GNGTS 2013 - Atti del 32° Convegno Nazionale

that the P-wave velocities inferred from the tomographic analysis are generally consistent with the expected velocities of litho-stratigraphic units for the studied area, however the variation with depth is smoother due to the low number of earthquake sources sampling the shallow layers of the model and the chosen parameterization which does not allow one to resolve length scales less than 1 km in depth (Fig. 3b). The Vp/Vs ratio shows a large lateral and vertical variability within the investigated crustal volume. It ranges from a value of 1-7-1.8 at shallow depths and increases up to 2-2.2 between 5 km and 12 km depth in the volume where most of present microseismicity occurs. Such high values of the Vp/Vs ratio are a determinant for the fluid saturation state of shallow crust, rock formations as well as of their inner pore pressure condition. The evidence for a predominant microearthquake activity confined within the highest Vp/Vs volume (Fig. 3c) and the combined interpretation of Vp/Vs ratio and Vp x Vs product confirms that in the explored low-magnitude earthquake range, fault lubrication processes occur and are driven by fluid pressurization along the fault zone, producing a substantial decrease of dynamic friction with a consequent increase in seismic radiation efficiency. These processes not only control the substantial concentration of background seismicity within a discrete, active fault-bounded block, but also the nucleation of large earthquakes such as the Ms 6.9, 1980 Irpinia earthquake through pore-pressure increase on fluid-filled cracks located within the damage zone volume surrounding the major active faults. References Aki, K. and W. Lee (1990). Determination of three-dimensional velocity anomalies under a seismic array using first p arrival times from local earthquakes, in a homogeneous initial model. Journal Geophys. Res. , 4381-99. Amoroso, O., N. Maercklin, and A. Zollo (20123). S-wave identification by polarization filtering and waveform coherence analysis. Bull. Seism. Soc. Am., Vol.102 No.2 doi:10.1785/0120110140. Aster, R. and C. Rowe (2002, Jun). Automatic phase-pick refinement and similar event association in large seismic data sets. In Advances in Seismic Event Location , Volume 92, pp. 231-263. eds. C. Thurber and N. Rabinowitz. Kluwe, Amsterdam, 1993. Chiao, L.-Y. and Kuo, B.-Y., 2001, Multiscale seismic tomography: Geophysical Journal International, v. 145(2), p. 517- 527. Cichowicz, A. (1993). An automatic s-phase picker. Bull. Seism. Soc. Am. 83 (1), 180-189. Crampin, S. (1977). A review of the effects of anisotropic layering on the propagation of seismic waves. Geophys J. R. ast. Soc. 49, 9-27. De Matteis, R., E. Matrullo, T. A. Stabile, L. A. Rivera, and A. Zollo (2012). Fault delineation and regional stress direction from the analysis of background microseismicity in southern Apennines, Italy. Bull. Seism. Soc. Am. , doi: 10.1785/0120110225. Diehl, T., N. Deichmann, E. Kissling, and S. Husen (2009). Automatic s-wave picker for local earthquake tomography. Bulletin of the Seismological Society of America 99 (3), 1906. Eberhart-Phillips, D. (1993). Local earthquake tomography: earthquake source regions In Seismic Tomography: theory and practice edited by H.M. Iyer and K. Hirahara- 1 st ed. Published by Chapmann and Hall, 2-6 Boundary Row, London SE1 8HN. Latorre, D., J. Virieux, T. Monfret, V. Monteiller, T. Vanorio, J.-L. Got, and H. Lyon-Caen (2004, Jan). A new seismic tomography of aigion area (gulf of corinth, greece) from the 1991 data set. Geophys. J. Int. 159 , 1013-1031. Lomax, A., J. Virieux, P. Volant, and B. Thierry (2000, Jun). Probabilistic earthquake location in 3d and layered models. in: Thurber c.h. and raibinowitz n. (eds.). Advances in Seismic Event Location. Modern Approaches in Geophysics 18 , 101-134. Matrullo E., R. De Matteis, C. Satriano, O. Amoroso and A. Zollo (2013 ) . An improved 1-D seismic velocity model for seismological studies in the Campania-Lucania region (Southern Italy). Geophys. J. Int. doi: 10.1093/gji/ggt224. Michelini, A. and A. Lomax (2004, Apr). The effect of velocity structure errors on double-difference earthquake location. Geophys. Res. Lett. , 1-4. Monteiller, V., J.-L. Got, J. Virieux, and P. Okubo (2005, Jan). An efficient algorithm for double-difference tomography and location in heterogeneous media, with an application to the kilauea volcano. J. Geophys. Res. 110 (B12), 1-22. Michael, A. J. and D. Eberhart-Phillips (1991). Relation among fault behavior, subsurface geology, and three-dimensional velocity models. Science , 651-654. Pavlis, G. L. (1986). Appraising earthquake hypocenter location errors: a complete pratical approach for single-event locations. Bull. Seism Soc. Am. 76 , 1699-1717. Rowe, C., R. Aster, B. Brochers, and C. Young (2002). An automatic, adaptive algorithm for refining phase picks in large seismic data sets. Bull. Seism. Soc. Am. 92 , 1660-1674. 10 GNGTS 2013 S essione 1.1