Digital terrain analysis of sea-land combined data on the Outer Zone of Southwest Japan including the Nankai Trough

Abstract

We analyzed drainage networks, profiles, and terrain types of Southwest Japan using a 490-m DEM which was a mosaic of land elevation by the Geospatial Information Authority and seafloor elevation by the Japan Coast Guard. Moreover, we visualized land topography and other data, i. e., seismic tomography data, distribution of seismic intensity, and plate boundary data that were published on the Internet. The visualization shows inhomogeneous structure of velocity below Japanese islands and a clear hot section leading out from deep underground to Osaka Bay. Land and shelf edges are quantitatively similar to each other in topography. However, there are almost no steep slopes with high valley density on the seafloor in contrast to land surfaces. Instead of high valley density slopes, long large steep slopes as in Holocene volcanos, such as Mt. Fuji, are widely distributed on the deeper seafloor. In addition, steeper parts of long large slopes are distributed around active faults in land, inner trench areas and outer ridges on the seafloor, in addition to tops of volcanos.