Petrological models of the oceanic lithosphere: Geophysical and geochemical tests     

Keisuke Ito 《Earth and Planetary Science Letters》1974,21(2):169-180

Petrological models of the oceanic lithosphere are tested to satisfy geophysical and geochemical constraints within the framework of plate tectonics. Quartz eclogite, olivine eclogite, peridotite and dunite are considered as the material of the lithosphere. The temperature at the base of the lithosphere is assumed to be the solidus temperature. This temperature, the thermal conductivity, and the heat flow and topography changes with age are used as the geophysical constraints. The compressional wave velocity-depth profile is used to select preferred models. Among geophysically successful models, high-temperature models are preferred to wet low-temperature models, because the low-temperature models have difficulties in explaining the mechanism of generation of oceanic basalt magmas. A preferred model is a two-layer model 70 km thick consisting of peridotite at the upper lithosphere and olivine eclogite at the lower lithosphere bounded at the base by the dry solidus.  相似文献   

Towards a new view of earthquake phenomena     

Keisuke Ito 《Pure and Applied Geophysics》1992,138(4):531-548

Recent advances in the theory of fracture and fragmentation are reviewed. Empirical laws in seismology are interpreted from a fractal perspective, and earthquakes are viewed as a self-organized critical phenomenon (SOC). Earthquakes occur as an energy dissipation process in the earth's crust to which the tectonic energy is continuously input. The crust self-organizes into the critical state and the temporal and spatial fractal structure emerges naturally. Power-law relations known in seismology are the expression of the critical state of the crust. An SOC model for earthquakes, which explains the Gutenberg-Richter relation, the Omori's formula of aftershocks and the fractal distribution of hypocenters, is presented. A new view of earthquake phenomena shares a common standpoint with other disciplines to study natural complex phenomena with a unified theory.  相似文献   

Noble gas isotopic composition of deep underground water in Osaka plain, central Japan: Evidence for mantle He and model for new volcanism     

Toshinori  Okada  Tetsumaru  Itaya  Mitsuo  Sato Keisuke  Nagao 《Island Arc》1994,3(3):221-231

Abstract Elemental and isotopic compositions of noble gases extracted from the bore hole water in Osaka plain, central Japan were examined. The water samples were collected from four shallow bore holes (180-450 m) and seven deep bore holes (600-1370 m) which have been used for an urban resort hot spring zone. The water temperatures of the deep bore holes were 22-50°C and that of the shallow bore holes, 13-23°C. The elemental abundance patterns show the progressive enrichment of the heavier noble gases compared with the atmospheric noble gas composition except for He, which is heavily enriched in deep bore hole water samples. ³He/⁴He ratios from the bore holes reaching the Ryoke granitic basement were higher than the atmospheric value (1.4 × 10⁻⁶), indicating a release of mantle He through the basement. The highest value of 8.2 × 10⁻⁶ is in the range of arc volcanism. On the other hand, the bore holes in sedimentary rocks overlying the basement release He enriched in radiogenic ⁴He, resulted in a low ³He/⁴He ratio of 0.5 × 10⁻⁶. ⁴He/²⁰Ne and ⁴⁰Ar/³⁶Ar ratios indicate that the air contamination is generally larger in shallow bore holes than in deep ones from each site. The helium enriched in mantle He is compatible with the previous work which suggested up-rising magma in 'Kinki Spot', the area of Osaka and western Wakayama, in spite of no volcanic activity in the area. A model to explain an initiation of magma generation beneath this area is presented.  相似文献   

A field study of the development of wind-waves     

Keisuke Taira 《Journal of Oceanography》1972,28(5):187-202

This paper presents the results of observation on the development of wind-waves which were generated in a lake water about 420 cm deep with a fetch 12 km long. Measurements of surface elevation were carried out at the end of an observational pier where the water depth was 80 cm. The wave momentum flux, i.e., the growth rate of the wave momentum, was estimated from both significant waves and power spectral densities for the wave records. The values obtained by the two ways accorded fairly well and they were 57 % as large as the wind stress measured simultaneously. The exponential growth rate of spectral densities for a frequency component was in good accord with that observed bySnyder andCox (1966) and by others. If these growth rates are applied to all the components of the spectrum, the wave momentum flux must exceed the wind stress. This cannot explain the experimental results nor can be physically accepted. The difference of spectral densities between the two successive runs showed that the increase of spectral densities was. limited in several bands of frequency. The phenomena are discussed in relation with the overshoot-undershoot effects studied byBarnett andSutherland (1968).Observational results suggest that the spectral growth of a certain component is closely related to the spectral densities of other components. Energy exchange among componented waves has not been considered in the theories for generation and development of wind-waves established by Phillips, Miles and others.New generation mechanism suggested byLonguet-Higgins (1969) was found to be able to describe the observed growth rates of the form(f)={(1/2)(t–t_1/2)}²: the spectral density(f) was proportional to the square of durationt. However, the mechanism can not explain the overshoot-undershoot effects peculiar to the equilibrium spectrum of windwaves.Three frequencies characterizing the discrete distributions of frequency bands where spectral densities increased were examined and three waves corresponding to these frequencies were found to be satisfying the resonance conditions for the wave-wave interactions among three sinusoidal wave trains as studied byPhillips (1960),Longuet-Higgins (1962) andBenny (1962). The interactions are suggested to predict well both the spectral growth proportional to squares of duration and the ceaseless oscillations of spectral densities in an equilibrium spectrum.  相似文献   

Geochronological characterization and petrogenesis of granitoids in the Ryoke belt, Southwest Japan Arc: constraints from K–Ar, Rb–Sr and Sm-Nd systematics     

Masaki Yuhara  Hiroo Kagami and  Keisuke Nagao 《Island Arc》2000,9(1):64-80

Abstract Rb–Sr and K–Ar chronological studies were carried out on granitic and metamorphic rocks in the Ina, Awaji Island and eastern Sanuki districts, Southwest Japan to investigate the timing of intrusion of the granitoids in the Ryoke belt. Intrusions of 'younger' Ryoke granitic magmas took place in the Ina district between 120 Ma and 70 Ma, and cooling began immediately after the emplacement of the youngest granitic bodies. Igneous activity in Awaji Island was initiated at 100 Ma and continued to 75 Ma. Along-arc variations of Rb–Sr whole-rock isochron ages suggest that magmatism began everywhere in the Ryoke and San-yo belts at almost the same time ( ca 120 Ma). The last magmatism took place in the eastern part of both belts. Rb–Sr and K–Ar mineral ages for the granitoids young eastwards. The age data suggest that the Ryoke belt was uplifted just after the termination of igneous activity. Initial Sr and Nd isotopic ratios for the Ryoke granitoids indicate that most were derived from magmas produced in the lower crust and/or upper mantle with uniform Sr and Nd isotopic compositions. Several granitoids, however, exhibit evidence of assimilation of Ryoke metamorphic rocks or older Precambrian crustal rocks beneath the Ryoke belt.  相似文献   

Diurnal variations in streamflow and water quality during the summer dry season     

Daiji Kobayashi  Yuji Kodama  Yoshiyuki Ishii  Yumiko Tanaka  Keisuke Suzuki 《水文研究》1995,9(7):833-841

In a headwater basin covered with boreal forest in northern Japan, the summer dry flow was monitored each summer from 1985 to 1993. Streamflow and specific electrical conductance fluctuated diurnally and these variations were attributed to daytime evapotranspiration. In 1989, the daytime reduction in streamflow and conductance were accompanied by a reduction in the HCO^?₃ concentration. The low flow hydrograph was separated into two components using HCO^?₃ and Cl^? concentrations in August 1989, assuming low flow to be a mixture of delayed subsurface flow and of quick shallow flow. The slight diurnal variation in the ratio of shallow flow to subsurface flow caused the diurnal variation in conductance by changing the HCO^?₃ concentration.  相似文献   

Assessment of rainfall-induced shallow landslides in Phetchabun and Krabi provinces,Thailand     

Keisuke Ono  So Kazama  Chaiwat Ekkawatpanit 《Natural Hazards》2014,74(3):2089-2107

Shallow landslides are a common type of rainfall-induced landslide, and various methods are currently used to predict their occurrence on a regional scale. Physically based models, such as the shallow landslide instability prediction (SLIP) model, have many advantages because these models can assess the hazards of shallow landslides dynamically, based on physical stability equations that consider rainfall as a triggering factor. The main objective of this research is to test the SLIP model’s potential to predict shallow landslide hazards in Thailand. To achieve this goal, the SLIP model was applied to two massive landslide events in Thailand. The results predicted by the SLIP model for the two study areas are outlined, and the model prediction capabilities were evaluated using the receiver operating characteristic plot. The Phetchabun results showed that the western part of the catchment had the lowest factor of safety (F _S) value, whereas the Krabi results showed that the slopes surrounding the peak of Khao Panom Mountain had the lowest F _S value, explaining the highest potentials for shallow landslides in each area. The SLIP model showed good performance: The global accuracies were 0.828 for the Phetchabun area and 0.824 for the Krabi area. The SLIP model predicted the daily time-varying percentage of unstable areas over the analyzed periods. The SLIP model simulated a negligible percentage of unstable areas over all considered periods, except for expected dates, suggesting that the prediction capability is reasonably accurate.  相似文献   

Holocene estuarine sediments as a source of arsenic in Pleistocene groundwater in suburbs of Hanoi,Vietnam     

Keisuke Kuroda  Takeshi Hayashi  Ayako Funabiki  An Thuan Do  Vu Duc Canh  Tran Thi Viet Nga  Satoshi Takizawa 《Hydrogeology Journal》2017,25(4):1137-1152

Groundwater pollution by arsenic is a major health threat in suburban areas of Hanoi, Vietnam. The present study evaluates the effect of the sedimentary environments of the Pleistocene and Holocene deposits, and the recharge systems, on the groundwater arsenic pollution in Hanoi suburbs distant from the Red River. At two study sites (Linh Dam and Tai Mo communes), undisturbed soil cores identified a Pleistocene confined aquifer (PCA) and Holocene unconfined aquifer (HUA) as major aquifers, and Holocene estuarine and deltaic sediments as an aquitard layer between the two aquifers. The Holocene estuarine sediments (approximately 25–40 m depth, 9.6–4.8 cal ka BP) contained notably high concentrations of arsenic and organic matter, both likely to have been accumulated by mangroves during the Holocene sea-level highstand. The pore waters in these particular sediments exhibited elevated levels of arsenic and dissolved organic carbon. Arsenic in groundwater was higher in the PCA (25–94 μg/L) than in the HUA (5.2–42 μg/L), in both the monitoring wells and neighboring household tubewells. Elevated arsenic concentration in the PCA groundwater was likely due to vertical infiltration through the arsenic-rich and organic-matter-rich overlying Holocene estuarine sediments, caused by massive groundwater abstraction from the PCA. Countermeasures to prevent arsenic pollution of the PCA groundwater may include seeking alternative water resources, reducing water consumption, and/or appropriate choice of aquifers for groundwater supply.  相似文献