EARTHQUAKE HISTORY OF THE TOPLIFF HILLS FAULT: EVIDENCE OF SIX EVENTS SINCE 69.3 KA

The Topliff Hills fault (THF) is a west-dipping normal fault with 1 – 8 m scarps on geomorphic surfaces of the western piedmont of the Topliff Hills and East Tintic Mountains in Utah. The paleoseismic history of the THF is important because it is near the populous Wasatch front and is structurally linked to the adjacent East Tintic and Southern Oquirrh Mountains Faults (SOMF) which, along with the Northern Oquirrh Fault (NOF) and Great Salt Lake Fault, comprise the second longest fault system in Utah (>250 km). We established the Topliff Hill Site (40.13, -112.203) to investigate the THF’s earthquake history and relate it to the nearby SOMF. In May 2019 we investigated two trenches: Trench 1 (T1: 50 m long, ~4 m deep) was cut across a ~7 m scarp in a late Pleistocene fan. Trench 2 (T2: 25 m long, ~1.5 m deep) was located 300 m north of T1, across a poorly defined 1 – 2 m scarp in a Holocene fan. Both trenches were situated topographically above the high shoreline of Lake Bonneville which is crossed by the fault ~1 km north of this site. In T1 we infer six events; two occurred during fan aggradation and four post-date fan deposition. The most recent four events resulted in 8 – 9 m of cumulative offset. In T2, a single 0.3 – 0.4 m displacement provides evidence for one recent earthquake. Offsets ranging from 0.3 – 3 m per event imply M6.5 to M7.2 earthquakes and surface rupture lengths from 25 to 70 km. Analyses of etched shorelines across the hanging and foot walls of the THF lead us to infer 2.1 ± 1.0 m of displacement since high shoreline abandonment. We collected OSL and ¹⁴C samples to constrain ages for the 6 inferred events. Preliminary OSL results allow us to infer that all six events occurred since 69.3 ka. The oldest three events occurred between 69.3 and 39.9 ka. The youngest three events occurred since 18.0 ka. These results are consistent with the interpretation that activity along this fault may be modulated by the presence of pluvial lakes like Bonneville. Initial comparisons show that the earthquake record differs significantly from the NOF and SOMF, which have evidence for only a single post-Bonneville event with large displacements (2.7 – 3.1 m) and longer inferred rupture lengths.