SCEC Award Number 20022 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Stress Field of the Southern Los Angeles Basin, California from Borehole Breakout and Drilling-Induced Fracture Analysis
Name Organization
Patricia Persaud Louisiana State University
Other Participants Justin Kain
SCEC Priorities 1d, 1c, 1e SCEC Groups SDOT, GM, FARM
Report Due Date 03/15/2021 Date Report Submitted 03/15/2021
Project Abstract
Previous work suggests that the Wilmington blind-thrust fault is capable of producing a multi-segment (Mw 6.3–6.4) earthquake rupture based on the Torrance, Wilmington blind-thrust, and Huntington Beach fault segments all rupturing together. This rupture model does not consider stress heterogeneity near these faults (~55 km total length) which may limit the size of earthquakes. To obtain a more realistic estimate of potential earthquake rupture, information on the stress field of the fault system is essential. We analyze digital oriented 4- and 6-arm caliper data from 36 wells in the region to provide constraints on the in situ stress. All wells have surface locations within 5 km of the Wilmington blind-thrust and are as close as ~500 m to the projected fault surface trace. Our caliper data set yields ~400 breakouts comprising ~1300 m in total length. Preliminary results in near-vertical borehole sections, indicate consistent SHmax orientations at some platforms, but significant SHmax variations between the groups. Group III’s near-vertical SHmax orientation is N4°W, Group IV’s is N7°E, and Group V’s is N22°W. This indicates a varying SHmax orientation along strike of the Wilmington blind-thrust. We also conducted a preliminary misfit analysis between the dataset and theoretical stress orientations for eight Group IV wells comprising ~650 m total breakout length to obtain a high confidence model of the stress state. Our initial best fit model has relative stress magnitudes of SHmax=2, Shmin=1.9 and SV=1 with a median SHmax orientation of N3°E giving a reverse faulting stress regime for the platform.
Intellectual Merit The Wilmington anticline has been the subject of recent research that posits the existence of a major blind-thrust fault located along the fold axis of the anticline that extends onshore to the northwest through the Torrance anticline (Figure 1; Wolfe et al., 2019). This fault zone is capable of hosting greater that magnitude 7 earthquakes (Wolfe et al., 2019). These new results are in contradiction to the long-held view that the region is characterized by a strike-slip stress regime similar to the nearby Newport-Inglewood fault zone. Previous studies on the stress field in this region are sparse, however, a strike-slip stress regime has been confirmed by in-situ stress analysis (e.g., Katahara et al., 1995). Our study provides a more detailed and comprehensive study of the stress regime surrounding the Wilmington blind-thrust fault, which is needed to determine the likelihood of large ruptures with reverse-slip occurring.
Broader Impacts This award has supported a PhD student and the research program of an early-career faculty. A manuscript was published with the methodology for the stress analysis and stress results for the Santa Barbara channel (Persaud et al., 2020). Preliminary project results for the Wilmington blind thrust fault were presented at the 2020 SCEC Annual Meeting with a second manuscript in preparation.
Exemplary Figure Figure 3. (a) Map showing preliminary maximum horizontal stress (SHmax) orientations near the Wilmington blind-thrust fault based on the analysis of near-vertical wellbore sections.