Active Fault Geometry and Crustal Deformation Along the San Andreas Fault System Through San Gorgonio Pass, California: The View in 3D From Seismicity

Craig Nicholson, Egill Hauksson, & Andreas Plesch

Published December 2012, SCEC Contribution #1737

Understanding the 3D geometry and deformation style of the San Andreas fault (SAF) is critical to accurate dynamic rupture and ground motion prediction models. We use 3D alignments of hypocenter and focal mechanism nodal planes within a relocated earthquake catalog (1981-2011) [Hauksson et al., 2012] to develop improved 3D fault models for active strands of the SAF and adjacent secondary structures. Through San Gorgonio Pass (SGP), earthquakes define a mechanically layered crust with predominantly high-angle strike-slip faults in the upper ~10 km, while at greater depth, intersecting sets of strike-slip, oblique slip and low-angle thrust faults define a wedge-shaped volume deformation of the lower crust. In some places, this interface between upper and lower crustal deformation may be an active detachment fault, and may have controlled the down-dip extent of recent fault rupture. Alignments of hypocenters and nodal planes define multiple principal slip surfaces through SGP, including a through-going steeply-dipping predominantly strike-slip Banning fault strand at depth that upward truncates a more moderately dipping (40°-50°) blind, oblique North Palm Springs fault. The North Palm Springs fault may be the active down-dip extension of the San Gorgonio Pass thrust offset at depth by the principal, through-going Banning strand. In the northern Coachella Valley, seismicity indicates that the Garnet Hill and Banning fault strands are most likely sub-parallel and steeply dipping (~70°NE) to depths of 8–10 km, where they intersect and merge with a stack of moderately dipping to low-angle oblique thrust faults. Gravity and water well data confirm that these faults are sub-parallel and near vertical in the upper 2–3 km. Although the dense wedge of deep seismicity below SGP and largely south of the SAF contains multiple secondary fault sets of different orientations, the predominant fault set appears to be a series of en echelon NW-striking oblique strike-slip faults that dip 55°-60°NE and strike slightly oblique to the SAF. Together with the multiple active fault strands of the SAF, these deeper, mostly blind faults also help accommodate plate boundary strain through SGP. The improved complex 3D fault models allow for multiple pathways of dynamic rupture along the SAF to propagate in the northern Coachella Valley and into SGP, implying that large earthquake ruptures at depth through SGP are indeed possible.

Nicholson, C., Hauksson, E., & Plesch, A. (2012). Active Fault Geometry and Crustal Deformation Along the San Andreas Fault System Through San Gorgonio Pass, California: The View in 3D From Seismicity. Eos, Transactions, American Geophysical Union, 93(52), T22C-03.