An episode of fault-valve behaviour during compressional inversion? — The 2004 MJ6.8 Mid-Niigata Prefecture, Japan, earthquake sequence

Principal ruptures of the 2004 Mid-Niigata Prefecture sequence involved close-to-pure reverse slip on a 50–60° WNW dipping fault system uplifting a Miocene rift basin on its hanging wall, consistent with geological evidence for ongoing compressional inversion in the region since the Late Pliocene. The MJ6.8 mainshock was followed by four other MJ > 6 shocks (all with hypocentres in the 7–12 km depth range) and a rich aftershock sequence defined a criss-crossing network of reverse-slip rupture planes dipping 50–60° WNW and 25–35° ESE. The steep rupture planes dip close to the maximum for reverse-fault earthquakes and are poorly oriented for frictional reactivation assuming horizontal maximum compressive stress. In contrast, the subsidiary ESE-dipping thrusts appear optimally oriented in the stress field. The combined reactivation of existing, unfavourably oriented reverse faults and optimally oriented thrusts requires high but probably variable fluid-overpressuring within the rock-mass. Evidence for overpressures in and around the seismogenic fault system comes from borehole measurements in the hanging-wall sedimentary basin plus a range of anomalous seismological and electrical characteristics which also suggest an overpressured mid-crust below the seismogenic zone. Minor postseismic effusion of anomalously warm, saline formation fluids was recorded from the aftershock area: similar discharges characterize the epicentral areas of historic earthquakes in the Niigata region. The rupturing of unfavourably oriented faults in a fluid-overpressured compressional regime – together with the swarm-like character of the aftershock activity and the evidence of postseismic discharge – are all suggestive of a ‘fault-valve’ episode involving upwards migration of fluids from compartmentalized fluid-overpressures within the seismogenic zone.