New Constraints on Stress Heterogeneity along High Risk Fault Systems in the Santa Barbara Channel, California from Borehole Breakouts

Edward H. Pritchard, Patricia Persaud, & Joann M. Stock

Published August 8, 2018, SCEC Contribution #8344, 2018 SCEC Annual Meeting Poster #153

The Santa Barbara Channel represents the offshore portion of the Ventura Basin in Southern California. Miocene extension and ongoing transpression related to a regional left step in the San Andreas Fault have led to the formation of highly complex, active structures beneath the channel. Recent studies have suggested that faults within the northern region of the channel could be capable of a multisegment rupture and producing a Mw 7.7–8.1, tsunamigenic earthquake. However, the level of structural complexity and stress heterogeneity that would limit earthquake rupture in this region are still undetermined.

We use oriented 4-arm caliper data from 18 deviated wells (depths < 1800 m) obtained from industry to identify borehole breakouts beneath the Holly and Gail oil platforms in the Channel. A misfit-based forward modeling technique is used to determine the best fit stress regime and provides quantitative constraints on the orientations and relative magnitudes of the three principal stresses beneath the platforms. At platform Gail, we determine a thrust faulting stress regime with an SHmax trending N44°E. Our results are consistent with local structures, which reflect deeper regional scale trends, and with similar studies onshore nearby. We also determine a thrust faulting stress regime at platform Holly, located in the structurally complex north channel region. Our SHmax orientation of N55°W differs from previous studies onshore and offshore to the west. But, both our results and those of previous studies are consistent with the complex changes in fault strike within the Pitas Point-North Channel-Red Mountain fault systems. One important finding from our study is that borehole breakouts may record stress on a localized scale, allowing stress heterogeneity to be recognized where other methods may fall short. Our results also provide ground truth for future modeling efforts seeking to test the influence of in-situ stress variability on estimates of maximum earthquake size.

Key Words
Borehole, breakouts, stress, offshore, santa barbara channel, fault

Citation
Pritchard, E. H., Persaud, P., & Stock, J. M. (2018, 08). New Constraints on Stress Heterogeneity along High Risk Fault Systems in the Santa Barbara Channel, California from Borehole Breakouts. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)