Kenji Nemoto

Morphology and Structure of the Seafloor in the Northern Part of the North Fiji Basin

A geophysical survey of the north central North Fiji Basin provides a high density data base for accurately charting seafloor morphology and defining major structural trends. The resulting bathymetric chart is believed to reliably portray the tectonic fabric of the basin and provide insight into the formation of the seafloor and development of associated structural features. The South Pandora Ridge (Hazel Holme Fracture Zone) appears to be a young or recently formed spreading ridge that bifurcates at its eastern end into the ENE-trending Rotuma Ridge and ESE-trending Tripartite Ridge. Southwest of South Pandora Ridge lies Pentecost Basin, interpreted to be originally formed along the western limb of the extinct central North Fiji Basin (CNFB) triple junction and now inundated, at its southwestern end, by sediment of the New Hebrides archipelagic apron. Southeast of the ridge lies Balmoral Basin, interpreted to be originally formed along the eastern limb of the old CNFB triple junction. Between the two basins lies a wedge of sediment-free ridge and trough terrain believed to be formed by a propagating rift. To the east, in the southern part of Balmoral Basin, Balmoral Ridge is interpreted to be the upraised leading edge of an overthrust block of North Fiji Basin crust, to the south of which lies the eastern end of the Fiji Fracture Zone, a presently active transform fault.

Evidence for erosion and deposition by the 2011 Tohoku-oki tsunami on the nearshore shelf of Sendai Bay, Japan

Ongoing geological research into processes operating on the nearshore continental shelf and beyond is vital to our understanding of modern tsunami-generated sediment transport and deposition. This paper investigates the southern part of Sendai Bay, Japan, by means of high-resolution seismic surveys, vibracoring, bathymetric data assimilation, and radioisotope analysis of a core. For the first time, it was possible to identify an erosional surface in the shallow subsurface, formed by both seafloor erosion and associated offshore-directed sediment transport caused by the 2011 Tohoku-oki tsunami. The area of erosion and deposition extends at least 1,100 m offshore from the shoreline down to water depths of 16.7 m. The tsunami-generated sedimentological signature reaches up to 1.2 m below the present seafloor, whereas bathymetric changes due to storm-related reworking over a period of 3 years following the tsunami event have been limited to the upper ~0.3 m, despite the fact that the study area is located on an open shelf facing the Pacific Ocean. Tsunami-generated erosion surfaces may thus be preserved for extended periods of time, and may even enter the rock record, because the depth of tsunami erosion can exceed the depth of storm erosion. This finding is also important for interpretation of modern submarine strata, since erosion surfaces in shallow (depths less than ~1 m) seismic records from open coast shelves have generally been interpreted as storm-generated surfaces or transgressive ravinement surfaces.

Distribution of Surface Deposits and Depositional Process off Shimizu Coast, Shizuoka Prefecuture

Coastal erosion has been one of the serious environmental problems in Japan. Numerous studies on sediment transport related to the coastal erosion for preservation of the coastal environment, have been performed on the basis of civil engineering research. To clarify modem sediment transportation related to the coastal erosion at the near shore zone, is very important for preservation of the coast at present and the future. In this study, we researched the surface deposits and sedimentary structures to discuss the process of sediment transportation off Shimizu coast, Shizuoka, Japan.