Estimation of Absolute Stress in the Hypocentral Region of the 2019 Ridgecrest, California, Earthquakes

Strength of the upper brittle part of the Earth's lithosphere controls deformation styles in tectonically active regions, surface topography, seismicity, and the occurrence of plate tectonics, yet it remains one of the most debated quantities in geophysics. Direct measurements of stresses acting at seismogenic depths are largely lacking. Seismic data (in particular, earthquake focal mechanisms) have been used to infer orientation of the principal stress axes. I show that the focal mechanism data can be combined with information from precise earthquake locations to place constraints not only on the orientation, but also on the magnitude of absolute stress at depth. The proposed method uses relative attitudes of conjugate faults to evaluate the amplitude and spatial heterogeneity of the deviatoric stress and frictional strength in the seismogenic zone. Relative fault orientations (dihedral angles) and sense of slip are determined using quasi‐planar clusters of seismicity and their composite focal mechanisms. The observed distribution of dihedral angles between active conjugate faults in the area of Ridgecrest (California, USA) that hosted a recent sequence of strong earthquakes suggests in situ coefficient of friction of 0.4–0.6, and depth‐averaged shear stress on the order of 25–40 MPa, intermediate between predictions of the “strong” and “weak” fault theories. Plain Language Summary Strength of the Earth's crust is one of the most important yet least known quantities in geophysics. It determines how high the mountains can grow and how large the earthquakes can be, but is notoriously difficult to measure. Only a handful of direct measurements are available from data collected in deep boreholes, and even fewer measurements were made in the vicinity of faults capable of large earthquakes. This study uses clusters of small earthquakes to map out active faults at depth, and determine their relative orientations. Such orientations (more precisely, the angle between faults having an opposite sense of slip “conjugate faults”) provide clues about how strong natural faults are, and what is the average level of shear stress supported by the seismogenic crust. Data from the Ridgecrest (California) area that hosted a sequence of large earthquakes in 2019 suggest that the newly formed faults progressively weaken as they mature and accumulate more slip. The average shear stress acting at the seismogenic death is found to be on the order of several tens of Megapascals, lo