ALBERT

Description: Abstract We present the largest dataset of anisotropic high‐quality parameters (~12000) for the Amatrice‐Visso‐Norcia seismic sequence and investigate the physical mechanisms causing crustal anisotropy and its relation with crustal deformation, stress field, fluids and earthquake generation. We performed shear wave splitting analysis on ~40000 aftershocks recorded at 31 seismic stations during the first six months of the sequence following the August 24th 2016 Mw 6.0, Amatrice mainshock. Automatic and manual‐revised P‐ and S‐ picking and high‐precision locations are used to delineate the fracturing pattern and spatio‐temporal variations in the anisotropic parameters: fast direction polarization (φ) and delay time (δt). The mean φ strikes N146°, parallel to the extensional Quaternary fault systems, and to the NW‐SE local active SHmax as proposed by the EDA model. Locally φ directions outline the pattern of micro and mesoscale structures that we relate to structural‐controlled anisotropy. Temporal variations of anisotropic parameters allowed us to deduce stress‐induced and pervasive fluid‐filled stress‐aligned crack systems as the prevalent anisotropic mechanisms in some sectors. Higher δt (〉 0.072 s) and higher anisotropy percentage (〉 1.5%‐2.0%) are found at the boundaries and in the western side of the activated fault systems, where heavily fractured carbonates are present. The fault networks along with the lithological heterogeneities present in the area may act as a structural barrier along which fluids are channeled or trapped, thus causing over‐pressured fluid zones. Observations of shear‐wave splitting parameters during a seismic sequence can monitor the buildup of stress before earthquakes and the stress release as earthquakes occur.