11:15 AM - 11:30 AM
[SCG40-08] Prevalence of potential mud volcanism in the Hyuga-nada area and its hydrogeological implications
Keywords:Hyuga-nada, Mud volcano, Seismic reflection, Ridge subduction
Submarine mud volcanism that discharges deep-sourced fluids and hydrocarbon gases on the seafloor is a common phenomenon on the forearc wedge of subduction zones and provides an important window into subsurface hydrogeological system. The Hyuga-nada subduction zone at the boundary between the Nankai Trough and the Ryukyu Trench became known to host a number of seafloor mud volcanoes after a pioneering study by Ujiie (2000). In recent years, geochemical studies utilizing core samples from the mud volcanoes have been revealing their tectonic and hydrologic processes (Ijiri et al., 2023; Mitsutome et al., 2023). One of the key findings from these studies is possible involvement of deep fluids in the formation of mud volcanoes in addition to clay mineral dehydration at shallow crustal depths, which is consistent with the seismic imaging studies that observed upper-plate fluid conduits linked to the fluid-rich plate boundary at over 10 km depths (Arai et al., 2023). Another study by Asada et al. (2023) presents bathymetric and seismic evidence of dozens more mud volcanoes in the region east off Miyazaki Prefecture, but the overall distribution of mud volcanoes in the Hyuga-nada area has not been constrained yet.
For comprehensive detection of mud volcanoes in the regional Hyuga-nada subduction zone, this study examines seismic reflection data on 11 lines JAMSTEC collected over the last quarter century in the region. We newly identified dozens of previously unknown diapiric structures indicating potential mud volcanoes (characterized by seismically transparent zones breaching horizontally stratified sedimentary layers). Interestingly, many of the diapiric structures remain buried in the sedimentary basins, suggesting possible subsurface accumulation of fluid and/or gas in the absence of any bathymetric signatures. Even if exposed on the seafloor, the diapiric structures are not always accompanied by a surface circular mound that is a typical morphological feature of mud volcanoes, and alternatively a majority of them are situated beneath linear ridges or fault scarps, which may reflect significant impacts on the upper-plate deformation by the subduction of Kyushu-Palau Ridge. We find that these potential mud volcanoes do not occur near the trench axis (within ~50 km from the trough) where shallow tectonic tremors actively occur (Yamashita et al., 2023) and are distributed on the downdip side of a large subducting seamount or on its flanks, but they are missing immediately above the subducting seamount. These results may indicate tectonic controls of the seamounts on the distribution of mud volcanoes and suggest that the overriding plate has been damaged too intensively above the seamount to sustain fluid overpressure that is necessary to result in forming mud volcanoes.
For comprehensive detection of mud volcanoes in the regional Hyuga-nada subduction zone, this study examines seismic reflection data on 11 lines JAMSTEC collected over the last quarter century in the region. We newly identified dozens of previously unknown diapiric structures indicating potential mud volcanoes (characterized by seismically transparent zones breaching horizontally stratified sedimentary layers). Interestingly, many of the diapiric structures remain buried in the sedimentary basins, suggesting possible subsurface accumulation of fluid and/or gas in the absence of any bathymetric signatures. Even if exposed on the seafloor, the diapiric structures are not always accompanied by a surface circular mound that is a typical morphological feature of mud volcanoes, and alternatively a majority of them are situated beneath linear ridges or fault scarps, which may reflect significant impacts on the upper-plate deformation by the subduction of Kyushu-Palau Ridge. We find that these potential mud volcanoes do not occur near the trench axis (within ~50 km from the trough) where shallow tectonic tremors actively occur (Yamashita et al., 2023) and are distributed on the downdip side of a large subducting seamount or on its flanks, but they are missing immediately above the subducting seamount. These results may indicate tectonic controls of the seamounts on the distribution of mud volcanoes and suggest that the overriding plate has been damaged too intensively above the seamount to sustain fluid overpressure that is necessary to result in forming mud volcanoes.