Large mid-Holocene and late Pleistocene earthquakes on the Oquirrh fault zone, Utah

Abstract The Oquirrh fault zone is a range-front normal fault that bounds the east side of Tooele Valley and it has long been recognized as a potential source for large earthquakes that pose a significant hazard to population centers along the Wasatch Front in central Utah. Scarps of the Oquirrh fault zone offset the Provo shoreline of Lake Bonneville and previous studies of scarp morphology suggested that the most recent surface-faulting earthquake occurred between 9000 and 13,500 years ago. Based on a potential rupture length of 12 to 21 km from previous mapping, moment magnitude (Mw) estimates for this event range from 6.3 to 6.6 In contrast, our results from detailed mapping and trench excavations at two sites indicate that the most-recent event actually occurred between 4300 and 6900 yr B.P. (4800 and 7900 cal B.P.) and net vertical displacements were 2.2 to 2.7 m, much larger than expected considering estimated rupture lengths for this event. Empirical relations between magnitude and displacement yield Mw 7.0 to 7.2. A few, short discontinuous fault scarps as far south as Stockton, Utah have been identified in a recent mapping investigation and our results suggest that they may be part of the Oquirrh fault zone, increasing the total fault length to 32 km. These results emphasize the importance of integrating stratigraphic and geomorphic information in fault investigations for earthquake hazard evaluations. At both the Big Canyon and Pole Canyon sites, trenches exposed faulted Lake Bonneville sediments and thick wedges of fault-scarp derived colluvium associated with the most-recent event. Bulk sediment samples from a faulted debris-flow deposit at the Big Canyon site yield radiocarbon ages of 7650 ± 90 yr B.P. and 6840 ± 100 yr B.P. (all lab errors are ±1α). A bulk sediment sample from unfaulted fluvial deposits that bury the fault scarp yield a radiocarbon age estimate of 4340 ± 60 yr B.P. Stratigraphic evidence for a pre-Bonneville lake cycle penultimate earthquake was exposed at the Pole Canyon site, and although displacement is not well constrained, the penultimate event colluvial wedge is comparable in size to the most-recent event wedges. Charcoal from a marsh deposit, which overlies the penultimate event colluvium and was deposited during the Bonneville lake cycle transgression, yields an AMS radiocarbon age of 20,370 ± 120 yr B.P. Multiple charcoal fragments from fluvial deposits faulted during the penultimate event yield an AMS radiocarbon age of 26,200 ± 200 yr B.P. Indirect stratigraphic evidence for an antepenultimate event was also exposed at Pole Canyon. Charcoal from fluvial sediments overlying the eroded free-face for this event yields an AMS age of 33,950 ± 1160 yr B.P., providing a minimum limiting age on the antepenultimate event. Ages for the past two events on the Oquirrh fault zone yield a recurrence interval of 13,300 to 22,100 radiocarbon years and estimated slip rates of 0.1 to 0.2 mm/yr. Temporal clustering of earthquakes on the nearby Wasatch fault zone in the late Holocene does not appear to have influenced activity on the Oquirrh fault zone. However, consistent with findings on the Wasatch fault zone and with some other Quaternary faults within the Bonneville basin, we found evidence for higher rates of activity during interpluvial periods than during the Bonneville lake cycle. If a causal relation between rates of strain release along faults and changes in loads imposed by the lake does exist, it may have implications for fault dips and mechanics. However, our data are only complete for one deep-lake cycle (the past 32,000 radiocarbon years), and whether this pattern persisted during the previous Cutler Dam and Little Valley deep-lake cycles is unknown.