The deformation style of the continental lithosphere is a relevant issue for geodynamics and seismic hazard perspectives. Here we show the first evidence of two well-distinct low-angle and SW-dipping individual reverse shear zones of the Italian Outer Thrust System in Central Italy. One corresponds to the down-dip prosecution of the Adriatic Basal Thrust with its major splay and the other to a hidden independent structure, illuminated at a depth between 25 and 60 km, for an along-strike extent of ~ 150 km. Combining geological information with high-quality seismological data, we unveil this novel configuration and reconstruct a detailed 3D geometric and kinematic fault model of the compressional system, active at upper crust to upper mantle depths. In addition, we report evidence of coexisting deformation volumes undergoing well-distinguished stress fields at different lithospheric depths. These results provide fundamental constraints for a forthcoming discussion on the Apennine fold-and-thrust system's geodynamic context as a shallow subduction zone or an intra-continental lithosphere shear zone.

<p>We present SEISC-3D, an ArcGIS geodatabase for 3D SEIsmic Source Characterization. It integrates multi-scale and multi-depth geological and seismological information in a compressional environment to build a detailed regional-scale, geometric and kinematic, 3D curvilinear fault model suitable for seismic hazard modelers and seismotectonic purposes and geodynamic modeling. This first release focuses on the late Pliocene-to-Quaternary arcuate and eastward convex fold-and-thrust belt still active along the Outer front of the Italian Apennines in eastern Central Italy. The near-surfaces, onshore, and offshore thrust faults represent the hanging-wall structures of a potentially seismogenic regional shear zone, known as Adriatic Basal Thrust, which develops from near-surface to MOHO depths (about 35 km).</p> <p>Three hierarchic levels of structural maps are provided with decreasing details moving from fold-and-thrusts traces, enveloping thrust, and regional thrust alignments.</p> <p>Different datasets (points, lines, surfaces) are unified, compiled, and held in a common ArcGIS<strong> </strong>file system folder and linked on the basis of relational models.</p> <p>SEISC is composed of:</p> <ul> <li>one dataset consisting of fold hinges traces (syncline and anticlines)</li> <li>one dataset consisting of individual fold-related thrust</li> <li>one dataset consisting of enveloping thrusts organized in hierarchic orders</li> <li>one dataset consisting of interconnected curvilinear fault surfaces built along the down-dip projection of the enveloping thrusts, segmented along-strike and along-dip</li> <li>one structural data set containing geometric and kinematic point data (attitude, dip-angle, slip-vector, rake, sense of movement) for the node of each triangulated mesh of each fault surface.</li> </ul> <p>A crucial point when dealing with compressional structures is the difficulty in adopting segmentation criteria suitable for a realistic earthquake-fault association. In our methodological approach, the along-strike segmentation is strongly driven by the en-echelon distribution of the fold-related thrusts and by sharp variation in strikes and bending of the enveloping thrusts. On the other hand, the down-dip segmentation is controlled by the mechanical crustal layering derived from earthquake distributions and the rheological investigation.</p>

<p>A recent paper showed the evidence of two well-distinct low-angle and SW-dipping individual reverse shear zones of the Italian Outer Thrust System in Central Italy (de Nardis et al., 2022). One, referred to as Thrust 1 (T1),  corresponds to the down-dip prosecution of the Adriatic Basal Thrust with its major splay; the other, referred to as Thrust 2 (T2), corresponds to a hidden independent structure, illuminated at a depth between 25 and 60 km, for an along-strike extent of ~150 km. Combining geological information with high-quality hypocentral locations and focal mechanisms, a detailed 3D geometric and kinematic fault model of the compressional system, active at upper crust to upper mantle depths, is built. In addition, evidence of coexisting deformation volumes undergoing a co-axial stress field at different lithospheric depths is reported.</p> <p>November 9, 2022, seismic sequence principally activated T1  at upper crustal depth with pure compressional kinematics. Two significant events (Mw 5.5 and 5.2) enucleated within 1 minute, at depths of about 5 km and 7.5 km, respectively, and ~8 km away in map view. The sequence also released a cluster of microseismic events at mid-crust depths along the up-dip prolongation of the T2, thus opening the questions on the possible stress interaction during an ongoing seismic sequence.</p> <p>In this paper, we further constrain and detail the T1 upper-crust geometry and investigate the likelihood of static stress interactions between T1 and T2. Considering that in historical and instrumental times, T1 has been responsible for earthquakes with Mw 6-6.5  at upper- and lower-crust depths, we create possible Coulomb stress transfer scenarios using the Coulomb code 3.4 (Lin and Stein, 2004; Toda et al., 2005).</p> <p>We build three seismic sources (C1, C2, C3) assuming an Mw 6.2 thrust event enucleated on T1 at variable depths (8 km, 15 km, 22 km). The section-view and map-view distribution of the positive lobes of the modeled Coulomb stress scenarios show that a hypothetical T1 earthquake of the above magnitude may well determine, although marginally, stress increase along the underlying T2 segment.</p> <p> </p> <p> </p> <p> </p>

<p>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.</p> <p>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.</p> <p>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.</p>