Pressure dependence of fluid transport properties of shallow fault systems in the Nankai subduction zone

Abstract

We measured fluid transport properties at an effective pressure of 40 MPa in core samples of sediments and fault rocks collected by the Integrated Ocean Drilling Program (IODP) NanTroSEIZE drilling project Expedition 316 from the megasplay fault system (site C0004) and the frontal thrust (site C0007) in the Nankai subduction zone. Permeability decreased with effective pressure as a power law function. Permeability values in the fault zones were 8 × 10 −18 m 2 at site C0004 and 9 × 10 −18 m 2 at site C0007. Stratigraphic variation in transport properties suggests that the megasplay fault zone may act as a barrier to fluid flow, but the frontal thrust fault zone might not. Depth variation in permeability at site C0007 is probably controlled by the mechanical compaction of sediment. Hydraulic diffusivity at shallow depths was approximately 1 × 10 −6 m 2 s −1 in both fault zones, which is small enough to lead to pore pressure generation that can cause dynamic fault weakening. However, absence of a very low permeable zone, which may have formed in the Japan Trench subduction zone, might prevent facilitation of huge shallow slips during Nankai subduction zone earthquakes. Porosity tests under dry conditions might have overestimated the porosity.

Nucleation speed limit on remote fluid-induced earthquakes

“…Fluids moving from a highly pressurized source through a porous medium, such as fault zone rocks, exhibit diffusive behavior ( 39 ) that takes the form of , where r is the distance from the high–fluid pressure source, c is a constant [derived in one dimension for fault zones as c = 2.32 ( 40 )], t is time, and D is the coefficient of diffusivity. We note the consistency between diffusion rate curves calculated with typical ( 41 – 45 ) fault zone diffusivity values and observed magnitude-dependent delay expressed as time versus critical nucleation dimension ( 37 ) ( Fig. 5 ).…”

Near-trench slip potential of megaquakes evaluated from fault properties and conditions

“…37 (run number, CCT299). Our measured values of transport properties agreed well with previous data of similar fault materials from the megasplay fault (approximately 10 –19 to 10 –17  m 2 )36374344.…”

“…However, because it cannot be predicted whether the upgoing rupture of the next large earthquake will branch out to the megasplay fault or continue propagating along the décollement, in this study we considered both cases. Because no sample from the décollement dark gouge in the Nankai Trough was available, we referred to previously published data to constrain its properties 35 36 37 .…”

“… 37 (run number, CCT299). Our measured values of transport properties agreed well with previous data of similar fault materials from the megasplay fault (approximately 10 –19 to 10 –17 m 2 ) 36 37 43 44 . Permeabilities of the megasplay fault and décollement materials in the Nankai Trough were previously reported to have similar values and to depend similarly on the confining stress 36 37 ; therefore, we used the transport properties obtained from our measurements of the megasplay fault for the décollement as well.…”

Evaluating Stress State, Physical Properties, and Rupturing Behavior of Seismogenic Faults through Scientific Drillings

“…Permeability of spray fault is one order of magnitude smaller than sediments below the fault zone, and the fault may act as a barrier for fluid migration from the depths. Hydraulic diffusivity of the splay fault is slightly smaller than that of the frontal thrust (Tanikawa et al, 2014a) . , and volumetric heat capacity (c) .…”