Stress heterogeneity and its relation to geology during the 2016-2017 central Italy earthquake sequence

Quantifying the stress field variability at different wavelengths and how it evolves over time has important implications for the development of earthquake sequences. As small earthquakes occur more frequently than larger ones, their focal mechanisms are essential to create larger catalogs, that allow to statistically quantify the kinematics of earthquake sequences within a region and the preferential orientations of co-seismic strain release. Inverting earthquake focal mechanisms is widely used to infer a number of stress-related parameters characterizing a crustal volume, including the orientation of the principal stress axes, maximum horizontal stress, and the relative magnitudes of the principal (stress ratio R) and horizontal stresses (A_PHI). If resolution allows, these stress parameters can be employed to quantify stress variability and heterogeneity in different rock volumes. Here we studied the 2016-2017 Central Italy seismic sequence that is characterized by the occurrence of three mainshocks: the M_w 6.0 Amatrice, M_w 5.9 Visso and M_w 6.5 Norcia. These major events nucleated on normal faults with kinematics consistent with the regional stress field. Taking advantage of a high-resolution catalog generated with deep learning and containing ~56.000 focal mechanisms, we calculated the distribution of stress parameters over small crustal volumes activated during the sequence and resolved the variability of the stress field during three different time periods punctuated by the three largest events. We found a change in the local trend of S_HMax with better defined orientations consistent with the regional stress field towards the SE of the Amatrice mainshock, and larger variability to the NW where also Visso and Norcia mainshocks ruptured. These changes in S_HMax trend also coincide with a contrast in the A_PHI parameter quantifying the relative magnitude of the horizontal stresses. The area between the Amatrice, Visso and Norcia epicenters displayed the lowest A_PHI (i.e. representing a normal faulting stress regime where S_V >> S_HMAX) and the lowest relative magnitude of the S₂ (S_HMax in the case of normal faulting). Furthermore, an increase in the A_PHI parameter is observed at depths below 8 km, reaching transtensional and strike-slip stress regimes in some local volumes. The area surrounding the rupture of the Amatrice mainshock displays the largest deviations from the regional stress over the entire analysed time period, indicating a stress anomaly driven by the properties of the medium or stress heterogeneities caused by static stress transfer that are persisting over the one-year time span of the catalogue. The observed local stress deviations from the regional stress field can help illuminating dominant local loadings affecting the deformation pattern.