The 50-km-long Thousand Lake fault (TLF) aligns with a series of west-dipping normal faults forming an ∼200-km-long structural discontinuity along the eastern edge of the Basin and Range in south-central Utah. Quantifying a fault’s slip rate and slip rate variations is important for characterizing earthquake hazards and understanding how deformation within fault networks is accommodated through time. This study uses cross-section analyses to determine that the central TLF has displaced Cenozoic volcanic rocks by 1200−2200 m since fault initiation at 15−10 Ma, implying a long-term slip rate of ∼0.1−0.2 mm/a. In contrast, displacement measurements across a Late Pleistocene fan in Bicknell, Utah, along with luminescence and 3He geochronology, allow us to infer that the fault slip rate is only 0.03−0.08 mm/a over the late Pleistocene. The TLF is an example of a Basin and Range normal fault that has experienced significant slip rate variation over geologic time. Trenching of the fan’s primary fault scarp demonstrates that the last two earthquakes have occurred since 52.8 ± 8.5 ka, and the most recent earthquake occurred after 19.7 ± 4.7 ka. Finally, lidar-based geomorphic and fault scarp mapping along the entire fault shows that the last earthquake clearly post-dates the Last Glacial Maximum, rupturing the ground surface with vertical displacements of ∼0.8−1.4 m for ∼32 km in an ∼Mw 6.8 earthquake.
Toké et al. (2026) studied this question.