Surface deformation associated with continental earthquake ruptures includes localized deformation on the faults, as well as deformation in the surrounding medium through distributed and/or diffuse processes. However, the role of the diffuse part of the surface deformation to the overall rupture process, as well as its underlying physical mechanisms are not yet well understood. In this study, we compute high-resolution near-fault displacement maps from optical image correlations for the 2021/05/21 Mw7.4 Maduo, Tibet, strike-slip earthquake, and measure the contributions of the different deformation components to the surface deformations for that event. Results show that surface slip along primary faults accommodates, on average, only ~25% of the total surface deformation. Majority of the surface coseismic deformation is in fact accommodated by diffuse deformation,especially in the epicentral area where no surface slip was observed. In fact, the contribution of the diffuse deformation increases as localized deformation on the fault decreases. Localized deformation also decreases with decreasing total surface displacement. These observations highlight a gradual localization of the surface coseismic deformation, from regions of diffuse low (0.1-0.3%) strain, to regions of highly localized (>1 %) strain, with increasing coseismic displacement. Using simple two-dimensional mechanical models we show that diffuse deformation may correspond to elastoplastic bulk yielding, accounting for the deficit in shallow fault slip in the regions of surface rupture gap.

The Ridgecrest sequence (Mw6.4 and Mw7.1, July 2019, California) is a cross-fault earthquake that has been observed using a wide range of geophysical and geological methods. The sequence ruptured consecutively two orthogonal cross-fault systems within 34 hours (northeast- and northwest-trending). It raised the question of the relation between the two systems of faults both at depth and at the surface, and its impact on the surface displacement pattern. Here we use high-resolution (50 cm) satellite optical image correlation to measure the 3D surface displacement field at 0.5 meters ground resolution for the two earthquakes. Because our images bracket the whole sequence, our displacement and deformation maps include both earthquakes. Our data allow for measuring series of slip profiles in the components parallel and perpendicular to the rupture, and in the vertical direction, to look at the correlation between slip distribution and rupture complexity at the surface. We point out significant differences with previous geodetic and geological-based measurements and show the essential role of distributed faulting and diffuse deformation in the comprehension of surface displacement patterns. We discuss the segmentation of the rupture regarding the fault geometry and along-strike slip variations. We image several surface deformation features with similar orientation to the deeply embedded fabric identified in seismic studies. This northeast-trending fabric influenced the surface deformation both during the foreshock and the mainshock earthquakes. We also derive strain fields from the horizontal displacement maps and show the predominant role of rotational and shear strains in the rupture process. We finally compare our results to kinematic inversions and show that the foreshock did influence the mainshock by clamping the fault and encouraging off-fault diffuse deformation rather than fault slip in some areas.