Rapid and frequent turbidite accumulation in the bottom of Izu-Ogasawara Trench: Chemical and radiochemical evidence   总被引：1，自引：0，他引：1  

Yoshiyuki Nozaki  Yoichi Ohta 《Earth and Planetary Science Letters》1993,120(3-4):345-360

Two sediment cores (pilot gravity and piston) were obtained from the bottom of the Izu-Ogasawara Trench at 9750 m and analyzed for various elements and radioisotopes. The results showed a history of complex and frequent turbidite deposition: In the gravity core, eight layers rich in manganese were observed, of which five are enriched in Cu and Co as well. The other three are also enriched in Mo but no other heavy metals, suggesting the presence of at least two mechanisms of formation. Trapping of iron manganese micronodules can account for the enrichment of Mn, Cu and Co. The other three layers rich in Mn and Mo appear to be formed by a post-depositional diagenetic process of Mn mobilization and redeposition in the sediment column. A strong correlation between ²²⁶Ra and Cu in the gravity core suggests that the ²²⁶Ra was also carried into the bottom of the trench in turbidites in association with Mn micronodules. Little excess of ²¹⁰Pb over ²²⁶Ra was found at the top but the excess was significant at mid-depths from 30 to 70 cm, indicating that those sediments were deposited within the last 200 y.

In the piston core there is a sharp discontinuity of chemical and radiochemical composition around a depth of 250 cm. Below that depth the sediments appear to be dominated by materials derived from terrestrial sources, as compared with those in the upper layer which are of contemporary marine origin. ²²⁶Ra is deficient relative to ²³⁰Th throughout the sediment column down to 6 m. This finding is consistent with the finding that the average rate of sediment accumulation is 1–2 orders of magnitude faster than that in the western North Pacific abyssal plain, suggesting the convergence of materials into the bottom of the trench.  相似文献   

Editorial     

T. Nakano 《GeoJournal》1981,5(6):518-520

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A fluidized landslide on a natural slope by artificial rainfall   总被引：9，自引：5，他引：4  

Hirotaka?Ochiai  Yasuhiko?OkadaEmail author  Gen?Furuya  Yoichi?Okura  Takuro?Matsui  Toshiaki?Sammori  Tomomi?Terajima  Kyoji?Sassa 《Landslides》2004,1(3):211-219

An experiment to induce a fluidized landslide by artificial rainfall was conducted on a natural slope at Mt. Kaba-san in the village of Yamato, Ibaraki Prefecture, Japan. The experimental slope was 30 m long, 5 m wide, and the average slope gradient was 33°. A landslide initiated 24,627.5 s (410 m/27.5 s) after the start of sprinkling at a rainfall intensity of 78 mm/h. The landslide mass was 14 m long and 1.2 m deep (at maximum). It first slid, then fluidized, and changed into a debris flow. The travel distance was up to 50 m in 17s. The apparent friction angle of the fluidized landslide was 16.7°. Formation of the sliding surface was detected by soil-strain probes. Motion of the surface of the failed landslide mass was determined by stereo photogrammetry.  相似文献   

Superconducting gravimeter observations without a tilt compensation system     

Sachiko Iwano  Yoichi Fukuda 《Physics of the Earth and Planetary Interiors》2004,147(4):343-351

This paper examines the possibility of using superconducting gravimeter (SG) observations without a tilt compensation system. SG data obtained at Syowa Station, Antarctica, were recorded without tilt compensation from April 5, 2001 to January 4, 2002, however, tilt signals were registered during this time period. A tilt correction was applied to gravity data from August 2, 2001 to January 4, 2002. After the tilt correction, errors of tidal parameters were dramatically reduced and tidal parameters themselves almost coincide with those from the same length of tilt-controlled gravity data recorded in 2000, when tilt compensation system was used. The noise level of the thermal leveler in the seismic band was also investigated. Averaged power spectra of gravity from 15 quiet days each in 2000 and 2001 recorded with and without the tilt compensation system, respectively, were compared. ‘Quiet’ in this case, means very low earthquake activity and calm wind conditions. No significant difference in the seismic band, except at the frequency of 2 mHz, was noticed. The difference at 2 mHz occurred because of room temperature variations caused by the water cooler, which cools down a compressor by automatically switching on and off.  相似文献   

A geophysical interpretation of the secular displacement and gravity rates observed at Ny-Ålesund, Svalbard in the Arctic—effects of post-glacial rebound and present-day ice melting     

Tadahiro Sato  Jun'ichi Okuno  Jacques Hinderer  Daniel S. MacMillan  Hans-Peter Plag †  Olivier Francis ‡  Reinhard Falk  Yoichi Fukuda 《Geophysical Journal International》2006,165(3):729-743

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The dust distribution in some smallHII regions     

M. Nakano  T. Kogure  S. Mizuno  K. Sakka  T. Sasaki  S. D. Wiramihardja 《Astrophysics and Space Science》1983,89(2):407-419

Based on the calibrated maps in the Hα andV bands, simple shell models for the distributions of ionized gas and dust are calculated for the smallHii regions S237 and S254–S257. In deriving the dust distribution from theV-band maps, it is assumed that scattering particles are made of dirty ice. The results of calculation show that a dust depletion zone should be placed in the central region of each of the observedHii regions. The formation of this dust depletion zone and the evolutionary state of theseHii regions are briefly discussed.  相似文献   

Source mechanism of long-period events at Kusatsu–Shirane Volcano, Japan, inferred from waveform inversion of the effective excitation functions     

Masaru Nakano  Hiroyuki Kumagai  Bernard A. Chouet 《Journal of Volcanology and Geothermal Research》2003,122(3-4):149-164

We investigate the source mechanism of long-period (LP) events observed at Kusatsu–Shirane Volcano, Japan, based on waveform inversions of their effective excitation functions. The effective excitation function, which represents the apparent excitation observed at individual receivers, is estimated by applying an autoregressive filter to the LP waveform. Assuming a point source, we apply this method to seven LP events the waveforms of which are characterized by simple decaying and nearly monochromatic oscillations with frequency in the range 1–3 Hz. The results of the waveform inversions show dominant volumetric change components accompanied by single force components, common to all the events analyzed, and suggesting a repeated activation of a sub-horizontal crack located 300 m beneath the summit crater lakes. Based on these results, we propose a model of the source process of LP seismicity, in which a gradual buildup of steam pressure in a hydrothermal crack in response to magmatic heat causes repeated discharges of steam from the crack. The rapid discharge of fluid causes the collapse of the fluid-filled crack and excites acoustic oscillations of the crack, which produce the characteristic waveforms observed in the LP events. The presence of a single force synchronous with the collapse of the crack is interpreted as the release of gravitational energy that occurs as the slug of steam ejected from the crack ascends toward the surface and is replaced by cooler water flowing downward in a fluid-filled conduit linking the crack and the base of the crater lake.  相似文献   

Geochemical Characteristics of the Akiyoshi Limestones, Japan and Their Bearing on Exploration for Blind Skarn Deposits     

Takanori Nakano  Shunso Ishihara 《Resource Geology》2003,53(1):29-36

Abstract. Carboniferous-Permian limestones of the Akiyoshi Plateau, in the Inner Zone of southwestern Japan, are composed of essentially pure calcium carbonate containing only small amounts of other elements, and they are accompanied by marble and copper skarn deposits near the contact with late Cretaceous granitoids. The δ¹⁸O values of the Akiyoshi limestones range widely from 7.6 to 28.3% and are mostly lower than those of other areas of the same age (23–29%), whereas the differences among the δ¹³C values are small. The δ¹⁸O values are negatively correlated with Mn and Fe contents. Samples with high δ¹⁸O (>25%) and δ¹³C (>2%) values do not contain Fe, Zn, or Pb, but those with low δ¹⁸O values tend to be rich in these elements, indicating that these elements were introduced by interaction with H₂O dominant fluids, possibly of magmatic origin. Potential scores for evaluating the degree of interaction with hydro thermal fluids were calculated for δ¹⁸O, δ¹³C, Fe, Mn, Zn, Pb, and Sr. Higher scores implying much hydrothermal interaction were evident in the Mt. Hananoyama area, where there are many skarn deposits, and along faults oriented mainly NNW-SSE. Therefore, these are promising areas for exploring for blind deposits. It is likely that the hydrothermal fluid traveled through the limestones along fractures at the time of the granitic intrusions. However, the potential scores here are much smaller than those in the Pb-Zn mineralized area of the Kamioka mine, so more detailed petrological and mineralogical investigations are necessary.  相似文献   

Petrochemistry of Volcanic Rocks in the Hishikari Mining Area of Southern Japan, with Implications for the Relative Contribution of Lower Crust and Mantle-derived Basalt     

Takahiro Hosono  Takanori Nakano 《Resource Geology》2003,53(4):239-259

Abstract. This study presents the petrographical, mineralogical, and geochemical characteristics of Late Pliocene‐Pleistocene volcanic rocks distributed in the Hishikari gold mining area of southern Kyushu, Japan, and discusses their origin and evolution. The Hishikari volcanic rocks (HVR), on the basis of age and chemical compositions, are divided into the Kurosonsan (2.4–1.0 Ma) and Shishimano (1.7–0.5 Ma) Groups, which occur in the northern and southern part of the area, respectively. Each group is composed of three andesites and one rhyodacite. HVR are characterized by high concentrations of incompatible elements compared with other volcanic rocks in southern Kyushu, and have low Sr/Nd and high Th/U, Th/Pb, and U/Pb ratios compared with typical subduction‐related arc volcanic rocks. Modal and whole‐rock compositions of the HVR change systematically with the age of the rocks. Mafic mineral and augite/hypersthene ratios of the andesites decrease with decreasing age in the Kurosonsan Group, whereas in the Shishimano Group, these ratios are higher in the youngest andesite. Similarly, major and trace element compositions of the younger andesites in the former group are enriched in felsic components, whereas in the latter group the youngest andesite is more mafic than older andesites. Moreover, the crystallization temperature of phenocryst minerals decreases with younger age in the former group, whereas the opposite trend is seen in the latter group. Another significant feature is that rhyodacite in the Shishimano Group is enriched in felsic minerals and incompatible elements, and exhibits higher crystallization temperatures of phenocryst minerals than the rhyodacite of the Kurosonsan Group. Geochemical attributes of the HVR and other volcanic rocks in southern Kyushu indicate that a lower subcontinental crust, characterized by so‐called EMI‐type Sr‐Nd and DUPAL anomaly‐like Pb isotopic compositions, is distributed beneath the upper to middle crust of the Shimanto Supergroup. The HVR would be more enriched in felsic materials derived from the lower crust by high‐alumina basaltic magma from the mantle than volcanic rocks in other areas of southern Kyushu. The Kurosonsan Group advanced the degree of the lower crust contribution with decreasing age from 51 %, through 61 and 66 % to 77 %. In the Shishimano Group, the younger rhyodacite and andesite are derived from hotter magmas with smaller amounts of lower crust component (58 and 57 %) than the older two andesites (65 % and 68 %). We suggest that the Shishimano rhyodacite, which is considered to be responsible for gold mineralization, was formed by large degree of fractional crystallization of hot basaltic andesite magma with less lower crustal component.  相似文献   

Magnetic and electric field observations during the 2000 activity of Miyake-jima volcano, Central Japan   总被引：1，自引：0，他引：1  

Yoichi Sasai  Makoto UyeshimaJacques Zlotnicki  Hisashi UtadaTsuneomi Kagiyama  Takeshi HashimotoYuji Takahashi 《Earth and Planetary Science Letters》2002,203(2):769-777

Magnetic and electric field variations associated with the 2000 eruption of Miyake-jima volcano are summarized. For about 1 week prior to the July 8 phreatic explosion, significant changes in the total intensity were observed at a few stations, which indicated uprising of a demagnetized area from a depth of 2 km towards the summit: this non-magnetic source can be regarded as a vacant space itself. Electric and magnetic field variations were observed simultaneously associated with the tilt-step event, which was the abrupt (∼50 s) inflation at a few km depth within the volcano followed by gradual recovery (∼several hours). The electric field is ascribed to the electrokinetic effect most probably due to forced injection of fluids from the source, while the magnetic field to the piezomagnetic effect due to increased pressure. Large magnetic variations amounting to a few tens of nT were observed at several stations since July 8, and they turned almost flat after the August 18 largest eruption. Magnetic changes are explained mostly by the vanishing of magnetic mass in the summit and additionally by the thermal demagnetization at a rather shallow depth. A large increase in the self-potential by 130 mV was also observed near the summit caldera associated with the August 18 eruption, which suggests that the hydrothermal circulation system sustained within the volcano for the past more than 10 years was destroyed by this eruption.  相似文献