The Pacific plate converges with northeastern Eurasia at a rate of 8-9 m per century along the Kamchatka, Kuril and Japan trenches. Along the southern Kuril trench, which faces the Japanese island of Hokkaido, this fast subduction has recurrently generated earthquakes with magnitudes of up to approximately 8 over the past two centuries. These historical events, on rupture segments 100-200 km long, have been considered characteristic of Hokkaido's plate-boundary earthquakes. But here we use deposits of prehistoric tsunamis to infer the infrequent occurrence of larger earthquakes generated from longer ruptures. Many of these tsunami deposits form sheets of sand that extend kilometres inland from the deposits of historical tsunamis. Stratigraphic series of extensive sand sheets, intercalated with dated volcanic-ash layers, show that such unusually large tsunamis occurred about every 500 years on average over the past 2,000-7,000 years, most recently approximately 350 years ago. Numerical simulations of these tsunamis are best explained by earthquakes that individually rupture multiple segments along the southern Kuril trench. We infer that such multi-segment earthquakes persistently recur among a larger number of single-segment events.

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Comprehensive geological surveys have revealed the physiographical and sedimentological characteristics of the Kushiro Submarine Canyon, one of the largest submarine canyons around Japan. The canyon indents the outer shelf along a generally straight, deeply excavated course of more than 230 km in length upon the active forearc slope of the Kuril Trench in the Northwest Pacifi c. The forearc slope has a convex-upward geometry that can be divided into upper and lower parts separated by an outer-arc high (3200-3500 m water depth). The upper slope consists of gently folded forearc sediments, and the lower slope is underlain by sedimentary rocks deformed by subduction-related processes. The upper reaches of the canyon (~3250 m of thalweg water depth) are developed on the upper slope, showing a weakly concave-upward longitudinal profi le with a gradual down-canyon increase in relief between the thalweg and the canyon rim. Although an infi ll of hemipelagic mud and the absence of turbidite deposits indicates that the upper part of the upper reaches of the canyon (~900 m thalweg water depth) is inactive, the lower part of the upper reaches (900-3250 m thalweg water depth) is considered to be an active conduit to the lower reaches, as determined from voluminous turbidites recovered in sediment cores (~76-yr intervals) and rockfalls observed in the canyon bottom by deep-sea camera. A number of gullies developed upon the northern slope of the lower part of the upper reaches might well provide a frequent supply of turbidity currents, giving rise to a down-canyon increase in the frequency of fl ow events. The down-canyon increase in fl ow occurrence is related to a gradual decrease in gradient, demonstrating an inverse power-law relationship between slope and drainage area. In contrast, the lower reaches of the canyon (3250-7000 m thalweg water depth) are characterized by a gradual decrease in relief, a high gradient, and extremely low sinuosity. The limited increase in drainage area down-canyon of the confl uence with the Hiroo Submarine Channel, which is the largest tributary of the main canyon, indicates that the erosional force of turbidity currents decreases down-canyon. The gradient of the lower reaches largely refl ects the morphology of the forearc slope along the canyon, which has been deformed by subduction-related tectonics. The lack of an inverse power-law relationship between gradient and drainage area in the lower canyon supports the hypothesis that the topography of the lower reaches is dominated by subduction-related tectonic deformation of the substrate rather than canyon erosion. Interrelationships between canyon erosion by currents and tectonic processes along the forearc slope are important in the development of the physiography of submarine canyons upon active forearc margins.