“…The storage and transport of fluids in fault zones is generally assumed to be highly partitioned owing to the structural complexity of fault‐related fracture systems (Bonnet et al, ; Kim et al, ; Sibson, ; Tchalenko, ) and sharp contrasts in hydraulic properties (Bense et al, ; Brace, , ; Caine et al, ; Evans & Goddard, ; Roques et al, ). This affects processes in the Earth's crust operating on a broad range of scales, from the emergence of flow pathways (Tsang & Neretnieks, , and references therein) channeling solute (Kang et al, ) and heat transport (De La Bernardie et al, ; Fox et al, ; Geiger & Emmanuel, ; Klepikova et al, ) to convecting instabilities (Murphy, ; Patterson et al, ), frictional heating (Mase & Smith, ; Rice, ; Vredevoogd et al, ), dynamic fault weakening (Byerlee, ), and earthquake sequences (Jansen et al, ; Miller et al, ; Noir et al, ; Nur & Booker, ; Ross et al, ; Shapiro et al, ; Wang et al, ). Tracking how fluids migrate in faults in situ can therefore provide insights on key geological and physical processes.…”