Subduction of a large seamount into the landward slope of the Japan Trench

Abstract

Daiiti‐Kasima Seamount is a flat‐topped and odd shaped seamount of 3,000–4,000 meters in relative height, rising in the axis of the Japan Trench, situated 220 kilometers east of Tokyo. Survey of this seamount had been conducted many times since 1934. Mogi and Nishizawa (1980) had provided a hypothesis that breakdown had taken place, and the seamount was divided into two parts by a large NNE‐SSW fault, while the western half of the seamount had subsided into the Japan Trench.

The Hydrographic Department of Japan carried out detailed surveys of the seamount in 1983 and 1985 with a narrow multi‐beam echo sounder (Sea Beam), a multi‐channel seismic profiler, and other geophysical instruments.

The seismic reflection profile across the Daiiti‐Kasima Seamount clearly shows its western flank of the western half of the seamount, under the beds of the landward slope of the trench. The steeply dipping NNE‐SSW trending slope of 1,600 m in relative height, which divides the seamount into eastern and western halves, is confirmed as a fault scarp from its linear topographic features. The well stratified feature of the multi‐channel seismic profile of the flat top of both the eastern and western halves of the seamount show similar reflective characteristics with each other.

After a three‐dimensional topographic correction was applied to the free‐air gravity anomaly of the seamount and the adjacent area, Bouguer gravity anomaly distribution across the seamount was studied. The result does not show any apparent negative Bouguer gravity anomaly around the seamount. This fact indicates that the seamount is not isostatic, or the seamount is an excess mass on the Pacific Plate, so that it can easily subside affected by tensional stress field of the Japan Trench.

These results confirm the subduction of Daiiti‐Kasima Seamount into the Japan Trench.