Group B, Poster #138, Fault and Rupture Mechanics (FARM)

Heterogeneous coseismic stress state describes off-fault fracture orientation in the 2019 Ridgecrest sequence

Enrico Milanese
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Poster Presentation

2022 SCEC Annual Meeting, Poster #138, SCEC Contribution #12205 VIEW PDF
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.
The Ridgecrest region in Southern California is an example of mechanically unfavorable orientations at multiple scales. The 2019 sequence is characterized by a M w 6.4 for...
eshock and a M w 7.1 mainshock that generated slip along two orthogonal faults. This unexpected orientation can be explained with rotation of the stress state and of the faults since their formation millions of years ago [1]. Yet, off-fault secondary fractures also formed at unfavorable angles coseismically. Here, we show that these off-fault fractures are actually consistent with the classic Mohr-Coulomb criterion when including coseismic stress changes.
We calculate the coseismic stress changes from published slip models [2, 3] and we superimpose them 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 [4].
Our results 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 which they formed.

[1] Fialko, Y. & Jin, Z., Nature Geoscience 14, 513–518 (2021).
[2] Ross, Z. E. et al., Science, 366, 346–351 (2019).
[3] Jin, Z. & Fialko, Y., Bulletin of the Seismological Society of America 110, 1660–1679 (2020).
[4] Ponti, D. J. et al., Seismological Research Letters 91, 2942–2959 (2020).