Investigating the interseismic coupling degree and the seismic moment accumulation rate of the Northern Apennines arc buried beneath the Po Plain in Emilia-Romagna region, (Italy)

Summary The Emilia-Romagna portion of the Po Plain (Italy) hosts part of the outermost sector of the Northern Apennines orogenic front. The low strain-rate and the shortening gradient acting in these areas are well documented by means of recent studies, although it is still unclear which fault segment plays the dominant role in accommodating the deformation during the interseismic period. A fraction of this strain was released during the seismic sequence in Emilia-Romagna with mainshocks the May 20 and 29, 2012 (Mw 6.0 and ∼5.8, respectively), which several authors primarily attributed to two major segments of the Ferrara Arc fault system. In this work, we propose a comprehensive kinematic block modeling approach that integrates available information on fault geometry and GNSS data to identify the fault segments that most effectively accommodate the ongoing contractional geodetic velocity gradient through long-term uniform slip-rates. Our solution shows that the modeled front reproduces the observed velocity gradient by predominant dip-slip rates (∼1.0 mm/yr) and subordinate strike-slip rates (∼0.5 mm/yr). We therefore discretize the proposed fault model using a 3D triangular dislocation element (TDE) mesh to estimate the spatial distribution of slip-deficit rate. This modeling allowed us to identify possible locked asperities, characterized by high interseismic coupling coefficient values, and to distinguish them from areas likely affected by aseismic behaviour and low coupling. A resolution estimate is provided on the obtained coupling model, highlighting areas that are better resolved than others due to the uneven distribution of GNSS data. Finally, we propose a set of end-members rupture scenarios based on different coupling thresholds. This type of study shows the importance of having as many high-precision geodetic measurements as possible to accurately estimate seismic hazard in areas with slow tectonic deformation rates.