How likely does shear wave splitting and moment tensor parameters interact for the “2016 -2018 central Italy seismic sequence” ?

Abstract

The “2016-2018 Central-Italy seismic sequence” is characterized by a Mw 6.0 mainshock that occured close to the towns of Accumoli and Amatrice at 1:36 a.m. (UTC) of August 24th 2016 and it is still ongoing. In 20 months activity, 63905

recorded earthquakes are spatially elongated NW- SE, the aftershocks are mainly concentrated

between 2 and 25 km of depth with minimum and maximum magnitude ranging from Mw 0.1 to 6.5, respectively. This study area is characterized by a complex geological and structural setting derived by multi-phased contractional and extensional deformation began in the Early Tertiary. In detail, the epicentral area is located in the central sector of Apennine chain characterized by several NW-SE oriented Quaternary fault systems and compressional fronts. Nowadays stress indicators (such as borehole breakouts, anisotropic parameters, earthquake focal mechanisms and active faults) point out that the axial part of the Apennine belt is characterized by a general extension about NE-SW oriented. Slip data from Amatrice fault structures show that roughly N–S trending leftlateral strike–slip and transtensional/normal faults are all kinematically consistent with the existence of a Late Quaternary. The remote stress field is characterized by a NE–SW-oriented minimum horizontal stress (Shmin, sigma3 or maximum extension) and by a NW–SE trending maximum horizontal stress (SHmax, sigma1 or maximum compression). In this preliminary study we define the possible correlations between shear wave splitting and moment tensor parameters during the “2016-2018 Central-Italy seismic sequence”. Shear Wave Splitting phenomenon is described by 2 parameters: fast polarization direction and delay time. Generally, fast component strikes as the orientation of the anisotropic symmetry axes, and delay time measures the anisotropic strength. In

this geo-structural context, for sedimentary rocks, according to the Extensive Dilatancy Anisotropy (EDA) model and its evolution in Anisotropic Poro Elasticity (APE) the local variations of the anisotropic parameters are both time and space dependent. As a consequence the fast direction will be parallel to the SHmax direction and the delay time will measure the intensity and/or thickness of the fracture field. Moreover, SHmax corresponds to sigma1 (thrust and strike-slip faulting regime) or sigma2 (normal faulting regime) and can be related to P, T and B axes orientations of seismic moment tensors. In this assumption, we analyze the spatial variations of local stress-field in term of SHmax which is strictly related to sigma, and fast polarization direction. After computing the parameters above described, we compare them with the stress field and geologic structures of the crust to explain the tectonic processes.