GNGTS 2013 - Atti del 32° Convegno Nazionale

fundamental mode has a minimum. We note how the osculation frequency, at which we observe the shift of energy between fundamental and first higher mode, is the same frequency at which fundamental and first higher mode have similar ellipticity. We described as the fundamental mode below that singular frequency has a predominantly horizontal motion, while first mode has a predominantly vertical motion. If we look only on the vertical component energy seems to shift from the fundamental to the first mode while, on the contrary, the use of horizontal components can help to avoid mode misidentification. This evidence has been experimentally demonstrated in Boaga et al. (2013). These synthetic results and the supporting theory indicate that the use of horizontal component receivers in multichannel arrays can allow a correct definition of the fundamental mode dispersion down to low frequencies, well below the osculation point. This is the reason to employ a Multi-components Analyses of Surface Wave (McASW). This approach can lead to the design of alternative operational practice that can avoid any overestimation of bedrock seismic velocity as a consequence of the osculation problem. Since one of the main use of shear wave profiles is for the seismic amplification analysis, and that bedrock velocity overestimation could lead to serious misleading, the value of McASW approach here proposed is evident. Tab. 1 – Properties of the synthetic models. Case 1 V p (m/s) V s (m/s) density (g/cm 3 ) thickness (m) top layer 600 300 1.8 5 bedrock 1000 500 1.9 - Case 2 V p (m/s) V s (m/s) density (g/cm 3 ) thickness [m] top layer 600 300 1.8 5 bedrock 2200 1100 2.2 - Tab. 2 – Subsoil model for the Belluno site. Belluno Site V p (m/s) V s (m/s) density (g/cm 3 ) thickness [m] Layer1 700 350 1.8 6 Layer2 2100 1050 1.9 - References Ampuero J.-P., 2008, SEM2DPACK: A spectral element method tool for 2D wave propagation and earthquake source dynamics - User’s Guide Version 2.3.4. Available at http://sourceforge.net/projects/sem2d/ Arai H. and K. Tokimatsu, 2004, S-Wave Velocity Profiling by Inversion of Microtremor H/V Spectrum, Bulletin of the Seismological Society of America; v. 94; no. 1; p. 53-63; DOI: 10.1785/0120030028Bard P.Y., 1998, “ Microtremor Measurements: A Tool For Site Effect Estimation?”, Manuscript for Proc. of 2nd International Symposium on the Effect of Surface Geology on Seismic Motion, Yokohama, Japan, 1-3 Dec, 1998. Boaga J., Cassiani G., Strobbia L.C. and G. Vignoli 2013. Mode misidentification in Rayleigh waves: Ellipticity as a cause and a cure, Geophysics, 78, 4, 2013 Boaga J., G. Vignoli and G. Cassiani, 2011, Shear wave profiles from surface wave inversion: the impact of uncertainty onto seismic site response analysis, Journal of Geophysics and Engineering, 8, 162-174, doi:10.1088/1742-2132/8/2/004. Boaga J., S. Renzi, G. Vignoli, R. Deiana and G. Cassiani, 2012a, From surface wave inversion to seismic site response prediction: beyond the 1D approach, Soil Dynamics and Earthquake Engineering, doi:10.1016/j.soildyn.2012.01.001. Boaga J., G. Vignoli, R. Deiana and G. Cassiani, 2012b, The influence of subsoil structure and acquisition parameters on surface wave mode contamination, submitted, J. of Applied Geophysics . Bonnefoy-Claudet, S., Koeler, A., Cornou, C., Wathelet, M., And Bard, P.-Y., 2008, Effects of Love waves on microtremor H/V ratio, Bull. Seismol. Soc. Am. 98, 288-300. Castellaro S. and Mulargia F., 2009, The Effect of Velocity Inversions on H/V. Pure and Applied Geophysiscs, Vol. 166, 4, 567-592. Cercato M., 2009, Addressing non-uniqueness in linearized multichannel surface wave inversion Geophysical Prospecting, Volume 57, Issue 1, pages 27-47. 27 GNGTS 2013 S essione 3.1