Abstract
Abstract The quality and quantity of geochronologic data used to constrain the history of major earthquakes in a region exerts a first-order control on the accuracy of seismic hazard assessments that affect millions of people. However, evaluations of geochronological data are limited by uncertainties related to inherently complex depositional processes that may vary spatially and temporally. To improve confidence in models of earthquake timing, we use a high-density suite of radiocarbon and optically stimulated luminescence (OSL) ages with a grid of 342 portable OSL samples to explore spatiotemporal trends in geochronological data across an exemplary normal fault colluvial wedge exposure. The data reveal a two-dimensional age map of the paleoseismic exposure and demonstrate how vertical and horizontal trends in age relate to dominant sedimentary facies and soil characteristics at the site. Portable OSL data provide critical context for the interpretation of 14C and OSL ages, show that geochronologic age boundaries between pre- and post-earthquake deposits do not match stratigraphic contacts, and provide the basis for selecting alternate Bayesian models of earthquake timing. Our results demonstrate the potential to use emergent, portable OSL methods to dramatically improve paleoseismic constraints on earthquake timing.
Highlights
Radiocarbon (14C) and luminescence ages provide geochronological context to paleoseismic investigations; constrain the timing and rates of sedimentary, pedogenic, and geomorphic processes; and provide the basis for quantifying earthquake timing, recurrence, and fault slip rate (e.g., Weldon et al, 2004; Dolan et al, 2016; Galli et al, 2019; Scharer and Yule, 2020; Nelson et al, 2021)
Our focus was on the depositional history of a normal fault scarp that formed in a late Holocene surface rupture of the Wasatch fault zone (Schwartz and Coppersmith, 1984; Machette et al, 1992) and was revealed in a natural exposure at the Deep Creek site (Utah, USA; Fig. 1)
The 0.5–0.7 ka portable optically stimulated luminescence (OSL) age contours have the greatest spatial overlap with the stratigraphic event horizon and are most consistent with the sedimentary and pedogenic processes interpreted across the exposure
Summary
Radiocarbon (14C) and luminescence ages provide geochronological context to paleoseismic investigations; constrain the timing and rates of sedimentary, pedogenic, and geomorphic processes; and provide the basis for quantifying earthquake timing, recurrence, and fault slip rate (e.g., Weldon et al, 2004; Dolan et al, 2016; Galli et al, 2019; Scharer and Yule, 2020; Nelson et al, 2021). We employ dense 14C and traditional optically stimulated luminescence (OSL) sampling with a new application of portable OSL (Gray et al, 2021) to explore spatiotemporal trends in geochronological data and develop a robust interpretation of the sedimentary, pedogenic, and tectonic processes that underlie a paleoseismic exposure.
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