Temporal and spatial variability of site response during the 2016-2017 Amatrice-Visso-Norcia seismic sequence

The site response is critical for accurately estimating  source parameters, as it directly influences the spectral characteristics of seismic signals. Achieving reliable estimates of these parameters requires clearly distinguishing between source effects, path attenuation, and site response, which are often interdependent and subject to significant trade-offs. As highlighted in recent studies, site response reflects the amplification or damping effects of the shallow subsurface layers on seismic waves, and its accurate characterization is essential to correct the observed spectra and to improve the source parameter estimations. By accurate analysis of the site response, we aim to mitigate these uncertainties and achieve a more robust parameterization of seismic events, particularly for low-magnitude earthquakes, where these effects are more pronounced.

We analyzed the site response of seismic events during the 2016 Amatrice-Visso-Norcia, Central Italy, seismic sequence focusing on earthquakes with local magnitude Ml<2. All seismograms, recorded by the huge INGV-BGS seismic network that consists of 155 recording sites accounting for permanent and temporary seismic stations, were sampled at 100 Hz. We therefore assume that  spectral decays are dominated by path attenuation, as corner frequencies are expected to be beyond 40 Hz, the upper limit of the observable frequency range. 

The study spans from August 24 to November 30, 2016, and focuses on data from five seismic stations (NRCA, LNSS, SMA1, CAMP, RM33) located within the area hit by the seismic sequence. We selected these stations since they are representative of the activated fault system and lay on different geological units that are separated by a peculiar tectonic line that crosses the epicentral area. We performed a preliminary fit for all events, calculating, for each spectrum that exceeded the signal-to-noise ratio (S/N) threshold, the low-frequency level (Ω₀) and the t* attenuation parameter. This analysis was conducted while neglecting the corner frequency (f_c​) and assuming that the quality factor (Q) is frequency-independent. The amplitude residuals between the observed and modeled spectra for P- and S- waves, derived using the spectral residual technique, were used to calculate the site responses at the five stations. We derive the S/P amplitude ratios in time to highlight also the evolution of the site response at discrete frequencies during an ongoing sequence.   

The results provide valuable insights into the spatial and temporal variability of site-specific attenuation effects, emphasizing the dynamic role of subsurface conditions in shaping seismic wave propagation. These findings enhance our understanding of subsurface dynamics in the central Apennines and contribute to more accurate seismic hazard assessments and improved modeling of regional seismicity.