GNGTS 2014 - Atti del 33° Convegno Nazionale

cancel all high amplitudes due to transient phenomena since the object of the study is the wave phase. This may be acquired through a 1-bit normalization (Derode et al. , 1999), which is a quite strong operation, but it has been demonstrated to allow reproducing the exact phase and amplitude information in the cross-correlation functions (Cupillard et al. , 2010, 2011). Finally it is possible to cross-correlate all recordings grouped by couples of stations. In order to perform an interferometric analysis it is necessary to define a reference cross- correlation function, which is indicative of the background state of the crust, and many current cross-correlation functions that have to be specific of different time periods. The easiest way is to define the reference function as the cross-correlation of the whole time series (or equivalently the sum of all 1-h cross-correlations, which is convenient in terms of computation times). After that, we find the time period, for the definition of the current functions, as a trade off between similarity and difference with the reference function. We chose this stacking period on the basis of the evolution of the correlation coefficient of the reference with respect to all current functions of increasing stacking length. As Passive Image Interferometry technique we adopt the methodology described first by Poupinet et al. (1984): the Multi Window Cross-Spectral analysis. Their application was on doublet codas, and then Brenguier et al. (2008a) adapted this technique to the ambient noise cross-correlations. The details of the methodology are described also in Clarke et al. (2011). We merge together all station couple information in order to obtain a more stable result (less dependent on the source variations). And finally we get a time series of the relative variations of seismic velocity of the medium inside the network and in a few km depth (depending on the penetration of the Rayleigh waves) at Campi Flegrei. This study has been conceived for testing the resolution capability of Passive Image Interferometry in a volcanic environment at the time of very slight changes of the stress field, possibly also due to the hydrothermal activity in the shallower superficial layers of crust in response to deeper magmatic injection. We expect that the soft material and very plastic behavior of the Campi Flegrei area will emphasize the small variations occurring at depth, although we expect that the high level of noise of anthropogenic origin may complicate the results. References Aki K., (1957). Space and time spectra of stationary stochastic waves with special reference to microtremors. Bull. Earthq. Res. Inst . 35, 415-456. Amoruso, A., Crescentini, L., Sabbetta, I., De Martino, P., Obrizzo, U., Tammaro, U. (2014). Clues to the cause of the 2011 - 2013 Campi Flegrei caldera unrest, Italy, from continuous GPS data. Geophys. Res. Lett . 41, 3081-3088, doi:10.1002/2014GL059539 . Aster, R.C., Meyer, R.P., De Natale, G., Zollo, A., Martini, M., Del Pezzo, E., Scarpa, R., Iannaccone, G., (1992). Seismic investigation of Campi Flegrei Caldera. In: Volcanic Seismology , Proc. Volcanol. Series III. Springer Verlag, New York. Bensen G.D., Ritzwoller M.H., Barmin M.P., Levshin A.L., Lin F., Moschetti M.P., Shapiro N.M., Yang Y., (2007). Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements. Geophys. J. Int. 169, 1239-1260. Brenguier F., Campillo M., Hadziioannou C., Shapiro N.M., Nadeau R.M., Larose E., (2008a). Postseismic relaxation along the San Andreas fault at Parkfield from continuous seismological observations. Science 321, 1478-1481. Brenguier F., Shapiro N.M., Campillo M., Ferrazzini V., Duputel Z., Coutant O., Nercessian A., (2008b). Towards forecasting volcanic eruptions using seismic noise. Nat. Geosci. 1, 126- 130. Campillo M., (2006). Phase and correlation in “������� ������� ������ ��� ��� �������������� �� ��� ����� ��������� random” seismic fields and the reconstruction of the Green function. Pure Appl. Geophys. 163, 475-502. Chiodini G., Caliro S., De Martino P., Avino R. and Gherardi F. (2012a) Early signals of new volcanic unrest at Campi Flegrei caldera? Insights from geochemical data and physical simula- tions. Geology 40, 943–���� ������������ 946, doi:10.1130/ G33251.1. Clarke D., Zaccarelli L., Shapiro N.M., Brenguier F., (2011). Assessment of resolution and accuracy of the Moving Window Cross Spectral technique for monitoring crustal temporal variations using ambient seismic noise. Geophys. J. Int. 186, 867-882. Cupillard P., Capdeville Y., (2010). On the amplitude of surface waves obtained by noise correlation and the capability to recover the attenuation: a numerical approach. Geophys. J. Int. , doi:10.1111/j.1365-246X.2010.04586.x. 234 GNGTS 2014 S essione 1.3