GEOLOGICAL CHARACTERIZATION OF DAMAGE ZONE STRUCTURE OF THE SOUTHERN SAN ANDREAS FAULT AT FERRUM AND IMPLICATIONS FOR COSEISMIC DEFORMATION

Strike-slip fault zones are complex regions of deformation generally centered around a centimeters-wide fault core that hosts intensely deformed fault gouge. Surrounding the fault core is the 10’s to 100’s of meters thick fault damage zone marked by predominantly brittle inelastic off-fault deformation (OFD). Different styles of 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 (>M_w 6.6-6.8) compared to smaller earthquakes; therefore, the ability to identify components of permanent OFD in seismogentic fault zones that are uniquely tied to coeseismic 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 (sSAF) along a 100m outcrop at Ferrum, CA, collocated with historical creepmeter and InSAR surface displacement fields. The damage zone is characterized by asymmetric folds associated with long-term transpressional deformation (consistent with prior work in the area), development of axial planar cleavage in clay-rich rocks 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. We interpret folding and cleavage development to represent long term, quasi-static OFD products, whereas we argue that jigsaw-style deformation bands and granitoid pulverization are uniquely coseismic deformation signatures. Therefore, we can use these OFD elements to signify the component of OFD that is active during large Mw7.9 earthquake events.