In the central part of Suruga Bay, the Suruga Trough, which developed in the north-south direction, exists. Senoumi-bank is located on the western side of the Suruga Trough and stepped steep slopes are observed on the eastern slope. On this steep slope, crushed sedimentary rocks are confirmed, and the 90% of the collected gravels have a broken shape. The presence of staircase topography and fractured gravel is thought to be caused by the activity of the thrust fault accompanied with the subduction of the Philippine Sea Plate beneath Eurasian Plate.

1 Introduction

Suruga Bay is located in the center of the Honshu arc, its east-west width near the mouth of the bay is 60 km wide, its length is over 80 km in the north-south direction and the depth of the bay exceeds the 2,500 m (Fig. 1 a).

In the central part of Suruga Bay there is a Suruga Trough that developed in a narrow width to the north and south. The Suruga Trough is located at the boundary between the Philippine Sea Plate and the Eurasia Plate, and in the past several large earthquakes such as the Meiou Earthquake, the Hoei Earthquake, and the Ansei Tokai Earthquake occurred. Senoumi-bank (SB) is located on the side of Shizuoka (Eurasia Plate) in the central part of Suruga Bay (Fig. 1b), and on the east side of it, steep slopes ranging from water depth of 50 m to water depth of 1800 m are developed. On this steep slope, Multi-stepped topography are developed, and the existence of a thrust faulting due to subduction is presumed.

Sandstone and conglomerate are collected from the eastern side slope of Senoumi-bank. Many of these conglomerate rocks are fractured and it is presumed that shear or splitting failure by fault movement occurred due to seismo-geological conditions. In this paper, we have discussed the origin of fractured gravel from the geomorphological, geological features and sediment descriptions around the Senoumi-bank.

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