GNGTS 2013 - Atti del 32° Convegno Nazionale

reflective series, probably related to the Adige River fluvial deposits which fill the more depressed portion of the valley, where at present there is the Adige River bed. The thickness of this sub-horizontal reflective series is about 100 m. Below CDP 25 the bedrock undergoes a sudden increase in dip towards the south and its apparent depth becomes ~200 m below CDP 150. From this point forward, the bedrock top is no longer visible on the seismic section, and reappears below CDP 350 at about the same depth of (i.e. ~150 m from surface) with an opposite dip, therefore decreasing its depth (~40-50 m) towards the end of the profile. Seismic tomography velocity information is perfect agreement with seismic reflection data. The contour line 4 km/s (interpreted as indicative of the bedrock top) shows indeed a clear and distinct trend to deepen rapidly towards the central part of the profile, with a maximum depth of more than 200m, between metric progressives 600-1150 m, below the surface location of Adige River (Fig.2). Between CDPs 0-250 and CDPs 325-479, the fluvial deposits, are covered by an area of poor reflectivity, similar to those present in the upper part of profiles “Lasa_1” and “Lasa_4”. Based also on the P-wave velocity form tomographic models these seismic facies can probably be associated with dry, nonreflective alluvial fan deposits (Fig. 2). Along tomographic profile “Lasa_4”, ray penetration is lower (on average between 50 and 100 m) because the basement is shallower than on the other profiles. Lasa_4 profile also shows the maximum vertical gradients of velocity, certainlycausedbya shallow bedrock (with the top at about 50 m from surface) that tends to deepen toward the east. These features are in agreement with the reflection image, which confirms both the depth of the bedrock top, and its clear dip towards the east. This dip of the top of the basement explains a step of ~35 m in Fig 2 affecting the top of bedrock between the adjacent parts of profiles “Lasa_1” and “Lasa_2_3”. In fact, by analyzing the depth migrated and tomography profiles of “Lasa_2_3” and comparing them with those of “Lasa_1” it is evident a 35 m vertical offset of the top of the bedrock. This vertical offset is well justified by the lateral offset (about 300 m) between the two profiles (see Fig. 1), and by the slope towards the east of the bedrock top on “Lasa_4” migrated profile. References Ackermann H. D., Pankratz L. W. and Dansereau D. (1986). Resolution of ambiguities of seismic refraction traveltime curves . Geophysics, Vol. 51, N. 2; P. 223-235. Agliardi, F., Zanchi, A. & Crosta, G. B. 2009b. Tectonic vs. gravitational morphostructures in the central Eastern Alps (Italy): constraints on the recent evolution of the mountain range. Tectonophysics, 474, 250 – 270. Brardinoni F., Church M., Simoni A., and Macconi P. 2012. Lithologic and glacially conditioned controls on regional debris-flow sediment dynamics . Geology, 40, 455-458. Bruno P.P., A. Castiello, F. Villani & L. Improta, 2013, High-resolution densely spaced wide-aperture seismic profiling as a tool to aid seismic hazards assessments of fault-bounded intramontane basins: application to Vallo di Diano, Southern Italy , Bull. Seismol. Soc. America, Vol. 103, No. 3, doi: 10.1785/0120120071. Bruno P.P, Castiello A., Improta L. (2010) - Ultrashallow seismic imaging of the causative fault of the 1980, M6.9, southern Italy earthquake by Pre-Stack Depth Migration of dense wide-aperture data , Geophys. Res. Lett., 37. Fig. 3 – Depth migrated section of profile Lasa_4 with respective tomographic model. 55 GNGTS 2013 S essione 3.1