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Poulaki, E. M.*; 安邊 啓明; 他28名*
no journal, ,
We analyze samples from IODP Expedition 402, which occurred from Feb-April 2024 and drilled the continent-ocean transition in the Tyrrhenian Sea offshore Italy. Two drill sites comprise a sequence of (1) variably deformed granitic gneisses intercalated with ~cm-scale slivers of peridotites and basalts, and (2) a heterogeneous section of primarily serpentinized peridotites and cm-thick granitoids emplaced between the ultramafics. All peridotites are serpentinized with moderate to high alteration. The presence of both peridotites and granitoids showcases a diverse lithological and rheological heterogeneity that has not been previously observed in in-situ mantle sections. Initial observations suggest strain localization in a ~5 m-thick zone. Preliminary zircon U-Pb ages and apatite U-Pb ages constraints coupled with the microstructures require rapid exhumation after the granitoids' emplacement. Collectively, the presence of granitic gneisses within the detachment fault, as well as the young age of felsic intrusions, suggests that the weak granitoids may have accommodated the exhumation of this mantle section to the surface.
Filina, I. Y.*; 安邊 啓明; 他25名*
no journal, ,
The IODP Expedition 402 occurred from February to April of 2024 in the Tyrrhenian Sea with the primary objective of sampling the transition between the continental and oceanic tectonic domains. We have drilled two sites on the conjugate Cornaglia and Campania terraces, and four sites in the Vavilov basin. In this paper, we summarize different rock types recovered during the IODP expedition 402 that range from felsic to ultramafic composition. We present the average values of bulk density, porosity, grain density, compressional velocity, magnetic susceptibility, gamma-radiation, and thermal conductivity for different types of rocks. In particular, we report physical properties for shales and muds of the post-Messinian section including ach intervals and volcaniclastic gravel encountered in all holes, evaporites (halite, gypsum and anhydrite) of Messinian interval, and pre-Messinian shales in the continental domains, as well as basalts and diorites, granitoids/gneisses and peridotites recovered from the Vavilov basin. The data gathered from the 6 sites drilled by Expedition 402 will allow us to study the origin, formation, evolution and tectonic history of the Tyrrhenian Sea and uncover the geological processes that were active in the area at different times. The physical properties reported in this paper provide mandatory constraints for all kinds of geomodelling, such as geological, geophysical, geochemical and geodynamical studies, as well as tectonic reconstructions, which eventually will shed light on the complex tectonics of this important region.
Cs in ocean considering migration process of Cs in a lower trophic level ecosystem池之上 翼; 中西 貴宏; 川村 英之
no journal, ,
2011年3月に発生した福島第一原子力発電所の事故により、海洋環境中に放射性核種が放出された。放出された放射性核種のうち、Csは大量に放出され、かつ半減期が長いため、海洋環境への影響が懸念されている。Csの大部分は海水中に溶解するが、一部は生物起源粒子(植物プランクトン、動物プランクトン、デトリタス)に取り込まれ、吸着する。本研究では、低次生態系におけるCsの移行過程を考慮し、海洋におけるCsの長期動態を予測することを目的とする。海洋拡散モデルに低次生態系モデルを結合する改良を行い、海洋中Csの長期シミュレーションを実施した。この改良により、生物起源粒子と有機海底堆積物のCs濃度の長期予測が可能となった。計算結果は、生物起源粒子の沈降過程よりも、海水から有機海底堆積物への吸着過程が、有機海底堆積物中のCs濃度にはるかに大きな影響を与えることを示唆した。
永田 祐太郎*; 恩田 裕一*; 平松 翼*; 高橋 純子*; 榊原 厚一*; 新里 忠史
no journal, ,
This study aims to investigate the effect of litter on the discharge of dissolved Cs in a forested headwater catchment. We sampled stream water, groundwater, and water passing through litter to measure dissolved Cs, ions, and DOC. Additionally, we examined changes in river water quality and dissolved Cs concentrations by removing litter near the river channel, simulating forest decontamination. Sampling was conducted monthly and during rainfall events with totals of 26 mm and 51.5 mm. Results indicated that at runoff peaks, dissolved Cs concentrations were over ten times higher than baseflow levels, with concurrent increases in DOC and K
concentrations. While Cs and DOC concentrations remained elevated the day after rainfall, K concentrations returned to baseflow levels within four hours after the runoff peak. This suggests that leaching from organic matter significantly contributes to the increase in dissolved Cs concentration during runoff, much more than ion exchange with K. The study also reports changes in water quality and Cs concentrations following litter removal experiment in the watershed.
安邊 啓明
no journal, ,
Dike orientation has been used to estimate regional stress as an indicator for tectonics. A periphery of the fluid supply source, such as a magma chamber and mud diapir, is suitable for estimating stress because of a greater number of dike intrusions. However, stress changes locally due to radial compression, and it is expected to establish the method for distinguishing the effects of regional and local stresses around the fluid supply source. Mud diapirs and associated conglomerate-bearing mudstone dikes intruded at the study area in the Miocene Tanabe Group, a paleo-forearc basin deposits in southwestern Japan. To reveal the spatial stress variation, this study measured the orientation of the dikes and performed stress inversion using the mixed Bingham distribution method (Yamaji and Sato, 2011) for each sub-area of tens to hundreds of meters. The general trend of E-W minimum compressional stress axes suggests regional E-W tension stress. Minimum compressional stress axes are relatively dispersed in the southern part of the study area. The density of dike intrusion increases from north to south. These suggest that mud diapir exists in the southern part. Stress regime changes spatially, as normal faulting, maximum compressional stress axis-plunging southward, and strike-slip faulting from south to north. This pattern corresponds to the stress change suggested by the stress field model around the spherical cavity in the semi-infinite elastic body. Based on the model, this study proposed a new stress field model by summing homogeneous regional stress. Then, the parameters of the proposed model for accounting stresses detected in the study area are estimated inversely by the Markov chain Monte Carlo method. As a result, regional stress was well constrained as E-W tension normal faulting stress.
西山 成哲; 中嶋 徹; 後藤 翠*; 安邊 啓明; 箱岩 寛晶; 石原 隆仙; 長田 充弘*; 島田 耕史; 丹羽 正和
no journal, ,
Earthquakes with magnitudes of 6-7 have been reported even in various active tectonic settings where fault deformation topography have not been detected. Therefore, delineating concealed active faults generating such earthquakes is necessary to reduce earthquake damage; however, few studies exist to provide its clues regarding such faults. The 1984 Western Nagano Earthquake in Japan was a main shock with a magnitude of Mj 6.8 and a depth of 2 km at the source. Solid bedrocks are well-exposed in the earthquake source region; however, no surface rupture have been identified, and the active fault is known to be concealed. We collected minor faults-slip data by geological survey around this area and estimated stresses in this area using the multiple inverse method, confirmed that stresses similar to acting faults in this area were detected in minor faults around the known concealed active fault (Nishiyama et al., 2024). In this presentation, we will discuss the stresses that affected the minor faults that develop around the concealed active fault, including newly detected stresses from expanded survey area. The results of the survey were similar to those of Nishiyama et al. (2024) in the area of the expanded survey area. This suggests that the minor faults might be part of the damage zone that has been developed around the concealed active fault. This study indicates the possibility of detecting concealed active faults in the bedrock by geological survey. This study was funded by the Ministry of Economy, Trade and Industry (METI), Japan as part of its R&D supporting program titled "Establishment of Advanced Technology for Evaluating the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (JPJ007597) (Fiscal Years 2020-2022)" and "Establishment of Technology for Comprehensive Evaluation of the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (JPJ007597) (Fiscal Years 2023-2024)".
田中 桐葉; 大橋 聖和*; 武藤 潤*; 岡 壽崇
no journal, ,
断層の電子スピン共鳴(ESR)年代測定は、鉱物中の捕獲電子が最新の地震時に起きた断層すべりで完全に放出された(ゼロイング)と仮定して、最新断層活動年代を推定する開発段階の手法である。しかし、地震時の断層すべりに伴う捕獲電子の挙動は十分に理解されていない。これまでの研究によると、摩擦発熱はゼロイングを引き起こす重要な要因とされている。本研究では、摩擦発熱に関連する母岩の熱的特性が地震性の断層すべりの捕獲電子の挙動に及ぼす影響について検討した。岩石の熱特性の影響を調べるため、ムライト(HVF I)とチタン合金(HVF II)の円筒供試体を用いた2種類の高速摩擦実験を人工石英粉末に対して行った。HVF IとIIでは、単位面積あたりの摩擦仕事率の大きさが異なるように実験条件を設定した。石英の代表的な捕捉電荷の数の変化と関連するメカニズムを調べるため、出発物質と模擬断層ガウジに対して、ESR測定と粒子特性評価を行った。結果として、すべての摩擦実験で捕獲電子が生成された。HVF IとIIでは、摩擦仕事率の範囲が異なるにもかかわらず、捕獲電子の生成量は摩擦仕事率の増加とともに単調に減少し、同程度の値になった。一方、摩擦仕事率の範囲を問わず、捕獲電子の生成量は実験中に試料表面で観察された最高到達温度が高くなるにつれて減少した。ムライトとチタン合金の熱伝導率はそれぞれ4.2W/m/Kと7.5W/m/Kである。したがって、ムライトはチタン合金よりも熱を保持しやすい。このことは、HVF IIよりもHVF Iの方が、摩擦発熱により捕獲電子をより効果的に放出することができ、その結果、上述したような生成量の減少をもたらしたことを示している。本研究は、母岩の熱的特性が摩擦発熱に伴う捕獲電子への影響、すなわち、ゼロイングを支配する重要な要素になり得ることを示唆している。
Malatesta, L.*; Weiss, N.-M.*; 塚本 すみ子*; 末岡 茂; 石村 大輔*; Gailleton, B.*; 西村 卓也*; 高橋 直也*; 片岡 香子*; 小松 哲也; et al.
no journal, ,
On January 1st, 2024, the Mw 7.5 Noto Peninsula earthquake ruptured on a series of coastal offshore reverse faults in the back arc of central Japan. Recent Holocene terraces mapped along the northern coast, where coseismic uplift was greatest on January 1st 2024, suggest that they may be attributable to similar past ruptures. The Peninsula itself is remarkable for its 4767 unique terraces ranging in age from Holocene to 1.02 Ma. We digitized all terraces and recorded the elevation of their landward edge, and derived a rock uplift rate for each based on their age and the corresponding original sea level. The southeast-tilting crustal deformation recorded by the terraces associated with the last two interglacial high stands strongly resembles that caused by the Mw 7.5 earthquake. Older terraces, on the contrary all record a spatially uniform rate of uplift across the Peninsula. We conclude that the faults that caused the most recent earthquake became the dominant structures on the Peninsula over a quarter million year ago. This onset of seismogenic activity informs us about the regional plate tectonics of the area. The boundary separating Eurasia from the North American Plate currently runs south of the Noto Peninsula, linking the cities of Niigata and Kobe along a northeast-striking Tectonic Zone (NKTZ). Immediately east of the Peninsula lies another north-striking Tectonic Line, from Itoigawa to Shizuoka (ISTL). East of the ISTL and north of the NKTZ is the Island of Sado, which terraces also suggest a change in deformation with strong southeast tilting synchronous and similar to Noto. Together, Noto and Sado act as a railroad switch, indicating which of the ISTL or the NKTZ is the active plate boundary. When the ISTL is active, the sites are on different tectonic plates and evolve differently. When the NKTZ takes over, Sado Island becomes part of Eurasia and is more likely to evolve similarly to the Noto Peninsula.