In this study the properties of the aftershock sequences in the first five months after the 17 August 1999 Izmit and 12 November 1999 Düzce earthquakes are investigated. For this purpose, the b-value of frequency-magnitude distribution of earthquakes and temporal decay rate of aftershocks as described by the p-value in the modified Omori law are calculated. The data taken from the website of the KOERI consist of 1841 events for the Izmit and 913 events for the Düzce activity, respectively. The b-value for the Izmit sequence is found as 1.10±0.03 with Mc = 2.6 and b = 1.16 ± 0.05 for the Düzce sequence with Mc = 2.8. Both b-values are close to 1 and typical for aftershock sequences. The p-values are calculated as 0.86 ± 0.05 for the Izmit and p = 1.34 ± 0.09 for Düzce aftershocks, respectively. Small p-value found in Izmit sequence may be a result of the background seismicity of the aftershock region. Because of the lower p-value for the Izmit activity than that of the Düzce sequence, the aftershocks show a relatively slow decay rate in the Izmit earthquake. Considerable spatial variability is found, b-values ranging from 0.8 to 1.6, and p-values ranging from 0.4 to 1.4. The spatial variations of b and p-values of an aftershock sequence may associate with the rupture mechanism and geological structure of an earthquake area. A good agreement between the slip and b-value for the Düzce sequence are found whereas we could not observe a linear relation between these parameters for the Izmit sequence. After the Izmit earthquake, the larger b-values correlate with the regions where the lower stress changes occurred whereas the lower b-values are in the areas with the larger stress variations. Also, the lower and higher b-values are related to the higher and lower P-wave velocity in general. Regarding the geological aspect, the larger b and p-values for the two sequences are related to Holocene alluvium structures which have low velocity. On the other hand, after the both earthquakes, the larger p-values correlate well with the regions on which the maximum slip is located and the lower p-values are related to the lower slip regions. However, the material properties seem more effective than the stress changes and slip distributions in the b-value variations and the slip distribution is the most significant factor on p-values in the both sequences.

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We propose a scheme for calculating the magnetic field in a spherical shell, based on Earth's outer core, using the finite element method (FEM). The two most difficult problems for magnetohydrodynamics (MHD) simulations in a rotating spherical shell with FEM are solving the magnetic field outside the fluid shell, and connecting the magnetic field in the fluid shell to the exterior potential field at the boundary. To solve these problems, we extend the finite element mesh beyond the fluid shell and compute the vector potential of the magnetic field. To verify the present scheme, we consider three test case. First, we compare the FEM model with an analytical solution of Laplace's equation outside the fluid. Second, we evaluate free decay of a dipole field and compare the results with a spectral solution. Finally, compare the results of a simple kinematic dynamo problem with a spectral solution. The results suggest that the accuracy of the dipole field depends on the radius of the simulation domain, and that this error becomes sufficiently small if the radius of the outer region is approximately 6 times larger than the radius of the fluid shell.

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Regional to continental scale magnetic anomaly maps are becoming increasingly available from airborne, ship-borne, and terrestrial surveys. Satellite data are commonly considered to fill the coverage gaps in regional compilations of these near-surface surveys. For the near-surface Antarctic magnetic anomaly map being produced by the Antarctic Digital Magnetic Anomaly Project (ADMAP), we show that near-surface magnetic anomaly estimation is greatly enhanced by the joint inversion of the near-surface data with Ørsted satellite observations compared to Magsat data that have order-of-magnitude greater measurement errors, albeit collected at much lower orbital altitudes. The CHAMP satellite is observing the geomagnetic field with the same measurement accuracy as the Ørsted mission, but at the lower orbital altitudes covered by Magsat. Hence, additional significant improvement in predicting near-surface magnetic anomalies can result as lithospheric magnetic anomaly data from the CHAMP mission become available. Our analysis also suggests that a further order-of-magnitude improvement in the accuracy of the magnetometer measurements at minimum orbital altitude may reveal considerable new insight into the magnetic properties of the lithosphere.

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Magnetization vectors of seamounts and edifices in the West Philippine Sea were determined with a direct searching method in the space domain using the topography and magnetic anomaly data at the sea surface. The data analyzed were surveyed by the Hydrographic and Oceanographic Department of Japan since the 1980s. Several seamounts in the eastern part of the Minami Daito Basin are magnetized in the N70°W to N80°W directions with negative shallow inclinations, whereas edifices of the Kyushu-Palau Ridge have directions of N30°W with downward inclinations. The paleolatitude of the Oki-Daito Ridge as a whole is 4.4°S±11.0° and that of the Amami Plateau and the Daito Ridge, 15.1°N±4.6° and 20.1°S±8.2°, respectively. These results are inconsistent with the proposed clockwise rotation of the West Philippine Sea exceeding 80°. Furthermore, the results also suggest that the Oki Daito Ridge was generated in the Southern Hemisphere and then migrated northward. The large shortening between the Oki-Daito Ridge and the Amami-Daito Ridge regions may suggest the convergence boundary between them.

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Representing lightning discharge current source by a Dirac delta function, Maxwell's equations are solved to derive the expression for wave-electric field as a function of frequency and distance including the effect of interparticle collisions. The exact time-dependence of the propagating non-monochromatic signal for the realistic magnetospheric model is computed for low latitude stations (in India). The computation is extended for the wave propagating through different regions of the magnetosphere and results are compared with the measured data. Points of agreements and differences are illuminated.

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Thermal plasma measurements performed on MAGION 5 (subsatellite of INTERBALL 1) in the plasmasphere of the Earth are analyzed in conjunction with simultaneous solar wind data and ground-based ionospheric measurements in quiet geomagnetic conditions, during and after geomagnetic storms. In situ satellite observations reveal the existence of long-lived (2-3 days) depleted regions (MLT width > 1.5-2 hours) in the plasmasphere that extend out from L - 3. These observations well correspond to 'notch' or 'bite-out' regions found by the IMAGE spacecraft. Possible reasons for the formation of low-density regions are discussed.

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We performed photometric and polarimetric observations, on November 8 and 9, 1999, of an M-type main belt asteroid, (216) Kleopatra by using the HBS spectropolarimeter installed at Dodaira observatory, National Astronomical Observatory of Japan (NAOJ). Photometric amplitude of lightcurve in the V band was 0.12 mag, and the averaged degree of polarization was -1.01±0.1%. It seems that the polarimetric data might also show a slight change in the degree of polarization (-0.2%) at the second minimum of the photometric lightcurve, but we could not confirm that the feature was real because of the large errors of data. With the assumption that the surface is uniform, we have carried out lightcurve simulations based on shape models by Ostro et al. (2000), Tanga et al. (2001) and Roche binary (Cellino et al., 1985). The results of simulations were compared to the configurations of lightcurves which had been obtained at different 4 geometric positions (1980, 1982, 1987 and 1999). The model by Cellino et al. (1985) reproduced almost all the data points without the 1987 observations within -0.05 mag., which is the best result among the 3 models. The model by Tanga et al. (2001) well reproduced the lightcurves, but failed in reproducing the 1982 amplitude (difference Δ_diff - 0.2 mag.). We also confirmed that the model by Ostro et al. (2000) could not explain the observed lightcurves.

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A laboratory simulation experiment was performed to observe the dynamical behavior of the earth's magnetosphere, based upon the earthward electric field measurement in the magnetotail. The simulation was examined to satisfy the MHD scaling laws. The earthward electric field, E_x, is the signature of the current density based on j×B force in the tail, described by Podgorny (1978). The effect of the solar wind dynamic pressure to the earthward electric field, E_x, was investigated. The solar wind density was changed while the other parameters were kept almost constant. It is found that the E_x is modulated by the change in the solar wind dynamic pressure. The result also shows that the current continued to flow in the near earth region even after the solar wind had stopped. This result shows a similar resemblance to that of the particle confinement in the radiation belt of the real magnetosphere.

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Annual GPS campaigns were carried out at Okmok volcano in the Aleutian Islands, Alaska, between 2000 and 2002. Surface deformation detected by these measurements reveals that Okmok volcano has been inflating over these 3 years at a variable inflation rate. The horizontal displacements show a radial outward pattern, and there has been significant uplift of the caldera center. The uplift of the caldera center relative to the caldera rim was -2.1 cm during 2000-2001, and -6.7 cm during 2001-2002. The latter rate is quite consistent with that deduced from InSAR measurements spanning 1997-2000, but the deformation rate during 2000-2001 was much slower than during the preceding and succeeding periods. Shallow pressure source was inferred at a depth of -3.1 km beneath the approximate center of the caldera. The location of the source, -5 km laterally from the active vent, is consistent with that inferred from InSAR data during 1997-1998. The total increase in volume during 2000-2002 of the inferred source is -0.44 × 10⁷m³, which is 3-8% of the amount of volume erupted in 1997. The GPS and InSAR data show that magma accumulation beneath Okmok was steady in rate and location during 1997-2002, except for a pause at some time between 2000 and 2001.

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