Seismic Activity in the Central Adriatic Offshore of Italy: A Review of the 1987 ML 5 Porto San Giorgio Earthquake

On 3 July 1987, a seismic sequence, with a mainshock of M(L )5, took place in the offshore Adriatic, close to the coast of Porto San Giorgio (PSG), Italy. We present an accurate relocation of the PSG seismic sequence using a nonlinear probabilistic approach (Lomax et al, 2000). The trade-off between the hypo-central location and the velocity model was exhaustively explored using six different velocity models available for the area provided by previous studies. Through numerous tests performed by relocating the mainshock, we selected the two best velocity models providing two different depths (2.0 and 18.0 km). To resolve this intrinsic ambiguity, we developed a technique that uses the macroseismic intensity field data based on a grid search of the magnitude-depth space. The results show that the mainshock has a depth of 5.7 km and a magnitude (M-L) equal to 5; moreover, the relocated seismic sequence (similar to 30 events) developed in the upper portion of the crust (at a depth less than 15 km), thus activating thrust faults, which is typical of the main geological features that characterize the outer Apennines thrust belt and the Adriatic foreland folds. Because the Adriatic Sea hosts several hydrocarbon (mainly gas) production fields located near active faults, with some of them in the area of this study, analyzing the instrumental seismicity is necessary to better understand the seismicity generated by these seismogenic faults and improve the assessment of the area's seismic hazards.