Shallow thermal structure constrained by seafloor temperature and heat flow estimated from BSRs in the Nankai subduction zone

Abstract:

The Nankai Trough is a plate convergent boundary where earthquakes with a magnitude of 8 take place repeatedly. Thermal structure in subduction zones affects pore pressure and diagenesis such as consolidation, dewatering and cementation, and constrains physical properties of a fault-slip plane. In the Nankai subduction zone, existence of methane hydrate is confirmed from acoustic reflectors called the Bottom Simulating Reflectors (BSRs) which parallel the seafloor on seismic reflection images with high-amplitude and reverse-polarity waveforms. As a depth of BSR is theoretically constrained by subseafloor profiles of temperature and pressure, the BSR depths effectively produce subseafloor geothermal information over a wide area without heat flow probe penetration or in-situ borehole temperature measurement that is fragmentary. In this study, we aim at calculating precise two-dimensional shallow thermal structure. First, we investigate detailed distribution of the BSRs in the Nankai area ranging from offshore Tokai to Hyuga using two-dimensional multi-channel seismic reflection data. The BSR depths are then forwarded to estimate heat flow values. Second, we use a simple two-dimensional thermal modeling of Blackwell et al. [1980] that takes into account topographical effects of the seafloor roughness. We also employ additional boundary conditions constrained by seafloor temperature and the heat flow estimated from BSR depths. In order to confirm reliability of the modeled thermal structure, we additionally estimate the base of gas hydrate stability zone which is proved to almost equal to observational BSR depths. We find in the modeled thermal structure that the convex portions that are subject to cooling by cold bottom water, while depressions are less subject to the cooling from observational BSRs and theoretical calculation. The thermal structure gained here provides essential data for seismic simulations in subduction zones and for laboratory experiments as analogues to seismic ruptures along plate boundary faults.