[1] We examined the physical properties of an exhumed and fossilized subduction zone megasplay fault by analyzing geophysical logging data obtained by the Nobeoka Thrust Drilling Project, which provide a high-resolution transect of properties across the main fault zone. The footwall cataclasite exhibits higher averages of neutron porosity (7.6%) and lower values of electric resistivity (232 Xm) compared to the hanging wall phyllite (4.8%, 453 Xm). This clear contrast between the hanging wall and footwall may account for the difference in maximum burial and structural variation. Despite the contrast observed between the hanging wall and footwall in macroscopic scale, the resistivity and porosity data from both the hanging wall and footwall can be fit with a single curve using Archie's law, suggesting the similarities in microstructures and mineralogy in this low porosity range. Above the main fault core of the Nobeoka Thrust a brittle damage zone in the hanging wall contains pseudotachylyte as evidence of the seismogenic slip and does not follow Archie's law. Damage zones in the hanging wall are also observed in the modern splay fault at shallow depth in the Nankai Trough but with much thicker width, whereas the footwall damage zone is more extensive in the Nobeoka Thrust. Splay faults may exhibit strong deformation in the hanging wall in the early stage, and as fault rocks get buried deeper and as displacement and physical property contrast increase across the fault, the damage effect may eventually be enlarged in the footwall.

Studying subduction zone fluid at shallow seismogenic depths is important to understand the nature of fault rocks at the updip limit of the seismogenic zone because fluid-rock interactions affect heat and mass transfer, and fault strength. In this study, we conducted detailed analyses of distribution of shear veins, and estimation of pressuretemperature conditions for shear vein formation for the Yokonami mélange, Shikoku, Southwest Japan, which is tectonic mélange zone in an on-land accretionary complex. We found a seismogenic fault at the upper boundary of the Yokonami mélange, indicating that the Yokonami mélange was active at seismogenic depth. The field-transect distribution of shear veins was examined. The frequency, the total and mean thicknesses of the shear veins were about 3.7 per meter, about 10 mm per meter, and about 3 mm per shear vein, respectively. Quartz within the shear veins shows elongate-blocky textures, suggesting precipitation from advective flow. The pressure and temperature conditions for shear vein formation were examined by fluid inclusion analysis, ranging 175-225°C and 143-215 MPa, respectively. The temperature is almost consistent with the paleotemperature determined from vitrinite reflectance, suggesting that the shear veins were formed at up to the maximum depth. The depth might be consistent with that where the seismogenic fault was formed. On the basis of the pressure and temperature conditions and the distribution of shear veins, we estimated the minimum volumetric ratio of fluid to host rocks, assuming that the shear veins had precipitated from advective flow. The estimated amount of fluid is about 106 m 3 per cubic meter of host rocks. The results suggest that a large amount of fluid migrates through mélange zones at shallow seismogenic depths. This fluid possibly originates from the dehydration of clay minerals from underthrusted sediments and an altered subducting slab.

A change in paleostress along a subduction zone plate interface in the shallow portion of a seismogenic zone was detected in an on-land accretionary complex, the Yokonami mélange in southwest Japan, using the microfault inversion method. Microfaults were classified into two groups based on location: those occurring throughout the Yokonami mélange and those occurring in the Goshikinohama fault zone, which is considered to be a fossil seismogenic fault and is located at the northern end of the Yokonami mélange. Stresses obtained for these classified microfaults indicate that two different stress states exist for each deformation feature: one is subhorizontal σ 1 and subvertical σ 3 with a smaller stress ratio, and the other is subvertical σ 1 and subhorizontal σ 3 with a larger stress ratio. The difference between these stress states could be related to the horizontal stress change by stress drop after large earthquakes that were recently observed after the Tohoku-Oki great earthquake.

Laboratory measurements for compressional and shear wave velocities (Vp and Vs, respectively) and porosity were conducted with core samples from the Nobeoka Thrust Drilling Project (NOBELL) under controlled effective pressure (5-65 MPa at 5 MPa intervals) and wet conditions. Samples were classified according to deformation texture as phyllite, foliated cataclasite, or non-foliated cataclasite. Measured values of Vp, Vs, and porosity are within a range of 5.17-5.57 km/s, 2.60-2.71 km/s, and 2.75-3.10 %, respectively, for phyllite; 4.89-5.23 km/s, 2.46-2.57 km/s, and 3.58-4.53 %, respectively, for foliated cataclasite; and 4.90-5.32 km/s, 2.51-2.63 km/s, and 3.79-4.60 %, respectively, for non-foliated cataclasite, which are all consistent with the previous laboratory experiments conducted with outcrop samples under dry conditions. However, our results also indicate higher Vp and Vs and lower porosity than those measured by the previous studies that adopted the wire-line logging methods. The variations in Vp, Vs, and porosity are controlled by deformation structure and are greater for phyllite and foliated cataclasite than for non-foliated cataclasite. K E Y W O R D S acoustic properties, Nobeoka thrust, Shimanto Belt