In situ stress magnitudes at the toe of the Nankai Trough accretionary prism, offshore Shikoku Island, Japan

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doi: 10.1002/2015JB012415
Author(s): Huffman, K. A.; Saffer, D. M.
Author Affiliation(s): Primary:
Pennsylvania State University, Department of Geosciences, University Park, PA, United States
Volume Title: Journal of Geophysical Research: Solid Earth
Source: Journal of Geophysical Research: Solid Earth, 121(2), p.1202-1217. Publisher: Wiley-Blackwell for American Geophysical Union, Washington, DC, United States. ISSN: 2169-9313
Note: In English. 85 refs.; illus., incl. sect., strat. col., sketch map
Summary: Quantifying the orientation and magnitude of tectonic stresses is essential toward understanding deformation and faulting in subduction zones. However, constraints on in situ horizontal stress magnitudes (Shmin and SHmax) are rare. We estimate Shmin and SHmax at Ocean Drilling Program Site 808 at the toe of the Nankai accretionary prism offshore Japan, using coupled constraints from (1) the width of wellbore breakouts together with estimates of rock strength and a model describing stress redistribution at the borehole wall and (2) limits on regional differential stress defined by failure on preexisting faults. Our analysis extends from 175 to 915 m below seafloor (mbsf) and spans the active frontal thrust. For an upper bound on rock unconfined compressive strength (UCS) and assuming hydrostatic formation pore pressure, Shmin and SHmax (referenced to the seafloor) increase from 6.5 MPa at 175 mbsf to 17.4 MPa at 915 mbsf, with the stress state gradually transitioning from a thrust or strike-slip faulting regime above 800 mbsf to a normal faulting regime below. For cases with higher formation pore pressure, horizontal stresses are slightly lower but follow a similar pattern. We show that estimated Shmin and SHmax are strongly dependent on UCS, breakout width, and friction coefficient, all of which are characterized by uncertainty. Our results suggest that the prism is near thrust failure in the upper ∼300 mbsf, but far from failure below. This may be reconciled with active thrusting if thrust faults are locally weaker than the surrounding rock or if SHmax fluctuates during the seismic cycle. Abstract Copyright (2016), . American Geophysical Union. All Rights Reserved.
Year of Publication: 2016
Research Program: ODP Ocean Drilling Program
Key Words: 16 Structural Geology; Accretionary wedges; Body waves; Borehole breakouts; Elastic waves; Equations; Failures; Faults; Geophysical methods; Geophysical profiles; Geophysical surveys; Hydrostatic pressure; In situ; Leg 131; Leg 196; Nankai Trough; Normal faults; North Pacific; Northwest Pacific; ODP Site 808; Ocean Drilling Program; Orientation; Overpressure; P-waves; Pacific Ocean; Pore pressure; Pressure; Quantitative analysis; Seismic methods; Seismic profiles; Seismic waves; Strength; Stress; Surveys; Velocity; West Pacific
Coordinates: N322105 N322111 E1345646 E1345634
Record ID: 2016098674
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from John Wiley & Sons, Chichester, United Kingdom