[1] Analyses of normal faults in the Kumano forearc basin of the Nankai Trough reveal multiple normal fault populations in a region generally thought to be under compression. Most faults have offsets of less than 20 m and dips of [60][61][62][63][64][65][66][67][68][69][70] and show no growth structures, indicating that the faults were active for short periods of time. The oldest generation of faults is older than~0.9 Ma and strikes~50-60. The next oldest faults strike~160-170 , are older than 0.44 Ma, and are related to local uplift along the western edge of the region. The youngest faults cut the seafloor; shallow faults near the SE margin of the basin curve from 100 in the middle of the survey area to~145 at the SE corner of the area. The pattern of the two youngest fault populations is consistent with the regional stress pattern (maximum horizontal stress subparallel to the trench). Orientations of older fault populations are caused by uplift of the underlying accretionary prism, implying that the forearc basin region is not as stable as previously thought. Reconstruction of displacements on the youngest faults shows that the overall horizontal extension is less than 2%, concentrated near the seaward edge of the basin. The active normal faults distributed throughout the basin support the idea that the horizontal stress parallel to the plate convergence direction does not reach the critical stress to activate or form thrust faults and produce horizontal shortening within the shallow portion of the inner wedge.

[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.