Long-term variability of the low-luminosity active galactic nucleus of M81     

Naoko Iyomoto  Kazuo Makishima 《Monthly notices of the Royal Astronomical Society》2001,321(4):767-775

Long-term X-ray variability of the low-luminosity active galactic nucleus of M81 was studied, using 16 ASCA observations spanning 5.5 yr. The object exhibits a factor of 3 variation over the 5.5 yr. The source intensity was relatively constant within each observation which lasted typically for one day, but intra-day variability by 30 per cent was detected on the 15th observation. The power-spectral density (PSD) was estimated in a 'forward' manner, over a frequency range of 10^−8.2–10^−4.3 Hz (period range of 0.25 d–5.5 yr), by utilizing the structure function and extensive Monte Carlo simulations in order to overcome the very sparse and uneven data samplings. When the PSD is assumed to be white below a 'break frequency' f _b and falls off as ∝  f ^{− α} above f _b, where f is frequency and α is a positive parameter, the M81 light curve is well described with 1/ f _b≥800 d and α =1.4±0.2.  相似文献   

Deformation mechanisms and fluid behavior in a shallow, brittle fault zone during coseismic and interseismic periods: Results from drill core penetrating the Nojima Fault, Japan   总被引：1，自引：0，他引：1  

Hidemi Tanaka  Shin-Ichiro Hinoki  Kazuo Kosaka  Aiming Lin  Keiji Takemura  Akihiro Murata  Takao Miyata 《Island Arc》2001,10(3-4):381-391

Abstract This paper describes the results of petrographical and meso- to microstructural observations of brittle fault rocks in cores obtained by drilling through the Nojima Fault at a drilling depth of 389.52 m. The zonation of deformation and alteration in the central zone of the fault is clearly seen in cores of granite from the hanging wall, in the following order: (i) host rock, which is characterized by some intragranular microcracks and in situ alteration of mafic minerals and feldspars; (ii) weakly deformed and altered rocks, which are characterized by transgranular cracks and the dissolution of mafic minerals, and by the precipitation of zeolites and iron hydroxide materials; (iii) random fabric fault breccia, which is characterized by fragmentation, by anastomosing networks of transgranular cracks, and by the precipitation of zeolites and iron hydroxide materials; and (iv) fault gouge, which is characterized by the precipitation of smectite and localized cataclastic flow. This zonation implies that the fault has been weakened gradually by fluid-related fracturing over time. In the footwall, a gouge layer measuring only 15 mm thick is present just below the surface of the Nojima Fault. These observations are the basis for a model of fluid behavior along the Nojima Fault. The model invokes the percolation of meteoric fluids through cracks in the hanging wall fault zone during interseismic periods, resulting in chemical reactions in the fault gouge layer to form smectite. The low permeability clay-rich gouge layer sealed the footwall. The fault gouge was brecciated during coseismic or postseismic periods, breaking the seal and allowing fluids to readily flow into the footwall, thus causing a slight alteration. Chemical reactions between fluids and the fault breccia and gouge generated new fault gouge, which resealed the footwall, resulting in a low fluid condition in the footwall during interseismic periods.  相似文献   

Critical damping of structures with elastically supported visco‐elastic dampers     

Yujin Lee  Izuru Takewaki  Koji Uetani  Kazuo Inoue 《地震工程与结构动力学》2002,31(2):481-486

This paper presents a new formulation for critical damping of structures with elastically supported visco‐elastic dampers.Owing to the great dependence of damper performance on the support stiffness, this model is inevitable for reliable modelling of structures with visco‐elastic dampers. It is shown that the governing equation of free vibration of this model is reduced to a third‐order differential equation and the conventional method for defining the critical damping for second‐order differential equations cannot be applied to the present model. It is demonstrated that the region of overdamped vibration is finite in contrast to that (semi‐infinite) for second‐order differential equations and multiple critical damping coefficients exist. However, it turns out that the smaller one is practically meaningful. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

Upper and middle crustal deformation of an arc–arc collision across Hokkaido, Japan, inferred from seismic refraction/wide-angle reflection experiments     

Takaya Iwasaki  Keiji Adachi  Takeo Moriya  Hiroki Miyamachi  Takeshi Matsushima  Kaoru Miyashita  Testsuya Takeda  Takaaki Taira  Tomoaki Yamada  Kazuo Ohtake 《Tectonophysics》2004,388(1-4):59

The Hidaka Collision Zone (HCZ), central Hokkaido, Japan, is a good target for studies of crustal evolution and deformation processes associated with an arc–arc collision. The collision of the Kuril Arc (KA) with the Northeast Japan Arc (NJA), which started in the middle Miocene, is considered to be a controlling factor for the formation of the Hidaka Mountains, the westward obduction of middle/lower crustal rocks of the KA (the Hidaka Metamorphic Belt (HMB)) and the development of the foreland fold-and-thrust belt on the NJA side. The “Hokkaido Transect” project undertaken from 1998 to 2000 was a multidisciplinary effort intended to reveal structural heterogeneity across this collision zone by integrated geophysical/geological research including seismic refraction/reflection surveys and earthquake observations. An E–W trending 227 km-long refraction/wide-angle reflection profile found a complicated structural variation from the KA to the NJA across the HCZ. In the east of the HCZ, the hinterland region is covered with 4–4.5 km thick highly undulated Neogene sedimentary layers, beneath which two eastward dipping reflectors were imaged in a depth range of 10–25 km, probably representing the layer boundaries of the obducting middle/lower crust of the KA. The HMB crops out on the westward extension of these reflectors with relatively high Vp (>6.0 km/s) and Vp/Vs (>1.80) consistent with middle/lower crustal rocks. Beneath these reflectors, more flat and westward dipping reflector sequences are situated at the 25–27 km depth, forming a wedge-like geometry. This distribution pattern indicates that the KA crust has been delaminated into more than two segments under our profile. In the western part of the transect, the structure of the fold-and-thrust belt is characterized by a very thick (5–8 km) sedimentary package with a velocity of 2.5–4.8 km/s. This package exhibits one or two velocity reversals in Paleogene sedimentary layers, probably formed by imbrication associated with the collision process. From the horizontal distribution of these velocity reversals and other geophysical/geological data, the rate of crustal shortening in this area is estimated to be greater than 3–4 mm/year, which corresponds to 40–50% of the total convergence rate between the NJA and the Eurasian Plate. This means that the fold-and-thrust belt west of the HCZ is absorbing a large amount of crustal deformation associated with plate interaction across Hokkaido Island.  相似文献   

Coexistence of a Fluid Phase with Granitic Magma: Geochemical Characteristics of REE and Cu in Miyako Granite and in Scheelite-bearing Aplitic Veins at the Yamaguchi Cu-W Skarn Deposit, Iwate, Japan     

Takeyuki OGATA  Daizo ISHIYAMA  Toshio MIZUTA  Kazuo NAKASHIMA 《Resource Geology》2002,52(2):173-186

Abstract: The North granitic body of the Miyako pluton is located in the Northern Kitakami belt, Northeast Japan. The formation of the scheelite–chalcopyrite–magnetite–bearing aplitic veins and scheelite–chalcopyrite–magnetite–bearing Yamaguchi skarn deposit was closely associated with the formation of the Miyako plutons. Petrographic facies of the North granitic body vary from quartz diorite in marginal zone (zone A), to tonalite and granodiorite (zone B), and to granite (zone C) in the central. The large numbers of aplitic veins distributed around the Yamaguchi mining area are divided into two groups: barren and scheelite–mag–netite–chalcopyrite–bearing aplitic veins. The latter cut massive clinopyroxene skarns of the Yamaguchi deposit, and are composed of plagioclase, K‐feldspar and titanite. Some plagioclase crystals have dusty cores with irregularly shaped K‐feldspar flakes, and clear rims of albite. Textures of plagioclase in the mineralized aplitic veins are different from the idiomorphic textures with sharp plagioclase crystal boundaries that occur in the North granitic body and barren aplitic veins. These textural data suggest that the mineralized aplitic veins were formed from hydrothermal fluid. Changes in the contents of major and minor (Rb, Sr, Sc, Co, Th, U) elements in the North Miyako granitic body are similar to those of zoned plutons formed by typical magmatic differentiation processes. On the other hand, concentrations of REE, especially middle to heavy REE, of granitic rocks in zone C and barren aplitic veins are significantly lower than those of granitic rocks in zones A and B. The hypothetical chondrite‐normalized REE patterns, calculated assuming fractional crystallization from zone B granitic melt, suggest that REE concentrations of the residual melt increased with the degree of fractional crystallization, and changed into a pattern with enriched LREE and strongly negative Eu anomaly. However, the REE patterns of granitic rocks in zone C are different from the hypothetical patterns. Moreover, the REE patterns of magnetite–scheelite–chalcopyrite aplitic veins are quite different from those of granitic rocks. The Cu contents of granitic rocks in the North Miyako body increase from zone A (5–26 ppm) to zone B (10–26 ppm), and then clearly decrease to zone C (5–7 ppm) and drastically increase to the barren aplitic veins (39–235 ppm). Concentrations of Cu in the mineralized aplitic veins are also higher than those of the granitic rocks in zone C. The decrease in REE and Cu contents of granitic rocks from zone B to zone C is not a result of simple magmatic fractional differentiation. Fluid inclusions in quartz from mineralized aplitic veins contain 3.3 wt% NaCl equivalent and 5.8 wt% CO₂. It was also demonstrated experimentally that the removal of MREE and HREE by fluid from melt enabled the formation of complexes of REE and ligands of OH^‐ and CO₃^2‐. Based on the possibility that the melt of the granitic rocks of zone C and the mineralized aplitic veins coexisted with CO₂‐bearing fluid, it is thought that REE were extracted from the melt to the CO₂‐bearing fluid, and that the REE in the mineralized aplitic veins were transported by the CO₂‐bearing fluid. It is likely that the low HREE and Cu contents of the granitic rocks in zone C could have been caused by the removal of those elements from the granitic melt by the fluid coexisting with the melt. The expelled materials could have been the sources of scheelite–magnetite–chalcopyrite–bearing aplitic veins and copper mineralization of the Yamaguchi Cu‐W skarn deposit.  相似文献   

Isotopic compositions of potassium and calcium in magnetic spherules from marine sediments     

Tadashi Shimamura  Shohei Yanagita  Kazuo Yamakoshi  Ken&#x;ichi Nogami  Okio Arai  Yuji Tazawa  Koichi Kobayashi 《Earth and Planetary Science Letters》1979,42(3):379-382

Isotopic compositions of potassium and calcium in individual magnetic spherules were determined. No significant anomaly was observed for potassium within twice the statistical error (2σ), although for calcium isotopes enrichments of⁴⁶Ca,⁴⁴Ca and⁴²Ca were observed in one spherule. The relative excess of⁴⁶Ca,⁴⁴Ca and⁴²Ca in the spherule agrees with the relative yield of spallogenic calcium isotopes observed in iron meteorites. This fact indicates that the enrichment in the calcium isotopes was caused by cosmic ray irradiation of the spherule in outer space.  相似文献   

Short geomagnetic episodes in the Matuyama epoch     

Tsutomu Sueishi  Takaharu Sato  Naoto Kawai  Kazuo Kobayashi 《Physics of the Earth and Planetary Interiors》1979,19(1):1-11

Two short geomagnetic episodes in the middle Matuyama epoch have been identified within thin (4.3-mm) sections of two deep-sea calcareous sediment cores taken in the western equatorial Pacific. Both cores are correlated by magnetostratigraphic and micropaleontological methods. Magnetic stability and paleomagnetic reliability are tested by alternating field demagnetization of natural remanent magnetization as well as by the ratios of intensities of anhysteretic remanent magnetization to those of saturation isothermal remanent magnetization. One episode is dated to be 1.06 m.y. BP. The other is identified to be about 1.94 m.y. BP, which is presumably in agreement with the Reunion event. Both episodes seem to be accompanied by conspicuous drops in field intensity. Possible correlation of field intensity with biological productivity in the ocean is also postulated from the present paleomagnetic results.  相似文献   

Cosmic-ray-produced ⁵⁹Ni in marine sediments     

Kazuo Yamakoshi  Shohei Yanagita 《Earth and Planetary Science Letters》1981,52(2):259-263

⁵⁹Ni in 0.4 g of nickel from deep-sea sediments was measured with an extremely low-level X-ray spectrometer. The obtained specific activity was (5.9 ± 1.8) × 10^?2 dpm/kg sediments. The⁵⁹Ni activity induced by proton and alpha particle irradiation in outer space can be estimated as 300 dpm/kg dust. Hence, the content of extraterrestrial dust in deep-sea sediments was not more than 200 ppm.  相似文献   

Normal faulting of the Daiichi-Kashima Seamount in the Japan Trench revealed by the Kaiko I cruise, Leg 3     

Kazuo Kobayashi  Jean-Paul Cadet  Jean Aubouin  Jacques Boulgue  Jacques Dubois  Roland von Huene  Laurent Jolivet  Toshihiko Kanazawa  Junzo Kasahara  Kin-ichiro Koizumi  Serge Lallemand  Yasuo Nakamura  Guy Pautot  Kiyoshi Suyehiro  Shin Tani  Hidekazu Tokuyama  Toshitsugu Yamazaki 《Earth and Planetary Science Letters》1987,83(1-4)

A detailed topographic and geophysical survey of the Daiichi-Kashima Seamount area in the southern Japan Trench, northwestern Pacific margin, clearly defines a high-angle normal fault which splits the seamount into two halves. A fan-shaped zone was investigated along 2–4 km spaced, 100 km long subparallel tracks using narrow multi-beam (Seabeam) echo-sounder with simultaneous measurements of gravity, magnetic total field and single-channel seismic reflection records. Vertical displacement of the inboard half was clearly mapped and its normal fault origin was supported. The northern and southern extensions of the normal fault beyond the flank of the seamount were delineated. Materials on the landward trench slope are displaced upward and to sideways away from the colliding seamount. Canyons observed in the upper landward slope terminate at the mid-slope terrace which has been uplifted since start of subduction of the seamount. Most of the landward slope except for the landward walls aside the seamount comprises only a landslide topography in a manner similar to the northern Japan Trench wall. This survey was conducted on R/V “Jean Charcot” as a part of the Kaiko I cruise, Leg 3, in July–August 1984 under the auspices of the French-Japanese scientific cooperative program.  相似文献   

Book review     

J. Kazuo Minami 《地震工程与结构动力学》1973,2(2):204-204

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