GEOMETRY OF MATURE STRIKE-SLIP FAULTS: EXAMPLES FROM THE SAN ANDREAS FAULT SYSTEM IN SOUTHERN CALIFORNIA

Understanding the 3D geometry of major strike-slip faults is critical to accurate dynamic rupture and ground motion prediction models. The San Andreas fault (SAF) is particularly important as some controversy still surrounds its 3D geometry along strike. Where seismicity is present, relocated hypocenters and aligned focal mechanism nodal planes reveal a consistent picture of a steep-to-near-vertical, largely planar SAF extending from Parkfield to the Salton Sea, including a through-going steeply-dipping predominantly strike-slip Banning strand at depth in San Gorgonio Pass (SGP). A major source of discrepancy for the SGP section is whether there is assumed to be only one active principal slip surface at depth (that must account for all the deformation) and can be modeled as non-planar and/or with moderate dips of 40°-60°, or alternatively, this section is in places multi-stranded and the inferred non-planar geometry and complex deformation are reflections of a predominantly planar, steeply-dipping SAF interacting with adjacent secondary faults that help accommodate oblique components of plate boundary strain. 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 ~65°NE. This geometry of complex interacting fault sets is further validated by sub-surface imaging and other independent data. Although it is well understood that cumulative SAF displacement varied through time along strike as the fault system lengthened and evolved, displacement can also vary on either side of the same fault section. For example, owing to its complex tectonic history, the Mojave section likely has >300 km of slip on the older NE side, yet <150 km of slip on the younger SW side. Thus, besides conforming to the strike-slip geometry of least-work, the persistence of a steep-to-vertical SAF geometry may be due to the 2-sided fault slip surface having to conform to the geometry defined by the side with the larger displacement and longer slip history.