Many studies have been made on the subduction of the Pacific slab and the magmatism in northeastern Japan, but not on the subduction of the Philippine Sea slab and the magmatism in southwestern Japan. Primary reasons may be that seismological networks in southwestern Japan were sparse as compared with those in northeastern Japan and that geology including volcanism of southwestern Japan is more complicated than that of northeastern Japan. However, recent instrumental development of dense seismological networks in the Japanese Islands has provided us with high quality data not only for northeastern Japan but for southwestern Japan. One of the outcomes from the development is the increase of accuracy of arrival time readings of P- and S-waves and resultant hypocenter determination. We attempt to elucidate fine image of the uppermost mantle structure beneath the Japanese Islands and to find evidence for the relation between the magmatism and subduction process. We apply travel time tomography to 216, 247 P- and 98, 207 S-wave arrival times observed at 1, 328 seismic stations from 5, 242 earthquakes in and around the Japanese Islands, and obtain three-dimensional variations of P- and S-wave velocity structure. In Chubu and Kyushu, the subducting Philippine Sea slab bends downward in the depth range of 50 to 70 km. In some nonvolcanic regions, remarkable anomalies of high Poisson's ratio (and low S-wave velocity) are seen in the depth range of 25 to 40 km near the upper boundary of the Philippine Sea slab or the Moho discontinuity, and approximately coincide with the hypocenter distribution of deep low-frequency earthquakes. The anomalies of high Poisson's ratio are also seen near the upper boundary of the Philippine Sea slab or the overlying mantle wedge down to a depth of about 60 km, but are not seen after the downward bending of the slab, in the forearc region. The anomalies are probably caused by separated fluid or hydrous minerals. These characteristics should be taken into account in numerical modelling of the subduction of young slabs (e. g., Philippine Sea slab) and associated phenomena (e. g., magmatism).

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The quantitative determination of the variable density contrasts for an intermediate horizontal layer has been demonstrated. In particular a sub-surface dipping dike with a priori depth-dependent density contrasts was adopted as a forward model to project gravity anomaly effect above the Earth surface. The sub-surface location and density contrasts in a series of intermediate horizontal layers in the causative dipping dike structure have been recovered by means of inversion analysis. Density contrasts recovery errors of less than 8.0 percent were realized to a depth of 2.00 km on a maximum synthetic gravity anomaly effect of 10.0 mGals that is better in comparison to constant density models. Finally to demonstrate the efficacy of the inverse analysis in the study, the entire process was successfully applied to real field data, i. e., residual gravity anomaly for a micro-gravimetry site and/or localized structures in Matsumoto Basin, Chubu District-Japan.

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A new technique is proposed for the rapid reconstruction of ionospheric electric fields from plasma drift measurements made by the Sondrestrom incoherent scatter radar. We utilized the adaptive distribution of electric charges over and beyond of the radar field-of-view using a well-known method of regularization developed for solving the ill-posed problems. In this approach, no a priori assumptions are required concerning the regime of tangential components in the plasma drifts (i. e., radial electric fields). The test model calculations and comparisons of the reconstructed electric fields with ground-based geomagnetic field variations, global ionospheric convection modeling, and direct satellite observations demonstrate good results and, therefore, usefulness of the developed technique. This technique could be implemented as a tool for operational monitoring of the overall ionospheric convection dynamics within the incoherent (or coherent) radars field-of-view.

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For the Western-Pacific region spread-F has been found to occur with delays after geomagnetic activity (GA) ranging from 5 to 10 days as station groups are considered from low midlatitudes to equatorial regions. The statistical (superposed-epoch) analyses also indicate that at the equator the spread-F, and therefore associated medium-scale traveling ionospheric disturbances (MS-TIDs) occur with additional delays around 16, 22 and 28 days representing a 6-day modulation of the delay period. These results are compared with similar delays, including the modulation, for D-region enhanced hydroxyl emission (Shefov, 1969). It is proposed that this similarity may be explained by MS-TIDs influencing both the F and D regions as they travel. Long delays of over 20 days are also found near the equator for airglow-measured MS-TIDs (Sobral et al., 1997). These are recorded infrequently and have equatorward motions, while normally eastward motions are measured at the equator. Also in midlatitudes D-region absorption events have been shown (statistically) to have similar long delays after GA. It is suggested that atmospheric gravity waves and associated MS-TIDs may be generated by some of the precipitations responsible for the absorption. The recording of the delayed spread-F events depends on the GA being well below the average levels around sunset on the nights of recording. This implies that lower upper-atmosphere neutral particle densities are necessary.

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Chondrule formation due to a nebular shock wave heating is considered. We calculate the apparent gravitational acceleration, internal flow, and deformation of a chondrule precursor molten sphere in the shocked nebular gas. The gravitational acceleration and the internal flow are caused by momentum flux of gas molecules incident on the surface of the sphere. The gravitational acceleration just behind the shock wave is 1.1 to 330 times the terrestrial one. The velocity of the internal flow reaches around 0.1 m s^-1 for the pre-shock nebular gas density and the shock wave velocity are 10^-6 kg m^-3 and 8.7 km s^-1, respectively, and then chondrule melt is stirred well by the flow. As a consequence, if there is oxygen heterogeneity in the precursor particle, it must be homogenized by the high speed circulative flow in the molten sphere within a few seconds. The momentum flux also deforms the sphere. The variation of the radius of the molten sphere due to the deformation is less than 1% of the original radius for mm-sized sphere when the sphere re-solidifies. Because we used the hydrodynamic solution with the linear approximation, the applicability of our result of the internal flow is restricted to some region of the parameter space of the shock velocity, the nebular gas density, radius and viscosity of chondrule melt sphere. For larger values of those parameters than typical ones, nonlinear calculations are needed, which is left for future works.

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Geological and seismic evidence suggests that nearly two thirds of the convergence between India and Eurasia is accommodated by the crustal deformation of Asia. Two competing mechanisms were proposed to describe this accommodation: distributed crustal thickening and lateral extrusion along main faults. The kinematics of the Altyn Tagh Fault (ATF) is critical in determining the relative importance of these two mechanisms, inasmuch as the ATF slip-rates predicted by hypotheses of these competing mechanisms are very different. Using a finite element formalism to construct a thin-sheet model, we seek a velocity solution approaching the current kinematics of the ATF. The GPS data in the Tibetan Plateau and neighboring regions are employed as constraint conditions, in successive steps. The predicted velocity distribution near the ATF fits well to the observations, with overall standard deviations of 2.3 mm/yr and 2.8 mm/yr for the northward and eastward components, respectively. The inferred average slip-rate of the ATF is (7.4 ± 1) mm/yr, with some variation along the fault. The slip-rate estimate of the ATF reported in this paper supports the distributed crustal thickening hypothesis for the crustal deformation of the Tibetan Plateau.

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