Holocene earthquake history and slip rate of the southern Teton fault, Wyoming, USA

Abstract

The 72-km-long Teton normal fault bounds the eastern base of the Teton Range in northwestern Wyoming, USA. Although geomorphic surfaces along the fault record latest Pleistocene to Holocene fault movement, the postglacial earthquake history of the fault has remained enigmatic. We excavated a paleoseismic trench at the Buffalo Bowl site along the southernmost part of the fault to determine its Holocene rupture history and slip rate. At the site, ∼6.3 m of displacement postdates an early Holocene (ca. 10.5 ka) alluvial-fan surface. We document evidence of three surface-faulting earthquakes based on packages of scarp-derived colluvium that postdate the alluvial-fan units. Bayesian modeling of radiocarbon and luminescence ages yields earthquake times of ca. 9.9 ka, ca. 7.1 ka, and ca. 4.6 ka, forming the longest, most complete paleoseismic record of the Teton fault. We integrate these data with a displaced deglacial surface 4 km NE at Granite Canyon to calculate a postglacial to mid-Holocene (14.4–4.6 ka) slip rate of ∼1.1 mm/yr. Our analysis also suggests that the postglacial to early Holocene (14.4–9.9 ka) slip rate exceeds the Holocene (9.9–4.6 ka) rate by a factor of ∼2 (maximum of 3); however, a uniform rate for the fault is possible considering the 95% slip-rate errors. The ∼5 k.y. elapsed time since the last rupture of the southernmost Teton fault implies a current slip deficit of ∼4–5 m, which is possibly explained by spatially/temporally incomplete paleoseismic data, irregular earthquake recurrence, and/or variable per-event displacement. Our study emphasizes the importance of minimizing slip-rate uncertainties by integrating paleoseismic and geomorphic data sets and capturing multiple earthquake cycles.

Seismic Hazard Analyses From Geologic and Geomorphic Data: Current and Future Challenges

“…For the purposes of slip rate estimation for PSHA, a successful trenching campaign will ideally expose diagnostic evidence of multiple paleoearthquakes, piercing points that allow per‐event displacement estimates, and an abundance of dateable material that can bracket event ages (McCalpin, 2009). Both incremental and cumulative fault slip rates can be estimated by dividing the measured codisplacements by one or more interevent times associated with that displacement (e.g., DuRoss et al, 2019). Interevent times used in slip rate calculations from paleoseismic trenching are commonly bracketed using the same Quaternary numerical dating techniques as those used to constrain the age of offset Quaternary markers (see section 3.2.3 for more information).…”

“…OxCal is often used to constrain probability distribution functions of earthquake ages using a variety of constraints, including any type of numerical age, stratigraphic information, trench structure, historic events, and any other relevant timing information (Bronk Ramsey, 2009; Lienkaemper & Ramsey, 2009). Incremental or cumulative slip rates are often calculated by dividing the per‐event offsets measured within the trench walls by the interevent constraints from OxCal (e.g., DuRoss et al, 2019). For example, Bennett et al (2018) use detrital radiocarbon samples ( ), OSL dating ( ), and detailed stratigraphy and structure within a paleoseismic trench to tightly constrain the ages of six earthquakes on the Wasatch fault, Utah, USA, using OxCal.…”

“…Although paleoseismic trenching studies can provide interevent times important for slip rate calculations, the numerous ways that numerical ages can either overestimate or underestimate the true age of a unit can often lead to a mix of sample ages within a single trench unit or sample ages that appear perplexingly out of stratigraphic order. Samples that are obviously out of stratigraphic order are often excluded from OxCal models designed to constrain interevent times (Bennett et al, 2018; DuRoss et al, 2019; Hatem et al, 2019; Morell et al, 2018), but it can sometimes be difficult to decide which ages most reliably represent the depositional age of the deposit and which are outliers that should be excluded from further analysis. These issues can result in varying earthquake ages and interevent times depending on which numeric ages are included in the analysis, and it can be difficult to accurately specify the uncertainty presented by this problem.…”

“…Studies that use several of these types of slip rate methods together (e.g., Behr et al, 2010; Harkins et al, 2010; Thompson et al, 2002) can provide insights into whether apparent changes in slip rate over time are related to natural phenomena or if they represent potential bias in the slip rate calculation (Mouslopoulou et al, 2009, 2012; Nicol et al, 2009). DuRoss et al (2019) present a number of late Quaternary slip rate estimates for the Teton fault, Utah, USA, using offsets recorded from well‐dated postglacial (< ∼14 ka) features (geomorphic slip rate) together with per‐event offsets and event timing constrained using radiocarbon and OSL dating from trench exposures (trench slip rate). Their closed‐interval incremental trench slip rates are suggestive of a slight decrease in slip rate over the late Quaternary, ranging from 0.7–2.1 mm/year for the time period from ∼14–10 ka to 0.5–1.1 mm/year for the ∼10‐ to 5‐ka time period (Figure 4).…”

Slip and Strain Accumulation Along the Sadie Creek Fault, Olympic Peninsula, Washington

“…Similarly, combination of the deglacial age with the range of dip‐slip displacements on glacially derived deposits along the NOFZ (0.7–6.5 m) yield a dip‐slip rate of 0.05–0.5 mm/yr. If we subtract the amount of slip accumulated in the last earthquake (4 ± 1 m) and instead only consider the amount of slip accumulated over the timespan of the dated earthquake cycles from 12.2 to 10.0 ka to 1.2–0.9 ka (11,900 to 8,800 years; Schermer et al., 2020), the strike slip rate is 1.3–3.1 mm/yr (e.g., DuRoss et al., 2019).…”