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June 17, 2021
Kyoto University
Kobe University
Kyusyu University
The University of Tokyo

High Fluid-Pressure Patches around the Source Regions of Slow Earthquakes along the Nankai Trough
–A step forward to understand the generation mechanisms of slow earthquakes–

1. Key points

Scientific ocean drilling had successfully penetrated around the source regions of slow earthquakes in the Nankai Trough off Cape Muroto.
A high-pressure aquifer with a lateral extent of hundreds of meters was first confirmed along the shallow plate boundary, which was adjacent to the areas where slow earthquakes have occurred.
Slow earthquakes may be caused by relatively small, scattered, high-pressurized subsurface aquifers, such as the one discovered by us.

2. Overview

A high-pressurized subsurface aquifer was confirmed around the source regions of slow earthquakes in the Nankai Trough by a research team led by Dr. Takehiro Hirose of the Japan Agency for Marine-Earth Science and Technology.

Fault zones hinder fluid flow, thus, forming aquifers, with high-pressurized fluid, in the vicinity of the fault. Pore fluid pressure plays a key role in the generation of subduction zone earthquakes; however, the pore fluid pressures around the subduction zone faults has not been quantified extensively until now. Through the International Ocean Discovery Program (IODP) Expedition 370, the Nankai Trough subduction zone off Cape Muroto (Japan), where slow earthquakes have been observed, was drilled (Fig. 1). During drilling, we observed a transient upwelling flow of drilling mud from the borehole (Fig. 2), thus, providing the first direct evidence of an overpressured aquifer around the plate boundary fault. To understand the aquifer properties, we modeled aquifers with varying properties; subsequently, we found that the upwelling mud at the drill site was probably generated by an aquifer with a lateral extent of hundreds of meters and with pore pressures considerably higher than the typical (hydrostatic) pore fluid pressure expected at that depth. Our results suggested that slow earthquakes occurring near Cape Muroto may be caused by relatively small, scattered, high pressurized subsurface aquifers, such as the one discovered during Expedition 370 (Fig. 3).

The results were published in the Journal of Geophysical Research; Solid Earth on June 17, 2021 (Japan time). This study was partially supported by the JSPS KAKENHI Grant Numbers 19H02006 and JP16H06476, and by DFG EXC-2077 390741603.

High Fluid-Pressure Patches beneath the Décollement: A Potential Source 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. B. Heuer6, Fumio Inagaki1, Yuki Morono1, and Yusuke Kubo1

URL: DOI: 10.1029/2021JB021831
1Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
2Graduate School of Engineering, Kyoto University
3Graduate School of Science, Kobe University
4Graduate School of Engineering, Kyusyu University
5Earthquake Research Institute, The University of Tokyo
6Center for Marine Environmental Sciences, University of Bremen


Figure 1. Location of the drilling site (C0023). Grey dots and green-white balls represent the epicenter locations of extremely low frequency earthquakes observed during 2003–2018 (Asano et al., 2008; Nakano et al., 2018; Takemura et al., 2019). The pink shaded rectangle represents the estimated area of the 2017–2018 slow slip event (Yokota and Ishikawa, 2020).


Figure 2. Seafloor image of the upwelling mud flow from the well head at Site C0023 on November 1, 2016 (water depth: 4,476 m). Also see movie from the link.


Figure 3. Conceptual illustration showing a positional relationship between the high-pressurized aquifers and seismogenic zones along the shallow plate boundary at the Nankai Trough.


(For this study)
Takehiro Hirose, Principal Researcher, Kochi Institute for Core Sample Research, JAMSTEC.
(For press release)
Public Relations Section, Marine Science and Technology Strategy Department, JAMSTEC.
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