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

Abstract

This paper presents an interpretation of crustal seismic refraction data from the northern sector of the Southern Apennines thrust belt, a region that in historical times experienced large destructive earthquakes. The data were acquired in 1992 along a seismic line 75 km long and parallel to the Apenninic 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 geophysical 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 simulations in order to check the reliability of the velocity model inferred by ray-tracing modelling. The interpretation of the velocity model is constrained by stratigraphic and sonic velocity logs from wells for oil exploration located close to the seismic line. Gravity data modelling allows to check 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 the modelled velocities are produced by the overlapping of thrust sheets formed by: (1) Cenozoic flyschoid cover and basinal successions that underlie the seismic profile with P-wave velocities in the 2.8–4.1 km/s range and thicknesses varying between 0.5 and 4.5 km; (2) Mesozoic basinal sequences with a velocity of 4.8 km/s and a depth of 1.5–2.1 km in the northern part of the profile; (3) Mesozoic limestones of the Western Carbonate Platform with 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 continuous seismic interface 3.0–4.5 km deep with a velocity of 6.0 km/s is interpreted as the top of the Meso-Cenozoic Carbonate Multilayer of the Apulia 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 hypothesized at depths ranging between 9.5 and 11 km. As inferred by commercial 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-Triassic clastic deposits drilled at the bottom of the Apulia Platform. Seismic data do not allow us to identify possible deeper seismic interfaces that could correspond to the top of the Paleozoic crystalline basement; this is probably due to the low-velocity layer at the bottom of the Carbonate Multilayer that reflects and attenuates a great part of the seismic energy. The joint interpretation of seismic refraction and well data, in accordance with gravity data, provides the first detailed P-wave velocity model of the upper crust of the northern sector of the Southern Apennines, which differs considerably from previous 1-D velocity models used to study the seismicity of the region, and reveals new information about the structure of the thrust belt