Proposal to IOD: Drilling and monitoring in Hyuga-Nada to unvail effects of ridge subduction on slow earthquakes

抄録

Shallow slow earthquakes, which last minutes to years, are important indicators of subduction megathrust slip behavior and future seismic and tsunami potential. Subducting plate roughness and seamounts have been proposed to promote slow earthquakes by inducing local geomechanical and hydrogeological anomalies. In the Hyuga-Nada region offshore Kyushu, Japan, slow earthquakes are repeatedly observed on and near the subducting Kyushu-Palau-Ridge, chain of seamounts thus providing excellent opportunities to explore the effects of seamounts on geomechanical/hydrological/thermal properties, and ultimately seismic coupling. Long-term monitoring enabled by a planned permanent network (N-net) will allow subsurface processes during frequent (~1 year) periodical slow earthquakes and ~M7 earthquakes (~20-30 year interval) to be captured with high fidelity. Our plan consists from drilling and installing observatories at three primary locations in Hyuga-Nada to address two hypotheses: 1) Seamount subduction modulates stress and pore pressure, creates fracture networks and influences the thermal and hydrological state of the margin. 2) The spatiotemporal distribution of slow earthquakes is strongly influenced by seamount subduction. We will drill three primary distinct sites relative to the seamount, to measure physical properties and describe deformation by LWD, APCT-3, and core analysis to characterize in-situ stress state, fracture density, heat flow, and pore fluid flow. Spatial variations in the upper plate disruption caused by seamount subduction will be revealed by comparing results from the three sites; and these will constrain geomechanical, hydrological, and thermal models. We will install a “Fiber-CORK” observatory equipped with conventional pressure and temperature sensors and cutting-edge fiber-optic sensors. The combination will fill a gap in slip durations currently observable in this region with seismic and geodetic instrumentation.