** Progress in Earth and Planetary Science is the official journal of the Japan Geoscience Union, published in collaboration with its society members.
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Research
Solid earth sciences
202502202502
Prevalence of mud diapirs in the Hyuga-nada subduction zone, southwest Japan, and its implications on hydrogeological processes
Ryuta Arai, Kazuya Shiraishi, Yasuyuki Nakamura, Gou Fujie, Arata Kioka, Masataka Kinoshita, Rina FukuchiRyuta Arai, Kazuya Shiraishi, Yasuyuki Nakamura, Gou Fujie, Arata Kioka, Masataka Kinoshita, Rina Fukuchi
Mud diapir, Seismic reflection, P-wave velocity, Methane concentration, Ridge subduction
Mud volcanoes, common seafloor features in subduction zone forearcs, provide crucial insights into deep hydrogeological systems associated with plate subduction. Despite their widespread occurrence, the factors governing their uneven distribution remain unclear, and their internal physical properties are poorly understood. Using multiple seismic reflection profiles and high-resolution P-wave velocity (Vp) models, we examined the distribution and physical properties of mud diapirs in the Hyuga-nada subduction zone, southwest Japan, impacted by the Kyushu-Palau ridge seamount chain entering the junction of the Nankai Trough and the Ryukyu Trench. Our seismic reflection data revealed more than 60 diapiric structures, some breaching the seafloor, while others remain buried beneath shallow sediments. These diapirs predominantly occur where the subducting slab is 10–20 km deep, mainly on the leading edge of a large subducting seamount. Notably, there is a significant absence of diapirs directly above the seamount and within approximately 50 km of the trench axis. The findings suggest that once the slab reaches depths greater than 10 km, thermogenic hydrocarbon gas production, which gains significant buoyancy, drives upwelling fluid flows in the overriding plate. The ridge subduction also contributes to diapir distribution by enhancing dehydration of underthrust sediments on the downdip side and creating a porous fracture network in the upper plate. The Vp structure of an exposed mud volcano indicates that it contains methane levels similar to those in the Kumano-nada region of the central Nankai Trough. Buried mud diapirs, rooted in fluid- and gas-rich reservoirs at 1–5 km depths below the seafloor, exhibit significantly lower seismic velocities (Vp < 3.0 km/s), indicating higher fluid and hydrocarbon gas concentration at depth. These observations suggest the forearc wedge in the Hyuga-nada area contains more extensive fluid/gas reservoirs than previously recognized.
