GNGTS 2013 - Atti del 32° Convegno Nazionale

agreement is obtained under the hypothesis n. 3 (body and surface waves) as shown in Fig. 3, where the red line with errors are the observed PE hr while the curves with other colors show the simulations for different depths. The best agreement is obtained for 1.4 km depth which coincides with the depth from surface at which UND array was located. Discussion and conclusion. The results of our analysis on observed coda waves (Coherence, propagation parameters, H/V ratio) indicate that the late coda of local and regional earthquakes recorded at about 1.4 km depth is composed of body waves in a diffusive regime for frequency greater than 3 Hz. As the lapse time increases along the coda the features of diffuse wavefield become more evident. This is particularly evident for regional earthquakes. For local earthquakes the observations are limited by the coda duration. The energy equipartition in the late coda is inferred by the results of PE hr . PE hr has been calculated in different coda windows for different frequency bands in order to assure the conditions of diffuse wavefield (time windows starting after twice S-wave travel time, low coherence and spectral amplitude of coda waves higher than noise). For the frequency band 8-20 Hz the observed PE hr is very close inside the errors to the expected value for equipartition in an homogeneous halfspace. However, a final proof of the energy equipartition would be achievable only by using a 3D underground array. That would allow for a complete separation of longitudinal and shear waves. By considering a stratified halfspace we have simulated energy partition in the case of diffusive wavefiled and simulated and observed energy partition curves show a qualitative agreement in the frequency range 0.5 - 5 Hz under the hypothesis that coda are composed by surface and body waves. We believe that the main cause of discrepancy between observed and simulated results is due to the flat layer model that does not fit the area of Gran Sasso massif. In future simulations it will be necessary to include the local topography. References Bath M., 1974; 1974: Spectral Analysis in Geophysics. Elsevier (Holland). Foster M. and Guinzy N.; 1967: The coefficient of coherence: its estimation and use in geophysical data processing. Geophysics, 32(4), 602–616. doi: 10.1190/1.1439878 Catalano G. P., Cavinato P. G., Salvini F., Tozzi M.; 1984: Analisi strutturale nei laboratori dell’I.N.F.N.del Gran Sasso d’Italia . Mem. Soc. Geol. It., 35, 647-655. Capuano P., De Luca G., Di Sena F., Gasparini P., Scarpa R.; 1998: The density of the rock covering Gran Sasso Laboratories in Central Apennines, Italy by underground gravity measurements . Journal of Applied Geophysics, 39, 25-33. Chiarabba C., AA. VV.; 2009: The 2009 L’Aquila (central Italy) M W 6.3 earthquake: Main shock and aftershocks . 2009, Geophysical Research Letters, vol. 36, doi:10.1029/2009GL039627. Formisano L.A., La Rocca M., Del Pezzo E., Galluzzo D., Fischione C., Scarpa R. (2012), Topography effects in the polarization of earthquake signals: a comparison between surface and deep recordings. Bollettino di Geofisica Teorica e Applicata, vol. 53, n. 4., pp 471-484, doi: 10.4430/bgta0055. Hennino R., Tregoures N., Shapiro N. M., Margerin L., Campillo M., van Tiggelen B. A., Weaver R. L.; 2004: Observation of Equipartition of Seismic Waves. Physical Review Letters, vol. 86, num. 15, 9 April 2001. La Rocca M., Del Pezzo E., Galluzzo D., Scarpa R.; 2013: Joint observation of coherent coda waves at surface and underground arrays . Geophys. J. Int., Express Letter, doi:10.1093/gji/ggt111. Margerin L.; 2009: Generalized eigenfunctions of layered elastic media and application to diffuse fields. J. acoust. Soc. Am., 125, 164-174. Margerin L., Campillo M., Van Tiggelen B. A., Hennino R.; 2009: Energy partition of seismic coda waves in layered media: theory and application to Pinyon Flats Observatory . Geophys. J. Int., 177, 571-585. Nakahara H., and Margerin L.; 2001: Testing equipartition for S-wave coda using borehole records of local earthquakes. Bull. Seismol. Soc. Am.,101, 2243-2251, 2011. Sanchez-Sesma F. J., Rodriguez-Castellanos A., Perton M., Luzon F., Ortiz-Aleman C.; 2011: Diffuse seismic waves and site effects . J. Geophys. Eng., 8, 109-114. Papanicolaou G. C., Ryzhik L. V., Keller J. B.; 1996: Stability of the P-to-S Energy Ratio in the Diffusive Regime . Bulletin of the Seismological Society of America, 86, n.4, 1107-115. Weaver R.; 1982: On diffuse waves in solid media . J. Acoustic. Soc. Am., 71(6), June 1982. Saccorotti G., Di Lieto B., Fischione C., Tronca F., Scarpa R.; 2006: Performances of the UNDERground SEISmic array for the analysis of seismicity in Central Italy . Annals of Geophysics, 49, 2006. Scarpa R., Muscente R., Tronca F., Fischione C., Rotella P., Abril M., Alguacil G., De Cesare W., Martini M.; 2004: UNDERSEIS: The underground seismic array . Seism. Res. Lett. 75, 4, 493-504, 2004. 68 GNGTS 2013 S essione 1.1