Attenuation P-S-waves tomography of the Amatrice-Norcia fault system from high densely recorded aftershock data

We perform the attenuation local earthquake tomography (P,S, waves) of the complex fault system that ruptured a wide portion of the central Apennines, during the long lasting seismic sequence that started in 2016. Three mainshocks (the 24 August Mw 6.1, the 26 October Mw 5.9, and the 30 October Mw 6.5) hit the towns of Amatrice, Norcia and Visso causing several casualties and diffuse damages. The ruptured faults spread over an 80 km north-northwest-elongated section of a normal-faulting system. A huge amount of seismic data has been collected by permanent and temporary seismic stations since the onset of the sequence.

About 230,000 seismic events have been analyzed to retrieve P and S seismic waves arrival times, which allowed us to compute 3-D velocity structure and precise earthquake locations.

For all of the earthquakes that meet strict selection criteria based on the signal to noise ratio, we computed the low frequency spectral level and the decay of the amplitude spectra (t*) of both P- and S-waves at about 150 recording sites. In order to avoid source complexity, we selected only events with M<2 for which the source corner frequency is beyond the analysed frequency band (1-40 Hz) and the spectral decay could be modeled only by attenuation effects. A preliminary fit of the observed spectra was used to compute the amplitude residuals between the observed and modeled spectra for P- and S- waves. For each station, the mean of the overall amplitude residuals, for each frequency, contribute to define the site response, a sort of site transfer function that is used to correct the observed spectra in a second round of fit. The high-frequency spectral decays (t*) are then computed again on the corrected spectra and they are used as input for tomography.

The applied method is a powerful tool to image the elastic properties of the medium in terms of seismic energy absorption, i.e. lower or higher attenuation of seismic waves. The retrieved pattern of attenuation gives useful insights on the physical state of the rocks in the crustal volume hosting the ruptured faults during the ongoing  seismic sequence.