SCEC Award Number 17121 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Reanalysis of California fault slip rates using new models for cosmogenic nuclide production and inheritance
Name Organization
Michael Oskin University of California, Davis
Other Participants Veronica Prush
SCEC Priorities 1a, 2e, 5c SCEC Groups EFP, SAFS, Geology
Report Due Date 06/15/2018 Date Report Submitted 11/14/2018
Project Abstract
Quantifying Quaternary fault slip rates requires constraints on the age of offset geomorphic features. Surface exposure age is commonly determined using cosmogenic nuclides (CNs), such as Beryllium-10 (10Be). Age determinations can be skewed by the effects of inheritance and erosion, which can result in clast ages that appear older or younger than the average clast age sampled from a surface. In a previously funded SCEC project (#15209), we demonstrated that the inherited CN component observed in surface clast datasets can be modeled using a generalized Pareto distribution (GPD). The three-parameter GPD fit solves for a location parameter as the zero-inheritance surface age, which tends to result in surface ages closer to the youngest sampled clast age, rather than the mean value of the clast-age dataset. Though this earlier project noted an expected increase in slip rates due to decreases in surface ages, we did not focus our efforts on evaluating this effect in detail. In light of the recent development of physics-based nuclide production-rate models, we investigate how the combination of our model with new production rates would affect slip rates in southern California. In general, we found that the reduction in ages found using our model is offset by a decrease in production rates that produces older ages. Individual site results vary considerably depending on how the original publication authors interpreted their exposure age results.
Intellectual Merit The concurrent development of new inheritance and production-rate models for cosmogenic nuclide concentrations in surface clasts motivates re-evaluation of slip rates based on these datasets. We reanalyze 22 sites across southern California where single-clast exposure ages dates were obtained for slip-rate analysis. We find that younger ages predicted by our inheritance model are countered by lower production rates that yield older clast ages. Significant revisions to slip rates are found in some locations, such as the Eastern California Shear Zone, suggesting that a site-by-site reanalysis of both slip and age are warranted. Though we did not observe a substantial change in regional slip rates, our methods do reduce age uncertainty, producing more precise slip-rate estimates. We also find through forward modeling of synthetic clast-age datasets that 19 out of 22 sites in California lack sufficient measurements (n < 14) to yield a robust age prediction. Hence additional clast-age measurements would be a good investment to increase confidence in these ages.
Broader Impacts Our modeling improves on the accuracy of widely applied surface-exposure age geochronology by explicitly modelling the inherited contribution as a probability distribution function. This project provided research fellowship support for University of California, Davis graduate student Veronica Prush, including travel to research conferences and analyses that will contribute to two research publications.
Exemplary Figure Figure 1. Summary of reported slip rate changes for sites in southern California [8-20] based on incorporation of the generalized Pareto distribution model of inheritance and physics-based CN production rate models. Green boxes indicate a slip rate increase. Red boxes indicate a slip rate decrease. Most new slip rates, outlined by the yellow boxes, remain unchanged within reported uncertainty.