[M-GI32] Drilling Earth Science

[MGI32-P03] High Fluid-Pressure Patches Spreading around Source Region of Slow Earthquakes in the Nankai Trough off Cape Muroto

*Takehiro Hirose¹, Yohei Hamada¹, Wataru Tanikawa¹, Nana Kamiya², Yuzuru Yamamoto³, Takeshi Tsuji⁴, Masataka Kinoshita⁵, Verena Heuer⁶, Fumio Inagaki¹, Yuki Morono¹, Yusuke Kubo¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Graduate School of Engineering, Kyoto University, 3.Graduate School of Science, Kobe University, 4.Graduate School of Engineering, Kyusyu University, 5.Earthquake Research Institute, The University of Tokyo, 6.University of Bremen)

Keywords:Pore fluid pressure, Nankai Trough, slow earthquake

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.