Site-Dependent Amplification on Topography during the 2016 Amatrice Seismic Sequence, Central Italy

Following the M-w 6.0 Amatrice earthquake on 24 August 2016 in central Italy, the Emersito task force of the Istituto Nazionale di Geofisica e Vulcanologia installed a temporary seismic network focusing on the investigation of amplification effects at municipalities located on topographic reliefs. Fourteen stations were installed at three sites: Amandola, Civitella del Tronto, and Montereale. During the operational period, stations recorded about 150 earthquakes with M-w up to 4.7. Recorded signals were analyzed calculating the horizontal-to-vertical spectral ratios at single station, using both ambient noise and earthquake waveforms, as well as standard spectral ratios (SSRs) to a reference site. To robustly estimate site amplification at each station of the site amplification effect at each station, the influence of backazimuth and epicentral distance is investigated. With the aim of reproducing the observed amplification pattern, 2D numerical simulations were performed on a section orthogonal to the topography major axis, constrained through in situ geological investigations and geophysical surveys. Although at Montereale site no clear amplification effects were observed, at Amandola site, all stations on the relief consistently detected significant peaks at about 4 Hz and along N120-150 azimuth. At Civitella del Tronto, a proper reference station is missing, implying a misleading of site response evaluation in terms of SSRs. Moreover, even if all stations show amplification in the frequency band 1-3 Hz, the direction of the maximum amplification varies from northeast to northwest. At the three sites, observations were successfully reproduced by 2D numerical models, the latter suggesting that topography alone cannot reproduce data, and the interplay with subsoil velocity structure is needed to produce a clear amplification effect. We conclude that according to the previous articles, rather than the sole topography convex shape, the geophysical structure has often a predominant role in controlling the observed amplification pattern on topography.