[M-GI32] 地球掘削科学

[MGI32-P03] 室戸岬沖南海トラフのスロー地震震源域近傍に拡がるパッチ状高圧間隙水帯

*廣瀬 丈洋¹、濱田 洋平¹、谷川 亘¹、神谷 奈々²、山本 由弦³、辻 健⁴、木下 正高⁵、Heuer Verena ⁶、稲垣 史生¹、諸野 祐樹¹、久保 雄介¹ (1.国立研究開発法人海洋研究開発機構、2.京都大学大学院 工学研究科、3.神戸大学大学院 理学研究科、4.九州大学 工学研究院、5.東京大学 地震研究所、6.ブレーメン大学)

キーワード：間隙水圧、南海トラフ、スロー地震

Pore fluid pressure plays an important role in the generation of earthquakes in subduction zones. However, quantitative constraints for its determination are quite limited. Here, we estimate the subsurface pore pressure by analyzing the transient upwelling flow of drilling mud from Hole C0023A of the International Ocean Discovery Program (IODP) Expedition 370, in the Nankai Trough off Cape Muroto. This upward flow provided the first direct evidence of an overpressured aquifer in the underthrust sediments off Cape Muroto. To estimate the pre-drilling pore pressure in the overpressured aquifer around a depth of 950–1050 meters below sea floor, we examined the measured porosities of core samples retrieved from nearby ODP wells (Holes 1174 & 808); we then proceeded to explain the observed time evolution of the flow rate of the upwelling flow by modeling various sized aquifers through solving a radial diffusion equation. It was observed that for a permeability of 10^–13 m², the aquifer possessed an initial excess pore pressure of ~5 to 10 MPa above the hydrostatic pressure, with a lateral dimension of several hundred meters and thickness of several tens of meters. The overpressure estimates from the porosity-depth profile at Site C0023 differ from those at Holes 1174 and 808, suggesting the possible existence of multiple overpressured aquifers with a patchy distribution in the underthrust sediments of the Nankai Trough. As pore pressure is relevant in governing fault stability, the overpressured aquifers may be the source of slow earthquakes that have been observed around the drilling site.