Extensional tectonics
Earthquake sequences play out on geologic fault and fracture systems, which are usually underconstrained by data. Modern deep learning earthquake detection and characterization methods now allow us to compute high‐sensitivity and high‐resolution seismicity catalogs, with which we can image at least the seismogenic parts of fault and fracture systems with much more detail than had been possible previously. Here, we use a convolutional neural network classifier and the SKHASH algorithm to compute a catalog of 16,600 well‐constrained focal mechanisms (FMs) for the exceptionally well‐monitored 2016 6.0 Amatrice– 5.9 Visso– 6.5 Norcia earthquake sequence in Italy. The resulting catalog paints a detailed picture of earthquake faulting kinematics in a fragmented extensional tectonic system. We observe that normal‐faulting mechanisms dominate the seismic activity only over the depth range of 2–9 km. At shallower depths, for which the overburden may be too low for normal faults to be elastically loaded, strike‐slip faulting is more common. The much‐debated basal shear zone—an extensive about 2 km wide near‐horizontal layer with distributed seismicity at 8–10 km depth—is characterized by much higher FM variability than the shallower parts of the crust, where the main normal faults host the largest earthquakes. In the north and south of the study region, the basal shear zone seismicity is sharply divided by the main normal faults, with predominantly normal faulting in the hanging wall, and predominantly strike‐slip faulting in the footwall. The FMs from this study provide insight into deformation processes at the intersection of this basal shear zone and the major normal faults, which is where both the Amatrice and Norcia events nucleated.
We analyze the evolution of stress parameters from the 2016–2017 central Italy seismic sequence taking advantage of ∼13,747 robust focal mechanisms from a deep learning catalog. The density of the catalog allows us to invert focal mechanisms over distances of a few km and different time periods. We inferred a number of stress-related parameters, including the fault plane variability, the orientation of principal stress axes and maximum horizontal stress, the relative magnitudes of principal stresses and the variability of the principal stress orientations with respect to the median. From the uniform regional stress field consistent with the extension of the Apennine Belt, we observe local stress heterogeneities that are driven by the structural features and the coseismic stress history. A variation of the principal stress magnitudes and regimes from pure normal faulting toward transtension with depth is observed. Stress differences at the 1–10 km wavelength are observed between each side of two of the main regional fault structures. The reported stress results suggest a partial mechanical coupling and a strong interaction between the shallow normal faults and the detachment horizon at depth. Furthermore, distinct trends are observed in the stress parameters after the largest mainshocks, and before the MW 6.5 Norcia mainshock, potentially indicating the high shear stress still available in well oriented faults after the MW 6.0 Amatrice earthquake. Our analysis holds implications toward (a) constraining stress magnitudes, (b) illuminating the interaction between the shallow normal faults and detachment horizons, and (c) tracking stress evolution during seismic sequence.
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