In the 1940's, several destructive earthquakes occurred in western Japan. Seismograms in this period were usually recorded on smoked paper and the quality was poor compared to modern digital data. But the recent development of image processing technology enabled us to reconstruct feasible waveform data, whereby we investigated source rupture processes of two devastating earthquakes: the Tonankai earthquake (M7.9) of December 7, 1944, and the Mikawa earthquake (M6.8) of January 13, 1945. The results for the Tonankai earthquake show that the source roughly consists of a single asperity with a length scale of 100 km, having no segment structure with a smaller length-scale. Such a feature seems to be reflected to the sea bottom topography above the source region. The main source parameters are as follows: the seismic moment = 1.0 × 10²¹ Nm (Mw=7.9); the fault area = 140 km × 80 km; (strike, dip, rake) = (225°, 15°, 79°); the maximum and averaged dislocations = 4.4 m and 3.0 m, respectively. The analysis of the seismograms for the Mikawa earthquake shows that the source is a reverse fault with a slight left-lateral component. The pressure axis is directed to ENE-WSW, which is a little rotated from the EW compression axis prevailing in western Japan. This fault can be regarded as the southern extension of the Nobi earthquake fault system. The main source parameters are as follows: the seismic moment = 1.0 × 10¹⁹ Nm (Mw=6.6); the fault area = 20 km × 15 km; (strike, dip, rake) = (135°, 30°, 65°); the maximum and averaged dislocations are 2.1 m and 1.1 m, respectively. The slip distribution mainly consists of two asperities: the one near the hypocenter and the other 10-15 km northwest from it. The heavily damaged area is well correlated with the northwestern asperity.

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The INTERMAGNET program publishes each year a CD-ROM containing homogeneous series from a number of magnetic observatories (76 in 1999). These series are definitive one-minute values of the three components of the geomagnetic field. We transform these series using a simple nonlinear analysis tool able to characterize the activity of a signal, and we obtain a remarkably simple activity field, whose space and time variables separate over a large part of the Earth. The time function is almost identical for all observatories, and might be interpreted as an activity index. The-almost stationary-field geometry exhibits a dipole-like structure everywhere except in high latitudes. Key words: Geomagnetic field, irregular variations, INTERMAGNET, absolute derivative.

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Focusing to the self-reversal of Thermo-Remanent Magnetization (TRM), we examined magnetic properties of 32 pieces of unoriented pumice samples, which were systematically collected from eight layers of pumice-fall and pumice-flow deposits at Haruna Volcano, Japan in the two stages of the eruption, i. e., Futatsu-dake stage (the 5th stage) and caldera-forming stage (the 4th stage). The magnetic behaviors of TRM well correlated with the primary chemical composition (TiO₂-content or x (molecular fraction value of ilmenite in xFeTiO₃- (1-x) Fe₂O₃) ) of the hemoilmenite phenocrysts in the samples. That is, the samples of which chemical composition of hemoilmenite is TiO₂ - 30 wt% (x - 0.582) showed various types of TRM including typical intense Self-Reversed Thermo-Remanent Magnetization (SRTRM), whereas the samples of which chemical composition of hemoilmenite is TiO₂ - 32 wt% (x - 0.620) showed one type of TRM; weak intensity of self-reversed component of TRM. This result harmonizes with the well-known diagram by Uyeda, where the intensity of SRTRM of hemoilmenite (quenched) is an explicit function of the chemical composition, and only in the restricted narrow range of the chemical composition, intense SRTRM is acquired. Moreover, the hemoilmenite samples of which chemical composition is less than TiO₂ - 31 wt% (x - 0.60) are capable to acquire various intensities of SRTRM on annealing according to its condition. In our TiO₂-poorer (or smaller-x) samples, the variety in the behavior of TRM is well explained with this annealing effect in this compositional range.

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Study of paleosecular variation was carried out to mostly andesite lavas from the late Quaternary Ontake Volcano which have detailed stratigraphy and accurate radiometric ages. Among 42 sites, some of the results from lava successions were combined due to possible equivalent ages, remaining 35 sites. After excluding two low latitude VGPs, angular dispersion of 15.6° was obtained for a period which is accurately confined between 20 ka and 90 ka. The obtained angular dispersion is not different from the typical value for the last 5 my, and this is not surprising if we consider that the global paleointensity was not necessarily low and rather oscillatory during the period of 40-100 ka which precedes the global low in 20-40 ka. Two low latitude VGPs from 48 ka lava (41.9°N, 196.2°E) and 80 ka lava (15.9°N, 183.3°E) indicate existence of excursions in Japan during the latest Pleistocene. Preliminary paleointensity experiments indicate that the 80 ka excursion is accompanied by a low paleointensity of about 5 μT while the 48 ka one is not. These two excursions are probably related to those reported previously not only from the Ontake Volcano and its proximity but also from other several sites in Japan, although they were not conclusive. The 48 ka excursion probably correlates to the Laschamp excursion. This is consistent with the fact that the VGP position from the 48 ka lava comes to the central Pacific region, similar to those from the Laschamp excursion found in New Zealand. Possible correlation of the 80 ka excursion is the one from Norwegian-Greenland Sea and Arctic Ocean, which are stratigraphically between the Laschamp and the Blake excursions.

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The propagation features of ion-cyclotron whistler waves through the hydrogen, helium and oxygen plasma are studied. Expression for temporal damping, spatial damping and the time development of wave amplitude is derived from the modified theory of cyclotron damping of ion-cyclotron whistler wave including thermal effect. It is shown that the temporal damping is dominant for wave frequencies closer to the ion gyrofrequencies. The variations in plasma conditions cause variation in sudden commencement of cyclotron damping, which occurs after finite time of propagation of ion whistlers. The results have been used to explain the sudden cut-off of the amplitude of ion whistler wave observed by rockets and satellites in the terrestrial ionosphere. It is shown that the present study can be used to estimate ion temperature of the plasma.

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The magnetic survey in China carried out in resent years is briefly introduced. On the basis of the magnetic data at 119 repeat stations, 39 permanent observatories and 20 grid values of the IGRF 2000, the magnetic charts and models of China for 2000, called China Geomagnetic Reference Field 2000 (or ChinaGRF 2000) have been compiled.

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Power spectra of a plasmaspheric cavity resonance (strictly, a plasmaspheric virtual resonance) in a longitudinally non-uniform plasmasphere are calculated. It is shown that the spectra depend on longitude. Therefore, a cavity resonance mode can have local time depending spectra when the plasmasphere is non-uniform in a longitudinal direction. This fact concludes that the local time dependent peak frequencies of the mid- and low-latitude Pi2 pulsations discussed by Kosaka et al. (2002) are also explained by the cavity resonance model. We also discuss that the surface eigenmode can be a possible generation mechanism for Pi2 pulsations localized in a longitudinal direction.

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Aeromagnetic data, covering 600, 000 km² of Central India, have been analysed for the first time to throw light on the various tectonic blocks of the region, ranging in age from the Archean to the present. Existing geotectonic models are based on inadequate data and studies of relatively small regions. From the aeromagnetic data, we use the analytic signal and Euler deconvolution, to elucidate the subsurface structure of the region and redefine the tectonic elements. Contrary to the belief that the Central Indian shear defines the edge of the Central Indian craton, we find that the Sukinda thrust merges with the Tan shear to demarcate the edge of the Bastar and Eastern Ghat blocks by a shear that extends for approximately 1000 km in length. We call this the Main Peninsular shear. Magnetic anomaly data can thus play a crucial role in understanding the tectonic elements of a region with large surface cover.

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Source modeling of recent shallow intraslab earthquakes is studied using strong-motion data. The total area of strong-motion generation areas obtained is smaller than the prediction from the empirical relationship for inland crustal earthquakes by Somerville and co-workers. Moreover, the ratio between the combined area of strong-motion generation areas obtained in this study and the area predicted by the empirical relationship, tends to decrease with focal depth. The stress drops on strong motion generation areas (or asperities) of shallow intraslab earthquakes increase with focal depth.

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