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Sub Kilometer-scale Variability in In-situ Stress Directions near the Newport-Inglewood Fault, Southern California

Patricia Persaud, Joann M. Stock, & Deborah E. Smith

Published 2015, SCEC Contribution #6066

This study highlights possible variations in stress directions near active faults in the Los Angeles basin at length-scales less than 1 km, and provides much needed observational constraints on in-situ stress, which are useful for validating the Community Stress Model (CSM). Our results have implications for seismic hazard estimates, dynamic earthquake rupture models, and earthquake simulations for Southern California, which rely on information on the stress field obtained from the CSM. Using a dataset of multiple-arm oriented caliper logs from 76 wellbores, we determined the orientation of stress-induced compressive failures or borehole breakouts, which are reliable indicators of the orientation of the maximum horizontal stress (SH) in the active Newport-Inglewood Fault (NIF) zone. The NIF zone is a series of right-lateral, left-stepping en echelon segments and associated anticlines that produced the 1933 Long Beach Mw 6.4 earthquake. Various oil companies, and oil field operators provided the well logs, which allowed us to investigate the variation of SH in the four sub-regions separated by as much as 30 km, each with a tight well spacing.

In the Inglewood oil field, a dense dataset of 24 wells in ~2 km², SH varies from N9°E to N32°E over a depth range of 1-3 km and within 400 m of the fault in the western fault block, with more variability occurring in wells father away. At depths below 2 km, SH takes on a more northerly orientation. In contrast, SH is oriented E-W in the eastern fault block, based on constraints from two wells. Similarly, in the Long Beach oil field, a dataset of 36 wells located in a narrow faulted anticline parallel to the NIF, SH varies from N8°W to N25°W in two wells separated by ~300 m. In the Long Beach oil field, where some wells are located less than 50 m apart and some extend to 4.2 km depth, significant variations in SH orientation occur at length-scales less than 1 km, with a prominent east of north SH orientation also present. In the Wilmington oil field located between the Thums-Huntington Beach Fault and the NIF, data from 11 deviated wells yields a pattern of elongation directions, which differs from the more complex pattern obtained for the Huntington Beach wells located ~12 km to the southeast. The short-length-scale variations in SH direction are attributed to the proximity to faults, fault segmentation, or fault overlap; and indicate the likely complexity that may be found in stress fields near other active faults.

Persaud, P., Stock, J. M., & Smith, D. E. (2015). Sub Kilometer-scale Variability in In-situ Stress Directions near the Newport-Inglewood Fault, Southern California. Poster Presentation at 2015 SCEC Annual Meeting.

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