ABSTRACT The Shikoku Basin is a back-arc basin located offshore southwest Japan. Sediments within the basin make up a key part of the subduction inputs to the Nankai Trough. A 19 m.y. history of sedimentation has been documented at Sites C0011 and C0012 of the Integrated Ocean Drilling Program (Kumano transect) and Sites 1173 and 1177 of the Ocean Drilling Program (Muroto and Ashizuri transects, respectively). This paper focuses on three noteworthy aspects of that history: (1) the onset of substantial pyroclastic influx, which shifted significantly along the strike length of the margin, from 3.3–3.9 Ma at Sites 1177 and 1173 to 7.6–7.8 Ma at Sites C0011 and C0012; (2) transport of sand by sediment gravity flows, which resulted in three discrete sand bodies during the Miocene (Kyushu, Daiichi Zenisu, and Daini Zenisu submarine fans); and (3) clay mineral assemblages within hemipelagic mudstones, which show systematic reduction of 3 wt% detrital smectite per 1 m.y. decrease in age. Collectively, these temporal and spatial adjustments of lithofacies and sediment composition have important implications for downdip and along-strike projections of frictional, geotechnical, and hydrogeological properties as strata enter the Nankai subduction zone. The stratigraphic positions of smectite-rich Miocene mudstones, for example, should match up with increases in the volume of fluid production by clay dehydration during subduction. The higher-permeability sand bodies (Kyushu and Zenisu submarine fans) should act as preferred conduits for focused fluid flow. The potential for buildup of fluid overpressures should increase above and laterally adjacent to stratigraphic pinch-outs of sand bodies, especially where the aquifers are inclined or confined between basement highs. These three-dimensional complexities set the Nankai-Shikoku system apart from other subduction zones (e.g., Japan Trench, Costa Rica) where inputs consist of comparatively homogeneous pelagic and hemipelagic deposits.

ABSTRACT Recent seismic reflection data across the Japan Trench show that frontal accretion involves offscraping sediments on top of horsts and scooping-up sediment from grabens. However, seismic profiling does not illuminate the structure within the accretionary prism, and thus the processes of accretion and prism growth are unknown. Key data from scientific drilling at Integrated Ocean Drilling Program Site C0019 that penetrated the prism in the region of large displacement during the 2011 Tohoku earthquake support a model in which frontal accretion occurs by imbricate thrusting, folding, and stacking of thrust sheets that are composed of semicoherent-sediment strata. Using palinspastic restoration techniques, we conclude that out-of-sequence thrusting and duplex development during the underthrusting of horsts can form and displace hanging-wall ramps along the plate-boundary detachment, which helps to explain the formation of some unexpected tectonostratigraphic relations at C0019, such as the emplacement of a thick section of the youngest sediments at the base of the accretionary prism, and numerous juxtapositions of different-age sediments within the basal plate-boundary fault zone.

Abstract Large earthquakes and related tsunamis serve as triggering mechanisms that generate turbidity currents which form turbidites. The event deposits from the recent 2011 Tohoku-oki earthquake and tsunami are observed throughout a wide area along the Pacific coast of Tohoku, northern Japan, extending from the coast through the shelf and slope, to the trench floor. Spatio-temporal correlation of turbidites and other tsunamigenic deposits, such as those generated in the 2011 event, can be used to reconstruct the recurrence history of large earthquakes and tsunamis. Here we use sediment cores and sub-bottom profiles to analyse the depositional setting along the Japan Trench, and show that the environment is ideal for preserving turbidites. The subducting Pacific Plate creates graben or basins along the trench floor that accommodate the episodic deposition of fine-grained turbidites; and interseismic hemipelagic deposits that form with high sedimentation rates along the Japan Trench effectively cover earthquake-induced turbidites and preserve the deposits as a geological record of large earthquakes. Therefore, small deep-sea basins with high sedimentation rates, such as in and around the Japan Trench floor, are favourable environments for studies of turbidite palaeoseismology.

Near-vertical multiple ScS (S waves reflected at the core-mantle boundary) phases are among the cleanest seismic phases traveling over several thousand kilometers in the Earth's mantle and are useful for constraining the average attenuation and shear wave speed in the whole mantle. However, the available multiple ScS pairs are limited. We took advantage of the recent dramatic increase in the number of global broadband stations and made a thorough computer-assisted search for high-quality data of multiple ScS pairs. We could find 220 station-event pairs which provided us with robust local estimates of average Q (quality factor) and two-way shear wave travel times. With the assumption that geometric focusing caused by lateral velocity heterogeneity does not seriously affect the amplitude measurements, the Q values exhibit strong short-range lateral variations, with very high and very low Q regions adjacent to each other. The mantle beneath seismic station KIP (Hawaii) has normal Q and shear wave speed, which supports the result of earlier studies. The mantle beneath station AFI (Samoa Islands) has a very high Q , possibly larger than 1400, and the slowest shear wave speed. The stations on the upper plate of the Tonga and Japan subduction zones yield average to low Q values. In contrast, the stations on the trenchward side of the upper plate of some subduction zones, e.g., station LVC (Chile) and station PET (Kamchatka, Russia), indicate high Q values, larger than 1000. We found no obvious correlation between Q and shear wave speed, which suggests that different factors like temperature, composition, anisotropy, etc., are controlling these properties in the mantle of different tectonic environments.