GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 2.2 471 highly uncertain interpretation. We identified features in the H/V curves that allow us to discern between H/V peaks due to 1D and 2D resonance. When 1D resonance is measured, the frequency information can be converted to depth by constraining the model with independent information from the reflection profiles (Pöyry, 2017). When 2D resonance is measured, instead, a depth model cannot be reconstructed. However, important information on the seismic response of the valley can still be inferred. 1D and 2D resonance: theory. It is well established (e.g. Fah et al. , 2001; Haghshenas et al. , 2008) that in the case of horizontally 1D layered geological structures, the H/V peak frequency is a good proxy to the SH-resonance frequency f 0 , which depends on the shear-wave velocity Vs and on the thickness of the resonating layer H. In the case of a single layer (e.g. sediment cover) overlaying a half-space (bedrock): f 0 = Vs/4H When local 1D resonance occurs and when spatial homogeneity of the sediment cover can be assumed, lateral changes of the resonance frequency will reflect changes of the sediment layer thickness, which explains the stratigraphic application of the H/V technique. When 2D resonance occurs, instead, the correlation between the observed resonance frequency and the sediment thickness is less easy to be assessed. Bard and Bouchon (1985) were the first to study numerically non-1D amplification phenomena in sediment-filled valleys. Depending on the valley geometry, they distinguished (1) shallow valleys, where 1D resonance and amplification due to laterally propagating surface waves occur and (2) deep valleys where 2D resonance patterns develop due to the focusing of body waves reflected at the sediment-bedrock interface towards the valley center. In this case, they identified two fundamental horizontal vibration modes, called SH- and SV- occurring at different frequencies (f SV > f SH ). While the Fig. 1 - a) b) c) examples of H/V curves recorded on the surveyed profiles. In a) and c) the green and blue dots mark the longitudinal and transversal 2D resonance peaks. In b) the pink dot marks the local minimum of the vertical component at the 1D resonance frequency. d) map of the study area (Digital Surface Model from Geoportale Alto Adige).