Abstract Detailed 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.

Abstract Between the Median Tectonic Line (MTL) and the Japan Sea, the western Chugoku region of SW Japan is cut by a series of N45°E first-order faults and oblique (N60°–N170°E) second-order faults. This fault network, probably formed during Late Cretaceous–Palaeocene times (70–60 Ma), defines a regional block structure. Pre-Plio-Quaternary kinematical indicators suggest left-lateral motion along the first-order faults and right-lateral motion along some of the second-order faults. Geomorphological evidence and earthquake focal mechanisms indicate that Plio-Quaternary slip senses are opposite to Pre-Plio-Quaternary ones. The overall fault pattern is geometrically and kinematically similar to patterns obtained by experimental modelling of simple shear deformation distributed at the base of a brittle layer analogue over its entire width. This similarity suggests the possibility of a mid-crustal, flat-lying partial attachment zone which could have controlled the formation of the western Chugoku fault network in Cretaceous to Palaeocene times. The zone, presently inactive, could correspond to the ‘proto-MTL’, a low-angle fault recently imaged by seismic reflection studies and whose trace approximately coincides with the present-day MTL. Reactivation of the system occurred twice after its formation: firstly in Miocene times, during the opening of the Japan Sea and concomitant clockwise rotation of the entire SW Japan arc; and secondly in Late Pliocene to Quaternary times, after a shift of the relative direction of convergence between the Philippine and Eurasia plates. Unlike the first reactivation, the second reactivation led to an inversion of the sense of slip along the faults.

Abstract The Okinawa trough is a back-arc basin currently forming above the subducting Philippine Sea plate by crustal stretching of the Eurasia lithosphere. Existing geophysical investigations have revealed that the northern part of the Okinawa trough consists of a series of en echelon left-stepping grabens or half-grabens, some of which were formed in Miocene times and are presently inactive. In the Kyushu island, three major zones of extensional deformation are recognized. They are characterized by 20–40 km long sediment-filled basins lying on the hanging-wall side of N60°- to N80°-trending, north– or NW–dipping normal faults. These basins display an en echelon left-stepping arrangement. The ages of faulted rocks, the ages of graben-filling sediments and radiometric ages newly obtained on pseudotachylites associated with normal faulting indicate that extension started at 13 Ma at the southern end of Kyushu and migrated northwards in the middle part of Kyushu (Beppu Bay), where it is still active today. The continentward-dipping, listric geometry inferred in depth for the fault systems is consistent with that of faults imaged by existing seismic profiles obtained off-shore in the Okinawa trough and across the Beppu Bay. The spatial association between Miocene or younger normal faults and pre-Miocene regional low-angle thrust faults suggests the possibility of a reactivation of some of the thrust faults as low-angle detachment faults merging in depth into a mid-crustal partial attachment zone.