[S-CG58] Science of slow earthquakes: Toward unified understandings of whole earthquake process

[SCG58-P13] High Fluid Pressure Patch beneath the Décollement as a Potential Source of Slow Earthquakes at the Nankai Trough

*Takehiro Hirose¹, Yohei Hamada¹, Wataru Tanikawa¹, Nana Kamiya², Yuzuru Yamamoto³, Takeshi Tsuji⁴, Masataka Kinoshita⁵ (1.Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, 2.Graduate School of Engineering, Kyoto University , 3.Japan Agency for Marine-Earth Science and Technology, 4.Department of Earth Resources Engineering, Kyushu University, 5.Earthquake Research Institute, the University of Tokyo)

Keywords:fluid pressure, slow earthuake, Nankai Trough

Pore fluid pressure is important for understanding generation of both megathrust and slow earthquakes at subduction zones. However, its occurrence and quantitative constraints are quite limited. Here, we report the estimate of pore pressure by the analysis of transient upwelling flow from the borehole, that was observed while drilling the underthrust sediments in the Nankai Trough off Cape Muroto during IODP Expedition 370. In order to interpret the observed velocity and duration of the flow, we have solved a radial diffusion equation to estimate pore pressure before penetrating an aquifer. The calculation yields that the pore pressure exceeded ~3 MPa above hydrostatic and the size scale of the aquifer is several hundred meters, in case of an aquifer permeability of 10^-13 m². Our result suggests that the underthrust sequence is currently composed of patchily-distributed high-pressure aquifers.

In the neighborhood of the drilling site, very low frequency (VLF) earthquakes have been reported (e.g., Obara and Kato, 2016). Seismic survey has suggested a possible linkage between high pore pressure zone and the distribution and generation of slow earthquakes (e.g., Kodaira et al., 2004). Furthermore, high temperature and pressure friction experiments by Sawai et al. (2016) suggested that a transition from stable to unstable slip behavior appears with increasing pore fluid pressure that is a prerequisite for the generation of slow earthquakes. Our result implies that the slow earthquakes at off Cape Muroto can be attributed with slip behaviors along not only décollement but also the patchily distributed high-pore-pressure aquifers in the underthrust sediments.