Abstract
Abstract. 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 ∘C, employing sliding velocities of 0.1–100 µm s−1 and using a conventional triaxial apparatus plus direct shear assembly. All samples showed marked slip-weakening behavior at shear displacements beyond ∼ 1–2 mm, from a peak friction coefficient approaching ∼0.5 to (nearly) steady-state values of ∼0.3, 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 ∼0.006. 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 >1 µm s−1 in the coal sample under vacuum-dry conditions but at >10 µm s−1 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.
Highlights
Carbonaceous materials are widely present in the lithosphere, including in several large fault zones over the world (Kaneki and Hirono, 2019), such as the Longmenshan thrust belt in China (Kuo et al, 2014), the Atotsugawa fault zone in Japan (Oohashi et al, 2012) and the Alpine fault zone (Kirilova et al, 2017)
The typical apparent friction coefficient (μ) versus displacement data obtained in velocity stepping experiments
The quasi-steady-state friction coefficient decreased slightly with displacement, reaching a new quasi-steady-state value at 4–6 mm. This slight weakening might be caused by the reduction of the loadsupporting area of the sample during shear deformation. To quantify this effect, we define μpeak as the peak friction coefficient obtained at 0.5–0.75 mm of shear displacement, and we take μss1 and μss2 to represent the nearsteady-state friction coefficient values obtained at ∼ 2.2 and ∼ 5.7 mm of shear displacement, respectively
Summary
Carbonaceous materials (e.g., amorphous carbon, graphite, organic matter) are widely present in the lithosphere, including in several large fault zones over the world (Kaneki and Hirono, 2019), such as the Longmenshan thrust belt in China (Kuo et al, 2014), the Atotsugawa fault zone in Japan (Oohashi et al, 2012) and the Alpine fault zone (Kirilova et al, 2017). Coal has been widely investigated because of its importance in fuel energy and industry (Guo et al, 2018; Chen et al, 2019), its frictional properties are not yet well determined and understood. This contribution addresses the frictional properties of coal and the likely mechanisms
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