GNGTS 2015 - Atti del 34° Convegno Nazionale

In our model (Fig. 3a), a positive value of P velocity around 7 km/s at a depth of about 12 km could be correlated to the geological bedrock. Moreover, it is also possible to distinguish some P velocity regions (with a range of P values between 6 and 6.5 km/s) which could be related to a portion of basement that is interposed in the Triassic dolomitic formations. The interpretation of geological section Dogna-Forni di Sopra (Ponton, 2010 modified) seems to be coherent with our model because the bedrock is posed at 12 km of depth and is interrupted by the ramps of dinaric’s thrusts as we can deduct from geophysical and geodynamical investigations in the Italian (Cati et al. , 1989) and Slovenian (Placer, 1999) area. Furthermore, the interpretation of the discontinuity of the two high P velocity anomalies of our model at a depth between 5 and 10 km, could reflect a high component of strike-slip fault of But-Chiarsò line detected by Ponton (2010). This fault system divides the section in two domains: the eastern part, that is moved in north direction along Fella-Sava line and the western part, which is moved in south direction, following Tagliamento line. In Fig. 3b, we report the model of the section M-N which goes from the village of Bate, near Nova Gorica, at the boundary between Italy and Slovenia, to the village of Zell in southern Austria. For the sector in western Slovenia, we do not have a lot of investigations to take a comparison, except for a work of Michelini et al. (1998), whose investigation touches in a very small part our area. The section M-N is perpendicular respect to the main active faults in Slovenia and crosses the dextral strike-slip system of Idrija fault. In general, in our model, we can observe P velocities basically higher compared to the values found for the Friuli Venezia Giulia area; in particular some high P velocity zones with values around 7 km/s which could be in the closeness of the main dislocations of the strike-slip fault. In the eastern part of this section, the lack of data below 10 km of depth is due to the poor ray’s coverage in that zone. Conclusions. The goal of this work was to provide a 3-D velocity model of the upper crust for the area between south-eastern Alps and External Dinarides with the technique of Local Earthquake Tomography based on first arrivals. The investigated zone has always been interested by moderate seismicity both in historical and instrumental time. In particular, the seismic sequence in the western Slovenia characterized by recent events, has shown that the geodynamical process which controls the crustal thickening is nowadays active. The Transfrontier network allowed the production of a 3-D velocity model without political boundaries even if the obtained results are highly influenced by inhomogeneity of rays’s distribution. Comparing the 3-D velocity model obtained with geological sections available in the Friuli Venezia Giulia area, in west-east direction, the velocity model shows a positive anomaly which could indicate the Paleozoic bedrock and the intrusions within the most recent geological formations. In general, the results have confirmed the existence of an anomalous body with P velocity values around 7 km/s beneath central Friuli at a depth between 4 and 8 km Tab. 1 - Geologic formations existing in the investigation area (Ponton 2010, modified). For the colors corresponding to name abbreviations, see the stratigraphic column reported at the bottom of Fig. 3a. Name Geologic period Geologic formations TR 3 Upper Triassic Fm. Monticello; Dol. Principale; Organic laminates; (Norico-Retico) Dolomia di Forni; Fm. Dachstein TR 2 Upper Triassic (Carnico) Fm. Santa Croce and Travenanzes TR 1 Lower-Upper Triassic Fm. Val Degano; Dol. Sciliar and Dol. Cassiana; Anisian-Carnic terrigenous; Anisian platform; Fm. Werfen PZ 2 Upper Permian Fm. a Bellerophon PZ 1 Upper Permian Arenaria della Val Gardena; Pemic-Carboniferous succession; Fm. Dimon; Fm. Hochwipfel; Ordovician-Devonian limestones; phyllites and schists 72 GNGTS 2015 S essione 1.2