10:45 AM - 12:15 PM
[SSS13-P06] Sensitivity of bedrock channel steepness to on-fault vertical slip rates
Keywords:Bedrock river, Active fault, Channel steepness, Uplift rate
Rates of erosion and uplift are essential for understanding landscape evolution in various spatiotemporal scales. However, limitations of appropriate sites, funding, and time make it difficult to estimate erosion and uplift rates at multiple locations. Given these limitations, bedrock channel steepness (ks: channel steepness) is widely used to infer erosion and uplift rates. Theoretical and observational evidence has shown that ks positively correlates with erosion rates. Under a topographic steady state where erosion rates are equivalent to uplift rates, ks can be used to infer relative uplift rates. However, relative uplift rates inferred from ks is deemed to contain considerable uncertainty because there are many geologic and climatic factors other than uplift rates affecting bedrock channel steepness. Nevertheless, it is worth studying the effects of uplift and other elements on ksbecause ks can be calculated easily and used in areas lacking clear geomorphic evidence of paleoearthquakes. In this study, I focus on bedrock channels across major active fault zones designated by the Japanese government (the headquarters for earthquake research promotion) and discuss a relationship between bedrock channel steepness and vertical slip rates of faults.
I selected 87 rivers in 29 fault zones based on the reliability of slip rates reported in previous studies and a relationship between channel gradient and catchment area. Assuming ks is a power function of uplift rates as predicted by a detachment limited incision model and a topographic steady state, I performed regression for the whole data, and the resulting R2 was ~0.1. Next, I divided the dataset according to fault types (normal, reverse, and strike-slip) and rock types (sedimentary, igneous, and metamorphic) and performed the same regression for each subset, yielding low R2 values of 0.08–0.32. I also performed regression for six fault zones where slip rates were estimated at many (6–10) sites and obtained R2 values of 0.02–0.32. These results re-emphasize the need to identify the controlling factors of ks and isolate their effects before discussing long-term uplift rates using ks. Recent studies have proposed methods to isolate lithologic and climatic influence on ks, which may improve the correlation of ks and uplift rates. However, the effects of channel width and sediment dynamics on ks are challenging to quantify, hindering the use of ks as a measure of uplift rates. Therefore, in addition to ks, it is important to test the relationship between uplift rates and more sophisticated indices of bedrock channel incision, such as basal shear stress and the excess Shields number, to develop alternative methods to estimate uplift rates.
I selected 87 rivers in 29 fault zones based on the reliability of slip rates reported in previous studies and a relationship between channel gradient and catchment area. Assuming ks is a power function of uplift rates as predicted by a detachment limited incision model and a topographic steady state, I performed regression for the whole data, and the resulting R2 was ~0.1. Next, I divided the dataset according to fault types (normal, reverse, and strike-slip) and rock types (sedimentary, igneous, and metamorphic) and performed the same regression for each subset, yielding low R2 values of 0.08–0.32. I also performed regression for six fault zones where slip rates were estimated at many (6–10) sites and obtained R2 values of 0.02–0.32. These results re-emphasize the need to identify the controlling factors of ks and isolate their effects before discussing long-term uplift rates using ks. Recent studies have proposed methods to isolate lithologic and climatic influence on ks, which may improve the correlation of ks and uplift rates. However, the effects of channel width and sediment dynamics on ks are challenging to quantify, hindering the use of ks as a measure of uplift rates. Therefore, in addition to ks, it is important to test the relationship between uplift rates and more sophisticated indices of bedrock channel incision, such as basal shear stress and the excess Shields number, to develop alternative methods to estimate uplift rates.