石垣島とその周辺海域を統合した変動地形の研究 : 沿岸域の変動地形研究の発展に向けた海底地形情報整備の必要性
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Deformation of Marine Terraces and the Seafloor on and around Ishigaki Island Based on the Analysis of a Digital Elevation Model
digital elevation model (DEM)
Topographic anaglyph images are viewed using red-cyan glasses, which enables us to identify topographic relief features easily. In general, anaglyphs produced from the digital elevation model (DEM) are an effective means of identifying the tectonic geomorphology of both inland and seafloor. Few studies of tectonic geomorphology along coastal areas have been conducted because detailed topographical maps that combine data from inland and seafloor measurements have been lacking.
This study aims to re-examine the tectonic geomorphology of the late Quaternary and present information related to the crustal deformation of the marine terraces and the seafloor on and around Ishigaki Island. This island is located in the southern part of the Nanseishoto Islands in Southwest Japan. Boulders transported by the giant tsunami in 1771 have remained on the coral reefs and marine terraces of this island. However, the source fault of the tsunami is unidentified.
In this study, we interpret the topographic anaglyph images produced from 5-m- and 10-m-mesh inland DEMs of Japan provided by the Geospatial Information Authority of Japan. In addition, we examine a 2-s-mesh DEM of the seafloor obtained from cloud point data of multi-beam echo-sounding devices provided by the Japan Agency for Marine-Earth Science and Technology as well as a 2.8-s-mesh DEM re-processed from digital bathymetric charts using 1-m- to 2-m-interval counters from the Japan Hydrographic Association.
The marine terraces in the southern part of Ishigaki Island were divided into three levels using the topographic anaglyph images. The heights of the old shorelines of these marine terraces in the east are higher than those in the west, suggesting that crustal movement tilted westward during the late Pleistocene.
On the seafloor, 17-km-long northeast-trending steep and straight slopes are identified off the southeast coast of the island. The active reverse fault probably causes these scarps because small slopes were observed on the southeast dipping young slope distributed to the northeast extension of the steep slopes. Moreover, the active fault that cut the Ryukyu groups of the Pleistocene sediment just beneath the steep slope was depicted in some areas of the published geological map. This fault is named the Shiraho-Oki fault in this study and may be the source of the tilting to the west on the island.
The resolution and accuracy of the DEM for the territory of Japan vary considerably between the inland and the seafloor. Detailed digital elevation data of the seafloor acquired by multi-beam echo-sounding devices should be obtained and stored to understand the tectonic geomorphology along the coastal areas.