GNGTS 2019 - Atti del 38° Convegno Nazionale

674 GNGTS 2019 S essione 3.2 PRELIMINARY RESULTS OBTAINED BY PASSIVE SEISMIC INVESTIGATIONS AT SAN LEO (NORTHERN ITALY) R. Iannucci 1,2 , L. Borgatti 3 , S. Martino 1,2 , A. Paciello 4 , D. Pistillo 1 1 Dipartimento di Scienze della Terra, Sapienza Università di Roma, Roma, Italy 2 Centro di Ricerca per i Rischi Geologici CERI, Sapienza Università di Roma, Roma, Italy 3 Dipartimento di Ingegneria Civile, Chimica, Ambientale e dei Materiali DICAM, Università di Bologna Alma Mater Studiorum, Bologna, Italy 4 ENEA, C.R. Casaccia, Roma, Italy Introduction. In the last decades, geophysical techniques have been applied for supporting and integrating engineering geological models. In particular, passive seismic has been widely used in seismic microzonation and local seismic response studies (Bour et al. , 1998; Haghshenas et al. , 2008) as well as for characterising and monitoring landslide processes (Galea et al. , 2014; Fiorucci et al. , 2017; Iannucci et al. , 2018). Preliminary results obtained by passive seismic investigations at the historic village of San Leo (Northern Italy) are here reported. On July 2018 and March 2019, passive seismic measurements at single station configuration and continuous seismic acquisition with an array of sensors were carried out. The seismic data were analysed to evaluate the seismic response of the hilltop where the village is built and monitoring the gravity-induced instability processes that involve its vertical edges. The San Leo case study. The historic village of San Leo is located in the Valmarecchia valley, in the Emilia-Romagna Region (Northern Italy). San Leo is a high interest tourist site due to its impressive history, architectural peculiarities and natural landscape. From a geological point of view, the village stands out at the top of a hill composed of a slab of limestones and sandstones, belonging to the “San Marino” and “Monte Fumaiolo” geologic formations, that lays over a scaly clay formation named “Argille Varicolori”. The rock slab is bordered by vertical and overhanging cliffs that have been affected by several gravity-induced instability processes in the recent past, as evidenced by the large debris deposits at their feet. The last major landslide event occurred on 27 th February 2014, when 0.30*10 6 m 3 of rock detached from the North-Eastern portion of the slab edge (Borgatti et al. , 2015). Data acquisition and analysis. Two types of passive seismic measurements were carried out in two field campaigns on July 2018 and March 2019: i) passive seismic measurements at single station configuration having duration of 1 h; ii) continuous seismic acquisition by a Seismic Navigating System (SNS) array. The seismic ambient noise measurements at single station were analysed by the Horizontal- to-Vertical Spectral Ratio (HVSR) technique (Nakamura, 1989) as well as by the WAVEPOL code (Burjánek et al. , 2012) for analysing the polarization of the particle motion. On the contrary, the SNS seismic data were analysed by the Weak Motion analysis (Lermo and Chavez- Garcia, 1993; Field and Jacob, 1995) for weak regional earthquakes and by the Nanoseismic Monitoring approach (Joswig, 2008) for characterising microseismic events related to the instability process evolution. Results. Several features of the seismic response of the rock slab were defined with the analysis of the seismic ambient noise measurements. As it resulted from the processed data, most of the measurements presented an HVSR peak at about 4.5 Hz; this peak seems to be related to the anthropic activities since it appears only during working hours and its damping results negligible over time. In addition, the measurements carried out in the village and in the fortress area do not reveal any resonance frequency. On the contrary, a HVSR peak between 5 and 9 Hz can be observed in the measurements carried out in the main parking lot of the village and in the former camping area; this peak could be related to a 1-D resonance due to the impedance contrast between a shallow colluvial deposit and the underlying “Monte Fumaiolo” sandstone. The measurements carried out in the North-East portion of the slab, where the 2014 landslide occurred, evidenced the presence of several HVSR peaks at 5-7 Hz characterised by