Mapping of absolute stresses around two California earthquakes reveals a very weak crust

Absolute amplitudes of shear stresses that drive crustal earthquakes are not well known. There is a long-standing divergence between the values inferred from lab experiments and stress changes during faulting. Two large earthquakes near Ridgecrest, California with M6.4 and 7.1 provide a natural laboratory to determine the in-situ average shear stress in the crust off the main faults. Here we use the change in faulting geometries of abundant small earthquakes together with stress changes imposed by doublet slip to determine full deviatoric stress tensors both before and after it. We first invert suites of focal mechanisms for stress orientations and ratios between eigenvalues. We then invert for the 3-D full deviatoric tensors constrained by the stress orientations, stress ratios, and the coseismic stress change due to the doublet. We applied this method using two doublet slip models and two endmember approaches: first dividing the region into 12 blocks surrounding the mainshock faults, and second performing 9,200 separate inversions offset by ~ 1 km. To obtain reliable results, we use the 3-D relationship rather than a common 2-D strike-slip simplification, define inversion regions that do not cross the main faults, and include only high-quality events a few km away from the main faults to avoid large heterogeneities in the co-seismic stress change. Deviatoric stresses are only a few percent of levels expected at seismogenic depths from Byerlee friction, except for regions near the doublet hypocenters where they are up to only ~ 7.5%. Our approach yields strong evidence for a very weak continental crust, which bears on earthquake and geodynamic modeling, as well as earthquake recurrence behavior and hazard, suggesting near-complete stress drops in the mainshock doublet and a low chance of imminent large slips there.