Constructing a 3D Smoothed Seismicity Model for the Seismic Hazard Assessment in the Adriatic Thrust Zone, Italy

In any probabilistic seismic hazard analysis (PSHA), the computation of earthquake forecasting models is a fundamental step. A widely used approach is the smoothed seismicity, which uses seismic catalogs to produce earthquake forecasts in time, space, and magnitude. Early smoothed seismicity models, called fixed smoothing, used spatially uniform smoothing parameters such that the kernels were invariant to spatial variations in seismicity rate. However, recently developed adaptive smoothing methods spatially adapt the smoothing parameters according to the earthquake density. All these fixed or adaptive methods are mainly used in regions with complex seismic source characterization since they do not rely on geological, tectonic, or geodetic information, and they overcome the difficulties in characterizing and segmenting complex geological set-ups. Nevertheless, the standard smoothed seismicity approaches may not properly present the seismicity rates for complex seismotectonic areas.

In this study, we propose an innovative 3D approach for fixed and adaptive smoothed seismicity methods that can be advantageously exploited in all contexts with available well-constrained 3D fault models derived from high-quality seismic catalogs. This approach presents a 3D kernel in the algorithm to smooth the location of earthquakes on a spatial grid by considering the earthquake's depth and spatial coordinates. This allows the use of a three-dimensional grid built on a 3D fault model to represent the depth variations of the structure and also provide the rupture parameters. We tested our method with the Adriatic Basal Thrust (ABT) in eastern Central Italy, a regional active contractional structure with a well-constrained 3D fault model and a related high-quality location catalog.

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 represents a perfect case study to test the 3D smoothed seismicity with fixed and adaptive methods. The 3D seismicity model was constructed for the ABT using a detailed catalog with completeness magnitude Mc ≥ 1.4 opportunely selected to identify ABT activity and declustered for the time-independent (Poisson) model.   We then applied the 3D kernel algorithm with the adaptive and fixed smoothed seismicity approaches to calculate the M ≥ 4.5 ABT earthquake rates. We also include a series of geological information regarding the depth, the fault strike, the dip angle, the seismogenic layers depth, and the rake of the slip direction that can be used for the seismic hazard analysis in the region. Finally, we presented the impact of the 3D smoothed seismicity model on PSHA in central Eastern Italy using OpenQuake software.