3:30 PM - 5:00 PM
[HGM02-P07] Evaluating the activity of the Tsubonuma and Enda faults, northeast Japan, based on the stream power model
Keywords:Active fault, Bedrock river, Stream power model, Quaternary
Activity assessment and historical surveys of active faults are generally concentrated along faults. However, the preservation of fault displacement topographies is not always good, therefore there are many cases in which data on cumulative displacement and age are not obtained. In addition, the importance of off-fault displacement has been realized by remote-sensing observation in recent years. It is difficult to obtain information about off-fault displacement by the traditional survey methodology. In either case, we may obtain data about fault movement and displacement by focusing on bedrock river response to uplift.
When the base level of erosion falls due to uplift caused by fault movement, the bedrock erosion rate increases in proportion to the rate of base level lowering. Therefore, when other conditions that affect the erosion rate, such as rock strength and climate, are the same, the bedrock incision rate and the amount of energy used for erosion (unit stream power), which is a factor that determine the erosion rate, are considered to be indicators of uplift rate. In particular, if a dynamic equilibrium state in which the erosion rate and uplift rate are in balance can be assumed, it may be possible to estimate the vertical displacement rate of an active fault from the bedrock incision rate. However, there are few examples in Japan where unit stream power has been used as a method for evaluating active faults.
The Tsubonuma fault and the Enda fault, which are reverse faults, are distributed between Natori City and Murata Town, Shibata County, Miyagi Prefecture. The Tsubonuma and Enda faults are considered to be important sections in the continuity of the Nagamachi–Rifu line and the Western Margin fault of the Fukushima Basin. However, along the Tsubonuma and Enda faults, there are few fault displacement topographies that can be used to evaluate the activity of the faults. There are rivers crossing the Tsubonuma and Enda faults, suggesting the activity of the Tsubonuma and Enda faults can be evaluated based on the distribution of unit stream power. Therefore, the purpose of this study is to investigate the unit stream power around the Tsubonuma and Enda faults and assess the activity of the faults.
We calculated unit stream power (ω) in rivers around the Tsubonuma and Enda faults and examined the vertical displacement rate of the faults based on the spatial distribution of ω. ω is the energy loss rate of flowing water per unit riverbed area, and a portion of the energy loss is considered to be used for erosion. Therefore, ω is used as an indicator of the erosive power of bedrock rivers, and the larger the value of ω, the stronger the erosive power. In order to obtain ω for bedrock rivers flowing on hangingwalls and footwalls of the Tsubonuma and Enda faults, we measured channel width, water depth, and riverbed gradient in the field. Measurements were made mainly with a TruPulse 360 (Laser Technology). The strength of exposed basement rocks was measured using a Schmidt hammer (model: GS-2) in order to examine the erodibility of the bedrock.
In the rivers around the Tsubonuma fault, andesite and basalt were mainly exposed, and the Schmidt hammer rebound values were similar in the entire study section. The ω at the hanging wall of the Tsubonuma fault is about twice that of the foot wall, suggesting that the fault-driven hangingwall uplift may have intensified the erosive force. In the hanging wall, there was no tendency for the value of ω to decrease with the distance from the Tsubonuma fault. Therefore, it is possible that other factors, such as the incision of the Natori River and the size of riverbed gravels, affect ω. In the rivers around the Enda fault, andesite was mainly exposed, and the Schmidt hammer rebound values were similar except for the sections where mudstone and tuff were exposed. The ω values of the river reaches where andesite was exposed were not significantly different across the fault. This suggests that around our field site, the vertical slip rate of the Enda fault is not large compared with the Tsubonuma Fault. Moreover, in order to think about the relationship between bedrock strength and ω, we examined the relationship between Schmidt hammer rebound values and ω. The larger the rebound values, the larger ω to be. However, even when the rebound values were similar, the variability of ω was large. This means that factors other than bedrock strength, such as the of riverbed gravels and uplift rate affect ω. In the future, it is necessary to consider the effect of riverbed gravels on ω in order to examine the relationship between uplift rate and ω with more details.
When the base level of erosion falls due to uplift caused by fault movement, the bedrock erosion rate increases in proportion to the rate of base level lowering. Therefore, when other conditions that affect the erosion rate, such as rock strength and climate, are the same, the bedrock incision rate and the amount of energy used for erosion (unit stream power), which is a factor that determine the erosion rate, are considered to be indicators of uplift rate. In particular, if a dynamic equilibrium state in which the erosion rate and uplift rate are in balance can be assumed, it may be possible to estimate the vertical displacement rate of an active fault from the bedrock incision rate. However, there are few examples in Japan where unit stream power has been used as a method for evaluating active faults.
The Tsubonuma fault and the Enda fault, which are reverse faults, are distributed between Natori City and Murata Town, Shibata County, Miyagi Prefecture. The Tsubonuma and Enda faults are considered to be important sections in the continuity of the Nagamachi–Rifu line and the Western Margin fault of the Fukushima Basin. However, along the Tsubonuma and Enda faults, there are few fault displacement topographies that can be used to evaluate the activity of the faults. There are rivers crossing the Tsubonuma and Enda faults, suggesting the activity of the Tsubonuma and Enda faults can be evaluated based on the distribution of unit stream power. Therefore, the purpose of this study is to investigate the unit stream power around the Tsubonuma and Enda faults and assess the activity of the faults.
We calculated unit stream power (ω) in rivers around the Tsubonuma and Enda faults and examined the vertical displacement rate of the faults based on the spatial distribution of ω. ω is the energy loss rate of flowing water per unit riverbed area, and a portion of the energy loss is considered to be used for erosion. Therefore, ω is used as an indicator of the erosive power of bedrock rivers, and the larger the value of ω, the stronger the erosive power. In order to obtain ω for bedrock rivers flowing on hangingwalls and footwalls of the Tsubonuma and Enda faults, we measured channel width, water depth, and riverbed gradient in the field. Measurements were made mainly with a TruPulse 360 (Laser Technology). The strength of exposed basement rocks was measured using a Schmidt hammer (model: GS-2) in order to examine the erodibility of the bedrock.
In the rivers around the Tsubonuma fault, andesite and basalt were mainly exposed, and the Schmidt hammer rebound values were similar in the entire study section. The ω at the hanging wall of the Tsubonuma fault is about twice that of the foot wall, suggesting that the fault-driven hangingwall uplift may have intensified the erosive force. In the hanging wall, there was no tendency for the value of ω to decrease with the distance from the Tsubonuma fault. Therefore, it is possible that other factors, such as the incision of the Natori River and the size of riverbed gravels, affect ω. In the rivers around the Enda fault, andesite was mainly exposed, and the Schmidt hammer rebound values were similar except for the sections where mudstone and tuff were exposed. The ω values of the river reaches where andesite was exposed were not significantly different across the fault. This suggests that around our field site, the vertical slip rate of the Enda fault is not large compared with the Tsubonuma Fault. Moreover, in order to think about the relationship between bedrock strength and ω, we examined the relationship between Schmidt hammer rebound values and ω. The larger the rebound values, the larger ω to be. However, even when the rebound values were similar, the variability of ω was large. This means that factors other than bedrock strength, such as the of riverbed gravels and uplift rate affect ω. In the future, it is necessary to consider the effect of riverbed gravels on ω in order to examine the relationship between uplift rate and ω with more details.