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Data: 1-mar-2012
Autori: Panzera, Francesco
Titolo: Approaches to earthquake scenarios validation using seismic site response
Abstract: A seismic hazard assessment was carried out for the Catania and Siracusa towns providing a comprehensive re-examination and re-processing of all the available seismic data. The site approach and the seismotectonic one were used and compared. The hazard assessment, using both methods, was performed following a logic-tree approach in order to consider and reduce the epistemic uncertainties. The combined use and comparison of these approaches is recommended since it allows to verify the robustness of the hazard estimates and allowed us to obtain useful elements to define the seismic hazard in Catania and Siracusa. Experimental data and numerical modeling were used to study the effect of local geology on the seismic response in the Catania area. Available boreholes data and elastic parameters were used to reconstruct a geotechnical model in order to perform 1D numerical modeling. Seismic urban scenarios were simulated considering destructive (Mw=7.0), strong (Mw=6.2) and moderate (Mw=5.7) earthquakes. PGA and spectral acceleration at different periods were obtained in the urban area through the equivalent linear numerical code EERA, and contour maps of different levels of shaking were drawn. Standard and horizontal-to-vertical spectral ratios were achieved making use of a dataset of 172 seismic events recorded at ten stations located on the main outcropping lithotypes. Spectral ratios inferred from earthquake data were compared with theoretical transfer functions. Both experimental and numerical results confirm the role of the geologic and morphologic setting of Catania. A study aimed to investigate on the dynamic properties of main lithotypes outcropping in the Siracusa area and their relationships with the local seismic response was performed. Non-invasive seismic prospecting techniques using the vertical component of surface waves (MASW and ReMi) were adopted, as well as ambient noise measurements, processed through the Nakamura technique. Moreover, a cluster analysis was performed to subdivide into homogeneous groups the experimentally obtained noise spectral ratios. Results pointed out that the use of combined different methods provides a more robust way to characterized the investigated soils and to reduce the problems linked to the non-uniqueness of solutions during the interpretation of geophysical data. The role of local geology and topography on the site response of a small hill, located in the northern part of Catania, was investigated. Ambient noise and earthquake data were processed through standard and horizontal-to-vertical spectral ratios. Directional effects were also investigated by computing the spectral ratios after rotating the horizontal components of motion and performing polarization analysis. Results of noise and earthquakes analysis, although show significant differences in amplitude, are comparable in frequency, especially in the sedimentary terrains. Pronounced directional effects are mostly observed on the slopes rather than at the hill top. Our findings appear linked to the complex wavefield generated by the lithologic heterogeneities existing in the area which seem to have a stronger influence with respect to the simple topographic effect. Seismic noise recorded by mobile stations in the Ortigia peninsula (downtown Siracusa) was analyzed through H/V spectral ratios, to investigate local site effects. Moreover, shear wave velocities were investigated through non-invasive techniques (MASW and ReMi) in order to assess the theoretical resonant frequency of the hill. Experimental results coming out from the spectral ratios show peaks in the frequency range 1.0-3.0 Hz which are consistent with the theoretical resonance frequency at Ortigia. The H/V azimuthal spectral analysis shows a clearly predominant E-W directional effect, transversal to the main axis of the peninsula, which is also confirmed by the polarization analysis in the time domain.
InArea 04 - Scienze della terra

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