We analyze an earthquake catalog and waveform data from a dense seismic array in western Nagano Prefecture in order to investigate seismic properties and source parameters. The catalog used here consists of about 30, 000 smaller magnitude earthquakes that were located within a source volume of order (10 km ) ³. Moving window analysis shows that the seismic rate is steady over time and the temporal change in the size distribution of earthquakes shows only statistical fluctuations. Spatial analyses, however, reveals that certain features of seismicity are depth-dependent. At shallow depths (<3 km), we find an increase in average stress drop with depth that may be related to the characteristic depth of seismicity (the depth where the frequency of earthquakes is maximum). The b-value is also found to be elevated throughout depths where the seismicity is most intense but then the b-value slowly decreases as depth increases. Another result from our analysis of waveform data is a clear relation between earthquake magnitude and stress-drop in the magnitude range -0.5 to 2.5.

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A source model of long-period volcanic earthquakes is presented. We consider that a fluid-filled spherical cavity surrounded by an infinite elastic medium is excited into resonance like the Earth's free oscillations. The eigenequation of this system is derived in a general manner, making use of the spherical harmonic and spherical Bessel expansions. The solution is given as a complex number; its real part is the eigenfrequency and the imaginary part represents the attenuation coefficient of the oscillation. The eigenmodes are classified into five groups: (1) the compressional modes in a fluid sphere, (2) the compressional modes in a solid medium, (3) the shear modes in a solid medium, (4) the Stoneley modes, and (5) the torsional modes. We apply them to the long-period volcanic earthquake observed at Asama volcano, Japan. Estimating the characteristic frequencies and attenuation coefficients of the observed vibrations and assuming that the primary component ( f = 1.73 Hz) corresponds to the fundamental translation mode of a fluid sphere as one of the compressional modes in fluid, we conclude that the resonator which is a spherical cavity of diameter 220 m filled with steam of temperature 500°C and pressure 170 atm is favorable.

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We carried out magnetotelluric measurements in the southeastern region of Mt. Ontake, where the 1984 Western Nagano Prefecture earthquake occurred and earthquake swarms have been observed since 1976. Most of the earthquakes have focal depths shallower than 10 km. The depth of the hypocenters increases towards the west. Our purpose is to delineate the resistivity structures down to the focal depths of the earthquakes, because the resistivity structure is very sensitive to the free water in the crust. The regional strike was estimated as N60°E. We carried out a two-dimensional analysis over two profiles across the earthquake fault of the 1984 Western Nagano Prefecture earthquake: one is in the low seismicity region of the fault (A-A′) and the other, in the high seismicity region (B-B′). The resistivity structure along the A-A′ profile is reasonably homogeneous and shows a high resistivity of more than 300 ohm-m. The resistivity structure along the B-B′ profile has a clear boundary at the center of the profile. This boundary divides the structure along the B-B′ profile into two resistivity blocks and its location coincides with that of the earthquake fault. A conductor is detected at depths greater than 8 km to the northwest of the earthquake fault. The depth to the upper boundary of the conductor coincides with those of the seismic reflectors. This structure seems to be formed by the free water dehydrated from the magma.

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The geomagnetic jerk amplitudes, which are defined as abruptness of changes in the trends of geomagnetic time series, are investigated with geomagnetic monthly means computed from hourly mean values at each local time. A statistical time series model in which the trend component is expressed by a second order spline function with variable knots is constructed for each time series. The optimum parameter values of the model including positions of knots are estimated by the maximum likelihood method, and the optimum number of parameters including the number of knots are determined based on the Akaike Information Criterion (AIC). The jerks are detected objectively and automatically by regarding the optimized positions of knots as the occurrence epochs. This analysis reveals that the spatial distributions of jerk amplitudes essentially do not depend on the local time, which indicates that the jerks cannot be explained by abrupt changes in intensities of latitudinally flowing external currents such as the field-aligned currents. Longitudinally flowing currents, on the other hand, such as the ring current could explain the distributions. The abrupt changes of the ring current intensity are estimated from the distributions of jerk amplitudes in the eastward component in 1969, 1978, and 1991 supposing that an abrupt change in the ring current intensity causes a jerk. However those estimated changes cannot consistently explain the distributions of the jerks in the northward and downward components. Therefore it is plausible that the jerks which occurred in 1969, 1978, and 1991 are not caused by external sources but internal ones. It is also confirmed that the occurrence epochs of jerks in the southern hemisphere are a few years after those of the 1969 and 1978 jerks in the northern hemisphere, and it is also found that the jerk in the southern hemisphere occurred a few years after the occurrence of the 1991 jerk in Europe. Taking these time lags in occurrence epochs into account, it can be said that the 1969, 1978, and 1991 jerks are global phenomena.

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We adopted the wavelet technique to analyze the GPS time-series data of daily co-ordinates from the continuous GPS network, GEONET, during a four-year period (1996-1999). In the present analysis we removed the data jumps associated with earthquakes and due to artificial errors using ARMA technique, and also removed seasonal variations and white noises using the wavelet technique to estimate the secular crustal deformation in central Japan. The secular displacement vectors obtained were used to calculate various crustal strain fields in the study area by means of the least squares prediction technique. The secular displacement field was different from that estimated from the raw GPS time-series data, and we would have overestimated the secular strain components without considering them. The main characteristic features of the secular strain fields were as follows: (1) there existed a compressive regime in the entire region; (2) there was a remarkable peak around Tokyo Bay in the maximum shear strain distribution; (3) along the Shinanogawa seismic zone there existed a distinguishing boundary zone in the several kinds of strain fields.

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The Global Positioning System (GPS) provides an alternative way to investigate ionospheric irregularities and their effects on the radio wave propagation. The method is based on fluctuations of the total electron content (TEC) resulted from the ionospheric plasma irregularities. Previous studies have showed the correlation between the radiowave intensity (including GPS signals) and ionospheric irregularities during magnetic storm periods. In this study, phase fluctuations derived from GPS signals are used to address aspects of the ionospheric storm events during the low irregularity activity months. We analyze data from seven GPS stations located in Central- and South- America during eight magnetic storms occurred from 1997 to 2000. It is found that, in general no significant feature in the phase fluctuation is observed during the low irregularity activity months, except during the 26 August 1998 and the 15 July 2000 storms. A detailed study shows that the GPS phase fluctuations develop when the Dst index begins to decrease significantly. This phenomenon cannot be compared directly to previous observations and model results due to the fundamental difference in the background levels of irregularity activity. To better understand the generation of ionospheric irregularities during the storm period of the low irregularity activity months, the temporal relationship between the magnetic Dst index, equatorial anomaly TEC, and the GPS phase fluctuations are examined and discussed.

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We analyzed the brightness of the sky background observed with a coronagraph at Norikura, Japan, in the period of 1951-1997. We discovered that the power spectrum shows a clear eleven-year periodicity of solar activity cycle, in addition to strong annual variations. The peaks in the eleven-year component are found in the declining phase of activity, 2-4 years after the sunspot number maximum. The brightness of the continuum corona is far fainter than the observed amplitude of the eleven-year component and cannot account for the observed phenomenon. A possible interpretation is that the solar activity modulates the contents of aerosols in the upper atmosphere of the earth, thus producing variations in scattering of sunlight. Why this effect is largest in the declining phase of solar activity is still unknown.

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The oblique whistler waves have been studied having k vector at an angle to magnetic field for a generalized distribution function reducible to bi-maxwellian and loss-cone. The dispersion relation and growth rate have been obtained for oblique whistler mode instability incorporating the trajectory of the particles, in the presence of perpendicular a-c electric field by method of characteristic solutions. The effects of distribution function and beam effect have been discussed for the space plasma at magnetospheric height and at 1 A.U. The results are compared with satellite observations and reported results obtained by other techniques. Excitation of two separate, but simultaneous left hand polarized whistler mode at 1 A.U. by electron been are demonstrated.

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We calculate normal modes for the Earth model, which has a slight rigidity layer at the base of the liquid outer core. We show that such a layer with thickness about 40 km and the shear wave velocity of 0.017 km/sec can produce a normal mode, which has a close eigenfrequency to that of liquid core model, without affecting fundamental modes and most of the higher modes. Our results indicate that the thin finite rigidity layer at the base of the outer core might explain the anomalous splitting of the Earth's normal modes, which has not been fully explained by the anisotropy in the inner core.

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