Hideki Mukoyoshi

Thermal conductivities under high pressure in core samples from IODP NanTroSEIZE drilling site C0001

We examined the effects of high pressure on thermal conductivity in core samples from the slope–apron facies and the upper part of the accretionary prism at site C0001 of the NanTroSEIZE drilling program and in other samples of five terrestrial rock types. Thermal conductivity clearly increased with increasing pressure for both wet (water saturated) and dry samples. We determined the rate of thermal conductivity change of the NanTroSEIZE sediments to be 0.014 Wm−1K−1/MPa when pressure was increased, and 0.01 Wm−1K−1/MPa when pressure was decreased. Using the rate determined for decreasing pressure, we estimated that thermal conductivities measured at atmospheric pressure rather than at in situ pressure may be underestimated by 7% for a core sample from around 1 km depth and by 20% for a core sample from around 3 km depth. In general, the rate of thermal conductivity change with pressure showed a positive correlation with porosity. However, the relationship of the rate of thermal conductivity change to porosity is also dependent on the fabric, mineral composition, and pore structure of the sediments and rocks. Furthermore, for two sandstones we tested, the effect of pressure on thermal conductivity for dry samples was greater than that for wet samples.

Frictional melting of clayey gouge during seismic fault slip: Experimental observation and implications

Clayey gouges are common in fault slip zones at shallow depths. Thus, the fault zone processes and frictional behaviors of the gouges are critical to understanding seismic slip at these depths. We conducted rotary shear tests on clayey gouge (~41 wt % clay minerals) at a seismic slip rate of 1.3 m/s. Here we report that the gouge was melted at 5 MPa of normal stress and room humidity conditions. The initial local melting was followed by melt layer formation. Clay minerals (e.g., smectite and illite) and plagioclase were melted and quenched to glass with numerous vesicles. Both flash heating and bulk temperature increases appear to be responsible for the melting. This observation of clayey gouge melting is comparable to that of natural faults (e.g., Chelungpu fault, Taiwan). Due to heterogeneous fault zone properties (e.g., permeability), frictional melting may be one of the important processes in clayey slip zones at shallow depths.

Release of mineral-bound water prior to subduction tied to shallow seismogenic slip off Sumatra

Sediments tell a tsunami story Trying to understand where major earthquakes and tsunamis might occur requires analysis of the sediments pouring into a subduction zone. Thick sediments were expected to limit earthquake and tsunami size in the Sumatran megathrust event in 2004, but the magnitude 9.2 earthquake defied expectations. Hüpers et al. analyzed sediments recovered from the Sumatran megathrust. They found evidence of sediment dehydration, which increased fault strength and allowed for the much larger earthquake to occur. Thus, models of other subduction zones, such as the Gulf of Alaska, may underestimate the maximum earthquake magnitude and tsunami risk. Science, this issue p. 841 Sediments drilled near the rupture of the 2004 great Sumatran earthquake provide insight into the unexpectedly large tsunami. Plate-boundary fault rupture during the 2004 Sumatra-Andaman subduction earthquake extended closer to the trench than expected, increasing earthquake and tsunami size. International Ocean Discovery Program Expedition 362 sampled incoming sediments offshore northern Sumatra, revealing recent release of fresh water within the deep sediments. Thermal modeling links this freshening to amorphous silica dehydration driven by rapid burial-induced temperature increases in the past 9 million years. Complete dehydration of silicates is expected before plate subduction, contrasting with prevailing models for subduction seismogenesis calling for fluid production during subduction. Shallow slip offshore Sumatra appears driven by diagenetic strengthening of deeply buried fault-forming sediments, contrasting with weakening proposed for the shallow Tohoku-Oki 2011 rupture, but our results are applicable to other thickly sedimented subduction zones including those with limited earthquake records.

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

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 m2 at site C0004 and 9 × 10−18 m2 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 m2 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.

Cathodoluminescence and fluid inclusion analyses of mineral veins within major thrusts in the Shimanto accretionary complex: evidence of hydraulic fracturing during thrusting

To elucidate fluid-rock interaction in a seismogenic zone along a plate-subduction boundary, we investigated the occurrence of mineral veins within the major thrusts in the Shimanto accretionary complex and examined their microstructures using a cathodoluminescence technique. We found discriminative structures, for example, a jigsaw-puzzle structure, within the quartz veins in the thrusts, which could indicate that hydraulic fracturing occurred by abnormal pore-fluid pressure during thrusting. Pore pressure values, estimated quantitatively by fluid inclusion analyses, were 3–27 MPa higher than the surrounding parts, which may be direct evidence of abnormal pore-fluid pressure. High pore-fluid pressures and subsequent hydraulic fracturing may play an important role within major thrusts along a plate-subduction boundary.