Suitable 3D Geometry for the San Andreas Fault ShakeOut Scenario 2.0 from Seismicity
Craig NicholsonSubmitted August 30, 2026, SCEC Contribution #15519, 2026 SCEC Annual Meeting Poster #080
A critical component of the San Andreas Fault (SAF) ShakeOut Scenario 2.0 rupture model is accurate and reliable 3D fault geometry. This geometry can be provided by the fault models in CFM5.3 that are based on surface mapping, seismicity and multiple data validation methods. Where seismicity is present, relocated hypocenters and aligned focal mechanism nodal planes define a consistent steep-to-near-vertical, largely planar SAF extending from Parkfield to the Salton Sea, including around the Big Bend and a through-going steeply-dipping predominantly strike-slip Banning strand at depth in San Gorgonio Pass [Nicholson and Hauksson, 2023]. In the northern Coachella Valley, seismicity indicates that the Garnet Hill and Banning strands are sub-parallel and steeply dipping (≥70°NE) to depths of >15 km, and interact with a stack of low-to-moderately dipping oblique thrust faults. The Mission Creek strand is also steep-to-near vertical (≥80°NE). Farther south, earthquakes define a near-vertical southern SAF and an adjacent sub-parallel Mecca Hills-Hidden Springs fault system that dips ~60°NE. This CFM5.3 3D geometry of complex interacting fault sets and SAF strands is further validated by independent subsurface imaging and other data. CFM5.3 can also provide suitable 3D geometry and proper context for the proposed large LA basin ShakeOut Scenario aftershock on the Hollywood-Raymond fault.
In contrast, several faults adopted in 2022 for CFM6.1/7.0 are actually older outdated alternative models not independently validated with data, or are mutually exclusive and thus incompatible with each other or with adjacent faults. In particular, CFM7.0 SAF models with low-to-moderate dips (37°-60°) from the fault surface trace are inconsistent with and thus invalidated by this persistent steep-to-near-vertical SAF seismicity and nodal plane alignment. These alternative SAF models are based on gravity, or presume the SAF is everywhere single stranded [Fuis et al., 2012]. The net result is that active adjacent secondary faults can be misidentified as the SAF principal slip surface, the CFM7.0 southern SAF is incompatible with the steeply dipping Mission Creek fault with which it merges, exhibits a major fault gap near Bombay Beach, and is not consistent with various subsequent, updated studies since 2012 of seismicity, finite fault displacements, seismic imaging, or gravity modeling, including those by the authors themselves [e.g., Fuis et al., 2017; Langenheim and Fuis, 2022].
Key Words
San Andreas fault, 3D fault geometry, ShakeOut scenario, CFM model comparison
Citation
Nicholson, C. (2026, 08). Suitable 3D Geometry for the San Andreas Fault ShakeOut Scenario 2.0 from Seismicity. Poster Presentation at 2026 SCEC Annual Meeting.
Related Projects & Working Groups
Community Earth Models (CEM)
