GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.1 GNGTS 2023 damaging earthquakes, such as the recent 9 November 2022 seismic sequence offshore Ancona, and forms an active part in a complex scenario. The eastern Central Italy seismotectonic set-up is characterized by contractional active regional thrusts, such as ABT, representing the outermost and still active front of the Apennine fold-and-thrust belt and by coaxial extensional faults observable along the axis of the Apennine Chain. This complex framework shows different kinematic seismogenic sources overlapping at different depths and acting over the same area. The co-existence of the extensional domain, bounded to the west by the low-angle east-dipping Altotiberina Fault (ATF), with a contractional double shear zone (de Nardis et al., 2022) represents a perfect case study to test the 3D adaptive smoothed seismicity. The 3D seismicity model is constructed for the ABT by using a detailed catalogue with Mc ≥ 1.4 opportunely selected to identify ABT activity and declustered for the time-independent (Poisson) model. The grid we used to smooth earthquake locations is built using the high-resolution 3D model of the ABT provided by de Nardis et al., (2022). We divided ABT’s regional surface into 183 grid points with a 0.1° x 0.1° spacing, and for each, we derived the depth, the fault strike, the dip angle, the seismogenic layers depth, and the rake of the slip direction from the 3D model. We then applied the 3D kernel algorithm in the adaptive method to smooth ABT catalogue distribution on the grid with a smoothing distance Sigma that changes for each earthquake concerning the number of its neighbouring events (NN). The final result is the three-dimensional earthquake rate model shown in Figure 1 that forecasts ABT rates from M ≥ 4.5 isolating its contribution from the other overlapping seismogenic structures. The model also provides rupture parameters for PSHA. We tested the efficacy of our new approach by computing the difference between each catalogue hypocentral depth with the depth of our 3D grid points and the average hypocentral depth usually used in the standard 2D approach. The comparison of the residuals shown in Figure 2 reveals how our approach better represents the depth distribution of the catalogue. In conclusion, the 3D smoothed approach is valid for 1) complex seismotectonic settings such as compressional environments to overcome the problem of overlapping structures that are not well-represented in a 2D grid, 2) regional sources with poor surface data of which a fault-based seismic source characterization is complex and for which a grid source parametrization based on a 3D model can be resolutive for hazard computations.