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)

Abstract

<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>