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
Type: Oral
Date: 4/28/2020
Time: 03:15 PM
Room: 240
Description: 

Paleoseismic records for slow slip rate and cryptic fault systems are typically spatially and/or temporally limited or incomplete. For these faults, few paleoseismic sites, surface-rupturing earthquakes and/or constraining ages provide the basis for estimates of earthquake timing, recurrence and rupture extent, and ultimately, estimates of rupture probability and 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 questions include 1) whether we can separate the signal of colluvial-wedge progradation over a surface soil from the noise of age scatter, 2) if soil age from charcoal positively correlates with depth within a soil A horizon and 3) the degree to which the number of samples and their stratigraphic context affect earthquake-timing uncertainty.

To address these questions, we investigated a natural exposure of the Wasatch fault in central Utah. The Deep Creek site records a single prehistoric surface-rupturing earthquake, as evidenced by a 0.3-m-thick soil A horizon formed in Holocene alluvial-fan gravel that is vertically displaced 1.8 m and buried by a ~1.5-m-thick deposit of colluvial sediment eroded from the footwall fault scarp. We extensively sampled the A horizon and scarp colluvium for both radiocarbon and portable OSL dating. Charcoal extracted from 20 bulk soil samples complement ~345 OSL samples evenly spaced across the soil-colluvium contact. Together, these samples will yield an age map of the exposure and resolve vertical (age versus depth) and horizontal (age versus distance from the fault) trends in the soil and colluvium. We expect that our results will have important implications for the sampling and dating of paleoseismic exposures, the use of portable OSL in the field and how sample quantity and stratigraphic context influence estimates of earthquake-timing uncertainty.

Presenting Author: Christopher B. DuRoss

Authors