“…The presence of carbonaceous materials may therefore act as a lubricant to play a key role in frictional properties and accordingly in promoting the instability of the fault (Oohashi et al, 2011(Oohashi et al, , 2013Kuo et al, 2014). Meanwhile, organic-rich rocks (such as coal, shale and clay), as main source rocks for (un)conventional natu-ral gas, may also play a role in induced seismicity upon gas production (e.g., Kohli and Zoback, 2013;, water injection (e.g., Ellsworth, 2013) and coal mining (e.g., Westbrook et al, 1980). Compared to graphite (Ruan and Bhushan, 1994;Moore and Lockner, 2004;Kirilova et al, 2018), however, limited experimental data on the frictional properties of organic-rich rocks , particularly coal, under in situ pressure and temperature (PT) conditions are reported.…”