Three-dimensional simulations of molecular cloud fragmentation regulated by magnetic fields and ambipolar diffusion     

Takahiro Kudoh  Shantanu Basu  Youichi Ogata  Takashi Yabe 《Monthly notices of the Royal Astronomical Society》2007,380(2):499-505

We employ the first fully three-dimensional simulation to study the role of magnetic fields and ion–neutral friction in regulating gravitationally driven fragmentation of molecular clouds. The cores in an initially subcritical cloud develop gradually over an ambipolar diffusion time while the cores in an initially supercritical cloud develop in a dynamical time. The infalling speeds on to cores are subsonic in the case of an initially subcritical cloud, while an extended (≳0.1 pc) region of supersonic infall exists in the case of an initially supercritical cloud. These results are consistent with previous two-dimensional simulations. We also found that a snapshot of the relation between density (ρ) and the strength of the magnetic field ( B ) at different spatial points of the cloud coincides with the evolutionary track of an individual core. When the density becomes large, both the relations tend to   B ∝ρ^0.5  .  相似文献   

A trial for monitoring temporal variation of seismic velocity using an ACROSS system   总被引：4，自引：0，他引：4  

Koshun Yamaoka  Takahiro Kunitomo  Koji Miyakawa  Kazunori Kobayashi  Mineo Kumazawa 《Island Arc》2001,10(3-4):336-347

Abstract The temporal variation of seismic velocity near the Nojima Fault, which ruptured during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake), was detected using an accurately controlled routine-operated seismic source (ACROSS). The source generates elastic waves by a centrifugal force of an eccentric mass rotating around an axis. The mass is driven with an AC servomotor whose angular position is accurately controlled with reference to a very accurate global positioning system (GPS) clock. The error of the mass' position is less than 0.002 radian and does not accumulate. As a result, the source generates sinusoidal waves of very narrow spectral peaks enabling their detection with an excellent signal-to-noise ratio. Although the stability of the rotation is quite excellent, a large daily variation was found, which seems to be caused by changes in atmospheric temperature. The daily variation was 10% in amplitude and 0.1 radian in phase of the signal observed at the 800 m borehole seismometer. A significant variation was found to be due to that of coupling between the rotational source and the foundation made of reinforced concrete in which the source was situated. In order to make a correction on the signal of the 800 m borehole seismometer, the vibration of the foundation was measured and modeled assuming a rigid body movement. The correction successfully reduced the daily variation by approximately 90%, resulting in a variation of 1% in amplitude and 0.01 radian in phase. The phase variation of 0.01 radian corresponds to 100 μs and less than 0.1% in velocity over 1000 m between the source and the receiver.  相似文献   

Episodic exhumation of accretionary complexes: Fission-track thermochronologic evidence from the Shimanto Belt and its vicinities, southwest Japan     

Takahiro  Tagami  Noriko  Hasebe Chica  Shimada 《Island Arc》1995,4(3):209-230

Abstract Apatite and zircon fission-track (FT) analyses of the Shimanto accretionary complex and its vicinities, southwest Japan, unraveled the episodic material migration of the deep interiors of the accretionary complex. Apatite data with 100°C closure temperature (T_e) generally indicate ~10 Ma cooling throughout the Shimanto complex. In contrast, zircon data with 260°C T_e exhibit a wide range of apparent ages as a consequence of paleotemperature increase to the zircon partial annealing zone. In the Muroto and Kyushu regions, maximum temperatures tend to have been higher in the northern, older part of the complex, with indistinguishable temperature differences between coherent and melange units adjacent to each other. It thus suggests, along with vitrinite reflectance data, that older accretionary units occurring to the north sustain greater maximum burial during the accretion-burial-exhumation process. Zircon data suggest two cooling episodes: ~70 Ma cooling at widespread localities in the Cretaceous Shimanto Belt and Sambagawa Belt, and ~15 Ma cooling in the central Kii Peninsula. The former is consistent with ⁴⁰Ar/³⁹Ar cooling ages from the Sambagawa Belt, whereas the latter slightly predates the widespread 10 Ma apatite cooling ages. These data imply that the extensive material migration and exhumation took place in and around the Shimanto complex in Late Cretaceous as well as in Middle Miocene. Considering tectonic factors to control evolution of accretionary complexes, the episodic migration is best explained by accelerated accretion of sediments due to increased sediment influx at the ancient Shimanto trench, probably derived from massive volcano-plutonic complexes contemporaneously placed inland. Available geo- and thermochronologic data suggest that extensive magmatism triggered regional exhumation twice in the past 100 Ma, shedding new light on the cordilleran orogeny and paired metamorphism concepts.  相似文献   

New age constraints on the cooling and unroofing history of the Trans-Himalayan Ladakh Batholith (Kargil area), N. W. India     

Rasoul B Sorkhabi  Arvind K Jain  Susumu Nishimura  Tetsumaru Itaya  Nand Lal  R M Manickavasagam  Takahiro Tagami 《Journal of Earth System Science》1994,103(1):83-97

Thermotectonic history of the Trans-Himalayan Ladakh Batholith in the Kargil area, N. W. India, is inferred from new age data obtained here in conjunction with previously published ages. Fission-track (FT) ages on apatite fall around 20±2 Ma recording cooling through temperatures of ∼100°C and indicating an unroofing of 4 km of the Ladakh Range since the Early Miocene. Coexisting apatite and zircon FT ages from two samples in Kargil show the rocks to have cooled at an average rate of 5–6°C/Ma in the past 40 Ma. Zircon FT ages together with mica K−Ar cooling ages from the Ladakh Batholith cluster around 40–50 Ma, probably indicating an Eocene phase of uplift and erosion that affected the bulk of the batholith after the continental collision of India with the Ladakh arc at 55 Ma. Components of the granitoids in Upper Eocene-Lower Oligocene sediments of the Indus Molasse in Ladakh supports this idea. Three hornblende K−Ar ages of 90 Ma, 55 Ma, and 35 Ma are also reported; these distinctly different ages probably reflect cooling through 500–550°C of three phases of I-type plutonism in Ladakh also evidenced by other available radiometric data: 102 Ma (mid-Cretaceous), 60 Ma (Palaeocene), and 40 Ma (Late Eocene); the last phase being localised sheet injections. The geodynamic implications of the age data for the India-Asia collision are discussed.  相似文献   

S-wave velocity structures of sediments estimated from array microtremor records and site responses in the near-fault region of the 1999 Chi-Chi,Taiwan earthquake     

Satoh?ToshimiEmail author  Hiroshi?Kawase  Tomotaka?Iwata  Sadanori?Higashi  Sato?Toshiaki  Huey-Chu?Huang 《Journal of Seismology》2004,8(4):545-558

The 1999 Chi-Chi, Taiwan earthquake, M_W = 7.6, caused severe damage in the near-fault region of the earthquake. In order to evaluate site effects in the near-field strong motions we estimate S-wave velocity structures of sediments at four sites using array records of microtremors. We also recalculated S-wave velocity structures at other four sites previously reported. To show the validity of the estimated S-wave velocity structures we separate empirical site responses from aftershock records using the generalized inversion method and show the agreement between empirical and theoretical site responses. We also show an observed fact that suggests soil nonlinearity during the Chi-Chi earthquake by comparing horizontal-to-vertical spectral ratios (HVRs) for main shock records with HVRs for aftershock records. Then we calculate one-dimensional equivalent-linear site responses using the estimated S-wave velocity structures and the main shock records observed on the surface. It is found that site amplification due to thick (about 6 km) sediments is one of the important factors for explaining the long-period velocity pulses of about 5 to 10 sec observed at sites in the footwall during the Chi-Chi earthquake. It is also found that the theoretical site responses of shallow soft sediments at sites that sustained severe damage in the hanging wall shows significant amplification around 1 sec. As the amplitude of velocity pulses with period around 1 sec is most critical in causing damage to ordinary buildings of moderate heights, our results suggest that the 1-sec period velocity pulses, amplified by the site response of shallow sediments should contribute to the severe damage during the Chi-Chi earthquake.  相似文献   

Interactions of uranium with bacteria and kaolinite clay     

Toshihiko Ohnuki  Takahiro Yoshida  Takuo Ozaki  Mohamad Samadfam  Naofumi Kozai  Kunio Yubuta  Toshiaki Mitsugashira  Takeshi Kasama  Arokiasamy J. Francis 《Chemical Geology》2005,220(3-4):237-243

We assessed the accumulation of uranium (VI) by a bacterium, Bacillus subtilis, suspended in a slurry of kaolinite clay, to elucidate the role of microbes on the mobility of U(VI). Various mixtures of bacteria and the koalinite were exposed to solutions of 8 × 10^− 6 M- and 4 × 10^− 4 M-U(VI) in 0.01 M NaCl at pH 4.7. After 48 h, the mixtures were separated from the solutions by centrifugation, and treated with a 1 M CH₃COOK for 24 h to determine the associations of U within the mixture. The mixture exposed to 4 × 10^− 4 M U was analyzed by transmission electron microscope (TEM) equipped with EDS. The accumulation of U by the mixture increased with an increase in the amount of B. subtilis cells present at both U concentrations. Treatment of kaolinite with CH₃COOK, removed approximately 80% of the associated uranium. However, in the presence of B. subtilis the amount of U removed was much less. TEM–EDS analysis confirmed that most of the U removed from solution was associated with B. subtilis. XANES analysis of the oxidation state of uranium associated with B. subtilis, kaolinite, and with the mixture containing both revealed that it was present as U(VI). These results suggest that the bacteria have a higher affinity for U than the kaolinite clay mineral under the experimental conditions tested, and that they can immobilize significant amount of uranium.  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