FRICTIONAL WEAKNESS OF THE WASATCH FAULT ZONE ILLUMINATED BY DEFORMATION EXPERIMENTS AND MICROSTRUCTURAL ANALYSES

The Wasatch fault zone (WFZ) spans a north-south urban corridor with ~80% of Utah’s population. It is one of the world’s longest active normal faults that is capable of producing M_w 7 earthquakes. Geophysical analyses indicate that the WFZ is listric, likely dipping at <30⁰ at seismogenic depths. However, seismic slip at such fault orientations, such as from the 2020 M_w 5.7 Magna Earthquake, is incompatible with Andersonian fault theory and unexpected unless the fault is significantly weak. We report on results from friction experiments on oriented, intact wafers of exhumed granitic gneiss from the WFZ and the comparatively undeformed footwall rocks. Our experiments and microstructural analyses show that the WFZ rocks at depth are intrinsically frictionally weaker (μ ~ 0.34) than the surrounding host rock (μ ~ 0.53). This difference likely reflects that experimental deformation in the WFZ exploited a penetrative ductile fabric overprinted by brittle fractures that are collectively absent in the footwall specimen. Further, we demonstrate that the WFZ rocks are frictionally unstable or conditionally stable over a range of slip rates, which is necessary to sustain multiple earthquakes. We also show via slide-hold-slide experiments that their healing rate permits recurrence intervals of 1000-2000 years, which is consistent with existing paleoseismic data. Our results reveal how the inherited ductile deformation fabric together with evolution of frictional properties due to successive earthquake ruptures can enable unstable slip at low angles on listric normal faults such as the WFZ.