GNGTS 2013 - Atti del 32° Convegno Nazionale

IN LINE HVSR MEASURE FOR MICROZONATION STUDY M. Compagnoni, M. Lualdi, F. Pergalani, G. Saccà DICA - Dipartimento di Ingegneria Civile e Ambientale, Politecnico di Milano, Italy Introduction. In the evaluation of the local seismic hazard and risk, the geological and geophysical studies are fundamental to carry out activities of prevention and reconstruction (Seed and Idriss, 1969; Seed et al. , 1988; Kramer, 1996), especially in Italy (Word Institute for Disaster Risk Management, 2004), where the hazard is usually connected to high building (Valente, 2012, 2013) vulnerability and high historic values of the urban centers. The aim of seismic microzonation study is the evaluation of the expected local seismic hazard, related to the geological and geomorphological characteristics of the subsoil (Bard and Bouchon, 1985; Bard and Gabriel, 1986; Jibson, 1993; Ashford and Sitar, 1994; Duncan, 1996; Pitilakis et al. , 1999; Ansal and Tonuk, 2007; Mandrescu et al. , 2007). These multidisciplinary studies can be performed with different levels of detail, in function of the objectives (TC4-ISSMGE, 1999; Mohanty et al. , 2007). The Italian Guidelines for the Seismic Microzonation (ICMS) (GdL MS, 2008) prescribes three approach levels: the Level 1 (L1) (qualitative approach) is dedicated to map the areas that can produce amplification and/or instability phenomena. The Level 2 (L2) (semi-quantitative approach) consist in the definition of the expected amplification factors by abacuses, starting from the definition of soils thickness and the equivalent average share waves velocity. The Level 3 (L3) (quantitative approach) requires a quantitative evaluation of the amplification and instability phenomena through numerical and experimental analyses. L1 is a prerequisite to carry through the successive levels. This cognitive framework necessary to obtain this level is based on pre-existing data collection: geological, geomorphological, geotechnical and geophysical data and some new surveys compatible with a low financial plan available for this step. This level allows to delimit qualitatively the seismic homogenous areas, defined as zones with similar stratigraphy, lithology and morphology, that can produce similar seismic surface effects during an earthquake. The synthesis of all available data allows to create different maps: the surveys one, the geolithotecnic correlated to geo-lithotechnic section and the seismic homogeneous areas map (MOPS). The MOPS can be used for urban planning, but provides also an important information for emergency planning (e.g. damage scenarios, taking into account the local geological and morphological condition) and for project design. In Piemonte Region, north Italy, we conducted the L1 studies in 4 municipalities. The aim of the paper is to describe the choices about the use of HVSR compatible with the low financial plan, to integrate the pre-existent data. HVSR. The HVSR (Horizontal to Vertical Spectral Ratio) technique popularized by Nakamura (1989), based on the spectral analysis of the noise recordings, has been widely used in the past years for seismic microzonation and site effect studies in sedimentary basins. In this framework, the HVSR technique was chosen because: it is not invasive, it is very fast and applicable almost everywhere unlike other seismic method. Assuming to investigate soft deposit above a base seismic bedrock it is possible to establish a relationship between the thickness of the soft layer h , the shear wave velocity ( V S ) and fundamental resonance frequency f of the site through the formula (Lanzo and Silvestri, 1999): (1) Through the f 0 value defined by the HVSR survey it is possible to estimate the thickness h or the shear wave velocity of the soft layer. The HVSR technique has been validated with both simulation and earthquake recordings (e.g. Lermo and Chàvez-Garcìa, 1993; Konno and Ohmachi, 1998). It has been tested in different studies (e.g Ibs-von Seht and Wohlenberg 1999; Delgado et al. , 2000a, 2000b; Parolai et al. , 2002; Gosar and Lenart, 2010; Benjumea et al. , 2011) as a fast and efficient way to obtain bedrock geometry. It is not allow to evaluate correctly the real amplitude of the seismic motion (e.g. Bonnefoy-Claudet et al. , 2006; Haghshenas et 480 GNGTS 2013 S essione 2.3