Frictional and microstructural evidence for a weak Wasatch fault zone

The Wasatch fault zone (WFZ), spanning a north-south urban corridor where ∼80% of Utah’s (USA) population resides, is one of the world’s longest active normal faults and capable of producing M_w 7 earthquakes. Geophysical analyses suggest that the WFZ is listric, dipping <30° at seismogenic depths. Seismic slip at such fault orientations is incompatible with Andersonian fault theory unless the fault is significantly weak. We performed friction experiments on oriented, intact wafers of exhumed granitic gneiss from the WFZ and comparatively undeformed footwall. Experiments and microstructural analyses demonstrate WFZ rocks at depth are intrinsically frictionally weaker (μ ∼ 0.34) than the surrounding host rock (μ ∼ 0.53). This difference reflects that experimental deformation in the WFZ sample exploited a penetrative ductile fabric overprinted by brittle fractures that are collectively lacking in the footwall specimen. WFZ rocks exhibit slip weakening at slip rates over 30 µm/s, indicating an ability to facilitate rupture propagation, and their healing rate permits recurrence intervals of 1000−2000 years, consistent with prior paleoseismic data. Our results reveal how inherited ductile deformation and overprinting brittle fault processes from successive earthquake ruptures can enable slip at low angles in quartzofeldspathic rocks dissected by listric normal faults.