17:15 〜 19:15
[SSS10-P18] Investigating Smectite Content in Neodani Fault Gouge: A Quantitative Approach to Understanding of Fault Mechanics
キーワード：Neodani, Fault-gouge, XRD-RockJock, Methylene Blue
The Neodani Fault, famous for its activity during the 1891 Nobi Earthquake, presents a unique opportunity to delve into the mineralogical and geochemical composition of fault gouges. The relevance of smectite in fault mechanics is underscored by its role in modulating the stability of fault zones and influencing the dynamics of seismic events. Furthermore, the spatial distribution and concentration of smectite within fault gouge can provide valuable information about the geological history and tectonic processes in a region, making it an essential focus of study in geomechanical research.
This study investigates the quantitative analysis of smectite in Neodani Fault gouge using two primary methods: the XRD RockJock method (Eberl, D.D., 2003) and the Methylene Blue Adsorption test (Miyoshi et al., 2016). Samples analyzed in this study were sourced from borehole NDFD-1-S1 drilled by the Nuclear Regulation Authority and an outcrop sample. The Methylene Blue Adsorption test is employed to measure the cation exchange capacity (CEC) of the smectite in the fault gouge. This test leverages the high adsorptive capacity of smectite for methylene blue dye, which quantitatively binds to cation exchange sites. The XRD RockJock method utilizes X-ray diffraction data analyzed through the RockJock software to quantify the mineralogical composition of the fault gouge. This method is particularly advantageous due to its accuracy and ability to handle small sample sizes, making it suitable for analyzing the often-limited quantities of fault gouge material available. Prior to analyzing these samples, both methods were calibrated using Kunigel V1 Bentonite samples with measured value range from 46% - 49 % Smectite content (Ito et. al, 1998). Calibration results indicated that RockJock program generally showed a slightly lower smectite content than the calculated value compared to the Methylene Blue Adsorption test which 2 out of 3 samples falls in the range of the calculated value, but both methods consistently indicated the presence of smectite.
The RockJock analysis revealed significant smectite presence, alongside primary minerals identified, including quartz, plagioclase, and potassium feldspar. For the outcrop sample, smectite content was determined to be 2.34% by RockJock and 3.841% by Methylene Blue test. Despite its low content, the presence of smectite suggests potential plasticity. For drilling samples from NDFD-1-S1, analyzed by XRD, fault gouge from the latest slip zone (basalt protolith) showed the highest smectite content, around 49.7%, compared to other gouges from older slip zones, which showed 32% and 35% (also basalt origin), and merely 4.38% for mudstone-derived gouge. The differing smectite content in drilling samples, even from the same lithological origin, can be attributed to differences in environmental conditions, hydrothermal alterations, and degrees of deformation.
The highest smectite content in the latest slip zone suggests that the basalt-origin fault gouge underwent extensive hydrothermal alteration and deformation, facilitating smectite formation, which may reduce shear strength and make it prone to slip. In older slip zones also of basalt origin, slightly lower smectite content (approximately 32% and 35%) implies less hydrothermal alteration or different deformation histories, potentially resulting in more frictional strength compared to the latest slip zone. In contrast, the lowest smectite content (around 4.38%) in mudstone-derived gouge indicates less alteration and plasticity, making these zones more brittle and prone to stick-slip behavior. Differences in smectite content based on lithology significantly impact fault mechanics. High smectite content in basalt-derived gouges suggests enhanced plasticity and lower shear strength. Conversely, the lower smectite content in mudstone-derived gouges may result in higher frictional strength. Understanding these variations can help predict fault behavior and inform earthquake hazard assessments.
This study investigates the quantitative analysis of smectite in Neodani Fault gouge using two primary methods: the XRD RockJock method (Eberl, D.D., 2003) and the Methylene Blue Adsorption test (Miyoshi et al., 2016). Samples analyzed in this study were sourced from borehole NDFD-1-S1 drilled by the Nuclear Regulation Authority and an outcrop sample. The Methylene Blue Adsorption test is employed to measure the cation exchange capacity (CEC) of the smectite in the fault gouge. This test leverages the high adsorptive capacity of smectite for methylene blue dye, which quantitatively binds to cation exchange sites. The XRD RockJock method utilizes X-ray diffraction data analyzed through the RockJock software to quantify the mineralogical composition of the fault gouge. This method is particularly advantageous due to its accuracy and ability to handle small sample sizes, making it suitable for analyzing the often-limited quantities of fault gouge material available. Prior to analyzing these samples, both methods were calibrated using Kunigel V1 Bentonite samples with measured value range from 46% - 49 % Smectite content (Ito et. al, 1998). Calibration results indicated that RockJock program generally showed a slightly lower smectite content than the calculated value compared to the Methylene Blue Adsorption test which 2 out of 3 samples falls in the range of the calculated value, but both methods consistently indicated the presence of smectite.
The RockJock analysis revealed significant smectite presence, alongside primary minerals identified, including quartz, plagioclase, and potassium feldspar. For the outcrop sample, smectite content was determined to be 2.34% by RockJock and 3.841% by Methylene Blue test. Despite its low content, the presence of smectite suggests potential plasticity. For drilling samples from NDFD-1-S1, analyzed by XRD, fault gouge from the latest slip zone (basalt protolith) showed the highest smectite content, around 49.7%, compared to other gouges from older slip zones, which showed 32% and 35% (also basalt origin), and merely 4.38% for mudstone-derived gouge. The differing smectite content in drilling samples, even from the same lithological origin, can be attributed to differences in environmental conditions, hydrothermal alterations, and degrees of deformation.
The highest smectite content in the latest slip zone suggests that the basalt-origin fault gouge underwent extensive hydrothermal alteration and deformation, facilitating smectite formation, which may reduce shear strength and make it prone to slip. In older slip zones also of basalt origin, slightly lower smectite content (approximately 32% and 35%) implies less hydrothermal alteration or different deformation histories, potentially resulting in more frictional strength compared to the latest slip zone. In contrast, the lowest smectite content (around 4.38%) in mudstone-derived gouge indicates less alteration and plasticity, making these zones more brittle and prone to stick-slip behavior. Differences in smectite content based on lithology significantly impact fault mechanics. High smectite content in basalt-derived gouges suggests enhanced plasticity and lower shear strength. Conversely, the lower smectite content in mudstone-derived gouges may result in higher frictional strength. Understanding these variations can help predict fault behavior and inform earthquake hazard assessments.