14:30 〜 14:45
[SCG48-14] 最終氷期以降における大阪湾の音響層序と地殻変動
キーワード：大阪湾、活断層、サブボトムプロファイラー
Osaka Bay is a closed sea area surrounded by several significant active fault zones, including the Uemachi Fault Zone, the Rokko-Awajishima Fault Zone, and the Median Tectonic Line Fault Zone. Notably, seismic events such as the 1995 Hyogo-ken Nanbu earthquake (Mj7.3) in the Rokko-Awajishima Fault Zone, a Mj6.3 earthquake on Awaji Island in 2013, and a Mj5.5 earthquake in northern Osaka Prefecture in 2018 have occurred in this seismically active area. Previous seismic reflection surveys have revealed the presence of the Osaka Bay Fault Zone, a reverse fault extending approximately 39 km in a northeast-southwest direction in western Osaka Bay, and several dozen other faults and bending structures in shallow sediments. The study of Nanayama et al. (2000) reported two events in the northern part of Osakabay Faults based on high-frequency acoustic surveys and drilling cores. The Alluvium in this study has been affected by crustal deformation, erosion, and sedimentation since the last glacial period. It is thus important to understand changes in sedimentation patterns and tectonic transitions in Osaka Bay since the beginning of the inflow of seawater into the bay. Additionally, given the potential for seafloor faults to trigger tsunamis, this information is valuable for assessing disaster risks in populous areas near Osaka Bay.
The objectives of this study involve creating areal thickness distributions of the Alluvium and each sedimentary unit in Osaka Bay, influenced by faulting activities since the last glacial period. The investigation aims to identify faults that cause bending within the Alluvium sediments, determine their distribution, and decipher their activity history. Data collected in 2022 and 2023, utilizing the Kongsberg sub-bottom profiler TOPAS PS18 aboard the Kobe University training ship Kaijin Maru, covers almost all of Osaka Bay in a grid pattern. The vertical resolution of TOPAS PS18 is smaller than 0.15 m in maximum, allowing for high-resolution data. High-quality data was obtained for the Alluvium at approximately 30 m below the seafloor, facilitating the identification of thin layers.
In the data analysis, the Alluvium above the Pleistocene was acoustically divided into three sedimentary units in total area and four sedimentary units in both the Northern and Southern parts of Osaka Bay, respectively. By delineating each unit's seafloor surface and boundary surface and calculating the depth difference, each sedimentary unit's thickness distribution on an areal scale was determined. These results, combined with information from previous drilling, were used to constrain the age of the sedimentary units. Identified faults on cross sections were mapped, revealing a bending structure near the Tsuna-oki Fault and the northernmost part of Osaka Bay Faults, suggesting recent activity that requires further investigation. The presentation aims to show the areal thickness distribution of sedimentary units and discuss the reconstruction of the past seafloor surface and sedimentary environment in Osaka Bay, examining their relationship to faulting activities.
The objectives of this study involve creating areal thickness distributions of the Alluvium and each sedimentary unit in Osaka Bay, influenced by faulting activities since the last glacial period. The investigation aims to identify faults that cause bending within the Alluvium sediments, determine their distribution, and decipher their activity history. Data collected in 2022 and 2023, utilizing the Kongsberg sub-bottom profiler TOPAS PS18 aboard the Kobe University training ship Kaijin Maru, covers almost all of Osaka Bay in a grid pattern. The vertical resolution of TOPAS PS18 is smaller than 0.15 m in maximum, allowing for high-resolution data. High-quality data was obtained for the Alluvium at approximately 30 m below the seafloor, facilitating the identification of thin layers.
In the data analysis, the Alluvium above the Pleistocene was acoustically divided into three sedimentary units in total area and four sedimentary units in both the Northern and Southern parts of Osaka Bay, respectively. By delineating each unit's seafloor surface and boundary surface and calculating the depth difference, each sedimentary unit's thickness distribution on an areal scale was determined. These results, combined with information from previous drilling, were used to constrain the age of the sedimentary units. Identified faults on cross sections were mapped, revealing a bending structure near the Tsuna-oki Fault and the northernmost part of Osaka Bay Faults, suggesting recent activity that requires further investigation. The presentation aims to show the areal thickness distribution of sedimentary units and discuss the reconstruction of the past seafloor surface and sedimentary environment in Osaka Bay, examining their relationship to faulting activities.