Recent Creep and Structural Evolution of the San Gregorio Fault at Pillar Point, California

Simone Yeager, Clarissa I. Smith, Patrick OConnor, Hudson Washburn, Tyler Ladinsky, Christie D. Rowe, & Kimberly Blisniuk

Submitted September 10, 2023, SCEC Contribution #13141, 2023 SCEC Annual Meeting Poster #078

Here we summarize and characterize newly identified fault splay exposures of the San Gregorio Fault, collectively referred to as the Middle Strand. This inferred Middle Strand is exposed along a landslide headscarp more than 100 m in length and up to 10 m in height and exhibits significant oblique slip. Our study applied an interdisciplinary approach that combines methods of stratigraphy, photogrammetry, and deformation characterization to a spectacular exposure of the San Gregorio Fault at Pillar Point Bluff in California, where it comes onshore and deforms bedrock of the Purisima Formation and young marine terrace sediments. The fault deforms Pliocene Purisima Formation bedrock, which forms a wavecut platform, and late Pleistocene marine terrace deposits. We further report the discovery of three or four dextral-oblique fault splays cutting marine sand deposits that reveals a positive flower structure along the San Gregorio Fault. These inferred fault splays have a complex interaction with active slumping along the seaward edge of Pillar Point Bluff, but dextral en echelon tension crack arrays and displacement of asphalt may indicate one or more strands is currently creeping. These observations bring into question previous assumptions of the fault structure within this exposed section of the San Gregorio Fault, indicating that subsurface fault geometry likely plays a significant role in surface morphology.

Key Words
San Gregorio Fault, Land slide, Creep, Pillar Point, Stratigraphy, Photogrammetry, Deformation

Citation
Yeager, S., Smith, C. I., OConnor, P., Washburn, H., Ladinsky, T., Rowe, C. D., & Blisniuk, K. (2023, 09). Recent Creep and Structural Evolution of the San Gregorio Fault at Pillar Point, California. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology