Spatio-temporal stress states estimated from seismicity rate changes in the Tokai region, central Japan

Since unprecedented large-scale silent slip was detected by GPS in 2001 in the Tokai region, evaluating whether such movement is uniquely connected to the expected Tokai earthquake or repeatedly occurs in this area becomes vitally important. Because of short history of GPS observations and the limited areal coverage surrounding the Suruga trough, we take advantage of continuously recorded seismicity that is presumed to be sensitive to the deformation at seismogenic depth. Together with the well-maintained NIED earthquake data, we employ the seismicity-to-stress inversion approach of rate/state friction to infer the spatio-temporal stress changes in and around the presumed hypocentral zone of the future Tokai earthquake. Mapping stress changes inverted from microseismicity year by year, we find that the stress under Lake Hamana, the western expected future Tokai source, has been decreasing since 1999, during which the GPS data showed a normal trend of plate coupling. In contrast, stresses in the surrounding regions are calculated to have increased by transfer from Lake Hamana region. We interpret that this continuous process is associated with the 2000–2004 Tokai slow slip event. The characteristic patterns related to aseismic stress-release are also identified in the early 1980s and during 1987–1989, when slow events are inferred to have occurred on the basis of conventional geodetic measurements. Revisiting the seismotectonics and taking into account the mechanical implications of the inversion results, we argue that the transition zone situated between a deep stable creeping zone and a locked zone undergoes episodic creep and plays an important role in the transfer of stress to the locked zone. Consequently, even though we speculate that the current (2000 to present-day) silent slip event might be one of the repeating events, the inferred enlargement of the stress releasing area is significant and possibly raises the likelihood of the next Tokai earthquake.     

Evolution of the composition of seawater through geologic time, and its influence on the evolution of life

The redox state of the surface environment of the early Earth is still controversial, and a detailed and quantitative estimate is still lacking. We carried out in-situ analyses of major, trace, and rare-earth elements of carbonate minerals in rocks with primary sedimentary structures in shallow and deep sea-deposits, in order to eliminate secondary carbonate and contamination of detrital materials, and to estimate the redox condition of seawater through time. Based on the Ce content and anomalies of the carbonate minerals at given parameters of atmospheric CO₂ content (pCO₂) and Ca content of seawater, we calculated the oxygen contents of shallow and deep seawater, respectively. The results show that the oxygen content of the deep sea was low and constant until at least 1.9 Ga. The oxygen content of shallow seawater increased after 2.7 Ga, but fluctuated. It became quite high at 2.5 and 2.3 Ga, but eventually increased after the Phanerozoic. In addition, the calculation of a high pCO₂ condition shows that seawater was more oxic even in the Archean than at present, suggesting a relatively low pCO₂ through geologic time.Our detailed calculations from compositions of carbonate minerals in Three Gorge area, south China show a low oxygen content of seawater after the Snowball Earth until the late Ediacaran, an increase in the late Ediacaran, and a significant decrease around the Precambrian–Cambrian and Nemakit/Daldynian–Tommotian boundaries. These variations were possibly caused by global regression and dissolution of methane hydrates.     

Izu-Bonin arc subduction under the Honshu island, Japan: Evidence from geological and seismological aspect

The Izu-Bonin intra-oceanic arc with 20–35 km thick continental crust is being subducted under the Honshu, presumably since 17 Ma. Tomographic image clearly demonstrates that the whole Izu-Bonin arc is subducting under the Honshu arc. Geologic cross section and the thickness of continental crust do not support the accretion of thick crust in spite of the continued subduction over 17 Ma.     

Naturally occurring radionuclides and rare earth elements in weathered Japanese soil samples

The activity concentrations of ²²⁶Ra and ²²⁸Ac in weathered Japanese soils from two selected prefectures have been measured using a γ-ray spectroscopy system with high purity germanium detector. The uranium, thorium, and rare earth elements (REEs) concentrations were determined from the same soil samples using inductively coupled plasma mass spectrometry (ICP-MS). For example, granitic rocks contain higher amounts of U, Th, and light REEs compared to other igneous rocks such as basalt and andesites. Therefore, it is necessary to understand the interaction between REEs and nature of soils since soils are complex heterogeneous mixture of organic and inorganic solids, water, and gases. In this paper, we will discuss about distribution pattern of ²³⁸U and ²³²Th along with REEs in soil samples of weathered acid rock (granite) collected from two prefectures of Japan: Hiroshima and Miyagi.     

Land-use and vegetation-cover mapping of an indigenous land area in the state of Mato Grosso (Brazil) based on spectral linear mixing model,segmentation and region classification

A spectral linear-mixing model using Landsat ETM+ imagery was undertaken to estimate fraction images of green vegetation, soil and shade in an indigenous land area in the state of Mato Grosso in the central-western region of Brazil. The fraction images were used to classify different types of land use and vegetation cover. The fraction images were classified by the following two methods: (a) application of a segmentation based on the region-growing technique; and (b) grouping of the regions segmented using the per-region unsupervised classifier named ISOSEG. Adopting a 75% threshold, ISOSEG generated 44 clusters that were grouped into eight land-use and vegetation-cover classes. The mapping achieved an average accuracy of 83%, showing that the methodology is efficient in mapping areas of great land-use and vegetation-cover diversity, such as that found in the Brazilian cerrado (savanna).     

中国西风区黄土常量元素地球化学行为与古环境

位于中国西风区新疆伊犁盆地黄土与古土壤样品的常量元素分析表明，其烧失量和SiO2含量与样品的碳酸盐含量正相关。黄土与古土壤SiO2含量没有明显差别，但Al、Fe、Mn和K在古土壤中发生一定的分异，代表了相对强的成壤作用。与黄土高原相比，伊犁盆地 末次间冰期古土壤富钛、铁、锰、镁和钙（个别样品除外），而黄土高原S1更加贫钠和钾，而且钙含量很低；末次冰期黄土富锰、镁和钙，贫纳和钾，而黄土高原L1以富钙贫钠、钾为特征。常量元素地球化学行为揭示，伊犁黄土与古土壤较黄土高原同期黄土与古土壤形成于更加干旱的气候环境中。     

Progressive metamorphism of the Taitao ophiolite; evidence for axial and off-axis hydrothermal alterations

We estimated metamorphic conditions for the  6 Ma Taitao ophiolite, associated with the Chile triple junction. The metamorphic grade of the ophiolite, estimated from secondary matrix minerals, changes stratigraphically downwards from the zeolite facies, through the prehnite–actinolite facies, greenschist facies and the greenschist–amphibolite transition, to the amphibolite facies. The metamorphic facies series corresponds to the low-pressure type. The metamorphic zone boundaries are subparallel to the internal lithological boundaries of the ophiolite, indicating that the metamorphism was due to axial hydrothermal alteration at a mid-ocean ridge.

Mineral assemblages and the compositions of veins systematically change from quartz-dominated, through epidote-dominated, to prehnite-dominated with increasing depth. Temperatures estimated from the vein assemblages range from  230 °C in the volcanic unit to  380 °C at the bottom of the gabbro unit, systematically  200 °C lower than estimates from the adjoining matrix minerals. The late development of veins and the systematically lower temperatures suggest that the vein-forming alteration was due to off-axis hydrothermal alteration.

Comparison between the Taitao ophiolite with its mid-ocean ridge (MOR) affinity, and other ophiolites and MOR crusts, suggests that the Taitao ophiolite has many hydrothermal alteration features similar to those of MOR crusts. This is consistent with the tectonic history that the Taitao ophiolite was formed at the South Chile ridge system near the South American continent (Anma, R., Armstrong, R., Danhara, T., Orihashi, Y. and Iwano, H., 2006. Zircon sensitive high mass-resolution ion microprobe U–Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications. The Island Arc, 15(1): 130–142).     

Primary productivity and solar radiation off Sanriku in May 1997

We measured the in situ primary production at four stations from the surface to 80 m off Sanriku in late May 1997. The depth-integrated daily primary production in the upper 80 m was estimated to be 391, 468, and 855 mgC m⁻²d⁻¹ in water from the Oyashio, and 336 mgC m⁻²d⁻¹ in the warm-core ring. The variation in the primary production was primarily due to the variation in phytoplankton activity (chlorophyll α-specific primary production). A combination of previous and present studies in water from the Oyashio and the warm-core ring suggested that phytoplankton activity is proportional to light intensity between 12 and 50 Ein m⁻²d⁻¹ which is close to the usual light condition (61–75 Ein m⁻²d⁻¹) off Sanriku in May and June. Light may be a limiting factor for phytoplankton off Sanriku in late spring and early summer.     

Suction dynamics-induced bubbly sand under groundwater table fluctuations

Bubbly sand is found in intertidal zones and has been considered to be closely related to swash and tides. Unlike bubble structures that arise from in situ methane gas production and bubble growth in river and marine sediments due to organic matter decomposition, the formation mechanism and conditions of bubbly sand remain unresolved. The present study aims to resolve them on the basis of a conceptual model and controlled laboratory experiments. The study demonstrates that bubbly sand is a consequence of the cyclic expansion of sand under groundwater table fluctuations. The varying intensity of dynamics of suction, which is sand moisture tension defined by negative pore water pressure relative to atmospheric pressure, controls the manifestation of a full spectrum of sand behaviour relevant to the formation of bubbly sand. Namely, with increasing intensity of suction dynamics under larger groundwater table fluctuations, the sand behaviour varies from enhanced cyclic contraction, to weak contraction, loosening and expansion resulting in bubbly sand. The suction dynamics-induced cyclic expansion of sand occurs under conditions where the maximum suction developed exceeds the air-entry suction of sand such that the degree of saturation becomes lower than 60%, while the groundwater tables cyclically rise to and fall from the sediment surface. Accordingly, the sand porosity changes remarkably from dense to super-loose states of bubbly sand. These findings account for previously unanswered questions with respect to bubbly sand, both qualitatively and quantitatively, and will lead to a renewed understanding of the geological record and morphological features at waterfronts that are subject to groundwater table fluctuations.     

Ecological aspects of hydrothermal vent animals in captivity at atmospheric pressure

Hydrothermal vent animals were kept and displayed at atmospheric pressure with an artificial hydrothermal vent system in Enoshima Aquarium, Fujisawa, Kanagawa, Japan. The artificial hydrothermal vent system was composed of a main rearing tank, a heating tank, a hot water outlet with added Na₂S as the source of H₂S, and added CO₂ for chemosynthetic bacteria and pH regulation. When the need arises, a dissolved oxygen control unit and submersible heaters can be attached. We are now rearing hydrothermal vent crabs (Austinograea yunohana), hydrothermal vent galatheid crabs (Shinkaia crosnieri), vestimentiferan tubeworms (Lamellibrachia satsuma), hydrothermal vent shrimp (Opaepele spp.), hydrothermal vent barnacles (Ashinkailepas seepiophilia and Neoverruca sp.), and tonguefish (Symphurus sp.). In the artificial hydrothermal vent tank, shrimp and crabs have been observed to cluster close to the artificial hydrothermal vent. In particular, large (adult) crabs needed a heat source to live in the aquarium over a long term. Additionally, some species (A. yunohana, S. crosnieri, Opaepele sp. and Symphurus sp.) have spawned and hatched in captivity. It is likely higher water temperatures are needed for egg and larval development compared with temperatures for adult requirements.