Vol.6 No.3 2014
Research paper : Marine geological mapping project in the Okinawa area (K. ARAI et al.)−160−Synthesiology - English edition Vol.6 No.3 (2013) distribution of potential aggregate materials such as sand and gravel, as well as for identifying submarine hydrothermal deposits, as discussed below.3 Geology and structure of the active Ryukyu Arc and Okinawa Trough back-arc basinGeological information regarding the islands of the Ryukyu Arc was obtained during the Okinawa Project. Previous studies of the geological stratigraphy of the Ryukyu Arc have concentrated on field surveys of a small land area of the arc, and consequently much of its tectonic history remains unclear. However, marine geological data provided by the Okinawa Project are expected to provide a more detailed understanding.The Ryukyu Arc is a chain of islands that extends for approximately 1,200 km between Kyushu and Taiwan. It is an arc-trench system created as a result of subduction of the Philippine Sea Plate along the Ryukyu Trench. The Ryukyu Trench runs in a northeast-southwest direction, almost parallel to the Ryukyu Island Arc, and reaches a maximum depth of over 6,000 m. The orientation of the Ryukyu Arc shifts to east-west in the southern region (Fig. 5). In contrast to the Nankai Trough subduction zone offshore of Shikoku and Honshu, the fore-arc slope of the Ryukyu Trench is extremely limited, and there is almost no fore-arc basin, and no significant outer-arc ridge. This is possibly due to the difference in the subduction mechanism of the Philippine Sea Plate. The lack of the outer-arc ridge is more conspicuous on the slopes of the Ryukyu Trench north of Okinawa-jima Island than in the middle of the northern region. The Ryukyu Arc can be divided into three regions, based on differences in zonal geological structure: North Ryukyu, Central Ryukyu, and South Ryukyu. The borders of these regions are defined by Tokara Strait and Kerama Gap, a depression over 1,000 m deep that runs northwest-southeast. These structures were formed by normal faulting oriented perpendicular to the axis of the Ryukyu Arc. A number of active normal faults were identified east of Okinawa-jima Island from data collected during the GH08 cruise, with subsequent analysis demonstrating that they developed in shallow areas close to land in the Ryukyu Arc and its upper fore-arc slope. Given that activity on these faults may cause tsunamis, it is necessary to investigate their distribution and activity in more detail. Historical records from Okinawa reveal little evidence of an earthquake and tsunami that caused significant damage. However, the Yaeyama Earthquake and Tsunami of Meiwa 8 (1771) caused major damage in the Yaeyama and Miyako Islands. This emphasizes the importance of evaluating the geological hazards present in the Ryukyu Arc as part of the Okinawa Project.Fig. 3 Schematic diagram of the marine geological survey methods (from Reference )Bathymetric, sub-bottom profiling, seismic reflection profiling, gravity, and magnetic surveys were conducted for creation of the 1:200,000 marine geology maps. Sampling mainly included grab sampling, core sampling, and dredging.1:1,000,000 Geological Map1:200,000 Sedimentological Map1:200,000 Geological Map46121178111314151718246912162537343236384142444551575653545860626668697071727613510302022242627282933354039484743464931525055596163646567737475HydrophoneGravimeterGPS surveyGPS surveyGeophysical observationGeophysical observationSamplingSamplingSoundingSeafloor profilingGravity profilingSeismic reflection profilingSeismic reflection profilingSeismic refraction methodMagnetic profilingSeabed coring systemDredgeCorerCameraGrabFree fall samplerRadio sono-buoyGI gunAir gunSide scan sonarProton magnetometerAcoustic sounder3.5 kHz sub-bottom profilingFig. 2 Publication status of the marine geology maps for ocean regions around JapanThe areas within blue lines are those for which 1:1,000,000 marine geology maps have been published, and the red blocks are areas covered by published 1:200,000 marine geology maps (from AIST URL).