Analysis of normal fault populations in the Kumano forearc basin, Nankai Trough, Japan: 2. Principal axes of stress and strain from inversion of fault orientations

Abstract

[1] We use a high-resolution 3-D seismic survey to map a population of recent normal faults within the Kumano Basin of the Nankai subduction zone, in order to quantify patterns of strain and stress state over the last 0.44 Myr. We identify distinct fault populations that define three phases of extension. Phases 1 and 2 comprise NW-SE striking faults located along the western basin edge and in the northwestern portion of the study area, respectively. The NE-SW striking faults of phase 3 comprise the largest population, and extend~20 km landward from the basin's seaward edge. Phase 2 faults typically terminate within a few reflectors of the seafloor, whereas most phase 3 faults form seafloor scarps. Inversion of the fault populations documents NE-SW extension during phases 1 and 2 and NW-SE extension during phase 3, consistent with both core-scale structures and horizontal stress orientations observed at Integrated Ocean Drilling Program (IODP) boreholes. Slip on phase 3 faults accommodates strain of up to~1-2%, concentrated near the basin's seaward edge. Inversion for a best-fit stress tensor yields a subvertical s 1 and subhorizontal s 2 and s 3 for all faulting phases. We find that during phase 3 in most portions of the basin, s 2 = s 3 (S Hmax = S hmin ), reflecting widely varying fault strikes. This contrasts with distinct S Hmax and S hmin magnitudes inferred from IODP borehole data; these observations may be reconciled if the orientation of maximum horizontal stress fluctuates due to variation of subduction parallel compression through the seismic cycle.

In situ stress magnitude and rock strength in the Nankai accretionary complex: a novel approach using paired constraints from downhole data in two wells

“…These previous analyses suggested a stress state that lies in the normal faulting regime within the Kumano basin and transitions to a strike-slip or reverse faulting regime in the accretionary prism. This interpretation is broadly consistent with observations of normal faults in the Kumano basin (Sacks et al 2013) and with major thrust splay faults within the prism (Moore et al 2009). However, the LOTs show that at least to ~2000 mbsf, S hmin is the least principal stress (i.e., S hmin is less than S v ), limiting stresses to a normal or strike-slip regime in this depth range both at Site C0002 (Strasser et al 2014;Tobin et al 2015) and at Site C0009 located ~20-km landward (Ito et al 2013;Saffer et al 2013).…”

“…We find that S Hmax ranges from 11.9 to 22.2 MPa at 900 mbsf to Our results suggest that the in situ stress state lies in a normal to strike-slip faulting regime. This is consistent with a range of independent observations, including widespread recent normal faulting in Kumano basin (e.g., Sacks et al 2013), and in situ wireline hydraulic fracturing measurements conducted while drilling ~20-km landward . Our results also indicate that empirically derived relations commonly used to estimate rock strength from P-wave velocity for shale may slightly overestimate UCS in mudstone and could lead to overestimates of far-field stresses determined from breakout widths.…”

“…In both cases, the leak-off pressures are less than the total vertical stress (S v ), indicating that S hmin is the minimum principal stress. This indicates that the stress state lies in either a normal or strike-slip faulting regime throughout the section, which is broadly consistent with observations of abundant normal faults in the Kumano basin in seismic reflection data (e.g., Sacks et al 2013) and with previous estimates of stress magnitudes from wellbore breakouts (Chang et al 2010). In the underlying accretionary prism, seismic data document thrust splay faults that were active until ~0.4 Ma, but these thrust faults do not disrupt younger sediments (Boston et al 2016).…”

“…4a) and the range of S hmin we consider (±2 MPa from LOT gradients). In total, the available observations suggest that if thrust faults in the wedge are active, then the stress regime must transition to one of thrusting (S hmin > S v ) at depths >2000 mbsf or must evolve in time to drive thrust faulting later in the seismic cycle (e.g., Wang and Hu 2006;Sacks et al 2013;Hardebeck 2012;Lin et al 2013;Huffman and Saffer 2016).…”

Distribution of stress state in the Nankai subduction zone, southwest Japan and a comparison with Japan Trench

“…2,3 and 4a). These regional studies were followed by more detailed core-based analyses and geophysical studies, including interpretation of high-resolution seismic reflection data and S-wave splitting that provided a three-dimensional understanding of the stress field and the evolution of stresses through time Tsuji et al, 2011a;Lewis et al, 2013;Moore et al, 2013;Sacks et al, 2013;Conin et al, 2014). Taken together, these results show that at all sites except C0004 and C0010, the maximum principal stress  is vertical at shallow levels and that the orientation of the intermediate principal stress  changes from trench perpendicular at C0006 and C0001 to trench parallel at C0002.…”