GNGTS 2015 - Atti del 34° Convegno Nazionale

provide an estimate of the curve of Rayleigh wave H/V ratios as function of frequency with an accuracy level depending on the signal-to-noise ratio snr . Test results suggest that, since the estimate accuracy changes with the choice of filtering and identification parameters, one should make trials with different parameter combinations, selecting the one providing the minimum scatter of H max /V around HVIP values, among the combinations that identifies a sufficiently high number of Rayleigh-type data samples (at least 200 for H/V peak values). Analyzing signals with snr = 3 or more, it was possible to correctly identify peak frequency and, when present, site response directivity orientation. The estimate of the H/V peak turned out affected by an error increasing with the noise level from 13% to 34%, the results being better for the case of directionally polarized signals in comparison to isotropic signals. Even when the background noise has an amplitude comparable to that of Rayleigh waves, it is still possible to recognize the presence of an H/V peak, its frequency and (if present) directivity, although the H/V peak ratio was considerably underestimated. Comparatively, applying the classical Nakamura method to the same signals, the estimate of the H/V curve was always found less accurate and, in case of very high noise level, the H/V peaks were totally unrecognizable. The lower performance of the Nakamura technique was mainly due to the poor capacity of the HVNR curve to reproduce the lower part of the H/V curve (especially for H/V < 1), which, on the contrary was always outlined by the HVIP curve with good or excellent approximation. References Bonnefoy-Claudet S., Cornou C., Bard P.-Y., Cotton F., Moczo P., Kristek J. and Fäh D.; 2006: H/V ratio: a tool for site effects evaluation. Results from 1-D noise simulations , Geophys. J. Int., 167 , 827–837. Bonnefoy-Claudet S., Köhler A., Cornou C., Wathelet M. and Bard P.-Y.; 2008: Effects of Love Waves on Microtremor H/V Ratio , B. Seismol. Soc. Am., 98 , 288–300. Castellaro S. and Mulargia F.; 2009: VS 30 Estimates Using Constrained H/V Measurements . B. Seismol. Soc. Am., 99 , 761–773. Del Gaudio V.; 2013: Analysis of ambient noise instantaneous polarisation for site response characterisation . Proceedings XXXII Congress GNGTS, Trieste, 19-21 November, 2013 , 226-234. Del Gaudio V., Wasowski J. and Muscillo S.; 2013: New developments in ambient noise analysis to characterise the seismic response of landslide prone slopes . Nat. Hazards Earth Sys. Sci. , 13 , 2075-2087, doi:10.5194/nhess-13- 2075-2013, 2013. Morozov I. B. and Smithson S. B.; 1996: Instantaneous polarization attributes and directional filtering . Geophysics, 61 , 872-881. Nakamura Y.; 1989: A method for dynamic characteristics estimation of subsurface using microtremors on the ground surface . Q. Rep. Rail. Tech. Res. Inst., 30 , 25–30. Nogoshi, M. and Igarashi T.; 1971: On the amplitude characteristics of microtremor (part 2) (in japa-nese with english abstract). J. Seismol. Soc. Jpn., 24 , 26-40. Schimmel M. and Gallart J.; 2003: The use of instantaneous polarization attributes for seismic signal detection and image enhancement . Geophys. J. Int., 155 , 653–668. Schimmel M. and Gallart J.; 2004: Degree of Polarization Filter for Frequency-Dependent Signal Enhancement Through Noise Suppression . B. Seismol. Soc. Am.�, 94 , 1016–1035. Quaternary sedimentary cover thickness versus seismic noise resonance frequency in Western L’Aquila Plain F. Del Monaco, F. Durante, L. Macerola, M. Tallini Dipartimento di Ingegneria Civile, Edile, Architettura e Ambientale, Università dell’Aquila, Italy Introduction. The thickness of Quaternary basins sedimentary cover(hereinafter z) isone of the basic datain the evaluation of the local seismic response within the scope of urban planning purposes. One of the possible approachesto evaluate zis to determine the empirical correlation GNGTS 2015 S essione 2.2 81