Problems and Inconsistencies with Major Preferred Faults in the Community Fault Model (CFM6.1)

Craig Nicholson

Published September 8, 2024, SCEC Contribution #13918, 2024 SCEC Annual Meeting Poster #231 (PDF)

From 2009-2021, the CFM strived to produce a comprehensive 3D fault set that is internally consistent and kinematically compatible, as well as predictive and well validated against data and observations [Nicholson et al., 2009-2021]. Such efforts to update, expand and improve the CFM are fundamental if we are to better understand this complex fault system. In 2022, a part of this CFM 3D fault set underwent community review, but this process seemed to be both severely limited and evidently flawed. Only 25 of these faults and their alternatives were selected for review, rather than the entire 3D fault set of 480 potential faults. Each fault was evaluated in isolation, preventing careful consideration as to which was more kinematically compatible with adjacent faults, and fault selection was based exclusively on statistical rankings of perceived model preference. Several previous preferred models were replaced by older outdated alternative models not validated with data, or which were mutually exclusive and thus incompatible with each other or with adjacent faults. The result was a preferred CFM6.0/6.1 fault set [Marshall et al., 2023] that no longer meets its previous standards of internal consistency and predictability. Several adopted models (San Andreas, Banning, Garnet Hill, Pitas Point, San Mateo-Carlsbad, etc.) are fundamentally inconsistent with decades of relocated hypocenters and aligned focal mechanism nodal planes, subsequent publications, and/or updated surface and seafloor trace maps, while others (e.g., Palos Verdes) are not mechanically feasible. Major sections of the Compton thrust connector under LA, Sierra Cucapah, and San Mateo-Carlsbad faults are currently missing, and the CFM6.1 Southern San Andreas fault that replaced the CFM6.0 version (which had an unexplained 15-km fault gap) is incompatible with the Mission Creek fault with which it merges, is not consistent with seismicity, seismic imaging or gravity modeling, and it too exhibits a fault gap near Bombay Beach. Alternative models previously selected as preferred faults for CFM5.3 [Nicholson et al., 2021] do not exhibit these same problems. These inconsistencies with specific CFM6.1 (and now CFM7.0?) faults have been well-documented for years, and so warrant not only a proper re-evaluation of the CFM6.1 preferred 3D fault set using data validation, not user surveys, as the primary selection criteria, but also a re-evaluation of the CFM6.0/6.1 preferred model selection process itself.

Key Words
3D fault geometry, model inconsistencies, data validation, CFM preferred model criteria

Citation
Nicholson, C. (2024, 09). Problems and Inconsistencies with Major Preferred Faults in the Community Fault Model (CFM6.1). Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Community Earth Models (CEM)