Permeability structure and co-seismic thermal pressurization on fault branches: insights from the Usukidani fault, Japan

Abstract

AbstractDetailed mapping of complex fault zones shows that secondary faults often branch off the principal slip zone. However, the effect of secondary branch faults on the hydrodynamic behaviour of fault zones has not yet been examined, largely because of a lack of hydraulic data and because numerical or analogue modelling of splay faulting is a complex issue. This contribution investigates the thermal pressurization process in cases of slip along a principal slip zone and along splay faults branching off the principal displacement zone. The study is based on porosity and permeability data presented in this paper from the principal and secondary slip zones of an active, clay-rich gouge-bearing strike–slip fault, the Usukidani fault of SW Japan. Modelling constrained by these data suggests that thermal pressurization is a viable process only as long as the rupture remains located in the central gouge zones or in mature splay fault gouge zones. Splaying of the rupture into surrounding microbreccias or into immature or newly generated splay faults of higher permeability will release fluid pressure or inhibit the generation of coseismic excess fluid pressures by thermal pressurization. The modelling results suggest that secondary fault branches can play a key role in controlling fluid pressurization during faulting. Hence, complete investigation of active fault zones needs to include secondary faults and their corresponding hydraulic behaviour, in order to establish the influence of such structures on earthquake mechanics.