GNGTS 2013 - Atti del 32° Convegno Nazionale

Surface and subsurface geology investigated by multidisciplinary approaches. Con- cerning the level one in microzoning studies, it must be remembered that the knowledge of the subsurface successions with respect to lithologies, their dipping and buried geometries and dynamic parameters shall be a compulsory step towards the performance of local site response analyses. To get a realistic model of those portions of territory hurt by seismic wave propaga- tion, in field investigations can be accomplished by means of integrating direct and indirect methods such as geophysical, geotechnical and geological approaches. The multidisciplinary approach allows to get twofold goals: 1) to increase the details on the sediment response to the shaking and to 2) calibrate those indirect investigation methods whose result quality relies on an adequate “dispersion curves” or “propagation curves” representing the subsurface geology. Thus, geophysical methods are generally widely used to investigate the subsurface deposit properties such as the compression and share wave velocities, V S and V P , the geometries of the “seismic layers”, the presence of buried “slip surfaces”, the characteristic frequency of the site. The results of indirect geophysical investigations shall be calibrated by means of direct local inspections of geological successions and geometries. Again, the preliminary selection of the sites to be investigated shall be led by the maximum felt intensity maps. Geological relieves alone can give neither the technicians nor the local administrators reliable information on the local seismic amplification of the sediments, instable mechanical conditions, liquefaction po- tential: such phenomena result from the complex interactions among many factors and site fea- tures that can be appreciated by integrated investigation methods. A good example comes from the VEL project: the supposed active fault under the Fivizzano hospital was correctly predicted not moving by combining geological survey, geophysical refraction lines and point borehole inspections. After the Lunigiana earthquake, the hospital is still working with no damages and many point structural interventions on buildings showed their effectiveness. These buildings were chosen by means of the multidisciplinary approach in those areas where possible am- plification or faulting effects were confirmed by a comprehensive approach: local historical documents, maximum felt intensity maps and in field investigations. Conclusions. In this paper, a working strategy for microzoning studies in near field areas, implemented by the writing authors during the VEL project and derived from recent seismic records can be drawn according to the following points: 1) the portions of the Italian territory where developing microzonation of level 3 shall be selected by means of maximum intensity maps as well as geologic, geomorphologic and geotechnical surveys; 2) the GMPEs shall be Fig. 2. a) Map of maximum felt Intensity in Italy (after Boschi et al. , 1995, modified); b) attenuation laws of peak ground acceleration (PGA) with the minimum distance to the fault (after Faccioli 2010): in grey the uncertainty bands of Faccioli’s attenuation law c) Aterno Valley, Onna sector: the topography in grey tonality; the recordings of Ml 3.2 aftershock (E-W component, origin time 2 May 2009, 2:21 UTC) are shown, normalized at the same scale (after Di Giulio 2011, modified). 328 GNGTS 2013 S essione 2.2