LOCAL SEISMIC RESPONSE IN ARQUATA DEL TRONTO AND BORGO HAMLETS (CENTRAL ITALY)

This work presents the results of numerical analyses for the evaluation of the seismic response of Arquata del Tronto and Borgo hamlets (Ascoli Piceno, Italy). The villages, located about 9 km northeast of the M 6.1 Central Italy August 24th 2016 epicenter, reported after the event an irregular damage distribution, although less than 500 meters away. In particular, damage mainly concentrated in Arquata del Tronto (VIII-IX MCS), whereas Borgo suffered damage to a less extent (VII MCS) (Galli et al., 2016), highlighting significant occurrence of ground motion amplification phenomena. Arquata del Tronto is a small village, developed on a WNW-ESE elongated ridge, elevated about 100 m higher than the underlying alluvial valley, where Borgo is built on. The geological bedrock of the study area is represented by the Miocene "pre-evaporitic member" of the Laga Formation, consisting of four lithofacies associations distinguished according to their sandstone /pelite ratio: pelitic-arenaceous (LAG4e), arenaceous-pelitic II (LAG4b) arenaceous- pelitic I (LAG4d) and arenaceous (LAG4c). Arquata del Tronto is erected on a ridge made of the alternation of LAG4c, d and b lithofacies partially fractured and with high angle dipping strata. The hamlet of Borgo is located about 200 m north of the Arquata del Tronto ridge, where the geological bedrock is covered by fluvial gravels and sands and/or eluvial/colluvial gravelly deposits, whose thickness is laterally variable in the range of 10-30 m. Between the end of November and the beginning of December 2016, in the immediate aftermath of the 2016 main events (24 August and 30 October), we investigated the study area and a tentative damage zonation was made following the scheme provided by the Department of Civil Protection (DPC) in Italy for post-earthquake reconnaissance. During our surveys, an increasing level of damage was observed moving from Borgo towards Arquata del Tronto ridge. In detail, minor damage observed in Borgo, mainly consisted in cracking of non-structural elements. Instead, Arquata del Tronto was highly damaged with several unreinforced masonry structures partially or fully collapsed. Extensive geological, geophysical and geotechnical surveys were performed in the study area as part of the activities coordinates by the Center for Seismic Microzonation and its applications (CentroMS) with the aim of performing the Level 3 seismic microzonation in the municipalities mostly struck by the 2016 Central Italy seismic events. In this framework, thirteen temporary stations (1 Hz Lennartz 3D sensors) were deployed by the National Institute of Oceanography and Applied Geophysics (OGS) in the Arquata del Tronto and Montegallo municipality. In particular, in this work we used data recorded by stations located on Arquata del Tronto ridge, Borgo alluvial valley and Uscerno hamlet. All stations were deployed on geological bedrock, but only the Uscerno one was identified as a true reference station being characterized by and almost flat H/V curve. The results of geophysical investigations performed in the area (HVSR, MASW, ERT, DH) has been used in order to constrain geometries and retrieve reliable Vs profiles. These information have then been used to implement the geological - geotechnical model for numerical analyses aimed at deriving the motion at the ground level in the study area. We used ground motion data recorded by Uscerno station as input motion and accelerometric data recorded by the OGS temporary stations at the ridge crest of Arquata and at the alluvial valley where Borgo is built on, to calibrate the numerical model by comparing numerical results and seismic data recorded. The numerical analyses were performed both in one-dimensional (1D) and twodimensional (2D) settings, adopting the equivalent linear STRATA and QUAD4M software respectively. Studying local seismic response in such geological complex sites is a crucial challenge as many factors can contribute to modify the expected ground motion. Indeed, effects related to local surface and buried morphology are coupled with those associated to stratigraphic and tectonic setting (i.e. weathering and jointing conditions of rock mass), making the reconstruction of a reliable and thorough geological/geotechnical subsoil model to be used in numerical simulation, more difficult This work mainly aims to assess the role played by site settings in the seismic response of the case study, in particular: i) distinguishing the contribution of the topographic effect from others; ii) understanding the influence of the different mechanical properties, depending on the greater or lesser presence of the politic component; and iii) assessing the role played by the rock mass jointing/weathering conditions, that might give rise to Arquata del Tronto larger amplification compared to Borgo. REFERENCES Galli P., Peronace E., Tertulliani A.; 2016. Rapporto sugli effetti macrosismici del terremoto del 24 Agosto 2016 di Amatrice in scala MCS. Roma, rapporto congiunto DPC, CNR IGAG, INGV, 15 pp. DOI: 10.5281/zenodo.161323.