GNGTS 2017 - 36° Convegno Nazionale

342 GNGTS 2017 S essione 2.2 SITE RESPONSE CHARACTERISTICS AT AMATRICE FROM AMBIENT NOISE ANALYSIS V. Del Gaudio 1 , S. P. Bruno 1 , A. Moretti 2 , G. Ferrini 2 , E. Ursini 3 , J. Wasowski 4 1 Dipartimento di Scienze della Terra e Geoambientali, Università degli Studi “Aldo Moro”, Bari, Italy 2 Dip. di Medicina Clinica, Sanità Pubblica, Scienze della Vita e dell’Ambiente, Università degli Studi dell’Aquila, Italy 3 Servizio Nazionale della Protezione Civile c/o Università degli Studi dell’Aquila, Italy 4 Istituto di Ricerca per la Protezione Idrogeologica, Consiglio Nazionale delle Ricerche, Bari, Italy The 6.0 Mw earthquake that hit central Italy on 24 August 2016 caused at Amatrice damage of unprecedented severity for an earthquake of such a magnitude. The Amatrice’s old town centre was literally razed to the ground, accounting for more than 3/4 of the 300 victims of the earthquake. Different factors could have contributed to such a large amount of fatalities, including building vulnerability and local amplification of ground motion. Therefore, in view of the planning of the post-earthquake rebuilding of the town, it is of the utmost importance to clarify the reasons of this disaster. One of the questions that need to be investigated is the role of site response in aggravating the earthquake damage. A first contribution to shed some light on this aspect can derive from the study of ambient noise, since the employment of single station measurements using a lightweight, compact sensor allows a quick acquisition of data even within the context of an area that has to cope with the problems of emergency management. Hence, a series of campaigns of ambient noise acquisition were arranged, using a set of 3 tromographs “Tromino”. The data were acquired keeping one of the sensor at a “reference” station, where ambient noise was recorded continuously, while the other two tromographs were used for measurement sessions of 30-46 minutes at different sites. In this way, it is possible to check if differences in ambient noise properties acquired by different stations at different times reflect spatial changes in site-specific characteristics of soil dynamic response rather than temporal changes of environmental conditions generating ground vibrations. Furthermore, this configuration of data acquisition offers the possibility of additional types of data processing (e.g. through the analysis of correlation between simultaneous recordings to derive constraints for subsoil velocity models). Noise recordings were acquired on three different dates (October and December 2016, April 2017) at sites located within the most damaged area (known as the “red zone”) and outside it. Ambient noise data acquired in this area were processed following the traditional Nakamura’s approach (Nakamura, 1989) and the one recently proposed by one of us (Del Gaudio, 2017). The traditional Nakamura’s approach calculates the average spectral ratios HVNR between horizontal and vertical component of noise recording. The new approach, through instantaneous polarization analysis, identifies packets of Rayleigh waves within the noise recording and determines their ellipticity (ratio between horizontal H and vertical V component of elliptical particle motion), together with the azimuth of the vertical plane containing the elliptical motion. This method can provide thousands of estimates of H/V ratios from instantaneous polarization properties of ground motion (hereafter named HVIP), isolating the properties of portions of noise recordings where Rayleigh waves are dominant and then inferring curves of Rayleigh wave ellipticity HVIP as function of frequency. Such curves can be related to site response properties like frequency resonance, directivity of site response and impedance contrast between surface layer and bedrock. Our initial results show that, at different sites of Amatrice, both data processing methods consistently pointed out more or less pronounced maxima of H/V ratios at similar frequencies, comprised between 2 and 4 Hz. The frequency upper bound was observed at station Ama1, at the NW limit of the red zone, the lower bound at Ama8, at the SE limit of the same zone, and an intermediate value at the “reference” station Ama2 located about 300 m away from the red zone (Figs. 1 and 2). The measurements carried out at different dates provided similar results at Ama2, apart from a slight rotation of the H/V maximum direction, whereas evident