A heating process of Kuchi-erabu-jima volcano,Japan, as inferred from geomagnetic field variations and electrical structure     

Wataru Kanda  Mitsuru Utsugi  Yoshikazu Tanaka  Takeshi Hashimoto  Ikuko Fujii  Toshiaki Hasenaka  Nobuaki Shigeno 《Journal of Volcanology and Geothermal Research》2010,189(1-2):158-171

Since August 2000, we have recorded the total intensity of the geomagnetic field at the summit area of Kuchi-erabu-jima volcano, where phreatic eruptions have repeatedly occurred. A time series analysis has shown that the variations in the geomagnetic field since 2001 have a strong relationship to an increase in volcanic activity. These variations indicate thermal demagnetization of the subsurface around the presently active crater. The demagnetization source for the early variations, until summer 2002, was estimated at about 200 m below sea level. For the variations since 2003, the source was modeled on the basis of the expansion of a uniformly magnetized ellipsoid. The modeling result showed that the source is located at 300 m above sea level beneath the crater. We carried out an audio-frequency magnetotelluric survey with the aim of obtaining a relation between the demagnetization source and the shallow structure of the volcano. A two-dimensional inversion applied to the data detected two good conductors, a shallow thin one which is restricted to a region around the summit area, while the other extends over the edifice at depths between 200 and 800 m. These conductors are regarded as clay-rich layers with low permeability, which were assumed to be generated through hydrothermal alteration. The demagnetization source for the early variations was possibly located at the lower part of the deep conductor and the source after 2003 lies between the two conductors, where groundwater is considered to be abundant. Based on these results, as well as on seismological, geodetic, and geochemical information, we propose a heating process of the Kuchi-erabu-jima volcano. In the initial stage, high-temperature volcanic gases supplied from the deep-seated magma remained temporarily at the level around the lower part of the less permeable deep conductor since the ascent path had not yet been established. Then, when the pathway developed as a result of repeated earthquakes, it became possible for a massive flux of volcanic gases to ascend through the conductor. The high temperature gases reached the aquifer located above the conductor and the heat was efficiently transported to the surrounding rocks through the groundwater. As a consequence, an abrupt increase of the gas flux and diffusion of the heat through the aquifer occurred and the high-temperature zone expanded. Since the high-temperature zone is located beneath another conductor, which acts as caprock, we assume that the energy of the phreatic explosion is accumulated there.  相似文献   

Preliminary report on the activity at Unzen Volcano (Japan), November 1990-November 1991: Dacite lava domes and pyroclastic flows     

Setsuya Nakada  Toshitsugu Fujii 《Journal of Volcanology and Geothermal Research》1993,54(3-4)

The eruption of Unzen Volcano commenced on 17 November 1990. Phreatic and phreatomagmatic eruptions occurred by early May 1991. No large-scale explosive eruptions preceded the extrusion of lava domes. Lava domes appeared in a summit crater on 20 May 1991, and they grew on the steep slope of Mt. Fugen at Unzen Volcano. Rockfalls from the margins of the domes frequently generated pyroclastic flows. Major pyroclastic flows occurred on 3 June, 8 June, and 15 September 1991. The 3 June pyroclastic flow killed forty-three persons. Many of the pyroclastic flows seem to have resulted from the simple rockfalls, except one flow on 8 June, which was accompanied by an explosion from the crater. Many of the rockfalls that generated pyroclastic flows were witnessed. As of November 1991. Unzen Volcano was still active with a nearly constant magma-supply rate of about 0.3 × 10⁶ m³/d. The total magma output exceeded 45 × 10⁶ m³ by the beginning of November 1991. The volume of the lava domes is more than 23 × 10⁶ m³.  相似文献   

Radiocarbon dates of elevated shorelines,Indonesia and Malaysia. Part 1     

H.D. Tjia  Shoji Fujii  Kunihiko Kigoshi  Arata Sugimura  Thomas Zakaria 《Quaternary Research》1972

Four new radiocarbon dates of elevated strandlines in tectonically active areas of eastern Indonesia and East Malaysia indicate average rates of uplift that range between 4.5 and 9 mm annually during the past 24,000 yr. These values are at least three times higher than former estimates from eastern Indonesia. Another radiocarbon date from the south arm of Sulawesi—also tectonically mobile—indicates a rate of uplift of 1.4–2.5 mm per year which corresponds with earlier determinations. This particular case, however, suggests that the sample was probably located close to a north-south axis about which southern Sulawesi was tilted during the Quaternary.  相似文献   

Short-range prediction experiments of the Kuroshio path variabilities south of Japan   总被引：1，自引：0，他引：1  

Norihisa Usui  Hiroyuki Tsujino  Yosuke Fujii  Masafumi Kamachi 《Ocean Dynamics》2006,56(5-6):607-623

Assimilation and prediction experiments of the Kuroshio path variability south of Japan were conducted to investigate the predictability of the Kuroshio path. The assimilation and prediction system is composed of an eddy-resolving model and a three-dimensional variational analysis scheme with vertical coupled temperature–salinity empirical orthogonal function modes. The sea surface height (SSH) variability and the variations of the Kuroshio path of the assimilation fields are in good agreement with those observed. The results of the assimilation are then used as the initial conditions for 138 cases of 90-day prediction experiments conducted from 1993 to 2004. The predictive limit of our system is assessed by the SSH anomaly in the assimilation field and is found to be around 40–60 days, which is much longer than that of the persistence. The prediction results show good performance in the transition stage from a straight to a meandering path. For example, a large meandering event that occurred in August 2004 is successfully predicted in a 2-month forecast. Two types of failure cases are investigated. One is a case where the eastward propagation speed of the meander is faster than a real state. The dynamical response of the model to the assimilation revealed that an initial shock, caused by the dynamically unbalanced initial condition, induces the fast eastward propagation of the meander. The other case exhibits an unrealistic meander. In this case, a cold anomaly at an intermediate layer in the initial condition grows rapidly and results in the unrealistic meander. This implies that the Kuroshio path south of Japan has a chaotic nature. These facts revealed by the failure cases give us some insight for improving the predictive skill of the Kuroshio path variability.  相似文献   

Composition of liquids coexisting with spinel lherzolite at 10 kbar and the genesis of MORBs     

Toshitsugu Fujii  Christopher M. Scarfe 《Contributions to Mineralogy and Petrology》1985,90(1):18-28

Because of the controversy over the nature of the parental magma for MORBs, experiments have been performed at 10 kbar in order to assess the effect of modal variations in the source peridotite and the effect of temperature (degree of partial melting) on the composition of partial melts. A peridotite-basalt sandwich method was used and a run duration of 72 h was found to be necessary to equilibrate basalt and peridotite. A range of melt compositions, coexisting with olivine, orthopyroxene, clinopyroxene and spinel, was produced at 10 kbar, indicating that partial melting of peridotite cannot be regarded as isobarically pseudoinvariant. On projections in the normative tetrahedron OL-PL-CPX-SIL, the liquids obtained in this study define an area, rather than a point or narrow band. The compositions of some liquids in this study are similar to magnesian MORBs (MgO>9.5 wt%), providing evidence in support of the derivation of magnesian MORBs by partial melting of mantle lherzolite at about 10 kbar.  相似文献   

Nonlinear surface ground analysis via statistical approach     

Izuru Takewaki  Nobuyoshi Fujii  Koji Uetani 《Soil Dynamics and Earthquake Engineering》2002,22(6)

A statistical approach is proposed for nonlinear surface ground analysis. In contrast to the conventional method which deals with only a single ground motion for equivalent linearization of soil properties, a design response spectrum defined at the upper level (bottom of the surface ground) of an engineering bedrock can be handled as the target design earthquake in the present paper. The effective shear strain in each soil layer is evaluated by means of a statistical procedure in which the mean peak shear strain is computed in terms of its standard deviation and the corresponding peak factor. The stiffness and damping ratio of each soil layer are obtained iteratively from the nonlinear relation of stiffness reduction factors and damping ratios with respect to the strain level. After the evaluation of the equivalent stiffness and damping ratio of every soil layer, the ground surface response spectrum is transformed from the design response spectrum defined at the upper level of the engineering bedrock via the one-dimensional wave propagation theory. The reliability and accuracy of the proposed analysis method is examined through the comparison with the results by the conventional method (represented by the program) for many simulated spectrum-compatible ground motions.  相似文献   

The 3D electromagnetic response of the Earth to ring current and auroral oval excitation   总被引：4，自引：0，他引：4  

Ikuko Fujii  Adam Schultz 《Geophysical Journal International》2002,151(3):689-709

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Mg-Fe partitioning between olivine and ultramafic melts at high pressures     

Kenji Mibe  Toshitsugu Fujii  Shigeaki Ono 《Geochimica et cosmochimica acta》2006,70(3):757-766

Precise determination of the partitioning of Mg and Fe²⁺ between olivine and ultramafic melt has been made at pressures from 5 to 13 GPa using a MA-8 type multi-anvil high-pressure apparatus (PREM) installed at Earthquake Research Institute, University of Tokyo. A very short rhenium capsule (<100 μm sample thickness) was adopted to minimize temperature variation within the sample container. Synthetic gels with the composition of the upper mantle peridotite were used as starting materials to promote the homogeneity. Analyses of quenched melts and coexisting olivines were made with an electron probe microanalyzer. The obtained partition coefficient, K_D [=(FeO/MgO)^ol/(FeO/MgO)^melt], decreases from 0.35 to 0.25 with increasing pressure from 5 to 13 GPa, suggesting a negative correlation between pressure and K_D above 5 GPa. Our result is consistent with a parabolic relationship between K_D and degree of polymerization (NBO/T) of melts reported by previous studies at lower pressures. The negative correlation between pressure and K_D suggests that olivine crystallizing in a magma ocean becomes more Mg-rich with depth and that primary magmas generated in the upper mantle become more Fe-rich with depth than previously estimated.  相似文献   

Time variations of hard X-rays from Sco X-1     

M. Matsuoka  M. Fujii  S. Miyamoto  J. Nishimura  M. Oda  Y. Ogawara  S. Hayakawa  I. Kasahara  F. Makino  Y. Tanaka  P. C. Agrawal  B. V. Sreekantan 《Astrophysics and Space Science》1972,18(2):472-490

Simultaneous hard X-ray and optical observations of Sco X-1 were carried out on 1971 May 1 at Hyderabad, India, when Sco X-1 was optically bright. The X-ray intensity observed by balloon-borne counter telescopes increased in coincidence with optical enhancements, while the plasma temperature derived by fitting the X-ray spectrum in the energy range 20–40 keV to the thermal bremsstrahlung spectrum did not appreciably change over the whole period of observation.  相似文献   

Ion upflow and downflow at the topside ionosphere observed by the EISCAT VHF radar   总被引：2，自引：0，他引：2  

M. Endo  R. Fujii  Y. Ogawa  S. C. Buchert  S. Nozawa  S. Watanabe  N. Yoshida 《Annales Geophysicae》2000,18(2):170-181

We have determined the MLT distribution and KpK_P dependence of the ion upflow and downflow of the thermal bulk oxygen ion population based on a data analysis using the EISCAT VHF radar CP-7 data obtained at Tromsø during the period between 1990 and 1996: (1) both ion upflow and downflow events can be observed at any local time (MLT), irrespective of dayside and nightside, and under any magnetic disturbance level, irrespective of quiet and disturbed levels; (2) these upflow and downflow events are more frequently observed in the nightside than in the dayside; (3) the upflow events are more frequently observed than the downflow events at any local time except midnight and at any K_P level and the difference of the occurrence frequencies between the upflow and downflow events is smaller around midnight; and (4) the occurrence frequencies of both the ion upflow and downflow events appear to increase with increasing KP level, while the occurrence frequency of the downflow appears to stop increasing at some KP level  相似文献