Evidence of sub Kilometer-scale Variability in Stress Directions near Active Faults: An Example from the Newport-Inglewood Fault, Southern California

Abstract:

The active Newport-Inglewood Fault (NIF) zone is a series of right-lateral, left-stepping en echelon segments and associated anticlines that produced the 1933 Long Beach M_w 6.4 earthquake. Seismic hazard estimates, dynamic earthquake rupture models, and earthquake simulations for Southern California rely on information on the stress field obtained from the Community Stress Model (CSM), though the latter still lacks observational constraints. This study provides much needed observational constraints on in-situ stress, which are useful for validating the CSM. Our results highlight the possibility of variations in stress directions near active faults at length-scales less than 1 km. We determined the orientation of stress-induced compressive failures or borehole breakouts, which are reliable indicators of the orientation of the maximum horizontal stress (S_H) in over 40 wellbores in the Los Angeles basin near the NIF. The compressional jogs along the fault have long been drilled for oil in this major metropolitan area, and so provide the dataset of oriented caliper logs. This allowed us to investigate the variation of S_H direction in three oil fields. In the Inglewood oil field, a dense dataset of 24 wells in ~2 km², S_H 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, S_H takes on a more northerly orientation. In contrast, S_H is oriented E-W in the eastern fault block, based on constraints from two wells. 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 S_H direction are attributed to the proximity to faults or fault segmentation, and indicate the likely complexity that may be found in stress fields near other active faults.