Ryuta Furukawa

Unusually large earthquakes inferred from tsunami deposits along the Kuril trench

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 ∼8 over the past two centuries. These historical events, on rupture segments 100–200 km long, have been considered characteristic of Hokkaidos 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 ∼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.

Physiographical and sedimentological characteristics of submarine canyons developed upon an active forearc slope: The Kushiro Submarine Canyon, northern Japan

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 Pacific. 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 profile with a gradual down-canyon increase in relief between the thalweg and the canyon rim. Although an infill 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 flow events. The down-canyon increase in flow 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 confluence 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 reflects 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.

Geological Study of Unusual Tsunami Deposits in the Kuril Subduction Zone for Mitigation of Tsunami Disasters

On 26 December 2004 a magnitude M 9.3 earthquake deformed the ocean floor 160 km off the coast of Sumatra, generating the Indian Ocean tsunami and thus causing large sediment transfers due to tsunami run-up in coastal lowlands around the Indian Ocean (e.g., Goff et al., 2006; Moore et al., 2006; Hori et al., 2007; Hawkes et al., 2007; Choowong et al., 2007, 2010). Sediment transfers of this scale are rare events historically. Only when an unusual tsunami strikes coastal lowlands does a large-scale sediment transfer occur, leaving a sedimentary record, that is, tsunami deposits, in the geological strata on shore (Dawson & Stewart, 2007). In this chapter, we seek to understand the run-up process of past unusual tsunamis by examining a series of tsunami deposits on the Pacific coast of eastern Hokkaido, northern Japan, and we estimate the average recurrence interval of such tsunamis from the geological record. Large earthquakes with M > ~8 in the Kuril subduction zone have historically generated tsunamis that caused damage in eastern Hokkaido between Nemuro and the Tokachi coast (Satake et al., 2005; Fig. 1). Most recently, the 1952 Tokachi-oki, the 1960 Chilean, the 1973 Nemuro-oki, and the 2003 Tokachi-oki tsunamis caused considerable damage and great loss of life in this district. Therefore, it is very important to estimate the likely timing and size of the next large, earthquake-generated tsunami. Information about historical earthquakes in the Kuril subduction zone is limited, however, and no documents from before the 19th century that might refer to tsunami events are available. The earliest written records from eastern Hokkaido are the “Nikkanki” series of documents from Kokutai-ji Temple, which was built by the Edo government at Akkeshi in 1805 (Soeda et al., 2004; Fig. 1). In the hope of finding traces of past giant tsunamis to use to evaluate the frequency and extent of past tsunami inundation in east Hokkaido, late Holocene coastal sediments such as peat beds and lagoon sediments have been studied since 1998 by our research group and other researchers (e.g., Hirakawa et al., 2000; Nishimura et al., 2000; Sawai, 2002; Nanayama et al., 2003; Soeda et al., 2004). Nanayama et al. (2003, 2007) and Sawai et al. (2009) have reported the general stratigraphy of unusual tsunami deposits due to “500-year earthquake”

Volcanic Debris-Avalanche as a Cause of a Historic Tsunami: The A.D. 1640 Eruption of the Hokkaido-Komagatake Volcano, Northern Japan

Abstract: Sidescan sonar and sub-bottom reflection surveys off Shikabe town, along the Pacific coast, show the extent and volume of the submarine debris-avalanche deposit caused by the AD 1640 eruption of the Hokkaido-Komagatake volcano. The avalanche deposit extends from the subaerial part of the Shikabe lobe to as far as 20 km seaward. The subaqueous deposit, which reaches to 80 m of water depth, is 15 km wide and covers 126 km 2 of the seafloor. Hummocks of the subaqueous deposit decrease in height and width with distance from the source. The margin of the deposit lacks hummocks. The ratio of the collapse height to the traveled distance (H/L) is 0.06, suggesting that the debris avalanche of origin was more mobile than those of most subaerial debris-avalanche deposits. The subaqueous volume of the avalanche deposits, estimated by extrapolating the pre-eruptive topography of the surrounding area, ranges from 0.92 to 1.20 km 3 . INTRODUCTION A volcano located along a coast can cause a tsunami by its eruption or by the collapse of the volcanic edifice. Historically, volcanoes have caused 4.6% of tsunamis and 9.1% of tsunami-related deaths, which total about 41,000 people (Bryant 2001). Eruption-related tsunamis account for 20–25% of casualties associated with volcanic eruptions during the last 1000 years (Latter 1981). Some well-known eruption-related tsunamis are the 1400 BC eruption of Santorini, Greece (Kastens and Cita 1981), the AD 1883 Krakatau volcano, Indonesia (Latter 1981), and the 1883 eruption of Mount