Chris Milliner, Saif Aati, Jean-Philippe Avouac 

The variation of stress on faults is important for our understanding of fault friction and the dynamics of earthquake ruptures. However, we still have little observational constraints on their absolute magnitude, or their variations in space and in time over the seismic cycle. Here we use a new geodetic imaging technique to measure the 3D coseismic slip vectors along the 2019 Ridgecrest surface ruptures and invert them for the coseismic stress state. We find that the coseismic stresses show an eastward rotation that becomes increasingly transtensional from south-to-north along the rupture, that matches the known background stress state. We find that the main fault near the Mw 7.1 mainshock hypocenter was critically stressed. Coseismic slip was maximum there and decreased gradually along strike as the fault became less optimally oriented due its curved geometry. The variations of slip and stress along the curved faults are used to infer the static and dynamic fault friction assuming Mohr-Coulomb failure. We find shear stresses of 4-9 MPa in the shallow crust (~1.3 km depth) and that fault friction drops from a static, Byerlee-type, value of 0.61 ± 0.14 to a dynamic value of 0.29 ± 0.04 during seismic slip. These values explain quantitatively the slip variations along a transpressional fault bend.