Geometry and stress interaction of a complex lithospheric-scale thrust system as unveiled by background seismicity and moderate seismic sequences - the Marche-Adriatic case (eastern Central Italy)

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.

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.

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).

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.