Enrico Milanese, Camilla Cattania

According to the classical Mohr-Coulomb-Anderson theory, faults form at an angle from the largest regional compressive stress that is approximately 30° for most rocks. However, real settings are more complex and faults often present orientations inconsistent with the angles predicted by the classical theory applied to the present-day regional stress field.
The Ridgecrest region hosts a young fault system that is part of the Eastern California Shear Zone, and the 2019 earthquake sequence unveiled orthogonal ruptures at multiple scales, apparently at odds with the classical brittle failure model. We use the Ridgecrest region as a case study and compare surface ruptures that developed during the 2019 earthquake sequence to the expected orientations derived from classical faulting theory and to observations from rock experiments. We focus on the off-fault secondary fractures that developed coseismically at the northern termination of the mainshock fault. We calculate coseismic stress changes from published slip models superimposed to a background stress field. We find that a combination of tectonic regional stresses oriented with the largest compressive stress at N14E and weak intensity of coseismic stresses best captures the orientation of off-fault fractures in the classical Mohr-Coulomb-Anderson framework, with an internal rock friction coefficient µ = 0.6. The secondary fractures also show a scale separation: long fractures are best compatible with shear failure, while short fractures cluster along the direction of the largest horizontal stress. The latter is compatible with either local normal faulting or early tensile failures that would later coalesce to form longer faults, consistent with growth of shear fractures in laboratory experiments. Finally, the different orientations of fractures that developed during and prior to the 2019 events suggest that the tectonic stress has rotated over geological timescales. When accounting for the specificity of the area, orthogonal faulting is thus compatible with brittle fracturing
with typical experimental values of rock friction coefficient.