SCEC Award Number 22113 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Development of faults through sand and the slip history of the San Gregorio Fault
Investigator(s)
Name Organization
Kimberly Blisniuk San Jose State University Christie Rowe McGill University (Canada)
Other Participants
SCEC Priorities 2c, 2e, 3b SCEC Groups Geology, SAFS, FARM
Report Due Date 03/15/2023 Date Report Submitted 08/11/2023
Project Abstract
 We present geomorphic, stratigraphic, and tectonic data collected along the San Gregorio Fault where it comes ashore along Pillar Point Bluff in San Mateo County, California. Several newly identified fault splay exposures of the San Gregorio Fault, collectively referred to as the Middle Strand, trend down the middle of Pillar Point Bluff. The Middle Strand is exposed along a landslide headscarp, which forms a wavecut platform, and late Pleistocene marine terrace deposits. Fault gouge and displacements in the overlying marine terrace deposits are used to develop a revised picture of the faulting style and role of this fault strand. These exposures and further mapping of surface deformationalong the San Gregorio Fault suggest changes in fault geometry along strike may be controlling surface morphology. We observe a positive flower structure forming along a restraining bend of the fault that transfers slip to the Seal Cove Fault through a releasing stepover. 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.
Intellectual Merit These exposures and further mapping of surface deformation and consultant trench-identified faults along the San Gregorio Fault suggest changes in fault geometry along strike may be controlling surface morphology. We observe a positive flower structure forming along a restraining bend of the fault that transfers slip to the Seal Cove Fault through a releasing stepover. These observations and initial dates 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. Here we report the new discovery of three or four dextral- oblique fault strands cutting marine sand deposits. These fault strands 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 are currently creeping. This research contributed positively to high-priority questions within the 2022 SCEC Science plan, including P2.c (Constrain the properties of rock damage in fault-zones and in the subsurface, as well as the factors, such as initial properties, loading conditions and pore fluid pressure, that are likely to influence it); P2.e (Describe how fault geometry and rock damage interact to determine the probability of rupture propagation through structural complexities, and determine how model-based hypotheses about these interactions can be tested by the observations of accumulated slip, paleoseismic chronologies, and other near-fault observations); and P3.b (Constrain the geometry of active faults across the full range of seismogenic depths, including structures that link and transfer deformation between faults).
Broader Impacts This research was completed by SJSU, a Hispanic Serving Institution (HSI) and Asian American and Native American Pacific Islander Serving Institution (AANAPISI), graduate students in an Advanced Active Tectonics course taught by PI Blisniuk. Students in the class learned how to apply real research problems while learning professional skills in skills in conducting research, collecting field data, applying LidAR, ArcGIS and photogrammetry. These skills will allow SJSU students to be competitive in a highly technical workforce, while increasing the number of STEM graduates, especially from underrepresented minorities. The research also directly involves landslide and seismic hazard, thereby broadening and improving hazard assessment in the region.
Exemplary Figure Figure 3 Western Scarp (A) Deformation mapped in red lines between the Trench site of the landslide scarp and the Distillery near Ocean Blvd. Dashed red lines with question marks represent deformation that was interpreted. Black lines represent the previously mapped San Gregorio Fault. The Middle Fault is labeled and interpreted to cut across the landslide from the trench site and step to the left at the Middle Fault Scarp.
Right-stepping extensional echelon fractures and subsidence are observed in the western scarp on its down side in the landslide. Green dots show coordinates where deformation was documented. (B-C) Left-stepping compressional echelon fractures rotated counterclockwise from the Western Scarp with the down direction to the east. (B) Western Scarp on Ocean Blvd. oriented ∼335°. (C) Western Scarp in landslide oriented ∼345°. (D) Southern end of the Western Scarp oriented ∼345° with the down direction to the east.
Created by Simone Yaeger, SJSU student