Paleoseismic History and Slip Rate of the Teton Fault at the Buffalo Bowl Site

Date: 4/24/2019

Time: 09:15 AM

Room: Vashon

The Teton normal fault spans the base of the Teton Range in northwestern Wyoming, within the northeastern Basin and Range Province. Despite geomorphic evidence of postglacial (latest Pleistocene to Holocene) surface ruptures, the paleoseismic history and slip rate of the fault remain only broadly constrained, contributing epistemic uncertainty to regional seismic hazard assessments. For example, previous work suggests that the fault’s latest Pleistocene to early Holocene postglacial slip rate (~2 mm/yr) significantly exceeds a Holocene rate (~0.5–0.7 mm/yr) calculated using two paleoseismic ruptures. To address open questions about prehistoric rupture timing, recurrence, and fault slip rate, we excavated a trench on the southernmost part of the fault. The Buffalo Bowl site records ~6.3 m of displacement in early Holocene (~10.5-ka) alluvial-fan sediments, yielding the longest and most complete Teton fault paleoseismic record. We interpret three Holocene surface-faulting earthquakes (BB3 to BB1) based on three packages of scarp-derived colluvium that postdate the alluvial-fan units. Bayesian models of radiocarbon (n=29) and luminescence (n=17) ages constrain the timing of earthquakes to ~5–10 ka and provide the basis for inter-event recurrence estimates of ~1.7–2.9 kyr and a mean recurrence of ~2.3 kyr. The Buffalo Bowl record confirms a previously documented mid-Holocene (~5 ka) most recent rupture of the southern Teton fault and provides evidence for an additional early Holocene earthquake. Earthquakes BB3–BB1 yield a Holocene closed-interval vertical slip rate of ~0.9 mm/yr, which is similar to a latest Pleistocene to mid-Holocene rate of ~1.1 mm/yr and latest Pleistocene to early Holocene rate of ~1.3 mm/yr that integrate the displacement of a deglacial surface at Granite Canyon (4 km north). Our results corroborate a late Holocene period of tectonic quiescence on the southern Teton fault, but do not support a significant slip rate change in the early Holocene.

Presenting Author: Christopher B. DuRoss

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