GNGTS 2017 - 36° Convegno Nazionale

344 GNGTS 2017 S essione 2.2 differences were found in H/V peak amplitude and in directivity at stations within the red zone. Comparatively, the differences appear stronger for the HVNR measurements than for the HVIP ones. This confirms what we had observed in previous tests i.e., within noise recording, the superimposition of different types of waves (Love and body waves) over Rayleigh waves enhances the variability of the results from HVNR calculations, depending on the efficiency with which certain wave types are excited under different environmental conditions. However, the HVIP-derived results show that, even isolating the contribution of Rayleigh waves to the H/V ratios, a certain variability is observed. Our previous study (Del Gaudio et al. , 2014) hypothesized that variations could derive from changes of the Poisson ratio of surface layers, related to seasonal changes of water content. The verification of such hypothesis, however, will require further measurement repetitions. At station Ama1, main differences from October 2016 to April 2017 measurements concern the enlargement of the most amplified frequency band and the passage from a more directional character of the H/V peak (oriented in NNW direction at October 2016) to an almost isotropy of the H/V maximum on April 2017. This suggests that the directivity observed in the first campaign at Ama1 might not reflect a property of the site response, being conditioned by changing environmental conditions (e.g., the presence and spatial distribution of sources of more or less polarized noise around the measurement station). More striking appears the variation at the station Ama11, located at the center of the old town, near the clock tower. The results obtained on April 2017 showed a strong generalized reduction of H/V ratios, so that on the basis of this measurement alone, one would totally exclude the presence of site amplification. However, it would appear that evidences of amplification are less pronounced at the sites in the central part of the red zone with respect to those near its limits. At present we do not have a straightforward explanation for these spatial differences in amplification. Nevertheless, in general, the amplification registered at Amatrice can be linked to the local geologic setting and in particular to the presence of the impedance contrast between few tens of meters thick surficial Quaternary alluvial deposits and the underlying flysch-like bedrock. We can speculate that the above-mentioned spatial variations in the amplification could result from local changes in the bedrock lithology, with more rocky (sandstones) flysch sites producing greater impedance contrast than more marly-argillaceous sites. Overall, the extension and repetition of noise measurements are necessary to achieve a better comprehension of the properties of site response in different part of theAmatrice town and to pin point possible correlations with the damage distribution. These additional measurements should help us to better distinguish noise characteristics controlled by site condition peculiarities from those depending on changes of noise sources. Acknowledgement We thank the Municipal Administration of Amatrice for allowing the access to the “red zone” area during the measurement campaigns. References Del Gaudio, V., 2017: Instantaneous polarization analysis of ambient noise recordings in site response investigations . Geophys. J. Int., 210 , 443–464, doi: 10.1093/gji/ggx175. Del Gaudio, V., Muscillo, S., Wasowski, J., 2014. What we can learn about slope response to earthquakes from ambient noise analysis: an overview . Eng. Geol., 182 , 182–200. Nakamura, Y., 1989: A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface . Q. Report Railway Tech. Res. Inst., 30 , 25–33.