Reconnaissance of landslide-prone slope response to seismic shaking from ambient noise analysis (SE Apennines, Italy)

Earthquakes and slope instabilities represent two major geohazards for many small hilltop towns of Italy. Here we report on how the susceptibility of urban/peri-urban slopes to earthquake-induced failure is being addressed within the ongoing seismic microzonation project of the Apulia region (SE Italy). We focus on the towns built on marginally stable slopes formed in tectonically disrupted flysch units, which are common in the Daunia Mountains, located along the SE front of the Apennine chain. Historical records show that this area has been repeatedly hit by moderate-large magnitude earthquakes generated by active seismogenic sources frequently activated in the Apennine chain and, less frequently but with comparable energy, in the foredeep-foreland zone of northern Apulia. The reconnaissance studies conducted in the initial stage of the seismic microzonation project produced a large amount of in-situ acquired data. These included over 600 recordings in more than 300 sites of 18 municipalities. Overall, about 40% measurements were made on landslides or at their margins. Ambient noise recordings were analysed through techniques based on the calculation of the ratios between the amplitude of horizontal and vertical components of non-seismic ground vibrations. Evidence of resonance phenomena was observed at over 90% of the landslide sites, most of which had a main resonance frequency below 3 Hz. H/V peak amplitudes were generally low (< 3), likely because the flysch substratum is intensely fractured and may not cause a strong impedance contrast with the overlying surficial materials. However, about 10% of sites showed evidence of greater amplification effects (H/V peak > 5). This information interpreted in the context of local geological conditions helps defining the areas that will be subjected to more comprehensive investigations in the subsequent stages of the seismic microzonation project. AcknowledgementsStudy conducted with the financial support of the National Department of Civil Protection and of the Civil Protection Office of the Apulian Regional Administration.