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How Many Samples Do You Need to Date That Paleoearthquake? A Field Test of Portable OSL Using 345 Samples From a Single Colluvial-Wedge Exposure

Session: Cryptic Faults: Assessing Seismic Hazard on Slow Slipping, Blind or Distributed Fault Systems I

Type: Oral

Date: 4/21/2021

Presentation Time: 10:30 AM Pacific

Description: 

Paleoseismic records for slow slip rate and cryptic fault systems are typically spatially and/or temporally limited. For these faults, few surface-rupturing earthquakes, geochronological constraints, and/or paleoseismic sites provide the basis for estimates of earthquake timing, recurrence, rupture extent, and ultimately, estimates of seismic hazard. With this in mind, we set out to test several aspects of normal fault colluvial sedimentation and surface burial using radiocarbon and portable optically stimulated luminescence (OSL) dating. Our principal goal is to explore spatial and temporal age variability, colluvial-wedge deposition and soil-burial processes, and the extent to which earthquake-timing uncertainties relate to sample quantity and stratigraphic context. To achieve this, we investigated the Wasatch fault in central Utah at the Deep Creek site. Here, a natural exposure records a single prehistoric surface-rupturing earthquake and includes alluvial-fan gravel and a paleosol that are vertically displaced ~1.8 m and buried by a ≤1.5-m-thick deposit of fault-scarp-derived colluvial sediment. We extensively sampled the ~3-m-long fault exposure for both radiocarbon and OSL dating. Radiocarbon ages for charcoal from 35 bulk soil samples together with 11 traditional OSL ages complement ~345 portable OSL samples evenly spaced across the soil-colluvium contact and yield a total density of ~70 samples/m². Together, these samples approximate an age map of the exposure and resolve vertical (age versus depth) and horizontal (age versus distance from the fault) trends in the alluvial fan, paleosol, and colluvium. Our results highlight processes of soil formation, colluvial-wedge deposition, and postdepositional disturbance, and have implications for the sampling and dating of paleoseismic exposures, the use of portable OSL in the field, and how sample quantity, spatial distribution, and stratigraphic context influence estimates of earthquake-timing uncertainty.

Presenting Author: Christopher B. DuRoss

Student Presenter: No

Authors