Determining Histories of Slip on Normal Faults With Bedrock Scarps Using Cosmogenic Nuclide Exposure Data

H J Goodall,L C Gregory,R P Shanks,R J Phillips,L N J Wedmore,K J W Mccaffrey,G P Roberts,R M J Amey,A Hooper

doi:10.1029/2020tc006457

H J Goodall, L C Gregory + Show 7 more

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https://doi.org/10.1029/2020tc006457

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Abstract

AbstractCosmogenic exposure data can be used to calculate time‐varying fault slip rates on normal faults with exposed bedrock scarps. The method relies on assumptions related to how the scarp is preserved, which should be consistent at multiple locations along the same fault. Previous work commonly relied on cosmogenic data from a single sample locality to determine the slip rate of a fault. Here we show that by applying strict sampling criteria and using geologically informed modeling parameters in a Bayesian‐inference Markov chain Monte Carlo method, similar patterns of slip rate changes can be modeled at multiple sites on the same fault. Consequently, cosmogenic data can be used to resolve along‐strike fault activity. We present cosmogenic 36Cl concentrations from seven sites on two faults in the Italian Apennines. The average slip rate varies between sites on the Campo Felice Fault (0.84 ± 0.23 to 1.61 ± 0.27 mm yr−1), and all sites experienced a period of higher than average slip rate between 0.5 and 2 ka and a period of lower than average slip rate before 3 ka. On the Roccapreturo fault, slip rate in the center of the fault is 0.55 ± 0.11 and 0.35 ± 0.05 mm yr−1 at the fault tip near a relay zone. The estimated time since the last earthquake is the same at each site along the same fault (631 ± 620 years at Campo Felice and 2,603 ± 1,355 years at Roccapreturo). These results highlight the potential for cosmogenic exposure data to reveal the detailed millennial history of earthquake slip on active normal faults.

Highlights

Fault activity constrained over multiple earthquake cycles and across fault systems helps to address fundamental questions of how faults interact (Cowie et al, 2017; Mueller, 2017; Nixon et al, 2016; Wedmore et al, 2017), how tectonic strain accumulates and is released on brittle faults (Ferry et al, 2011; Hergert & Heibach, 2010), and how fault slip varies in time and space (Dolan et al, 2016; Nicol et al, 2010)
36Cl data are plotted as cosmogenic isotope concentration versus sample height on Figure 5 for the Campo Felice and Roccapreturo faults
The agreement we find between the two techniques, which have been applied in such close proximity on the Roccapreturo fault, provides further evidence for the reliability of slip histories derived from modeling of 36Cl on bedrock fault scarps, and the potential for these two techniques to be combined for more informed seismic hazard analysis

Summary

Introduction

Fault slip rates can be measured or inferred using a variety of tools, including geodesy (Bendick et al, 2000; Hussain et al, 2016; Walters et al, 2013; Williams et al, 2020), palaeoseismology and historical records (Cinti et al, 2019; Galli et al, 2008; Pantosti et al, 1996), and dating of offset geological, geomorphological, and man-made features Despite the range of techniques, there are still discrepancies between long-term average slip rates and geodetic strain rates, which in part may be due to methodological uncertainties and problems related to the preservation of earthquake surface deformation in the geological or geomorphological record O. Gold et al, 2013; Rockwell & Klinger, 2013; Walters et al, 2018; Wesnousky, 2008), and if patterns of variable displacement persist over multiple earthquake cycles, cumulative Quaternary displacement and slip rate will be different along the fault

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