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 E-W trending en-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 borehole-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 stress-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; S-V = 1.0) and an S-Hmax azimuth of N45 degrees 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 similar to N36 degrees W to similar to N57 degrees E occurs across similar to 100 m depth in a single well and differs from nearby results, indicating that short-length scale (<10 km laterally and <1 km in depth) stress heterogeneity is associated with complex changes in fault geometry. Plain Language Summary Studies have suggested that faults within the Santa Barbara Channel in Southern California could be capable of producing a Mw 7.7 -8.1 earthquake and tsunami, which would pose a major hazard to nearby populated areas. Information on the stress field in this region is one of the key ingredients needed to produce realistic computer models that allow us to determine the likelihood of a large earthquake rupture occurring and to estimate the ground motions that could result from such rupture. We use oil industry data from 19 boreholes drilled beneath the Santa Barbara Channel to provide information on the stress field for earthquake hazard purposes. We addressed the challenge of working with boreholes that are not drilled straight down into the earth but have complex paths by comparing our data to theoretical predictions. We find that the stress field is more complex and variable than previously documented with important changes in the stress field occurring across short distances of just a few kilometers. These changes coincide with changes in the geometry of the major fault systems and may lead to a reevaluation of whether multiple faults are likely to connect up to produce a large earthquake rupture in this region.