Scales of Stress Heterogeneity Near Active Faults in the Santa Barbara Channel, Southern California

The Santa Barbara Channel represents the offshore portion of the Ventura Basin in Southern California. Ongoing transpression related to a regional left step in the San Andreas Fault has led to the formation of Eâ€W trending enâ€echelon fault systems that accommodate localized shortening across the basin. Recent studies have suggested that faults within the channel could be capable of a multisegment rupture and producing a M_w 7.7–8.1 tsunamigenic earthquake. However, dynamic rupture models producing these results do not account for stress heterogeneity. With only sparse information available on the stress field in this region, further boreholeâ€derived stress constraints are essential for obtaining a more comprehensive understanding of the hazards related to the complex fault systems. We used caliper logs from 19 wells obtained from industry to identify stressâ€induced borehole breakouts beneath the Holly and Gail oil platforms in the channel. Our newly developed forward modeling technique provides constraints on the orientations and relative magnitudes of the three principal stresses. At Gail, we determine a reverse faulting stress regime (S_(Hmax) = 1.7; S_(hmin) = 1.6; SV = 1.0) and an S_(Hmax) azimuth of N45°E. Our results are consistent with local structures, which reflect deeper regional scale trends, and with similar studies onshore nearby. At Holly, an S_(Hmax) rotation from ~N36°W to ~N57°E occurs across ~100 m depth in a single well and differs from nearby results, indicating that shortâ€length scale (<10 km laterally and <1 km in depth) stress heterogeneity is associated with complex changes in fault geometry. ; © 2020 American Geophysical Union. Received 30 SEP 2019; Accepted 30 DEC 2019; Accepted article online 3 JAN 2020. We thank three anonymous reviewers, the Editor, Thorsten Becker, and the Editor in Chief, Claudio Faccenna, for their insightful comments and suggestions, which significantly improved this manuscript. The raw dataset of digital well curves used in this study was ...