Surface deformation associated with continental earthquake ruptures includes localized deformation on the faults, as well as deformation in the surrounding medium though distributed and/or diffuse processes. However, the connection 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. Computing high-resolution optical image correlations for the 2021/05/21 Mw7.4 Maduo, Tibet, rupture, we highlight a correlation between the presence of faults and fractures at the surface, and variations in the across-fault displacement gradient, fault zone width, and amplitude of surface displacement. We show that surface slip along primary faults is systematically associated with gradients greater than 1%, and is dominant in regions of greater coseismic surface displacement. Conversely, the diffuse deformation is associated with gradients ≤0.3%, and is dominant in regions of lesser surface displacement. The distributed deformation then occurs for intermediate gradients of 0.3-1%, and at the transition between the localized and diffuse deformation regions. Such patterns of deformation are also described in laboratory experiments of rock deformation, themself supported by field observations. Comparing these experiments to our observations, we demonstrate that the diffuse deformation along the 2021 Maduo rupture corresponds to kilometer-wide plastic yielding of the bulk medium occurring in regions where surface rupture is generally missing. Along the 2021 Maduo rupture, diffuse deformation occurs primarily in the epicentral region, where the dynamic stresses associated with the nascent pulse-like rupture could not overcome the shallow fault zone frictional strength.

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.