Subsurface Lithologic Controls on Off-Fault Deformation and Multi-Fault Slip During the 2016 Mw 6.5 Norcia Earthquake Revealed by Satellite Geodesy

Understanding the mechanisms controlling deformation localization is crucial for our understanding of fault mechanics and improving seismic hazard assessment, but has not been extensively studied for normal-faulting earthquakes. Here, we present a thorough analysis of the 2016 Mw 6.5 Norcia, Italy, earthquake using high-resolution satellite geodesy. We investigate the degree of deformation localization, evaluate its controlling factors and the link with the distribution of slip with depth. Using the optical image correlation technique with an innovative method for noise correction, we measure the near-field 3D displacements associated with the Norcia event. Based on these measurements, we quantify the amount of off-fault deformation (OFD) and evaluate how it varies with external factors. We find 46% (25 cm) of OFD on average and a strong correlation with local topographic slope and near-surface lithology, with increased distributed deformation where ruptures traverse unconsolidated sediments and areas of gentler slopes. In contrast, the correlation between OFD and the fault segment orientation relative to the regional stress field is weak. We develop a comprehensive slip model accounting for complex multi-segmented fault geometry, topography, and 3D elastic structure through a joint inversion of optical, InSAR, and GPS data. The inversion reveals a highly heterogeneous slip distribution characterized by large slip (up to 3.5 m) at depth, and several shallow slip patches. We find a pronounced average shallow slip deficit (SSD) of 72%, with no along-strike correlation between SSD and OFD. This suggests that the OFD primarily reflects surficial inelastic processes occurring within the shallow soil.