An integrated geophysical investigation of the upper crust in the epicentral area of the 1980, Ms=6.9, Irpinia earthquake (Southern Italy)

In this paper we investigate the upper crustal structure of the Irpinia region, Southern Apennines thrust-belt, Italy, through analysis and joint interpretation of gravity data, seismic reflection lines and subsurface information from many deep wells. The investigated region includes the epicentral area of the 1980 (Ms=6.9) Irpinia earthquake and is one of the Italian regions with the highest seismic hazard. The upper crustal structure is imaged by modeling a series of 15 SW-trending gravity profiles, about 5 km spaced, plentifully constrained by seismic reflection lines and wells, thus reducing the inherent ambiguity of the gravity modeling. Despite of the complexity of the modeled Bouguer anomalies, the application of a calibrating procedure to constrain the range of variability of the density values, as well as the use of geometric constraints, result in a good level of stability in the final density cross-sections, which appear in fact coherent both in the density values and in the geometrical features. The inferred model shows important lateral density variations, which can be mostly related to NW-trending geologic structures. High density bodies delineate carbonate platform thrust-sheets and broad antiforms involving Mesozoic basinal rocks, while low-density shallow bodies are associated with Pliocene basins. In addition, important density (i.e. lithological) variations are evident along the strike of the range, the most relevant being an abrupt deepening of the Apulia Carbonate Platform in the southeastern part of the investigated region. In the epicentral region of the 1980 event, we find that the geometry of the high-density, high-velocity carbonates of the Apulia Platform appears correlated with the distribution of the aftershocks and with the P-wave velocity anomaly pattern as inferred from a previous local earthquake tomography. The structural highs of the Apulia Platform correspond to high-velocity regions, where aftershocks and coseismic slip of the mainshock concentrated. This correlation suggests that the Apulia Carbonate Platform geometry played an important role in the rupture propagation and in the aftershock distributio