleration spectra, consistent with similar observations from records of microseismic noise. The non‐linear inversion of acceleration spectra (which have been corrected for an average site response) yields estimates of apparent corner frequencies that are approximately constant with moment, with a mean value of 2 Hz. A significant increase with seismic moment of the spectral decay parameter at high frequencies is also observed. The anomalous moment dependence of these spectral parameters is found to be related to the existence of a cut‐off high frequency limit, close to the smallest event corner frequency, which could be originated by combined site and travel path effects rather than source characteristics in the frequency range of observations (0.5–15 Hz). Independent geological, well soundings and seismic data for this site in the Apennines, suggest that the spectral shape on short‐period records is mainly controlled by the damping and resonance effects of a surficial layer of alluvium Quaternary sediments (100–200 m thick) characterized by low values of shear velocity and attenuation parameter.