ID: 878 7140 0680
Strike-slip ruptures generally occur on complex fault systems, with a deformation at the surface that distributes over wide areas through secondary faulting and diffuse deformation. As a consequence, the total surface displacement associated with an earthquake is particularly difficult to quantify. In this study, we use sub-pixel correlation of 0.5-m resolution optical satellite images to capture the detail of the 3D near-fault displacement field associated, respectively, with a 30-km long area of the 2013, Mw7.7, Baluchistan, Pakistan, rupture, and with the 2019, Mw6.4 and Mw7.1, Ridgecrest, California, earthquake ruptures. Our results reveal complex fault geometries and surface displacement patterns, raising the issues of what parameters control the distribution of the surface deformation, and what are the deformation mechanisms involved? To answer these questions, we perform a detailed analysis of the slip patterns across the ruptures. We quantify with a precision of 0.10 to 0.20 m the distribution of surface displacements along the fault systems, and we evaluate the total slip budget, including variations of on- and off-fault slip. We show that the deformation can be either very localized in a 20- to 30-m wide fault zone, or distributed over hundreds of meters to kilometers off the fault plane through off-fault cracks or continuous gradients of displacement of the order of 10- 3. The continuous deformation gradients are referred to as diffuse deformation, and represent on average 30% of the total displacement budget along the Ridgecrest ruptures. Along the study area of the Baluchistan rupture, off-fault deformation (OFD), including both off-fault cracks and diffuse deformation, represents on average 16% of the total displacement budget. We show that this difference in OFD proportions is primarily due to differences in the fault geometry, with a larger proportion of linear and well-defined fault sections along the Baluchistan rupture. Along the Ridgecrest ruptures, the entire fault system is complex, and we highlight a segmentation of the rupture that is correlated with variations in the fault geometry, the slip pattern at depth, and the slip distribution at the surface. We discuss the role of the geological context in controlling the fault geometry, and show that, along the Baluchistan rupture, and to the exception of the few large geometrical complexities, most of the fault slip reaches the surface along simple fault sections which correlate with the location of a long-lived interface within the folded sediments forming the Makran mountain range. Conversely, the Ridgecrest ruptures occurred within an area of heterogeneous lithology and fabric, which favored a complex fault geometry and a distributed surface deformation across the fault system. We show that our observations of wide diffuse deformation zones are supported by seismic data that highlight kilometric-scale areas of diffuse aftershocks and background seismicity around faults, interpreted as wide areas of inelastic rock deformation. This has consequences for the elastic properties of the host rock, for the accumulation of interseismic strain and for the rupture dynamics. In a final step, we use the high-resolution optical displacement maps, combined with existing GNSS and InSAR data, to better constrain the kinematic fault slip models, and improve our understanding of the rupture process at depth. Knowledge about the rupture processes and fault mechanics, in addition to tectonic observations on the distribution of surface deformation, are critical for estimating the seismic cycle on a fault, and the probabilities of fault displacement and ground motions.