GNGTS 2013 - Atti del 32° Convegno Nazionale

From some preliminary tests carried out at a site located on a slope affected by landslides, only a small fraction of data samples (in the order of 1%) was found to satisfy the criteria of Rayleigh type wave identification: this is likely due the fact that most of the noise recording was characterised by an overlapping of signals with different kinds of polarisation. Thus, it was possible to recognise Rayleigh polarisation only when the energy of this kind of wave was prevalent. However, on a sufficiently long recording one can obtain a considerably high number of estimates of H/V amplitude and azimuth. The test results showed that, on sites where directional resonance properties had been verified through the analysis of seismic event recordings, more coherent observations were obtained for H/V ratios and directivity by selecting Rayleigh type data samples, rather than analysing the entire data set or SH-type wave packets. This can increase the confidence in the inferences derivable from noise analysis and, in addition, in view of the employment of nu- merical simulations for data interpretations, offers the possibility of reducing the uncertainties related to the influence of the nature of the noise wavefield. References Albarello D. and Lunedei E.; 2009: Alternative interpretations of horizontal to vertical spectral ratios of ambient vibrations: new insights from theoretical modeling . B. Earthq. Eng., 8 , 519-534 doi: 10.1007/s10518-009-9110-0. Bard P.-Y.; 1999: Microtremor measurements: a tool for site effect estimation? In: Irikura K, Kudo K, Okada H, Sasatani T (eds) The effects of surface geology on seismic motion. Balkema, Rotterdam, 1251-1279. Bonnefoy-Claudet S., Cotton F. and Bard, P.-Y.; 2006: The nature of seismic noise wavefield and its implications for site effects studies – A literature review . Earth Sci. Rev., 79 , 205-227. Castellaro S. and Mulargia F.; 2009: VS 30 Estimates Using Constrained H/V Measurements . B. Seismol. Soc. Am., 99 , 761-773. Del Gaudio V. and Wasowski J.; 2007: Directivity of slope dynamic response to seismic shaking . Geophys. Res. Lett., 34 , L12301, Doi: 10.1029/GL029842. Del Gaudio V. and Wasowski J.; 2011: Advances and problems in understanding the seismic response of potentially unstable slopes . Eng. Geol., 122 , 73-83. Del Gaudio V., Coccia S., Wasowski J., Gallipoli M. R. and Mucciarelli M.; 2008: Detection of directivity in seismic site response from microtremor spectral analysis . Nat. Hazards Earth Sys. Sci., 8 , 751-762. 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. Fäh D., Kind F. and Giardini D.; 2001: A theoretical investigation of average H/V ratios . Geophys. J. Int., 145 , 535-549. Haghshenas E., Bard P.-Y., Theodulis N. and SESAME WP04 Team; 2008: Empirical evaluation of microtremor H/V spectral ratio . B. Earthq. Eng., 6 , 75-108. 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. Poggi V., Fäh D., Burjánek J. and Giardini D.; 2012: The use of Rayleigh-wave ellipticity for site-specific hazard assessment and microzonation: application to the city of Lucerne, Switzerland . Geophys. J. Int., 188 , 1154-1172. 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. 234 GNGTS 2013 S essione 2.2