Geological and Frictional Characterization of Damage Zone Structure of the Southern San Andreas Fault at Ferrum and Implications for Coseismic Off-Fault Deformation

William A. Griffith, Aidan Fullriede, Thomas K. Rockwell, & Anthony Torma

Submitted September 7, 2025, SCEC Contribution #14479, 2025 SCEC Annual Meeting Poster #TBD

A spectrum of slip behaviors occurs in active faultzones, ranging between localized slip on centimeters-thick fault principal slip zone to delocalized shear and dilatational deformation across the fault damage zones. Different styles of off-fault deformation (OFD) are related to specific parts of the earthquake cycle, some of which are the result of dynamic, coseismic processes while others are produced by long-term, quasistatic processes. Dynamic processes produce brief, strongly perturbed stress fields surrounding the fault core leading to a unique damage signature. The fracture energy estimated for natural earthquakes compared to the breakdown work during high-speed rock friction experiments suggests that OFD constitutes an outsized proportion of the work budget of large earthquakes (>Mw 6.6-6.8) when compared to smaller earthquakes. Therefore, the ability to identify components of permanent OFD in seismogenic fault zones that are uniquely tied to coseismic rupture processes would be extremely valuable in seismic hazard assessment. We report geologic mapping and damage zone characterization (e.g., fracture and fabric analysis, XRD, clay mineralogy) on the southern San Andreas Fault along a 100m outcrop at Ferrum, CA, collocated with a creepmeter and InSAR surface displacement fields. The fault zone consists of a thin (<0.5m) clay rich-fault core containing ~5% talc, surrounded by a 10s of m thick foliated red clay and variably damaged sandstone. The damage zone is characterized by asymmetric folds associated with long-term transpressional deformation, development of axial planar cleavage in the red clays adjacent to the fault core, pulverized granite ~30m SW from the fault core, parallel deformation bands in sandstones on the NE side of the fault core, and jigsaw-patterned deformation bands in rocks adjacent to the fault core and extending for about 20m SW of the fault core. Preliminary rock friction experiments under saturated conditions indicate that the fault core has steady state friction coefficient of <0.1. Weakness of the fault core promotes localization of simple shear in the fault core, whereas pure shear deformation is primarily accommodated by OFD. We argue that jigsaw-style deformation bands and granitoid pulverization are uniquely coseismic deformation signatures related to dynamic rupture in the fault core. Therefore, we can use these OFD elements to signify the component of OFD that is active during large ≥ Mw7.3-7.4 earthquake events.

Key Words
fault damage zone, off fault deformation, San Andreas Fault

Citation
Griffith, W. A., Fullriede, A., Rockwell, T. K., & Torma, A. (2025, 09). Geological and Frictional Characterization of Damage Zone Structure of the Southern San Andreas Fault at Ferrum and Implications for Coseismic Off-Fault Deformation. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology