Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates
<p>Previous studies show that organic-rich fault patches may play an important role in promoting unstable fault slip. However, the frictional properties of rock materials with nearly 100 % organic content, e.g., coal, and the controlling microscale mechanisms remain unclear. Here, w...
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doaj-02ad7e6d612341fc9f3daff5d188e07c2020-11-25T03:45:04ZengCopernicus PublicationsSolid Earth1869-95101869-95292020-07-01111399142210.5194/se-11-1399-2020Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip ratesC. Fan0J. Liu1J. Liu2J. Liu3L. B. Hunfeld4C. J. Spiers5School of Earth Sciences and Engineering, Sun Yat-Sen University, Guangzhou, 510275, ChinaSchool of Earth Sciences and Engineering, Sun Yat-Sen University, Guangzhou, 510275, ChinaGuangdong Provincial Key Lab of Geodynamics and Geohazards, Sun Yat-Sen University, Zhuhai, 519082, ChinaSouthern Marine Science and Engineering Guangdong Laboratory, Zhuhai, 519082, ChinaDepartment of Earth Sciences, Utrecht University, Utrecht, 3584 CB, the NetherlandsDepartment of Earth Sciences, Utrecht University, Utrecht, 3584 CB, the Netherlands<p>Previous studies show that organic-rich fault patches may play an important role in promoting unstable fault slip. However, the frictional properties of rock materials with nearly 100 % organic content, e.g., coal, and the controlling microscale mechanisms remain unclear. Here, we report seven velocity stepping (VS) experiments and one slide–hold–slide (SHS) friction experiment performed on simulated fault gouges prepared from bituminous coal collected from the upper Silesian Basin of Poland. These experiments were performed at 25–45 MPa effective normal stress and 100 <span class="inline-formula"><sup>∘</sup></span>C, employing sliding velocities of 0.1–100 <span class="inline-formula">µ</span>m s<span class="inline-formula"><sup>−1</sup></span> and using a conventional triaxial apparatus plus direct shear assembly. All samples showed marked slip-weakening behavior at shear displacements beyond <span class="inline-formula">∼</span> 1–2 mm, from a peak friction coefficient approaching <span class="inline-formula">∼0.5</span> to (nearly) steady-state values of <span class="inline-formula">∼0.3</span>, regardless of effective normal stress or whether vacuum-dry or flooded with distilled (DI) water at 15 MPa pore fluid pressure. Analysis of both unsheared and sheared samples by means of microstructural observation, micro-area X-ray diffraction (XRD) and Raman spectroscopy suggests that the marked slip-weakening behavior can be attributed to the development of R-, B- and Y-shear bands, with internal shear-enhanced coal crystallinity development. The SHS experiment performed showed a transient peak healing (restrengthening) effect that increased with the logarithm of hold time at a linearized rate of <span class="inline-formula">∼0.006</span>. We also determined the rate dependence of steady-state friction for all VS samples using a full rate and state friction approach. This showed a transition from velocity strengthening to velocity weakening at slip velocities <span class="inline-formula">>1</span> <span class="inline-formula">µ</span>m s<span class="inline-formula"><sup>−1</sup></span> in the coal sample under vacuum-dry conditions but at <span class="inline-formula">>10</span> <span class="inline-formula">µ</span>m s<span class="inline-formula"><sup>−1</sup></span> in coal samples exposed to DI water at 15 MPa pore pressure. The observed behavior may be controlled by competition between dilatant granular flow and compaction enhanced by the presence of water. Together with our previous work on the frictional properties of coal–shale mixtures, our results imply that the presence of a weak, coal-dominated patch on faults that cut or smear out coal seams may promote unstable, seismogenic slip behavior, though the importance of this in enhancing either induced or natural seismicity depends on local conditions.</p>https://se.copernicus.org/articles/11/1399/2020/se-11-1399-2020.pdf |
collection |
DOAJ |
language |
English |
format |
Article |
sources |
DOAJ |
author |
C. Fan J. Liu J. Liu J. Liu L. B. Hunfeld C. J. Spiers |
spellingShingle |
C. Fan J. Liu J. Liu J. Liu L. B. Hunfeld C. J. Spiers Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates Solid Earth |
author_facet |
C. Fan J. Liu J. Liu J. Liu L. B. Hunfeld C. J. Spiers |
author_sort |
C. Fan |
title |
Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates |
title_short |
Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates |
title_full |
Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates |
title_fullStr |
Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates |
title_full_unstemmed |
Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates |
title_sort |
frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates |
publisher |
Copernicus Publications |
series |
Solid Earth |
issn |
1869-9510 1869-9529 |
publishDate |
2020-07-01 |
description |
<p>Previous studies show that organic-rich fault patches may
play an important role in promoting unstable fault slip. However, the
frictional properties of rock materials with nearly 100 % organic content,
e.g., coal, and the controlling microscale mechanisms remain unclear. Here,
we report seven velocity stepping (VS) experiments and one slide–hold–slide (SHS)
friction experiment performed on simulated fault gouges prepared from
bituminous coal collected from the upper Silesian Basin of Poland. These
experiments were performed at 25–45 MPa effective normal stress and 100 <span class="inline-formula"><sup>∘</sup></span>C, employing sliding velocities of 0.1–100 <span class="inline-formula">µ</span>m s<span class="inline-formula"><sup>−1</sup></span> and using
a conventional triaxial apparatus plus direct shear assembly. All samples
showed marked slip-weakening behavior at shear displacements beyond
<span class="inline-formula">∼</span> 1–2 mm, from a peak friction coefficient approaching
<span class="inline-formula">∼0.5</span> to (nearly) steady-state values of <span class="inline-formula">∼0.3</span>,
regardless of effective normal stress or whether vacuum-dry or flooded with
distilled (DI) water at 15 MPa pore fluid pressure. Analysis of both
unsheared and sheared samples by means of microstructural observation,
micro-area X-ray diffraction (XRD) and Raman spectroscopy suggests that the
marked slip-weakening behavior can be attributed to the development of R-,
B- and Y-shear bands, with internal shear-enhanced coal crystallinity
development. The SHS experiment performed showed a transient peak healing
(restrengthening) effect that increased with the logarithm of hold time at a
linearized rate of <span class="inline-formula">∼0.006</span>. We also determined the
rate dependence of steady-state friction for all VS samples using a full
rate and state friction approach. This showed a transition from velocity
strengthening to velocity weakening at slip velocities <span class="inline-formula">>1</span> <span class="inline-formula">µ</span>m s<span class="inline-formula"><sup>−1</sup></span> in the coal sample under vacuum-dry conditions but at
<span class="inline-formula">>10</span> <span class="inline-formula">µ</span>m s<span class="inline-formula"><sup>−1</sup></span> in coal samples exposed to DI water at 15 MPa pore pressure. The observed behavior may be controlled by competition
between dilatant granular flow and compaction enhanced by the presence of
water. Together with our previous work on the frictional properties of
coal–shale mixtures, our results imply that the presence of a weak,
coal-dominated patch on faults that cut or smear out coal seams may promote
unstable, seismogenic slip behavior, though the importance of this in
enhancing either induced or natural seismicity depends on local conditions.</p> |
url |
https://se.copernicus.org/articles/11/1399/2020/se-11-1399-2020.pdf |
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