Contrasts in physical properties between the hanging wall and footwall of an exhumed seismogenic megasplay fault in a subduction zone

We examined the physical properties of an exhumed and fossilized subduction zone megasplay fault from analysis of from the geophysical logging data and preliminary results obtained by the Nobeoka Thrust Drilling Project that provide a high-resolution transect of properties across the main fault zone. The footwall cataclasite presents higher averages of neutron porosity (~7.6%) and lower values of electric resistivity (~232 Ωm) and P-wave velocity (~4.2 km/s) compared to those of the hanging wall phyllite (~4.8%, ~453 Ωm, ~4.8 km/s). Porosity, resistivity, natural gamma rays and spontaneous potential are particularly low in the hanging wall damage zone and the fault core (~3.6%, ~160 Ωm, ~4.4 km/s, ~108 API, ~39 mV). The clear contrast in the damage effect at the hanging wall and the footwall suggest differences in hydrologic properties and primary rock strength. The hanging wall damage zone implies compaction toward the upper fault core due to accumulated strain, associated with a low resistivity zone which might be derived by deformation and crack connectivity. The lower portion of the fault core of random fabric cataclasite gradually transforms to the foliated cataclasite of the footwall damage zone, which extends throughout the footwall. Despite the macroscopic contrast found between the hanging wall and the footwall, the seismogenic faulting has likely produced a continuous damage zone across the Nobeoka Thrust. The contrast in the physical properties and deformation of the Nobeoka Thrust may provide insights into the nature and fault mechanism of the active megasplay fault.