Wide-Field Camera for Gravitational Microlensing Survey: MOA-cam2

We have constructed a large, mosaic CCD camera called MOA-cam2 which has 4096 × 6144-pixelsto search for gravitational microlensing events. MOA-cam2 has three4096 × 2048-pixel SITe CCD chips, which have a very high quantum efficiency (nearly 80% in the wave region 500 to 800 nm),and three buttable sides. We have placed the threechips side by side with 100 m dead space. MOA-cam2 has been installed on the 61 cm Boller and Chivens telescope of the MOA collaboration at the Mt. John University Observatory (MJUO) in NewZealand since July 1998. The field coverage is 0.92° × 1.38° per exposure. The technical details of MOA-cam2 and the first images obtained with the Boller and Chivens telescope are presented. MOA-cam2 introduces a second phase of research on gravitational microlensing by the MOA collaboration.     

Tectonic implications of damage zone-related fault-fracture networks revealed in drill core through the Nojima fault, Japan

1800 m of drill core through the Nojima fault zone, Japan, reveals subsidiary fault and fracture networks that developed in the fault zone that triggered the 1995 Ms 7.2 Kobe earthquake. The subsidiary fault zones contain a fault gouge of < 1 cm bounded by thin zones of foliated cataclasite or breccia. Fractures are filled with calcite veins, calcite-cemented breccias, clay, and iron-oxide and carbonate alternation of the granitic host rock. These features are typical of extensional fractures that form the conduit network for fluid flux close to a major fault zone. The zone of distributed deformation surrounding the main fault is 50 m in width, and the dip of the Nojima fault at > 1 km depth is 75°. The fault-fracture networks associated with the Nojima fault zone are coseismic and were filled with carbonate and fine-grained material during repeated seismic-related infiltration of the fault zone by carbonate-bearing subsurface water. This study shows that fault-related fracture networks plays an important role as fluid flow conduits within seismically active faults, and can change in character from zones of high permeability to low permeability due to cementation and/or pore collapse.     

Seasonal and interannual variations in the East Sakhalin current revealed by TOPEX/POSEIDON altimeter data

Seasonal and interannual variations in the East Sakhalin Current (ESC) are investigated using ten-year records of the sea level anomaly (SLA) observed by the TOPEX/POSEIDON (T/P) altimeter. The T/P SLA clearly documents seasonal and interannual variations in the ESC along the east coast of Sakhalin Island, although sea ice masks the region from January to April. Estimates of surface current velocity anomaly derived from T/P SLA are in good agreement with drifting buoy observations. The ESC is strong in winter, with a typical current velocity of 30–40 cm s⁻¹ in December, and almost disappears in summer. Southward flow of the ESC is confined to the shelf and slope region and consists of two velocity cores. These features of the ESC are consistent with short-term observations reported in previous studies. Analysis of the ten-year records of T/P SLA confirms that the structure of the ESC is maintained each winter and the seasonal cycle is repeated every year, although the strength of the ESC shows large interannual variations. Seasonal and interannual variations in the ESC are discussed in relation to wind-driven circulation in the Sea of Okhotsk, using wind stress and wind stress curl fields derived from European Centre for Medium Range Weather Forecasts (ECMWF) reanalysis data and a scatterometer-derived wind product. Seasonal and interannual variations of the anticyclonic eddy in the Kuril Basin are also revealed using T/P SLA.     

Variations in turbulent energy dissipation and water column stratification at the entrance of a tidally energetic strait

We observed tidal currents, turbulent energy dissipation and water column stratification at the entrance of a narrow strait (Neko Seto) in the Seto Inland Sea, Japan, using a free-falling turbulent microstructure profiler (TurboMAP) and acoustic Doppler current profiler (ADCP). The variation in turbulent energy dissipation at the entrance of the strait was not at quarter-diurnal frequency but at semi-diurnal frequency; turbulent energy dissipation was enhanced during the ebb tide, although it was moderate during the flood tide. This result is consistent with the results of Takasugi (1993), which showed the asymmetry of tidal energy loss during a semidiurnal tidal cycle using control volume analysis. It is suggested that significant turbulent energy dissipation is generated in the strait, which influences the properties of water outside the strait when tidal currents flow out from the strait.     

Interactive behavior of soil–pile-superstructure system in transient state to liquefaction by means of large shake table tests

Shaking table tests were conducted by means of a large-scale laminar box with 4 m in length, 2 m in width and 2 m in height in order to investigate behavior of a soil-pile-superstructure system in liquefiable ground. A model two-storey structure, supported by a pile group, was set in a saturated sand deposit, and subjected to a sinusoidal base motion with increasing amplitude. Discussions are focused on the transient behavior until soil liquefaction occurs. Main interests are characteristics of springs used in a sway-rocking model and a multi-freedom lumped mass (MFLM) model that are frequently used in soil–pile interaction analysis. The spring constant in the sway-rocking model is represented by restoring force characteristics at the pile head, and that in the MFLM system is represented by an interaction spring connecting the pile to the free field. The transient state prior to soil liquefaction is shown to be important in the design of a pile because dynamic earth pressure shows peak response in this state. The reduction of the stiffness due to excess porewater generation and strain dependent nonlinear behavior is evaluated.     

Minor and trace element incorporation into branching coral Acropora nobilis skeleton

Chemical and isotopic compositions of the Acropora nobilis skeleton were analyzed at various spatial resolutions to investigate the mechanism by which elements are incorporated into the skeleton. Chemical and isotopic profiles along growth axes of axial and radial corallites did not show seasonal variation, with the exception of the δ¹⁸O profile of the axial corallite. Detailed observations of the skeletal structure revealed that the skeletal density increased with distance from the tip because secondarily precipitated aragonite (here called the “infilling” skeleton) filled pore spaces in the “framework” skeleton. Microscale element analyses revealed that main part of the infilling skeleton had lower Mg/Ca and higher Sr/Ca and U/Ca than the framework skeleton. At microscale, Sr/Ca and U/Ca were positively correlated with each other, and negatively correlated with Mg/Ca but only weakly. The results showed that the infilling skeleton differed significantly from the adjacent framework skeleton in terms of not only formation chronology but also chemical composition, and that the bulk composition was influenced by the infilling/framework skeletal ratio. In order to use the Acropora skeleton as a paleoclimate archive, the relationship between environmental factors and the chemical composition of each skeletal component needs to be established.     

Statistical study on the local equilibrium between wind and wind waves by using data from ocean data buoy stations

The local equilibrium between the wind and wind waves, which is defined by a range of the coefficient of the 3/2-power law between the non-dimensional significant wave height and period, is statistically investigated by using wind and wave data obtained at four ocean data buoy stations in the seas near Japan. The friction velocity is calculated from the wind speed measured at one height together with the significant wave period by using formulas of the wave dependent drag coefficient proposed by Tobaet al. (1990). The data for small waves or for weak winds indicate that the waves do not satisfy the criterion for the local equilibrium, because they may be affected by changing winds or remotely generated swells. In the seas near Japan, the data which satisfy the local equilibrium are about 6% through a year. Otherwise swells are dominant in most situations. Changing winds also cause deviations from the local equilibrium. The degree of satisfaction of the local equilibrium can be classified by ranges of the significant wave height. As the significant wave height exceeds 4 m, the local equilibrium is more frequently satisfied.     

In situ measurement of incoming solar radiation by voluntary ships in the western Pacific

We have been performingin situ measurement of downward short wave radiation (solar radiation) in the western Pacific Ocean in cooperation with voluntary ships since autumn 1990 in order to obtain much more precise knowledge of downward short wave radiation at the sea's surface than before. Preliminary result of the observation from autumn 1990 through spring 1992 is shown in this paper. The comparison of observed daily mean downward short wave radiation with that estimated from observed cloudiness by using Reed (1977) formula is also presented to show the necessity ofin situ measurement in the study of the downward short wave radiation at the sea's surface.     

VSOP Space VLBI observations of Pearson-Readhead survey sources

The VSOP Space VLBI mission uses the HALCA spacecraft, launched from Japan in February 1997, in conjunction with ground radio observatories around the world to create a high resolution radio-wavelength imaging facility. We are using this unique facility to observe a sub-sample of Pearson-Readhead Survey sources at 4.8 GHz to determine core brightness temperatures and pc-scale jet properties. We will highlight one of the sources that has been observed using a combination of the HALCA spacecraft and the EVN, 1642+690, and describe the preliminary brightness temperature distribution for the sub-sample, based on all data analyzed to date.     

Metallicity Distributions of Elliptical Galaxies and Globular Cluster Systems

Gradients of absorption line indices are studied and mean stellar metallicities are estimated for 46 elliptical galaxies. The mean stellar metallicities range from 〈 [Fe/H] 〉 ≃ =0.8 to +0.2 and ellipticals with smaller central velocity dispersions tend to have lower 〈 [Fe/H] 〉 thus the mass-metallicity relation holds not only for the galaxy center but also for the whole part of the galaxy. There is an evidence that the magnesium is enhanced systematically in all ellipticals by 0.2 dex with respect to the iron. Giant elliptical galaxies show lack of metal-poor stars (the G-dwarf problem). Metal-poor globular clusters of ellipticals formed well in advance of the formation of metal-rich ones which formed simultaneously with the bulk of stars of mother galaxies under the influence of galaxy chemical enrichment. The bimodal [Fe/H] distribution of globular clusters does not necessarily mean that elliptical galaxies formed by the mergers of disc galaxies. This revised version was published online in July 2006 with corrections to the Cover Date.