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

Abstract

On 3 July 1987, a seismic sequence, with a mainshock of ML 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 hypocentral 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 (ML) equal to 5; moreover, the relocated seismic sequence (∼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.

Probabilistic Assessment of Slip Rates and Their Variability Over Time of Offshore Buried Thrusts: A Case Study in the Northern Adriatic Sea

“…Nonetheless, considering the distribution and clustering of stress data in the region, robust stress inversions indicate NE-SW to ENE-WSW directions of the maximum horizontal stress, which are almost orthogonal to the strike of the major contractional structures in the area (Carafa and Barba, 2013;Carafa et al, 2015b). The analysis of the seismological data of the largest past earthquake in the region (Senigallia, 1930, Mw 5.9) is consistent with a compressive stress field (Vannoli et al, 2015), as also confirmed by the focal mechanisms of the smaller seismic sequence (max Ml 5.0) recorded in the offshore of Porto San Giorgio (south of the study area) in 1987 (Riguzzi et al, 1989;Battimelli et al, 2019). This evidence suggests that the mechanism that produced the weak but not negligible deformation observed for the post-hor-PS3c (0.4 Ma) period is coherent with the current stress regime.…”

Lithospheric double shear zone unveiled by microseismicity in a region of slow deformation

“…The prevalent deformation regime (reverse or wrench faulting) of the fold-and-thrust belt at the ABT hanging wall is still debated in the literature. Although active shortening perpendicular to the ABT is well supported by morphotectonic [67][68][69] and GPS data 25,70,71 , constraints from focal mechanisms 42,43,72 are more controversial due to the presence of strike-slip mechanisms for some of the major sequences (i.e., Cesena-Forlì 1993 and Ancona 1972). The presence of these FMs has been variously interpreted in the literature.…”

Revitalizing vintage seismic reflection profiles by converting into SEG-Y format: case studies from publicly available data on the Italian territory

“…In these areas, during the Pleistocene, the sedimentation rates were faster than the tectonic rates by one order of magnitude, leading to the complete burial of the frontal thrust of the Apennines [Maesano and D'Ambrogi, 2017]. Nonetheless, the geometry of the main intra-Pleistocene horizons both in the Adriatic and in the Po Plain [Amadori et al, 2019] has recorded the eventual activity of the underlying faults [Panara et al, 2021], and also the relocation of the seismicity in the area suggest that the frontal thrusts are still active and seismogenic [Battimelli et al, 2019]. The use of subsurface information thus may be crucial to refining the knowledge of the recent compressive deformation at the Apennines front.…”