Potential fluid migration process inferred from integrated active-source seismic imaging in the Nankai Trough subduction zone off Cape Muroto, Japan

Abstract

We investigate the potential fluid migration process in a plate subduction zone on the basis of integrated active-source seismic imaging from multichannel seismic (MCS) reflection and ocean-bottom seismograph (OBS) wide-angle seismic surveys along a 100 km-long line in the area offshore of Cape Muroto in the central Nankai Trough. First, to overcome poor reflection imaging in a thick part of the accretionary wedge and discrepancies between the results of conventional analyses from different datasets, we apply a signal enhancement technique in addition to standard preprocessing for the MCS reflection data and prestack depth migration based on a high-resolution seismic velocity model derived by full waveform inversion (FWI) using wide-angle OBS data. The improved MCS reflection profiles clarify the detailed geological architecture consistent with the seismic velocity variations with significant low-velocity zones in the accretionary wedge. A low-velocity band elongated immediately above the oceanic crust implies possible high pore fluid pressure in the underthrust sequence. A low-velocity column passing through the thick accretionary wedge could suggest the existence of gas-rich fluid in fault/fracture zones caused by subsequent deformation due to the oblique subduction of the Philippine Sea plate accompanied by multiple seamounts and topographic relief. We propose a schematic model of potential fluid migration in the central Nankai Trough subduction zone. The hemipelagic muddy sediments and turbidites containing organic carbon deposited in the Shikoku Basin were transported downward by underthrusting. High pore fluid pressure, which is caused by rapid loading and clay mineral dehydration within the underthrust sequence, may lead upward fluid migration through fault zones. Thermogenic methane generated at depth is a driving force of buoyancy for upwelling through permeable parts of fractures or dipping strata. In addition to biogenic methane, migrated thermogenic methane is also a source of widely distributed gas hydrates.