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We examined the possible factors affecting the spatial distribution of very low frequency earthquakes and tremors in the shallow megathrust of Nankai Trough (<30 km) using a dense network of prestack depth migrated profiles at the frontal wedge. Geometrical parameters examined were decollement roughness, taper angle, and underthrust thickness. Physical properties such as effective basal friction (μb) and pore pressure ratio (λ*) were calculated from the taper angle and p‐wave velocity. Regions of low λ* (0.39 ± 0.08) and smooth decollement showed no slow earthquake activity. In contrast, high activity of slow earthquakes was observed in areas with a rough decollement due to the presence of subducted seamounts or bathymetric highs. The low taper angle (3.8°) off Muroto where slow earthquakes also occur translates to a wide zone of low μb (0.21 ± 0.06) and high λ* (0.66 ± 0.06). However, our results also show that slow earthquakes don't always occur in areas with high λ*. Plain Language Summary: Slow earthquakes such as very low frequency earthquakes and tremors are different than typical earthquakes because they occur longer and can last for more than several days. They have been reported in numerous subduction zones around the world and the study of slow earthquakes has recently gained attention because they have been suggested as precursors to larger earthquakes albeit on rare occasions. Slow earthquakes show a clustered distribution in the Nankai Trough. We used a dense network of seismic reflection data acquired at the shallow portion of the Nankai Trough to examine why some areas have high activity and some areas have no activity. Our results showed that areas with no slow earthquake activity have a smooth decollement and low pore fluid pressure. On the other hand, areas with high slow earthquake activity have a rough decollement. The occurrence of slow earthquakes has been typically associated with high pore fluid pressure because it allows the two slabs of rock to slide easier. However, our results suggest that high pore fluid pressure may not always be a prerequisite for slow earthquakes to occur. Key Points: A smooth decollement due to absence of subducted seamounts or bathymetric highs, and low pore fluid pressure results in a slow earthquake gapSlow earthquakes don't always occur in areas of high pore pressure but in combination with other variables like decollement roughnessThe low taper angle off Muroto indicates a wide zone of low friction and high pore pressure possibly associated with subducted seamounts [ABSTRACT FROM AUTHOR]

[1] Nobeoka Thrust in Kyushu, southwest Japan, was investigated to understand the relationship between the seismogenic out-of-sequence thrust (OST) and fluid flow in accretionary prisms. The Nobeoka Thrust is a fossilized OST, being active at seismogenic depth. The hanging wall exhibits a penetrative plastic deformation, while a brittle, cataclastic melange-like occurrence characterizes the footwall, although both of them have same shale and sandstone-dominant protolith. Vitrinite reflectance analyses indicate that the maximum temperatures of the hanging wall and footwall are approximately 320 and 250[degrees]C, respectively. This thermal gap across the thrust corresponds to 8.6-14.4 km of displacement assuming a 28-47[degrees]C/km geothermal gradient. The brittle damage zone of the thrust is asymmetric: only 2 m for hanging wall side and 100 m for footwall. Three types of mineral veins, quartz, and carbonate are well developed, especially in the damaged footwall: the tension crack-filling vein, the fault-filling vein, and post-melange one. The first is harmonious with fabric, perpendicular to the P surface. Fluid inclusion geothermobarometry indicates the P-T of fluid in the intensively damaged zone of the footwall is ~300[degrees]C, 230-250 MPa, higher than that from vitrinite reflectance, which suggests that hydrothermal fluid flow is associated with deformation. The same type vein in the lowest portion of the footwall-damaged zone includes a much lower P-T fluid. This difference suggests that continuous underplating caused the damaged zone to propagate downward with cooling and shallowing, which differs from faults characterized by shear localization and might be unique to aquiferous OST in accretionary complexes. Citation: Kondo, H., G. Kimura, H. Masago, K. Ohmori-Ikehara, Y. Kitamura, E. Ikesawa, A. Sakaguchi, A. Yamaguchi, and S. Okamoto (2005), Deformation and fluid flow of a major out-of-sequence thrust located at seismogenic depth in an accretionary complex: Nobeoka Thrust in the Shimanto Belt, Kyushu, Japan, Tectonics, 24, TC6008, doi:10.1029/2004TC001655.

[1] The Mugi Melange located in western Shikoku of the Shimanto Belt shows systematic Y-P deformation fabrics formed by microshear and pressure solution that penetrate throughout the melange pile. Magnetic susceptibility ellipsoids obtained from the anisotropy of magnetic susceptibility (AMS) are highly oblate. Maximum and minimum axes of the ellipsoids are consistent with the shear orientation of the melange and the mean pole of P surfaces, respectively. This agreement suggests that the Mugi Me1ange was formed as a result of underthrusting of trench filling sediment. Vitrinite reflectance ranges from 2.52% to 3.08%, which corresponds to a maximum paleotemperature of ~180-200[degrees]C. Pseudotachylyte, evidence of a seismogenic slip, was found in the upper boundary roof fault of the Mugi Me1ange. However, there is not a thermal gap between the me1ange and the overlying coherent piles, and the temperature from vitrinite reflectance gradually rises downward from the coherent piles to the me1ange beyond the boundary fault, which suggests that paleoisotherms parallel the boundary fault orientation. The isotherms in the seismogenic zone are estimated as subparallel to the plate boundary decollement. Therefore the setting of the cataclastic boundary fault, which includes pseudotachylyte, appears to be a major plate boundary thrust or a subhorizontal splay fault. A probable geologic setting that accounts for the Mugi Me1ange and the seismogenic roof fault is partitioning of the slip along the plate boundary fault in space and time: interseismic slip in the me1ange and seismic slip along the roof fault. Citation: Kitamura, Y., et al. (2005), Melange and its seismogenic roof decollement: A plate boundary fault rock in the subduction zone--An example from the Shimanto Belt, Japan, Tectonics, 24, TC5012, doi: 10.1029/2004TC001635.

Systematic changes in the fabric of melanges in the accretionary complex of the Shimanto belt, Japan, are due to a change in relative convergence between subducting and overriding plates in the Pacific rim. Most likely, the Kula-Pacific ridge was subducted sometime after 43 Ma, in the late Cretaceous to Early Tertiary western Pacific margin. This would explain the change in relative convergence from left to right. Details of field observations of recurring structures in the tectonic melange are given.

Thin slabs of oceanic pillow basalt formed at ridge axes with associated pelagic sediments are common features in ancient accretionary complexes. Estimates of the average thicknesses of scraped-off basaltic sections of oceanic crust in accretionary prisms vary from a few tens of metres to 300 m, with a mean thickness of 80-100 m in the complexes studied. The permeability contrast within the uppermost part of the oceanic crust is an important factor governing the thickness of delaminated oceanic crust. This mechanism is an effective means of separating oceanic crust altered at low temperatures from the remainder of the downgoing slab and thus controlling geochemical budgets in subduction zones.

Trajectory analysis and paleomagnetic examinations help identify the original location of the Sorachi Plateau on the Izanagi Plate. Trajectory analysis of accumulated fragments in northern Japan and Sakhalin show the origin of these fragments in mid-Panthalassa around the equator, which has a positive residual geoid indicating a periodic plume upwelling in mid-Panthalassa. Paleomagnetic study proves the presence of coral reef limestone, and this corresponds to the results of trajectory analysis.

[1] Using colored clay, we examined geometrical evolution of the three-dimensional basement structure in Taiwan due to relative plate motion under the following assumptions: the motion of the Philippine Sea plate relative to the Eurasian plate has been constant during the last 15 Myr, the Ryukyu and Manila trenches had been connected by a transform fault before the collision between the Luzon arc and Eurasian continental margin, and the collision started at 5 Ma. The basement structure obtained from the clay model was fully consistent with observed deep seismicity, which led us to the following conclusions. There are no oceanic slabs beneath central Taiwan. Instead, the Eurasian continental margin is underthrusting beneath it. The Philippine Sea slab extends northwestward from the Ryukyu trench, while the South China Sea slab extends southeastward from the Manila trench. The downdip length of the South China Sea slab becomes shorter from south to north. Citation: Takada, Y., Y. Fukahata, A. Hashima, T. Terakawa, K. Fukui, T. Yanagisawa, Y. Ikeda, G. Kimura, and M. Matsu'ura (2007), Development of three-dimensional basement structure in Taiwan deduced from past plate motion: Consistency with the present seismicity, Tectonics, 26, TC3004, doi: 10.1029/2006TC001957.