地震性地殻変動と大規模ラハールによって規制された開析谷埋積シークエンス; 南海トラフ沿岸,宿毛臨海低地において採取されたSKMコアの解析例

Depositional sequence of the Post-LGM incised-valley fill controlled by seismic crustal deformation and large-scale lahars; An Example of SKM core obtained from the Sukumo coastal lowland along the Nankai Trough, Japan

七山 太*; 山口 龍彦*; 中西 利典; 辻 智大*; 池田 倫治*; 近藤 康生*; 三輪 美智子*; 杉山 真二*; 木村 一成*

Nanayama, Futoshi*; Yamaguchi, Tatsuhiko*; Nakanishi, Toshimichi; Tsuji, Tomohiro*; Ikeda, Michiharu*; Kondo, Yasuo*; Miwa, Michiko*; Sugiyama, Shinji*; Kimura, Kazunari*

南海トラフ巨大地震によって沈降が予測されている宿毛臨海低地において沖積コアを採取し、LGMの開析谷を埋積する沖積層の特徴と堆積シークエンスの検討を実施した。松田川開析谷はLGMに形成され、その後の後氷期海進により、9.8kaに標高-30mに海水が到達し、エスチュアリー環境へと変化した。その後も海水準は上昇し続けて内湾泥底環境となり、7.5kaに最高水深時となった。7.3kaに起こった南九州の鬼界カルデラ噴火により、給源に近い宿毛湾周辺においてもK-Ah火山灰が厚く降灰し、その直後に大規模なラハールが発生した。その直後から水中二次堆積物が急激に堆積した。7.0ka以降にデルタの成長が他の地域に先行して活発化したが、これは大規模なK-Ah火山灰の影響と考えられる。SKMコアから得られた過去1万年間の海面変動情報に基づくならば、宿毛湾地域は南海トラフ巨大地震によって一時的に地盤沈下するものの、長期的に見るとそれらの沈降量は相殺されると理解される。

The characteristics of the post-LGM incised valley fills and the depositional sequence were examined the SKM core collected in the Sukumo coastal lowland, where is expected to huge seismic subsidence due to the Nankai Trough great earthquakes. Our sedimentological, radiocarbon dating and paleoenvironmental results are as bellows. Sediments of the SKM core clearly show a succession influenced by post-glacial sea level change. The Matsuda River incised valley was formed in LGM and filled by fluvial sand and gravels in late Pleistocene. After the postglacial transgression, sea level reached -30 m (a.s.l.) at 9.8 ka and the incised valley changed to an estuary environment. The sea level continued to rise and it became an inner bay mud bottom environment, and reaching a maximum water depth was at 7.5 ka. The 7.3 ka Kikai caldera eruption in southern Kyushu caused heavy K-Ah ash fall in southwestern Shikoku, and then large-scale lahars frequently occurred immediately after the ash fall because of the vicinity of volcanic source. After ash fall, the K-Ah secondary sediments rapidly deposited on the inner bay environment and caused forced regression. After 7.0 ka, the growth of the delta became active ahead of the other regions, which may be due to the large K-Ah ash fall. At 5 ka, the sea level reached + 2.5 m (a.s.l.) estimated by the Sukumo midden and this altitude is recognized as the Holocene marine limit in this area. The information on relative sea level change during the past 10000 years has revealed that the Sukumo Bay area has not subsided due to seismic crustal deformation.