Does Earthquake Rupturing Initiate in Fluid-Overpressured Crust? - The Case for Scientific Drilling in NE Honshu

Inland earthquakes in Japan arise from rupturing within an upper crustal seismogenic zone that is typically 10-20 km deep. Because such events may occur in close proximity to cities or critical facilities, giving rise to particularly intense ground motions, they contribute significantly to the aggregate seismic hazard. Since 2003, five strong crustal earthquakes (6.3 < M < 6.9) have ruptured steep reverse faults (dips > 45°) both west and east of the Ou Backbone Range hosting the volcanic front in NE Honshu. The earthquakes generally nucleated within the lower seismogenic zone at depths of 5 - 15 km. Several earlier events in the region (e.g. 1964 M7.5 Niigata earthquake) are of similar character. These steep reverse ruptures appear to be part of the ongoing compressional inversion of Miocene rift basins associated with arc-normal shortening that began at c. 3.5 Ma. Hazard from such compressional inversion earthquakes is difficult to assess because potential seismogenic faults (often with low net displacement) tend to be blanketed by post-rift deposition within sedimentary basins (e.g. the 2004 M6.6 Mid-Niigata and M6.6 Niigata-Chuetsu earthquake sequences). The compressional regime of NE Honshu is an optimal setting for ‘holding-in’ fluid overpressure. Frictional mechanics suggests that reactivation of inherited normal faults as steep reverse faults requires pore-fluid pressure elevated above hydrostatic to near-lithostatic pressures at the depth of rupture initiation. Oil-field drilling has shown that aqueous overpressures above hydrostatic exist at depths > 2-3 km in the Niigata sedimentary basin which has hosted several of the rupture sequences. In addition, local geophysical anomalies (high electrical conductivity, seismic low velocities, bright-spot S-wave reflectors, Vp/Vs) in NE Honshu point to heterogeneous fluid overpressuring in the vicinity of the active faults in the lower seismogenic zone. Whether or not earthquake ruptures initiate in fluid overpressured crust is important because cycling of fluid-pressure and fault frictional strength through fault-valve action (postseismic discharge along rupture zones from overpressured portions of the crust) likely affects the nucleation and recurrence of successive earthquakes. A program of investigatory scientific drilling coupled to high-resolution geophysical investigations is proposed to target the lower seismogenic zone at depths of 5-10 km where larger inland earthquakes commonly initiate. It would aim to establish: (1) whether overpressuring extends throughout the full depth of sedimentary basins adjacent to active fault structures; (2) whether overpressures also exist in underlying basement assemblages; (3) whether overpressures are localized around the active fault structures; and, (4) the calibration of physical conditions responsible for observed geophysical anomalies. Unequivocal demonstration of overpressured pore fluids in basement rocks adjacent to an active fault would highlight the role of fluids in fault processes, providing important insights into the balance between stress-driven and fluid-driven failure, and critical variables affecting rupture nucleation and recurrence. Borehole measurements would also help to calibrate geophysical anomalies attributed to fluid overpressuring.