New field observations reveal two previously unmapped fault strands in the San Gorgonio Pass region of the San Andreas fault system

Patrick OConnor, Kim D. Blisniuk, & Michae Musick

Submitted August 30, 2026, SCEC Contribution #15293, 2026 SCEC Annual Meeting Poster #103

This study presents results of new bedrock and geomorphic mapping from field observations and high-resolution airborne LiDAR topographic scanning, undertaken in the San Gorgonio Pass region of the San Andreas fault system, in the study area south of Yucaipa Ridge, between Oak Glen, Mile High Ranch and Banning Canyon. Our new field observations reveal at least two previously unmapped fault strands through the study area, identified through a series of deformed surface features such as outcrops of gouge and brecciated bedrock, displaced landforms, scarps in Quaternary sediments and offset drainages and ridges, that all together align into continuous fault traces spanning several kilometers. We present updated digital maps of the geology and geomorphology in the vicinity of the San Gorgonio Pass section of the San Andreas fault identifying the two fault strands and discuss the observed features that provided evidence for their location. Estimates of Quaternary offsets are presented from field measurements and high resolution topographic digital elevation models. Initial analysis of the results suggests that at least one of these fault strands could be a primary active strand of the Mission Creek Fault that may accommodate slip at this latitude of the plate boundary. Further work is currently underway to date the offset landforms to better characterize the slip history on the two fault strands, in order to evaluate how this slip feeds into the overall slip budget of the region.

Key Words
field, geology, surface morphology, earthquake history, seismic deformation, active tectonics, active faulting

Citation
OConnor, P., Blisniuk, K. D., & Musick, M. (2026, 08). New field observations reveal two previously unmapped fault strands in the San Gorgonio Pass region of the San Andreas fault system. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology