Geological Society of America | GEOLOGY | Volume 50 | Number 11 | www.gsapubs.org 1229 Manuscript received 28 August 2021 Revised manuscript received 7 June 2022 Manuscript accepted 15 June 2022 https://doi.org/10.1130/G49639.1 © 2022 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. CITATION: Tobin, H.J., et al., 2022, Direct constraints on in situ stress state from deep drilling into the Nankai subduction zone, Japan: Geology, v. 50, p. 1229–1233, https://doi.org/10.1130/G49639.1 Direct constraints on in situ stress state from deep drilling into the Nankai subduction zone, Japan Harold J. Tobin 1 , Demian M. Saffer 2 , David A. Castillo 3 and Takehiro Hirose 4 1 Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195, USA 2 Institute for Geophysics and Department of Geological Sciences, University of Texas at Austin, 10601 Exploration Way, Austin, Texas 78758, USA 3 Insight Geomechanics, 8 Joffre Road, Trigg, Western Australia 6029, Australia 4 Kochi Institute for Core Sample Research (X-star), Japan Agency for Marine-Earth Science and Technology, 200 Monobe-otsu Nankoku, Kochi 783-8502, Japan ABSTRACT Stress state is a long-sought but poorly known parameter on subduction megathrusts and in overlying accretionary wedges in general. We used direct observations made during drilling of Integrated Ocean Drilling Program (IODP) borehole C0002 to a depth of 3058 m below the seafoor (mbsf) in the Nankai subduction zone of southwestern Japan to constrain in situ pore pressure and stress state in the deep interior of an accretionary wedge for the frst time. These data included downhole pressure, active pumping tests, and logging and sample measurements. We found a nearly linear gradient in minimum horizontal principal stress (S hmin ) and show that it remained consistently smaller than the vertical stress (S v ), defnitively ruling out a thrust-faulting stress regime to at least 3 km depth, and to within ∼2 km above the subduction megathrust. At 3000 mbsf, the estimated effective stresses were: S v = 33 MPa, S Hmax = 25–36 MPa, and S hmin = 18.5–21 MPa. We therefore interpret that the stress state throughout the drilled interval, which lies entirely in the hanging wall of the active mega- thrust, lies in a normal or strike-slip faulting regime (S v ≥ S Hmax > S hmin ). Total differential stresses are below ∼18 MPa. We conclude that (1) basal traction along the megathrust must be small in order to permit both locking (and frictional sliding at failure) of the décollement and such low differential stresses deep within the upper plate; and (2) although differential stresses may remain low all the way to the plate boundary at ∼5000 mbsf, S Hmax must transi- tion to become greater than the vertical stress—either spatially below the base of the borehole or temporally leading up to megathrust fault rupture—in order to drive thrust motion along the plate boundary as observed in great earthquakes and in recurring very low-frequency earthquakes and slow-slip events. ULTRADEEP SCIENTIFIC DRILLING IN A SUBDUCTION ZONE The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a comprehensive investigation of subduction zone faulting and stress conditions (Tobin and Kinoshita, 2006; Tobin et al., 2019). NanTroSEIZE has combined seismic imaging with Integrated Ocean Drilling Program (IODP) drilling for direct sampling, in situ measurements, and long-term borehole monitoring to better understand the nature of the megathrust seismic cycle, fault locking, and the spectrum of fault slip. A transect of bore- holes (Fig. 1) was drilled on a series of IODP expeditions from 2007 through 2019 (Tobin et al., 2019). The centerpiece is ultradeep drill- ing at IODP Site C0002 (Fig. 1B), which was targeted to cross and sample a seismic refector interpreted as the main plate-boundary fault (the “megathrust”), which lies at ∼5000 m below the seafoor (mbsf) based on refection and refrac- tion depth imaging (Moore et al., 2007; Bangs et al., 2009; Kamei et al., 2012). The principal borehole (Fig. 2) at this site (Hole C0002F/N/P) was drilled by the riser drilling vessel Chikyu to 3058 mbsf, with steel casing cemented in place to a depth of 2922 mbsf (Tobin et al., 2015a, 2019; Strasser et al., 2014). One key objective at Site C0002 is to characterize the present-day state of stress in the inner accretionary wedge, which forms the upper plate of the primary plate-boundary fault (Fig. 1B). The orientations and absolute magni- tudes of the three principal stresses—and their temporal evolution—drive fault strength, slip, and earthquakes (e.g., Scholz, 1998; Brodsky et al., 2020) throughout the seismic cycle (e.g., Magee and Zoback, 1993; Wang and Hu, 2006). Decades of effort to measure stress and pore- fuid pressure conditions, either directly or indi- rectly, however, have met with limited success (summarized in Saffer and Tobin, 2011), and the quantitative state of stress at depth in any subduction setting is not known with confdence. Previous NanTroSEIZE work using borehole breakouts and induced tensile fracture orien- tations from resistivity log imaging has estab- lished the orientation of the principal stress axes but not their magnitudes (Chang et al., 2010; Lin et al., 2015). Unlike nearly all other scientifc ocean drill- ing, this hole was drilled with a riser system, using a closed loop of drilling mud designed to clean the hole and provide pressure support to the borehole, permitting a number of otherwise impossible stress- and pore pressure-related observations (e.g., Saffer et al., 2013). We ana- lyzed a suite of data sets collected during drilling that, taken together, provide quantitative con- straints on the in situ stress tensor, including all three principal stresses and pore-fuid pressure. ESTIMATES OF IN SITU STRESS STATE We determined the vertical stress (S v ) directly using bulk density measurements from core samples and borehole cuttings returns (Kitajima et al., 2017). Values of bulk density ranged from ∼1500 kg/m 3 near the seafoor to ∼2400 kg/m 3 Published online 5 September 2022 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/50/11/1229/5720636/g49639.1.pdf by guest on 29 October 2022