FRICTIONAL PROPERTIES ASSOCIATED WITH ACOUSTIC EMISSION CHARACTERISTICS OF SIMULATED GRANITE FAULT GOUGES: EFFECTS OF NORMAL STRESS AND WATER CONTENT

doi:10.3969/j.issn.0253-4967.2024.06.007

Abstract

Abstract:

The Xinfengjiang Water Reservoir in Guangdong, China, is one of the reservoirs that has triggered earthquakes of magnitudes greater than 6. Numerous earthquakes have occurred since the reservoir's impoundment, making it one of the most active seismic zones in Guangdong. Disturbance in effective stress and water content in existing fault zones upon the water reservoir may play a critical role in induced earthquakes.

Here, we report 17 friction experiments performed on simulated granite fault gouges(28%quartz, 25% albite, and 44%microcline, particle size <0.25mm, 2mm thickness)collected from fault zones near Xinfengjiang Reservoir to investigate the effect of effective normal stress and water content. This was achieved using a direct shear apparatus associated with two acoustic emission(AE)sensors. Microstructure observation was also performed on post-deformed samples to discuss the possible mechanism of reservoir-induced earthquakes. Regarding the analysis method of acoustic emission data, we adopt a crack cumulative summation curve method, following the RA-AF crack classification method, which can successfully inverse and distinguish the possible different microcracking processes in deformed rock from the pure tensile microcrack development(k=1)to the pure shear microcrack development (k=-1) according to the variance of curve slope k value(i.e., -1~1). The velocity stepping(1-10-50~100μm/s)and slide-hold-slide(at a constant sliding velocity of 1 mm/s with hold intervals of 10-30-100-300-1 000-3 000 seconds)experiments were conducted at the fixed normal stresses of 0.5~20MPa(10%water content)effective normal stress and 0%~25%(10MPa effective normal stress)water content under drained conditions at room temperature. Acoustic emission signals were also recorded. The shear displacement of both types of experiments is~7mm.

(1)Velocity stepping experiments show that the wet samples with 10%water content exhibit a transition from velocity weakening to velocity strengthening at the normal stress of 10MPa, and the apparent dilatancy was observed for the samples showing velocity strengthening utilizing the changes in axial displacement during sliding. At the normal stress of 10MPa, the room dry sample also shows velocity strengthening accompanied by dilatancy. Still, the wet sample with 25%water content shows velocity weakening, though dilatancy was also observed. This may indicate the negative effect of water on velocity strengthening. In addition, AE analysis indicates the development of tensile microcracks(60%~80%)dominated for velocity weakening and the appearance of strongly developed shear microcracks dominated for velocity strengthening, which was consistent with the microstructural observation performed on the deformed samples. It clearly illustrates that the tensile fracture(T) zones developed in the samples showed velocity weakening, while the Ridel shear fracture(R) zones developed in the samples showed velocity strengthening.

(2)Slide-hold-slide experiments show a nonlinear relation between transient peak healing(Δμ_pk) and the logarithm of hold time(log(t_h)), or “non-Dieterich” healing behavior. By contrast, postpeak weakening(Δμ_w, a stress drop measured as the difference between the peak frictional strength and the steady-state friction of reshearing)shows a direct, near-linear relation with log(t_h). In general, Δμ_w increased with increasing log(t_h) and water content but reduced with increasing normal stress. This may indicate the lower normal stress, the higher water content and the longer hold time the more likely for the unsteady slip of the faults. AE analysis showed that the stress drop is positively correlated with the proportion of tensile cracks.

Based on the effects of the effective normal stress and water content on frictional properties, we suggest that the effect of hydrochemical weakening and increase of stress drop after healing may be the mechanisms responsible for a transition from velocity weakening to velocity weakening for granite gouges. The CNS model can well explain the “non-Dieterich” healing behavior of granite gouges. Combined with reservoir-induced earthquakes, it is indicated that the fault zone under Xinfengjiang Reservoir may have unstable slip below 5MPa effective normal stress and 25%water content, resulting in reservoir-induced earthquakes. Importantly, the weak rate weakening behavior under high water content is conducive to generating slow slip seismic events. The experimental results have certain indicative significance for reservoir-induced earthquakes.

Key words: Granite gouges, velocity weakening, frictional healing, acoustic emission, microcrack development, reservoir-induced earthquakes, slow slip

摘要：

正应力和含水率的扰动可能诱发断层不稳定滑动。文中对采自新丰江水库附近断裂带的花岗岩样品开展了速率阶跃(1~100μm/s)和滑动-保持-滑动(10~3 000s)摩擦实验, 且全程进行了声发射观测, 研究了不同有效正应力(0.5~20MPa)和含水率(0%~25%)对花岗岩断层泥摩擦特性的影响。实验结果表明, 花岗岩断层泥随有效正应力增大由速率弱化(5MPa以下)向速率强化转变, 应力降减小; 随着含水率升高, 速率强化程度减弱, 当含水率为25%时出现速率弱化行为, 应力降增大。速率弱化样品的声发射数据以拉张微裂纹发育为主, 显微结构中出现大角度张性断裂带, 推测可能是水的化学弱化作用导致。实验结果可能对理解新丰江水库诱发地震提供新的认识: 低有效正应力下的速率弱化行为可能致使水库诱发地震, 而高含水率下的微弱速率弱化行为有利于慢滑移地震事件的产生。

关键词: 花岗岩断层泥, 速率弱化, 摩擦愈合, 声发射, 微裂纹发育, 水库诱发地震, 慢滑移

SHAO Kang, LIU Jin-feng, XIE Bin, ZHU Min-jie. FRICTIONAL PROPERTIES ASSOCIATED WITH ACOUSTIC EMISSION CHARACTERISTICS OF SIMULATED GRANITE FAULT GOUGES: EFFECTS OF NORMAL STRESS AND WATER CONTENT[J]. SEISMOLOGY AND GEOLOGY, 2024, 46(6): 1332-1356.

邵康, 刘金锋, 谢彬, 朱民杰. 正应力和含水率对花岗岩断层泥摩擦特性和声发射特征的影响[J]. 地震地质, 2024, 46(6): 1332-1356.

Figures/Tables 16

Fig. 1 Schematic diagram of friction constitutive law.

Table1 Semi-quantitative analysis of mineral composition in granite gouge

矿物种类	石英	钠长石	微斜长石	白云母	其他矿物
花岗岩断层泥/wt%	28	25	44	1	2

Fig. 2 Schematic diagram of the sample assembly used in direct shear experiments.

Fig. 3 The best fitting results for the VS-10MPa-10% experiment during the step phase at a rate of 10~50μm/s.

Fig. 4 Schematic diagram showing RA-AF crack classification(a) and Schematic diagram of crack cumulative summation curve showing inversed five types of microcrack developments with different k values(b) (indexed from FAN Cai-yuan et al., 2023).

Fig. 5 The relationship curves of apparent friction coefficient, normal displacement, and shear displacement for granite gouge(VS experiment).

Fig. 6 The relationship curves of apparent friction coefficient, normal displacement, and shear displacement for granite fault gouge(Slide-Hold-Slide experiment).

Table2 Summary of friction constitutive parameters of granite gouge

有效正应力/MPa	含水率/%	$μ s s$ ^①	a-b^②	a	b	滑动模式
0.5	10	0.569	-4.58×10^-3	1.26×10^-3	5.84×10^-3	速率弱化,10μm/s准静态振荡
1	10	0.591	-4.20×10^-3	2.10×10^-3	6.30×10^-3	速率弱化,10μm/s准静态振荡
2	10	0.632	-3.52×10^-3	2.65×10^-3	6.17×10^-3	速率弱化
5	10	0.607	-1.68×10^-3	4.02×10^-3	5.70×10^-3	速率弱化
10	10	0.561	1.53×10^-3	3.94×10^-3	2.41×10^-3	速率强化
20	10	0.549	1.69×10^-3	4.37×10^-3	2.68×10^-3	速率强化
10	0	0.646	0.70×10^-3	4.59×10^-3	3.89×10^-3	速率强化
10	20	0.583	0.18×10^-3	4.97×10^-3	4.79×10^-3	速率强化
10	25	0.561	-0.52×10^-3	2.43×10^-3	2.95×10^-3	速率弱化

Table2 Summary of friction constitutive parameters of granite gouge

有效正应力/MPa	含水率/%	$μ s s$ ^①	a-b^②	a	b	滑动模式
0.5	10	0.569	-4.58×10^-3	1.26×10^-3	5.84×10^-3	速率弱化,10μm/s准静态振荡
1	10	0.591	-4.20×10^-3	2.10×10^-3	6.30×10^-3	速率弱化,10μm/s准静态振荡
2	10	0.632	-3.52×10^-3	2.65×10^-3	6.17×10^-3	速率弱化
5	10	0.607	-1.68×10^-3	4.02×10^-3	5.70×10^-3	速率弱化
10	10	0.561	1.53×10^-3	3.94×10^-3	2.41×10^-3	速率强化
20	10	0.549	1.69×10^-3	4.37×10^-3	2.68×10^-3	速率强化
10	0	0.646	0.70×10^-3	4.59×10^-3	3.89×10^-3	速率强化
10	20	0.583	0.18×10^-3	4.97×10^-3	4.79×10^-3	速率强化
10	25	0.561	-0.52×10^-3	2.43×10^-3	2.95×10^-3	速率弱化

Fig. 7 The variation of the rate-dependent parameter a-b with effective normal stress and water content.

Table3 Summary of frictional healing parameters of granite gouge

有效正应力/MPa	含水率/%	$μ s s$ ^①	保持时间阈值^②/s	$β 1$ ^③	$β 2$ ^④	$β 3$ ^⑤
0.5	10	0.576	1000	0.019	0.063	0.022
1	10	0.621	1000	0.015	0.034	0.012
5	10	0.621	1000	0.011	0.036	0.009
10	10	0.604	300	0.0063	0.017	0.0055
20	10	0.607	300	0.0054	0.016	0.0049
10	0	0.665	1000	0.0067	0.0024	0.0061
10	20	0.595	300	0.0078	0.018	0.0059
10	25	0.584	300	0.0073	0.019	0.0077

Table3 Summary of frictional healing parameters of granite gouge

有效正应力/MPa	含水率/%	$μ s s$ ^①	保持时间阈值^②/s	$β 1$ ^③	$β 2$ ^④	$β 3$ ^⑤
0.5	10	0.576	1000	0.019	0.063	0.022
1	10	0.621	1000	0.015	0.034	0.012
5	10	0.621	1000	0.011	0.036	0.009
10	10	0.604	300	0.0063	0.017	0.0055
20	10	0.607	300	0.0054	0.016	0.0049
10	0	0.665	1000	0.0067	0.0024	0.0061
10	20	0.595	300	0.0078	0.018	0.0059
10	25	0.584	300	0.0073	0.019	0.0077

Fig. 8 The variations of the friction healing parameters Δμp and Δμw with holding time.

Fig. 9 Calculation of microcrack signals for granite gouge in velocity stepping experiments.

Fig. 10 Crack cumulative summation curve of granite gouge at effective normal stress of 10MPa. a VS-10MPa-20%; b VS-10MPa-25%; c SHS-10MPa-20%; d SHS-10MPa-25%

Fig. 11 Fabrics of granite gouge after shear deformation as seen in OM image. a VS-1MPa-10%; b VS-2MPa-10%; c VS-10MPa-10%; d VS-20MPa-10%

Fig. 12 Fabrics of granite gouge at 2MPa effective normal stress and 10%water content after velocity stepping experiment as seen in SEM image.

Fig. 13 Frictional healing parameters Δ μ r plotted as functions of the logarithm of hold time.

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