Heterogeneous Coseismic Stress State Describes Off-fault Fractures Orientation in the 2019 Ridgecrest Sequence

The origins of faults are commonly described by the classical Mohr-Coulomb-Anderson theory: the tectonic regional stress state brings the crust to its shear failure, and a fault forms at an angle from the largest principal stress. This angle depends on the internal friction of the rock, and laboratory experiments show that it corresponds to approximately 30° for most rocks. However, faults often present orientations inconsistent with the angles predicted by the classical theory applied to the regional stress field. Discrepancies can indicate the presence of an heterogeneous local stress field, or that faults formed in an earlier stress field with a different orientation.The Ridgecrest region in Southern California is an example of mechanically unfavorable orientations at multiple scales. While several mechanisms have been suggested to explain the orientations of the main faults [1-3], here we focus on explaining the off-fault pattern of secondary fractures that developed coseismically during the Mw 7.1 mainshock of the 2019 Ridgecrest earthquake sequence.We calculate coseismic stress changes from published slip models [1,4] superimposed to a background stress field. We then estimate fractures orientation based on the classical Mohr-Coulomb-Anderson theory and we investigate if other mechanical quantities (e.g. strain energy density, proximity to the plastic limit, average stress state) provide insights in the spatial density of the fractures. Finally, we compare our estimation with the damage maps derived from field observations and geodetic data [5]. We find that off-fault fractures are unfavorably oriented in the regional stress field, but they are consistent with the Coulomb-Mohr criterion when including coseismic stress changes.Our results then show that off-fault fractures orientations only apparently contradict the classical theory of brittle rock failure: the orientation of faults near a major fault actually reflects the highly heterogeneous stress field in whichthey formed.

References:

[1] Ross et al, Science 366, 346-351 (2019).

[2] Fialko & Jin, Nat. Geosci. 14, 513-518 (2021).

[3] Scholz & Choi, Earth and Planet. Sci. Lett. 577, 117273 (2022).

[4] Jin & Fialko, Bull. Seismol. Soc. Am. 110, 1660-1679 (2020).

[5] Ponti et al, Seismol. Res. Lett. 91, 2942-2959 (2020).