GNGTS 2013 - Atti del 32° Convegno Nazionale

in depth between the two earthquake families, fault heterogeneity, variation of rheological properties, and pore pressure variation appear reasonable causes for b -value variation observed in this application. Fig. 2 (top panel) shows the preliminary results obtained for the Lunigiana sequence. The locations of the events obtained by using HypoDD indicate the existence of three different clusters. In particular, two of them could be associated to an anti-Apenninc system of faults oriented NE-SW, compatibly with the trend of the main shallow strike-slip faults linking the graben basins in the northern Apennines The third group of events, which is located near the FIVI station, seems to follow a NW-SE tectonic lineament (Apenninic direction). Looking at the NE-SW seismic cross-sections in Fig. 2, the anti-Apenninic sources present low-angle dipping planes, in accordance with the tectonic setting of the area. These observations are confirmed by the results of the waveform similarity analysis (Fig. 2, bottom panel), again showing the existence of different clusters of seismicity oriented both NE-SW and NW- SE. However, different families of earthquakes are identified along the same lineament. For instance, the southern NE-SW cluster appears to be characterized by three distinct families, indicating a very complex fault system. Summarizing all previous observations, the work has strengthened the results of various research studies, many of which cited in the bibliography, about the capability of the methods applied here in characterizing the seismogenic process of an area. Seismismogram cross- correlation and double-difference relocation, which are here applied to areas with different seismotectonic settings, have been once more proved to be effective tools for the definition of the seismogenic structures in terms of location and orientation. These methods are of great importance to characterize surface fault lineaments but assume a greater importance in areas, such as those investigated here, where seismicity is triggered by deep structures with no clear geological evidences. Although limited to the case study of the Sampeyre swarm, our work has also pointed out that investigating changes of the b -value within the rock volume involved in the seismic process may help in identifying structural and rheological heterogeneities. References Astiz L., Shearer P. M. and Agnew D. C.; 2000: Precise relocations and stress change calculations for the Upland earthquake sequence in Southern California . J. Geophys. Res., 105 , pp. 2937-2953. Augliera P., Cattaneo M., and Eva, C.; 1995: Seismic multiplet analysis and its implication in seismotectonics . Tectonophysics, 248 , pp. 219-234. Avigad D., Chopin C. and Le Bayon R.; 2003: Thrusting and extension in the southern Dora-Maira ultra-high-pressure massif (Western Alps): view from below the coesite-bearing unit . J. Geol., 111 , pp. 57-70. Balestro G., Cadoppi P., Di Martino L. and Sacchi R.; 1995: Il settore meridionale del Massiccio Dora-Maira (Valli Maira e Varaita): inquadramento, carta geologica e guida a un’escursione . In: Polino, R., Sacchi, R. (Eds.), Accademia Nazionale delle Scienze, Scritti e Documenti, 14 , pp. 501-529. Barani. S., Scafidi D. and Eva C.; 2010: S train rates in Northwestern Italy from spatially smoothed seismicity . J. Geophys. Res., 115 , B07302, doi: 10.1029/2009JB006637. Bartolini C. and Bortolotti V.; 1971: Studi di geomorfologia e neotettonica, 1. I depositi continentali dell’Alta Garfagnana in relazione alla tettonica plio-pleistocenica . Mem. Soc. Geol. It., 10 , pp. 203-245. Brozzetti F., Boncio P., Di Naccio D., Lavecchia G., Tinari P., Torelli L., Bernini M., Eva E. and Solarino S.; 2007: A Multidisciplinary approach to the seismotectonics of the Lunigiana and Garfagnana extensional basins (Northern Tuscany – Italy) . Convegno in Memoria dei Prof. G. Giglia e A. Decandia: “Alps and Apennines: a natural laboratory for structural geology and geodynamics”. Calais E., Nocquet J.-M., Jouanne F. and Tardy M.; 2002: Current strain regime in the Western Alps from continuous Global Positioning System Measurements, 1996-2001 . Geology 30 , 651-654. Castaldini D., Genevois R., Panizza M., Puccinelli A., Berti M. and Simoni A.; 1998: An integrated approach for analysing earthquake-indiced surface effects: a case study from the Northern Apennines, Italy . J. Geodynamics, 26 , pp. 413-441. Cattaneo M., Augliera P., Spallarossa D. and Eva C.; 1997: Reconstruction of a seismogenetic structures by multiplet analysis: an example of Western Liguria, Italy . Bull. Seism. Soc. Am., 87 , pp. 971-986. Cattaneo M., Augliera P., Spallarossa D. and Lanza V.; 1999: A waveform similarity approach to investigate seismicity patterns . Natural Hazards, 19 , pp. 123-138. Cattaneo M., Eva C., Giglia G. and Merlanti F.; 1983: Seismic hazard in the Northwestern Apennines . Pageoph., 121 , pp. 221-245. 22 GNGTS 2013 S essione 1.1