In situ stress within the Nankai accretionary prism determined from borehole breakouts

Quantifying the orientations and magnitudes of stress at tectonically active margins and along major fault systems is integral to understanding the mechanics of faulting and earthquakes. Here we use data collected as part of the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment to constrain in-situ stress magnitudes within the Nankai accretionary prism from wellbore breakouts identified in logging while drilling (LWD) resistivity images. During drilling, concentration of stresses around the borehole wall cause breakouts to form, with widths that depend on both the rock strength and far field stress state. Using estimates of rock strength derived from p-wave velocity together with measured breakout widths, we place bounds on the far field tectonic stresses. We focus on two regions: (1) Sites C0006 and C0024, located within a few km of the trench and penetrating the plate boundary décollement; and (2) Sites C0004 and C0010, ~25km landward of the trench and spanning a major out of sequence thrust fault (termed the megasplay). The vertical stress (Sv) is defined by density logs, and the minimum (Shmin) and maximum (SHmax) horizontal stresses are constrained by our analysis of the breakouts.

We find that the stress state along the megasplay fault lies in a thrusting regime (SHmax > Shmin > Sv), whereas at the toe of the prism stress state is near-isotropic, with small differential stresses. Stress state remains consistent from the hanging wall to the footwall, with the exception of Site C0010 where SHmax decreases across the fault. For our best estimates of rock strength, the resolved shear stress (𝜏) on the décollement is 0.2 - 0.4 MPa and the effective normal stress (σn') is 7 - 8 MPa. In order for sliding to occur along the décollement, assuming a friction coefficient (μ) of 0.2, either (1) SHmax would need to increase by > 7 MPa, (2) the rock strength would need to be significantly higher than estimated, or (3) there would need to be elevated pore pressure localized along the fault. In contrast, for the megasplay, 𝜏 ≅ 1.15 MPa and σn' ≅ 2.6 - 3.6 MPa. These stresses are consistent with slip on the megasplay in the current stress state, for reported frictional strength of the fault zone material (0.36 < μ < 0.46), with the implication that the fault is near failure.