Long-period ground motions from the 2003 Tokachi-oki earthquake

We study characteristics of long-period ground motions from the 2003 Tokachi-oki earthquake (Mj 8.0), a large interplate earthquake, based on spatial distribution maps and attenuation relationships for four kinds of peak ground velocity (PGV) value. The first kind (PGV(WB)) is obtained from a maximal value of vector sum of the three-component, wide-band velocity seismograms, and the other three kinds (PGV(BP10), PGV(BP20), and PGV(BP30)) are obtained from a maximal value of vector sum of the three-component, narrow band-pass filtered velocity seismograms (the central periods are 10, 20, and 30 s). The spatial distribution maps for all kinds of PGV value show azimuth dependence; the PGV values in Hokkaido, northern side of the epicenter are larger than those in Tohoku, southwestern side of the epicenter, when compared at a comparable distance. We find that the features result from the radiation pattern of long-period surface waves, that is, the source effect. The attenuation relationships show the following trends: The PGV(WB) values are larger than the sum of the PGV(BP10), PGV(BP20), and PGV(BP30) at distances (D) less than 200 km, while the PGV(WB) values are comparable to the sum of the PGV(BP20) and PGV(BP30) at D > 200 km. This indicates that the PGV(WB) values at D < 200 km are affected by ground motions with periods less than 10 s, while long-period surface waves mainly contribute to the PGV(WB) values at D > 200 km. The basin site effects generate a patchy pattern in the spatial distribution maps and a large scattering in the attenuation relationships for the PGV(WB) and PGV(BP10) values. Finally, we conclude that the PGV(WB) values from the 2003 Tokachi-oki earthquake are controlled by the radiation pattern of long-period S and surface waves and various basin site effects.     

A diagnostic study of the impact of El Niño on the precipitation in China

The impact of El Niño on the precipitation in China for different seasons are investigated diagnostically. It is found that El Niño can influence the precipitation in China significantly during its mature phase. In the Northern winter, spring and autumn, the positive precipitation anomalies are found in the southern part of China during the El Niño mature phase. In the Northern summer, the patterns of the precipitation anomalies in the El Niño mature phase are different from those in the other seasons. The negative precipitation anomalies appear in both southern and northern parts of China, while in between around the lower reaches of the Yangtze River and the Huaihe River valleys the precipitation anomalies tend to be positive. In the Northern winter, spring and autumn, the physical process by which El Niño affects the precipitation in the southern part of China can be explained by the features of the circulation anomalies over East Asia during the El Niño mature phase (Zhang et al., 1996). The appearance of an anticyclonic anomaly to the north of the maritime continent in the lower troposphere during the El Niño mature phase intensifies the subtropical high in the western Pacific and makes it shift westward. The associated southwesterly flow is responsible for the positive precipitation anomalies in the southern part of China. In the Northern summer, the intensified western Pacific subtropical high covers the southeastern periphery of China so that the precipitation there becomes less. In addition, the weakening of the Indian monsoon provides less moisture inflow to the northern part of China.     

Thermal metamorphism of the C,G, B,and F asteroids seen from the 0.7 μm, 3 μm,and UV absorption strengths in comparison with carbonaceous chondrites

Abstract Thermal metamorphism study of the C, G, B, and F asteroids has been revisited using their UV, visible, NIR, and 3 μm reflectance spectra. High-quality reflectance spectra of seven selected C, G, B, and F asteroids have been compared with spectra for 29 carbonaceous chondrites, including thermally-metamorphosed CI/CM meteorites. There are three sets of spectral counterparts, among which 511 Davida and B-7904 are the most similar to each other in terms of both spectral shape and brightness. By comparing the 0.7 μm and 3 μm absorption strengths of 21 C, G, B, and F asteroids and heated Murchison samples, these asteroids have been grouped into three heating-temperature ranges. These correspond to (1) <400 °C: phyllosilicate-rich; (2) 400–600 °C: phyllosilicates transformed to anhydrous silicates; and (3) >600 °C: fully anhydrous. A good correlation between the UV and 3 μm absorption strengths has been confirmed for the C, G, B, and F asteroids and the CI, CM, and CR meteorites. A plot of the UV absorption strength vs. the IRAS diameter for 142 C, G, B, and F asteroids shows that the maximum UV absorption strength decreases as the diameter increases for the asteroids >60 km, with a notable exception, Ceres. These relationships suggest that some of the larger asteroids may be the heated inner portions of once larger bodies and that common CI/CM meteorites may have come from the lost outer portions, which escaped extensive late-stage heating events.     

Interactions of uranium with bacteria and kaolinite clay

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.     

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

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.     

Improving the prospects for detecting extrasolar planets in gravitational microlensing events in 2002

Gravitational microlensing events of high magnification have been shown to be promising targets for detecting extrasolar planets. However, only a few events of high magnification have been found using conventional survey techniques. Here we demonstrate that high-magnification events can be readily found in microlensing surveys using a strategy that combines high-frequency sampling of target fields with on-line difference imaging analysis. We present 10 microlensing events with peak magnifications greater than 40 that were detected in real-time towards the Galactic bulge during 2001 by the Microlensing Observations in Astrophysics (MOA) project. We show that Earth-mass planets can be detected in future events such as these through intensive follow-up observations around the event peaks. We report this result with urgency as a similar number of such events are expected in 2002.     

Grain size of the surface regolith of asteroid 4 Vesta estimated from its reflectance spectrum in comparison with HED meteorites

Abstract— The grain-size distribution of the regolith of asteroid 4 Vesta has been estimated by comparing its reflectance spectra (0.3–2.6 μm) with those of HED meteorites. The finest grain-size separate (<25 μm) of a particular howardite has a reflectance spectrum most similar to Vesta's. In order to better simulate Vesta's surface mineralogy, reflectance spectra of those finest HED meteorite powders were linearly combined, and Vesta's spectrum was scaled for the best fit between them. Both the albedo and the shape of reflectance spectrum of Vesta were well reproduced by regional mixtures of the finest (<25 μm) powders of HED meteorites. The result suggests the heterogeneity of Vesta's surface and provides an estimate of the visible reflectance of Vesta that is close to its IRAS albedo. Thus, this suggests that fine grains can be generated and retained by relatively small bodies (Vesta is approximately 500 km in diameter).     

Numerical Study of Formation of Dichothermal Water in the Bering Sea

A one-dimensional numerical model with a level-2.5 turbulent closure scheme to provide vertical mixing coefficients has been used to investigate the process by which the dichothermal water is formed in the Bering Sea, the density of which is about 26.6 sigma-theta. The water column to be simulated is assumed to move along a predetermined path. That is, the present model is of the Lagrangian-type. Surface boundary conditions are given using the climatologies of heat, freshwater and momentum fluxes. In order to obtain a plausible moving speed of the water column along the path, pre-liminary experiments were done using the surface fluxes in the central part of the Bering Sea for the initial temperature and salinity profiles at the entrance of the Sea. As a result, it was found that the temperature minimum layer, i.e., the dichothermal water with temperature similar to the climatology at the exit of the Bering Sea, was formed after about two years of integration. Based on the result, the movement speed of the water column along the path was set as 4.5 cm/s in the standard run. It was found that this model could plausibly reproduce the subsurface temperature minimum layer. That is, the dichothermal water was formed in the winter mixed layer process in the Bering Sea. The existence of the subsurface halocline (pycnocline) prohibited the deeper penetration of the winter mixed layer, and therefore water with a temperature colder than that under the mixed layer was formed in the mixed layer due to wintertime surface cooling. In the warming season this water remains as the subsurface temperature minimum layer between the upper seasonal thermocline and the lower halocline. This revised version was published online in August 2006 with corrections to the Cover Date.     

Reliability of ADCP data detided with a numerical model on the East China Sea shelf

The reliability of a numerical tide model for detiding acoustic Doppler current profiler (ADCP) data is examined on the East China Sea shelf. The process is adopted for the ADCP data obtained on 12–13 May 2003. The ocean model accompanied by the most precise harmonic constants available to public is used to compute external tides. The root mean square difference is less than 10 cm/s between the detided currents and those using the least squares method, and so the detiding process using the numerical model is available to detect the East China Sea shelf circulation faster than this speed.     

Winter Mixed Layer and Formation of Dichothermal Water in the Bering Sea

The temperature minimum layer, called “dichothermal water”, is a characteristic feature of the North Pacific subarctic gyre. In particular, dichothermal water having a density of approximately 26.6 sigma-theta (σ_θ), which corresponds to the densest water outcropping in winter in the North Pacific, is seen in the Bering Sea. In order to clarify the water properties, and the area in which and the process by which the dichothermal water is formed, a new seasonal mean gridded climatological dataset with a fine resolution for the Bering Sea and adjacent seas has been prepared using historically accumulated hydrographic data. Although the waters of the Alaskan Stream have temperature minimum layers, their temperature inversions are very weak in climatologies and the core densities of the temperature minimum layers are much lighter than 26.6σ_θ. On the other hand, in the Bering Sea one can see the robust structure of temperature minimum layers, the core density of the dichothermal water being around 26.6σ_θ. In addition, it has been found that the properties of the dichothermal water observed in the warming season are almost the same as those in the winter mixed layer. That is, the dichothermal waters are formed in the winter mixed layer in the Bering Sea. Since these waters are found in the Kamchatka Strait, i.e., the main exit of the Bering Sea waters, it can be supposed that the dichothermal waters are exported from the Bering Sea to the Pacific Ocean by the Kamchatka Current. This revised version was published online in July 2006 with corrections to the Cover Date.