I have analyzed teleseismic body waves from the 2003 Tokachi-oki earthquake (M_w 8.1), and inferred the slip distribution. Two simple fault models are assumed for estimating the effect of fault geometry on derived slip distributions. One is a single planar fault with a dip of 20° and the other is a compound fault having a shallow plane with a dip of 5° and deeper, landward plane with a dip of 20°. The compound-fault model is preferable because it explains the initial part of the observed P-waves better. It is found that the planar fault has one asperity (patch of large slip) near the hypocenter and the other asperity to the landward side of the hypocenter. The compound-fault model shares the landward asperity with the planar-fault model, but does not have the asperity near the hypocenter. The other asperity on the compound fault is found far from the hypocenter. This difference of the slip distributions suggests the importance of accurate modeling of the fault dip angle when deducing the slip distribution from teleseismic body waves.

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A series of numerical simulation has been carried out to explore the influence of two major phase transitions at 410 km and 660 km phase boundaries on mantle convection with self-consistently moving and subducting plates, that is, on the “plate-like regime” of mantle convection. The degree of Clausius-Clapeyron slope at the 660 km phase boundary is systematically changed within the range estimated by high pressure experiments. On the plate-like regime where moving plates continuously subduct, as the Clausius-Clapeyron slope is steepened, the upwelling plumes originating from the bottom thermal boundary layer are less buoyant owing to the increase of average mantle temperature, so that the upwelling plumes are hard to penetrate through the 660 km phase boundary. Investigations reveal that three types of small upwelling plumes in the upper mantle are found on the plate-like regime: (1) The secondary plumes directly derived from the upwelling plumes from the lower mantle, (2) the passive upwellings from the shallow parts of the lower mantle due to the diffused return flow by continuously subducting plates, and (3) the secondary plumes originating from the 660 km phase boundary caused by the development of the small-scale convection cells confined in the upper mantle.

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This paper describes GPS multi-antenna device (one GPS receiver links multiple antennas) developed by authors, and the experimental results are presented. GPS has already proven to be an efficient tool for monitoring dam deformations and stability of high-risk slopes. It offers greater accuracy than other surveying techniques. However, GPS has its disadvantages when employing for slope and dam monitoring. The major drawback has been high cost due to large-scale GPS deployments are required in monitoring sites. The conventional GPS monitoring methods, where a permanent GPS receiver must be located at each point, have significant limitations of the cost. A new approach that a single GPS receiver links multiple antennas mounted at the monitoring points, has been employed to solve these problems in this paper. A dedicated switching device has been developed by authors for this approach. Field testing results show that the dedicated switching device for GPS multi-antenna system has excellent performances. Post-processing positioning accuracy is around 1-2 mm for the deformation monitoring of the Xiaolangdi dam on the Yellow River.

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We carried out a series of VLBI observations of Nozomi by using a dedicated narrow bandwidth VLBI system. The three carrier waves with frequency interval of 515 kHz were recorded in 3 channels of the system and correlated by a software method. As a result of the correlation, the residual fringe phases of the main carrier wave are obtained for every 1.3 seconds. We can also continuously track them for 100 minutes. The variation of the residual fringe phase is +/- 150 degrees. Moreover, we can derive succesively the group delay for every 100 seconds by using these three carrier waves. The RMS of the group delays is 13 nsec and its average is well accorded with the delay determined by the range and Doppler measurements within an error of 2 nsec. Consequently, we confirmed the validity of the narrow bandwidth VLBI system, and it could be expected that this system, in addition to range and Doppler measurements, can be applied to three-dimensional tracking of a spacecraft and the precise gravity measurement of the Moon and the planets.

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Continuous and repeated geomagnetic observations have been performed at 8 stations in the eastern part of Hokkaido, NE Japan, to confirm a detailed picture of geomagnetic secular changes. The observation delineated anomalously large secular changes of about 1 nT/year that have lasted at least for about 3 to 30 years (depending on the period of observations) at 3 stations situated in the remarkable geomagnetic anomaly region. Contributions from the earth's core or ionospheric origin are ruled out as source mechanisms because of the local distribution of the anomalous stations. Heat-triggered volcanomagnetic effect cannot be the origin of such secular changes because the stations are quite far from the volcanoes. Instead, we propose the changes originate from stress-induced tectonomagnetic effect (piezomagnetism). We performed piezomagnetic modeling under the condition that observed regional tectonic stress has been applied to the highly magnetized rock bodies inferred from the analysis of the observed geomagnetic anomalies. The modeling explained well the secular changes by assuming the stress sensitivity of the order of 10^-2 MPa^-1, which is one order larger than the ordinarily used value.

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The initiation of giant electrical discharges called as “gigantic jets” connecting thunderclouds to the ionosphere is investigated by numerical simulation method in this paper. Using similarity relations, the triggering conditions of streamer formation in laboratory situations are extended to form a criterion of initiation of gigantic jets. The energy source causing a gigantic jet is considered due to the quasi-electrostatic field generated by thunderclouds. The quasi-electrostatic field is assumed to be axisymmetrical. We calculate the electric fields for different thundercloud charges. The electron dynamics from ionization threshold to streamer initiation are simulated by the Monte Carlo technique. It is found that gigantic jets are initiated at a height of -18-24 km. This is in agreement with the observations. The distributions of electron positions and electron energies at different initiation heights are presented. The method presented in this paper could be also applied to the analysis of the initiation of other discharges such as blue jets and red sprites.

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Different type of discrete VLF emissions recorded at low-latitude ground stations Varanasi (geomag. lat., 14°55′N, geomag. long., 154°E, L = 1.07), Jammu (geomag. lat., 22°26′N, L = 1.17) and Gulmarg (geomag. lat., 24°26′N, geomag. long., 147°9′E, L = 1.28) are reported and spectral analysis of these emissions have been carried out. To explain various temporal and spectral features of these emissions, a possible generation mechanism has been presented based on the backward wave oscillator (BWO) regime of the magnetospheric cyclotron maser. On the basis of this model, we have computed various discrete emission parameters as well as some magnetospheric parameters relevant to the generation process. A comparison of the observed discrete characteristics with the proposed generation mechanism shows a good agreement with the BWO model.

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We calculate the mode structure of magnetospheric MHD waves on a meridional plane. We have added the effect of ionospheric dissipation to the three-dimensional dipole field MHD model of Lee and Lysak (1999); this model allows a realistic Alfven speed profile for the plasmasphere and realistic boundary conditions at the outer boundaries that vary with respect to local time. Using power spectra and plots of spatial mode structure, we show that the two-dimensional transverse modes on the dipolar meridian are strongly affected by ionospheric damping, but the compressional modes are not. The location of field line resonances spreads wide as the damping increases, but the compressional mode structure remains stable.

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Midcrustal low-frequency earthquakes (MLFs) have been observed at seismic stations around Mt. Fuji, Japan. In September-December 2000 and April-May 2001, abnormally high numbers of MLFs occurred. We located hypocenters for the 80 MLFs during 1998-2003 by using the hypoDD earthquake location program (Waldhauser and Ellsworth, 2000). The MLF hypocenters define an ellipsoidal volume some 5 km in diameter ranging from 11 to 16 km in focal depth. This volume is centered 3 km northeast of the summit and its long axis is directed NW-SE. The direction of the axis coincides with the major axis of tectonic compression around Mt. Fuji. The center of the MLF epicenters gradually migrated upward and 2-3 km from southeast to northwest during 1998-2001. We interpret that the hypocentral migration of MLFs reflects magma movement associated with a NW-SE oriented dike beneath Mt. Fuji.

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