Inferences on the upper crustal structure of Southern Apennines (Italy) from seismic refraction investigations and subsurface data

This paper presents an interpretation of crustal seismic refraction data fr om the northern sector of the Southern Apennines thrust belt, a region that in historical times experienced large destructive earthquakes. The data we re acquired in 1992 along a seismic line 75 km long and parallel to the Ape nninic chain, in order to determine a detailed 2-D P-wave velocity model of the upper crust in an area that had not been deeply investigated by geophy sical methods previously. We have used a 2-D ray tracing technique based on asymptotic ray theory to model travel times of first and reflected P-wave arrivals. Synthetic seismograms have been produced by finite difference sim ulations in order to check the reliability of the velocity model inferred b y ray-tracing modelling. The interpretation of the velocity model is constr ained by stratigraphic and sonic velocity logs from wells for oil explorati on located close to the seismic line. Gravity data modelling allows to chec k the velocity model and to extend the structural interpretation in 3-D. In the shallow crust, up to a depth of 3-4 km, strong lateral variations of t he modelled velocities are produced by the overlapping of thrust sheets for med by: (1) Cenozoic flyschoid cover and basinal successions that underlie the seismic profile with P-wave velocities ill the 2.8-4.1 km/s range and t hicknesses varying between 0.5 and 4.5 km; (2) Mesozoic basinal sequences w ith a velocity of 4.8 km/s and a depth of 1.5-2.1 km in the northern part o f the profile; (3) Mesozoic limestones of the Western Carbonate Platform wi th a velocity of 6.0 km/s and a depth of 0.1-0.8 km in the southern part of the profile. At a greater depth, the model becomes more homogeneous. A con tinuous seismic interface 3.0-4.5 km deep with a velocity of 6.0 km/s is in terpreted as the top of the Meso-Cenozoic Carbonate Multilayer of the Apuli a Platform, characterized by an increase in seismic velocity from 6.2 to 6. 6 km/s at depths of 6-7 km, A lower P-wave velocity (about 5.0 km/s) is hyp othesized at depths ranging between 9.5 and 11 km. As inferred by commercia l seismic lines and data from two deep wells located in the Apulia foreland and Bradano foredeep, this low-velocity layer can be related to Permo-Tria ssic elastic deposits drilled at the bottom of the Apulia Platform. Seismic data do not allow us to identify possible deeper seismic interfaces that c ould correspond to the top of the Paleozoic crystalline basement; this is p robably due to the low-velocity layer at the bottom of the Carbonate Multil ayer that reflects and attenuates a great part of the seismic energy. The j oint interpretation of seismic refraction and well data, in accordance with gravity data, provides the first detailed P-wave velocity model of the upp er crust of the northern sector of the Southern Apennines, which differs co nsiderably from previous 1-D velocity models used to study the seismicity o f the region, and reveals new information about the structure of the thrust brit. (C) 2000 Elsevier Science B.V. All rights reserved.