HOW INHERITED DEFORMATION AND FABRICS AFFECT THE STRENGTH OF THE WASATCH FAULT: INSIGHTS FROM FRICTION EXPERIMENTS AND MICROSTRUCTURAL ANALYSES ON THE FARMINGTON CANYON COMPLEX DAMAGE ZONE

The Wasatch Fault (WF) is a seismically active normal fault with a 43% chance of a M_w ≥ 6.75 earthquake to occur in the next 50 years, greatly impacting the 80% of Utah’s population who live adjacent to the WF. Earthquake rupture models and ground-motion prediction simulations aim to quantify the effect earthquake hazards have on these populations. However, these models are commonly limited by a lack of fault-specific mechanical and frictional properties and behaviors. This structure dips at low angles at depth (listric) as highlighted by the 2020 M_w 5.7 Magna earthquake sequence. Slip on a listric structure is mechanically unexpected and contradicts the Andersonian theory of faulting that suggests normal faulting occurs at ~60° to the minimum principal stress. We hypothesize the WF is frictionally weak at seismogenic depths due to inherited deformation in the form of microfractures and grain-size reduction from prior earthquakes enabling it to slip at low angles. Here we target a key lithology along the WF, the Farmington Canyon complex (Xfc), a granodioritic gneiss in the footwall of the southern Brigham City Segment that comprises dominantly quartz, feldspar, hornblende, and lesser biotite. We focus on a sample from the exhumed footwall damage zone that is a proxy for the WF at depth. This sample is dissected by multiple hematite fault mirrors, exhibits local foliation truncated by cataclasite, and pervasive fractures that cut foliation and cataclasite. Initial scanning electron microscopy (SEM) of our sample reveals intense, pervasive fracturing within quartz and feldspar grains and across grain boundaries, mirroring field and hand-sample scale observations. Ongoing friction experiments using a benchtop direct shear apparatus and post-experiment SEM analysis of intact rock wafers and powdered gouge of the same Xfc material will characterize the frictional strength, quantify the structural versus mineralogical controls on the strength, and identify specific weakening mechanisms. Our data will be compared to frictional properties and weakening mechanisms identified in prior work on two Xfc samples that exhibit (1) strong ductile foliation in the footwall damage zone and (2) comparatively undamaged rock from the footwall to determine the role of prior brittle deformation on the frictional behavior of the WF.