5:15 PM - 6:30 PM
[SSS10-P13] Stress field deduced from the deformation of the submarine and marine terraces in the middle part of Nanseishoto islands
Keywords:topographic anaglyph, tectonic geomorphology, submarine terrace, Iejima island, DEM
This paper aims to re-examine tectonic geomorphology and to present new tectonic maps for the late Quaternary in and around the Iejima island west off the coast of Motobu peninsula, the northern part of Okinawa island, based on the interpretation of the topographic anaglyph derived from the 6-m mesh digital surface model (DSM) of onshore as well as the 1.44-s to 2-s-mesh DEM of the seafloor. The DSM was produced from vintage aerial photographs. The DEM of the seafloor is processed from cloud point data of multi-beam echo-sounding devices provided by the Japan Coast guard as well as the Japan Agency for Marine-Earth Science and Technology.
As a result, four marine terraces were identified on the Iejima island. The paleo-shoreline heights on each emerged coral reef in the northern part of the island are higher than those in the south. The northeastern part of Iejima island is estimated to tilt to the south during the Late Quaternary. We also identified three submarine terrace surfaces around the Iejima island. However, it is hard to identify the deformation on these surfaces' paleo-shoreline depth. The tilting ratio of marine terraces on Iejima island is calculated to approximately 5-10‰, using the measured paleo-shoreline height of the marine terraces. The tilting rate is determined to be approximately 4-5×10^-5/kyr, assuming that these marine terraces were formed during MIS5 and MIS7 (Koike and Machida eds, 2001). It is considerably slower than the tilting of submarine terraces around the Iheyajima island with the rates of 1×10^-4/kyr, which would be caused by the nearby NE-SW trending thrust fault. In addition, the width of the tilting around Iejima island is narrower than those around the Iheyajima island, suggesting that it would be caused by the WNW-ESE trending normal faults distributed north off the coast of the island.
The geodetic studies (Tadokoro et al., 2016) and geophysical investigations (Arai et al., 2018) suggest that this area is under the tensional stress field related to the Okinawa Trough. However, these deformations around Iejima and Iheyajima islands can be explained if the horizontal compressive stress axis with the time scale of Late Quaternary is perpendicular to the island shelfs oriented NW-SE. Moreover, these results show that submarine and marine terraces can preserve valuable information for understanding the Late Quaternary tectonic deformation, especially in isolated, small islands.
As a result, four marine terraces were identified on the Iejima island. The paleo-shoreline heights on each emerged coral reef in the northern part of the island are higher than those in the south. The northeastern part of Iejima island is estimated to tilt to the south during the Late Quaternary. We also identified three submarine terrace surfaces around the Iejima island. However, it is hard to identify the deformation on these surfaces' paleo-shoreline depth. The tilting ratio of marine terraces on Iejima island is calculated to approximately 5-10‰, using the measured paleo-shoreline height of the marine terraces. The tilting rate is determined to be approximately 4-5×10^-5/kyr, assuming that these marine terraces were formed during MIS5 and MIS7 (Koike and Machida eds, 2001). It is considerably slower than the tilting of submarine terraces around the Iheyajima island with the rates of 1×10^-4/kyr, which would be caused by the nearby NE-SW trending thrust fault. In addition, the width of the tilting around Iejima island is narrower than those around the Iheyajima island, suggesting that it would be caused by the WNW-ESE trending normal faults distributed north off the coast of the island.
The geodetic studies (Tadokoro et al., 2016) and geophysical investigations (Arai et al., 2018) suggest that this area is under the tensional stress field related to the Okinawa Trough. However, these deformations around Iejima and Iheyajima islands can be explained if the horizontal compressive stress axis with the time scale of Late Quaternary is perpendicular to the island shelfs oriented NW-SE. Moreover, these results show that submarine and marine terraces can preserve valuable information for understanding the Late Quaternary tectonic deformation, especially in isolated, small islands.