High-resolution receiver function (RF) images of the upper mantle structure beneath the Japan Islands are obtained by RF analysis of the P-wave coda portions of 389 teleseismic events observed at 138 high-density broadband stations. We construct RFs through frequency-domain division using a water level of 0.01 and Gaussian low-pass filters of 1.0, 0.5, 0.3, and 0.1 Hz, and produce depth-migrated RFs using the one-dimensional IASP91 velocity model. The RF images clearly resolve the subducting Pacific Plate to a depth of 200 km, and reveal a local 30 km elevation of the 410 km discontinuity within the subducting plate. The 660 km discontinuity is also found to exhibit a broad 50 km depression under the influence of the stagnating slab. From analysis of the frequency dependence of the RFs, the thickness of the 410 km discontinuity is inferred to vary on a relatively local scale, whereas the 660 km transition is sharp throughout the Japan Islands.

View full abstract

To examine the effects of temperature-dependent lattice thermal conductivity (lattice-k) on the thermal convection with a temperature-dependent viscous fluid, particularly on the upper thermal boundary layer, we have studied numerically 2-D thermal convective flows with both constant thermal conductivity (constant-k) and lattice-k models. Numerical experiments with large viscosity contrasts, greater than a million, produce a cooler and thinner upper thermal boundary layer for the lattice-k compared with those for the constant-k, implying that thermal convection with lattice-k produces a much sharper boundary between the lithosphere and asthenosphere. The differences between the constant-k and lattice-k can be reasonably explained by the following two causes: (i) the decreasing lattice-k with depth increases an effective Rayleigh number around the bottom of the thermal boundary layer, and (ii) the distribution of lattice-k and uniform vertical heat flux within the thermal boundary layer determine the temperature distribution. The predicted sharper boundary, i.e. sharper vertical viscosity gradient near the bottom of the lithosphere, may play an important role on controlling the amount of lithospheric deformation associated with the downwelling.

View full abstract

A detailed paleomagnetic and rock magnetic investigation was carried out on calcareous nodule layers embedded in the loess-paleosol sequences near Baoji city, southern Chinese Loess Plateau. The anisotropy of low-field magnetic susceptibility confirms that calcareous nodules retain the primary sedimentary fabric of original deposits. Rock magnetic properties demonstrate that calcareous nodules have the same mineralogy and grain sizes as loess-paleosol deposits. Characteristic remanent magnetizations of calcareous nodules have directions consistent with those of adjacent loess and paleosol deposits. The Punaruu event, previously found from loess deposits in the upper part of loess unit L13 in Weinan, southern Chinese Loess Plateau, is observed at the corresponding stratigraphic level in the calcareous nodule layer. These observations indicate that the calcareous nodule layers preserve primary remanent magnetizations of original deposits before calcification, and that they can contribute to paleomagnetic studies in the Chinese Loess Plateau.

View full abstract

The numerical two-mode electromagnetic model of an ionospheric dynamo is constructed taking into account a skin effect of a field in an anisotropic plasma, i.e. a non-potential electric field of Sq-variations in the ionosphere. It is shown that a three-dimensional current system generated by the dynamo action of winds is located at altitudes less than about 200 km basically. It does not depend on an ionosphere condition in upper layers and weaker field-aligned currents between the northern and southern hemispheres practically, and an almost complete closed current circuit is generated in the dynamo layer.

View full abstract

VHF amplitude scintillations were recorded at Tirunelveli (8.7°N, 77.7°E; 0.6°N dip Latitude), Pondicherry (12°N, 79.1°E; 4.4°N dip Latitude) and Mumbai, (19°N, 73°E; 13.5°N dip Latitude) for the years 1992-1996 using the 250 MHz radio beacon from the geostationary satellite FLEETSAT (73°E). The recorded digital scintillation data for few nights are analyzed to estimate scintillation index (S₄), fade rates, auto-correlation functions and power spectral densities for every 2.5 minute sample during the period of the scintillation activity. The power spectral slopes are shallower for the scintillation at the generation phase and steeper towards the decay phase, which indicates the erosion of smaller scale sizes towards the decay phase.

View full abstract

A newly installed meteor radar has been installed to measure winds in the mesosphere and lower thermosphere (MLT) over Wuhan (114.4°E, 30.6°N). In the present study, a database of the first 25 months (February 2002-February 2004) of observations has been analyzed to investigate the climatology of mean winds and tides. The daily average zonal wind is charactered by a strong shear in solstices and an intense eastward flow in summer. The daily average meridional wind is northward in winter and southward in other seasons. There are some discrepancies between the radar mean winds and the HWM93 model winds. The summer zonal winds and meridional winds from the model are obviously weaker than our observations. The analysis on tides indicates that the diurnal tide is dominant at Wuhan. The seasonal variability is observed in both the diurnal and semidiurnal tidal amplitudes with the maximum values occurring usually near the equinoxes. Compared with the Global Scale Wave Model (GSWM00), the observed results generally show a smaller diurnal tidal amplitude and a larger semidiurnal tidal amplitude.

View full abstract

We studied the effects of the surface roughness and porosity of a sample layer on its light-scattering properties in laboratory experiments using fly ash, iron, graphite, and olivine powders. Three types of surface structure were prepared: compacted, knocked, and fluffy surfaces. The surface roughness is represented by the mean slope angle of small facets on the surface. We found a positive correlation between the surface roughness and the porosity of the layer. The bidirectional reflectance of the surface at the wavelength of a He-Ne laser (633 nm) was measured to illustrate the influence of surface structure on scattering properties, with the incidence angle fixed at 0°, while varying the phase angle from 2 to 80°. The reflectance of a relatively rough surface was lower than that of a relatively smooth one for all of the materials measured. The reflectance measured at 30° in phase angle decreased by between -25 and -60%. This effect may explain the discrepancy between the absolute reflectance in previous laboratory results and the observed results for C class asteroids (Kamei and Nakamura, 2002; Nakamura et al., 2002).

View full abstract

We confirmed existence of a buried large block beneath seafloor off Cape Shionomisaki, Kii Peninsula, southwestern Japan, by improving marine gravity data. We found that the 1944 Tonankai and 1946 Nankai earthquakes occurred close to this block. The block is characterized by high gravity anomalies of 80 mGal larger than surrounding area with a diameter of about 40 km. Center of the anomaly is located 30 km off Cape Shionomisaki, southern end of the Kii Peninsula. This high gravity anomaly can be explained by existence of a higher density body with approximate thickness of 8 km just beneath the seafloor. This block coincides with the reported high velocity zone from recent seismic refraction survey in this area. Judging from seismic velocity, density, and geologic evidences, the block presumably consists of intruded acidic rocks, not seamount.

View full abstract