The 2012 may 20 and 29, emilia earthquakes (northern italy) and the main aftershocks: S-wave attenuation, acceleration source functions and site effects

Abstract

We used strong-motion records from the 2012 May 20 and 29 Emilia-Romagna earthquakes (Mw 6.1 and 5.9, respectively) and four aftershocks with magnitudes ranging between 4.9 and 5.5 to analyse the S-wave spectral amplitude decay with distance and estimate acceleration source functions and site effects. The data set consists of six earthquakes, 44 stations and 248 records with hypocentral distances in the range 10 rx with rx = 60 km and normalizing at 15 km (the recording distance where the attenuation functions start to decay), we find that the average Q for SH waves can be approximated by QSH = 82 ± 1 f 1.2±0.02 and by QSV = 79 ± 1 f 1.24±0.03 for SV waves in the frequency range 0.10 ≤ f ≤ 10.7Hz. At higher frequencies, 11.8≤f≤40 Hz, the frequency dependence of Q weakens and is approximated by QSH = 301 ± 1 f 0.36±0.04 and QSV = 384 ± 1 f 0.28±0.04. These results indicate that the S-wave attenuation is radially isotropic at local distances in the epicentral area. Nevertheless, we used these attenuation parameters separately to correct the radial (with QSV) and transverse (with QSH) components of the acceleration spectra and to separate source and site effects using a non-parametric spectral inversion scheme.We found that the source function of the main event and the bigger aftershocks show enhanced low frequency radiation between 0.4 and 3.0 Hz. We converted the source functions into far-field source acceleration spectra and interpreted the resulting source spectra in terms of Brune's model. The stress drops obtained range between approximately 0.9 and 2.9MPa. Although all the recording stations used are located in the Po Plain, the site functions obtained from the spectral inversion show important amplification variability between the sites. We compared these site functions with the average horizontal to vertical spectral ratios calculated for each station, and we found consistent results for most stations. © The Authors 2013 Published by Oxford University Press on behalf of The Royal Astronomical Society.