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

Abstract

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 M J6.8 mainshock was followed by four other M J > 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.