Spatial Variations in the Stress Field Near the Wilmington Blind-Thrust, an Earthquake Source Beneath Los Angeles, California

A newly identified blind-thrust fault in the southern Los Angeles basin may have substantial implications for earthquakes in the region. A previous study suggests that the Wilmington blind-thrust may link up with nearby faults to produce a multisegment (Mw 6.6–7.4) earthquake rupture. These estimates are based on the joint rupturing of the Wilmington blind-thrust, Compton, Newport Inglewood, and the Huntington Beach fault segments (~85 km total length). However, stress heterogeneity near these faults may limit the size and intensity of earthquakes when incorporated into earthquake hazard modeling. This study presents a detailed model of the stress field in this region to facilitate understanding the potential of a multisegment rupture scenario. The dataset includes caliper data from 36 wells and resistivity image logs from 15 wells with logged sections all within 5 km of the Wilmington blind-thrust. The wells extend ~12 km along strike and are as close as ~400 m to the fault surface. To provide constraints on the orientation of the maximum horizontal principal stress, SHmax, and the stress regime, caliper data are used to identify borehole breakouts and resistivity image logs are also used to identify drilling-induced tensile fractures (DITFs), which in near-vertical wellbores indicate the orientation of the minimum (Shmin) and maximum (SHmax) horizontal principal stress, respectively. These analyses yield ~195 breakout zones (~1480 m total length in 26 wells) and 20 DITFs zones (~140 m total length in 4 wells). Along 6 km of the Wilmington blind-thrust, interpretations suggest changes in SHmax orientation from N5°E at well 26, fault block VI, to N26°W at Well 16 in fault block 90S. Stress feature rotations are identified in several image logs, where the localized change in SHmax orientation is 10° to 40°. The misfit analysis, which compares observed breakout results with theoretical breakout orientations from varying stress regimes, provides relative principal stress magnitudes of Shmin = 1.9,SHmax = 2, and SV = 1 at Chaffee and Freeman, with average SHmax orientations of N6°E and N14°W, respectively. Indicating that while the stress regime is reverse, variations in SHmax should be incorporated into earthquake hazard modeling