17:15 〜 19:15
[MIS20-P03] Source Depth of Sediments in Mud Volcanoes in the Ryukyu Trench to Nankai Trough
キーワード：泥火山、間隙率、ピストンコア試料、マルチプルコア試料、炭質物ビトリナイト反射率
The frequent slow earthquakes in the Hyuga-nada region, located off the Miyazaki, are believed to be related to the subduction of topographic highs, such as seamounts chain from the Kyushu-Palau Ridge. On August 8, 2024, a magnitude 7.1 earthquake occurred in Hyuga-nada at a depth of 31 km, near the plate boundary. This earthquake also took place near a seamount, in a region where numerous mud volcanoes are distributed on the seafloor. Additionally, Hyuga-nada is an area where gas fields producing hydrocarbon gases extend into onland Miyazaki.
Mud volcanoes are geomorphological features formed by the eruption of muddy sediments containing water and gases from deep seafloor to the subsurface, sometimes forming conical mounds. Mud volcanoes are considered surface indicators of oil and gas fields. The discovery of methane hydrate off Tanegashima suggests that they may contribute to hydrocarbon resource exploration. Moreover, changes in their activity are thought to indirectly indicate subsurface changes caused by earthquakes.
This study aims to estimate the depth of origin of the mud volcanoes in Hyuga-nada. During the KH-23-4 cruise using R/V Hakuho-maru, we collected mud volcano samples from Hyuga-nada, as well as from areas off Kikai Island and Tanegashima. We analyzed the porosity of the matrix and clasts in the multiple and piston core samples, measured the grain size and the thermal maturity using vitrinite reflectance analysis of carbonaceous material of the matrix.
The results showed significant variations in sediment matrix porosity among different sites, ranging from 0.3 to 0.7. The porosity of the clasts that ascended with the matrix also varied by lithology, location, mud volcano itself, ranging from 0.07 to 0.65. Grain size analysis revealed that the matrix was primarily composed of clay to fine silt, with some coarse silt to very fine sand mixed in. In certain cases, medium to coarse sand-sized particles were also present. The number of coarse particles differed by site, reflecting the subsurface lithology at each location.
Vitrinite reflectance value indicated 0.3–0.5%, similar to those or more higher than observed at approximately 3 km below the seafloor in the Nankai Trough off-Kumano. This suggests that while the depth of origin of the mud volcano deposits varies, in deeper cases, it could be more than 3 km below the seafloor. This finding is also consistent with the results from mud volcano carbonaceous material in off Tanegashima.
Mud volcanoes are geomorphological features formed by the eruption of muddy sediments containing water and gases from deep seafloor to the subsurface, sometimes forming conical mounds. Mud volcanoes are considered surface indicators of oil and gas fields. The discovery of methane hydrate off Tanegashima suggests that they may contribute to hydrocarbon resource exploration. Moreover, changes in their activity are thought to indirectly indicate subsurface changes caused by earthquakes.
This study aims to estimate the depth of origin of the mud volcanoes in Hyuga-nada. During the KH-23-4 cruise using R/V Hakuho-maru, we collected mud volcano samples from Hyuga-nada, as well as from areas off Kikai Island and Tanegashima. We analyzed the porosity of the matrix and clasts in the multiple and piston core samples, measured the grain size and the thermal maturity using vitrinite reflectance analysis of carbonaceous material of the matrix.
The results showed significant variations in sediment matrix porosity among different sites, ranging from 0.3 to 0.7. The porosity of the clasts that ascended with the matrix also varied by lithology, location, mud volcano itself, ranging from 0.07 to 0.65. Grain size analysis revealed that the matrix was primarily composed of clay to fine silt, with some coarse silt to very fine sand mixed in. In certain cases, medium to coarse sand-sized particles were also present. The number of coarse particles differed by site, reflecting the subsurface lithology at each location.
Vitrinite reflectance value indicated 0.3–0.5%, similar to those or more higher than observed at approximately 3 km below the seafloor in the Nankai Trough off-Kumano. This suggests that while the depth of origin of the mud volcano deposits varies, in deeper cases, it could be more than 3 km below the seafloor. This finding is also consistent with the results from mud volcano carbonaceous material in off Tanegashima.