The Fault Damage Zone of the Shallow Japan Trench Megathrust

Abstract:

The Mw 9.0 Tohoku-oki earthquake resulted in an unprecedented coseismic slip of >50 m in the shallow portion of the Japan Trench subduction zone. We present analyses of core recovered during IODP Expedition 343/343T (JFAST) that define structures surrounding the inferred plate boundary décollement, and use the results to constrain the fault’s long-term strength. The plate boundary fault is centered at 821.5 m below the sea floor, with a damage zone extending 15.5 m below and 51.5 m above. The damage zone is defined by shear fractures, subsidiary faults, deformation bands, mode I fractures, breccia zones, and sediment-filled veins. Orientations of mutually crosscutting shear fractures decrease in dip angle nearing the fault in the hanging wall, from 67° at 50 m above the fault down to 25°. In the footwall, dips range from 88° at 9.6 m below the fault to 30° at 11 m below. The damage zone characteristics were established using a set of criteria to eliminate drilling-, coring-, and handling-induced damage in core. Core-scale fracture density increases from 21 fractures/m at 51 m above the fault to 247 fractures/m adjacent to the fault in the hanging wall, and from 28 fractures/m at 11 m below the fault to 254 fractures/m adjacent to the fault in the footwall. The fall-off in fracture density is fit by power law functions in the hanging wall and footwall, with decay exponent n values of 0.70 and 1.45, respectively. Microstructures include shear fractures, veins, crystallographic preferred orientation bands, shear zones, and undifferentiated fractures. Microstructure density in the footwall increases from 0.32 fractures/mm 10 m below the fault to 2.04 fractures/mm adjacent to the fault, and is fit by a power law function with n = 1.27. Orientations of shear fractures have attitudes consistent with normal and reverse faults, indicating the stress field underwent significant reorientations multiple times. This is consistent with the inferred stress field changes accompanying great earthquakes measured from borehole breakouts (Lin et al., 2013). Compared to similar offset faults, the décollement has a steep fall-off in damage, which we interpret to indicate the fault is weak over geologic time; hence small changes in stress magnitudes could result in large stress field rotations.