The factors controlling earthquake swarm duration are remain unclear, especially in the long-living ones. A severe earthquake swarm struck the tip of the Noto peninsula, Japan. Ten M > 4.0 earthquakes occurred, and the sequence has continued more than four years. We investigated the spatiotemporal characteristics of the swarm using relocated hypocenters to elucidate the factors causing this long duration. The swarm consists of four seismic clusters-northern, northeastern, western, and southern-the latter of which began first. Diffusive hypocenter migrations were observed in the western, northern, and northeastern clusters with moderate to low diffusivities, implying a low-permeability environment. Rapid diffusive migration associated with intermittent seismicity deep within the southern cluster suggests the presence of a highly pressurized fluid supply. We conclude that the nature of this fluid supply combined with intermittent seismicity from the southern cluster and a low-permeability environment are the key causes of this long-living swarm.

Ancient exhumed accretionary complexes are sometimes associated with high-pressure–low-temperature (HP–LT) metamorphic rocks, such as psammitic schists, which are derived from sandy trench-fill sediment. At accretionary margins, sandy trench-fill sediments are rarely subducted to the depth of HP metamorphism because they are commonly scraped off at the frontal wedge. This study uses sandbox analogue experiments to investigate the role of seafloor topography in the transport of trench-fill sediment to depth during subduction. The experiments were conducted with a detached, rigid backstop to allow a topographic high (representing a seamount) to be subducted through a subduction channel. In experiments without topographic relief, progressive thickening of the accretionary wedge pushed the backstop down, leading to a stepping down of the décollement, narrowing the subduction channel, and underplating the wedge with subducting sediment. In contrast, in experiments with a topographic high, the subduction of the topographic high raised the backstop, leading to a stepping up of the décollement and widening of the subduction channel. These results suggest that the subduction of topographic relief is a possible mechanism for the transport of trench-fill sediment from the trench to HP environments through a subduction channel. A sufficient supply of sediment to the trench and topographic relief on the subducting oceanic plate might enable trench-fill sediment to be accreted at various depths and deeply subducted to become the protoliths of HP–LT metamorphic rocks.

In the 2011 Tohoku-Oki earthquake, the rupture in the subduction megathrust reached the trench axis and triggered a large tsunami. The shallow portion of the subduction megathrust fault was regarded as an aseismic stable zone. The frictional properties along the shallow subduction plate boundary are an important foundation for understanding the cause of the dynamic fault rupture in the earthquake near the trench. The critical taper model of a sedimentary wedge best describes the first-order mechanics of a subduction zone wedge. The tapered wedge geometry (slope angle α and basal dip angle β) is responsible for the strength of a shallow megathrust. However, to apply the critical taper model for the investigation of spatial heterogeneity, we need to improve handling β, since β is derived from the subsurface structure and its value depends on the number of accurately depth-converted seismic profiles. Here, the effect of décollement dip angle β in the critical taper model of a sedimentary wedge is examined. The effect is negligible for a high pore fluid pressure ratio, allowing the frictional variation to be obtained with only bathymetry data. We applied the model to the Japan Trench. The frictional variation indicates that a smaller frictional area corresponds to an area with a larger coseismic shallow rupture during the 2011 earthquake than those of the southern and northern areas. The method can be applied to other trenches to predict seismic potential.

We conducted sandbox analogue experiments for subduction of trench-fill sediments beneath accretionary wedge and backstop in order to explain how protoliths of high-pressure/low-temperature (HP-LT) metamorphic rocks are transported to high pressure environment. At accretionary-type subduction zones, it is commonly difficult that coarse-grained sandy trench-fill deposits subduct deeper than high pressure environment (>10 km in depth), because they are accreted at the shallower part of the wedge (<5 km) in association with stepping down of decollement due to progressive dewatering under the accretionary wedge. However, ancient exhumed accretionary complexes sometimes accompany with low-grade accretionary rocks from trench-fill turbidites and HP-LT metamorphic rocks including psammitic and even conglomeratic schists, whose provenance and depositional ages are similar to each other. Therefore, we need a model to explain growth of accretionary wedge and subduction of coarse-grained trench-fill sediments beneath the wedge at the same time. In this study, we attempt to identify an importance of seafloor roughness for transportation of trench-fill sediments to deep during subduction. For this purpose, we performed sandbox analogue experiments by using an unfixed rigid backstop on a subduction channel with the cases of smooth surface (Exp. A) and rough surface representing a seamount or ridge on subducting lower plate (Exp. B). The results of Exp. A showed progressive thickening of the accretionary wedge pushed the backstop down, meaning stepping down of the decollement and narrowing the subduction channel. On the other hand, Exp. B showed a subducting seamount lifted up the backstop, stepped up the decollement, and then widened the subduction channel. Subduction of a rigid material like seamounts is a possible mechanism to open subduction channels for transportation of terrigenous sediments from the trench to high-pressure condition. Significant sediment supply to the trench and rough surface of subducting oceanic plate are required to enable subduction of protolith of HP-LT metamorphic rocks and accretion of trench-fill sediments at the shallow part.

Forearc basin stratigraphy is expected to record a detailed history of the deformation and growth pattern of an accretionary wedge. However, the relationship between syntectonic basin sedimentation and growth of a wedge remains poorly understood, including (1) how deformation of the wedge modifies the basin stratigraphy and (2) how syntectonic sedimentation influences deformation of the wedge. In this study, we conducted scaled analogue sandbox experiments to reproduce accretionary wedges with and without syntectonic sedimentation. The results show that basin stratigraphy varied with the growth pattern of the accretionary wedge. In the case that wedge growth was dominated by trenchward accretion, the depositional area migrated landward. In contrast, prolonged underthrusting caused the sediment layers to be tilted landward and the depocenter to migrate landward. The occurrence of two types of basin stratigraphy (i.e., trenchward and landward migration of the depocenter) reflects a contrast in strength of the basal shear resistance between the inner and outer parts of the wedge due to sedimentation on the wedge. A change in the magnitude of normal stress acting on the wedge base likely influenced the mode of deformation of the wedge. A phase dominated by underthrusting can result in the combining a retro-wedge basin with a wedge-top basin, and yield a wide area of accommodation space in the forearc basin. These results suggest that forearc basin stratigraphy is influenced by the growth pattern of an accretionary wedge that is affected by syntectonic sedimentation.