Local earthquake tomography in the junction between south-eastern Alps and External Dinarides using the seismic data of the CE3RN network

Introduction. The main force responsible of tectonic activity in south-eastern Alps is given by the collision between Adriatic microplate and Eurasian plate. This motion has a fundamental role in geodynamic evolution due to the convergence rate between Adria and Eurasian plate estimated greater than 2 mm/yr (Platt et al., 1989). The largest historical earthquakes occurred in the region between Italy and Slovenia are reported in Burrato et al. (2008) and Galadini et al. (2005). Several large earthquakes hit the studied region in historical times, most of them were distributed in the Friuli Venezia Giulia region, as two events recorded in Tramonti in 1776 and 1794, but the most important earthquake that spread its effects in wide region was the 1511 Idrija event that represents the most destructive event that occurred so far in the region of the junction between south-eastern Alps and the External Dinarides, as reported in the catologues (Ribarič, 1982). In general, the area of Friuli Venezia Giulia region is characterized by a moderate seismicity, in fact the most important instrumental seismic event recorded in the area was the destructive 1976 Friuli earthquake with Ms=6.5 (Aoudia et al., 2000), that was widely studied thanks to the large amount of collected seismometric data. In Veneto region, seismicity normally decreases and the only destructive earthquake instrumentally recorded was the Cansiglio event with Mw=5.9 (Sirovich and Pettenati, 2004). Evidences that the collision is still active nowadays and the main seismogenic structures are located in the proximity of the political boundaries between Italy and Slovenia is also shown by the main shock of Bovec in April 1998 with Ms=5.7 (Bajc et al., 2001) and, later, in July 2004, with the event of Kobarid (Ms=4.9), which generated a sequence that lasted until the end of November 2004 (Bressan et al., 2009). It is evident that, in an area with complex seismogenic structures, the use of 1-D velocity model might not be sufficient, so defining a 3-D velocity model represents a better solution to understand the inner structure of the Earth. For this reason, we used the travel-time tomography technique to obtain an accurate 3-D velocity model: in this work, we used a Cat 3-D software to perform the travel time tomography and a non-linear location tool to define the position of the events.