We present a method to determine the complex travel times of impulses in the time domain on the basis of an autoregressive (AR) modeling of superimposed sinusoids in a finite complex series in the frequency domain. We assume that the complex frequency series consists of signals represented by a complex AR equation with additional noise. The AR model in the frequency domain corresponds to a complex Lorentzian in the time domain. In a similar way to the Sompi or extended Prony method, the complex travel times are given by solutions of a characteristic equation of complex AR coefficients, which are obtained as the eigenvector corresponding to a minimum eigenvalue in an eigenvalue problem of non-Toeplitz autocovariance matrix of the complex series. Our method is tested for synthetic frequency series of transfer functions, which show that (1) the complex travel times of closely adjacent pulses in the time domain are clearly resolved, and that (2) the frequency dependence of the complex travel times for physical and structural dispersions is precisely determined by the analysis within a narrow frequency window. These results demonstrate the usefulness of our method with high resolvability and accuracy in the analysis of impulse sequences.

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Severe, 10-cm level, elevation angle cutoff dependence of the height solution is detected in the analysis of sample baselines from GSI/GEONET (Geographical Survey Institute/GPS Earth Observation NETwork). It is inferred that station and monument specific differences in the radome and/or multipath environment are responsible for this effect, because it can be observed over short sample baselines for identical antennas mounted atop different monument types. Our test results show that both the radome and multipath from the metal plate at the top of the pillar affect the baseline solutions. A calibration experiment was carried out by observing short (<10 m) known baselines to obtain phase correction maps for the typical GEONET monuments. The antenna/monument phase centers of GEONET monument replicas with the same attachment and radomes but shortened in height, were determined relative to a tripod-mounted TRM29659.00 antenna. Phase maps were obtained with the BERNESE software for 6 typical combinations of 3 antenna types, 3 monument types, and 3 radome types commonly used in GEONET. We find monument/antenna specific phase differences up to 1 cm. These phase differences can result in more than 10-cm station height biases when tropospheric delay parameters are estimated, which is consistent with the height errors observed for GEONET.

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Phase maps for GSI/GEONET (Geographical Survey Institute/GPS Earth Observation NETwork) monuments and antennas obtained in Hatanaka et al. (2001) are evaluated by applying them to the analysis of GEONET. We used the same strategy as the GEONET routine analysis except for the antenna phase model. The coordinate solutions change by more than 10 cm in height when we apply the new phase maps. A scale change of up to 20 ppb is also observed for one of the sub-networks. The height bias is not constant but changing daily and seasonably, which implies that seasonal variations in the GPS solution are related to mis-modeling of the phase characteristics and that other environmental or geometric factors are coupled to the phase mis-modeling. Two more checks are done by comparing tropospheric delay estimates, and by conducting elevation angle cutoff tests. Both tests show dramatic improvement when the new phase maps are applied, as compared to applying the standard antenna phase maps. It is concluded from this experiment with almost 1000 GEONET sites, that monument/antenna specific phase characteristic calibrations are essential for any application of GPS to achieve the highest accuracy for Earth science applications.

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Paleomagnetic data are mostly given in the form of field directions (inclinations and declinations) which depend nonlinearly on the model parameters (Gauss coefficients). Because of this nonlinearity, the means of the data are affected not only by the means of the parameters but also by their fluctuations. Defining the mean directions by the Fisher method decreases this effect but does not completely eliminate it. For various mean fields, we evaluate the effect of secular variation on the means of Fisher-averaged directions by the analytical (Taylor expansion to the second order) as well as by the numerical (Monte Carlo) method. It was shown that a significant amount of offset occurs in the field values because of the fluctuation caused by secular variation. In the case of an inclination anomaly, the effect of secular variation as a function of latitude is antisymmetric about the equator, similar to that of the axial octupole term (g⁰₃). We also show that the measurement errors do not induce biases in the mean field data, provided that they are random and isotropic.

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Zircon fission-track (F. T.) ages and paleomagnetic directions were measured on the Neogene series of the Oki-Dogo Island. The results of F.T. dating on the the Tokibariyama Formation show ages of 23.7 ± 1.3 and 20.0 ± 2.3 Ma, indicating the early Early Miocene. Paleomagnetic measurements of the Tokibariyama, Kori Formations (24-17 Ma) and the Oki Trachyte-Rhyolite (6.8 Ma) were done to reveal the movement of Oki-Dogo Island during the Japan Sea opening. The Tokibariyama and Kori Formations show an average declination of 39.9°. The Oki Trachyte-Rhyolite has the same average geomagnetic direction as the present day value. These results show that a clockwise rotation of 40° ± 15° of the Oki-Dogo Island occurred between 17 and 6.8 Ma which is similar to the main land of southwest Japan.

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We report on our observation of pulse-like electromagnetic signals in the frequency range 1-10 kHz that we associate with earthquakes. The severest difficulty in separating earthquake-associated VLF signals from those originating in lightning discharges stems from the circumstance that the latter signals are overwhelming in number compared with the former. While claims have often been made of observation of electromagnetic signals in association with earthquakes, most of the claims, excepting a few, heavily rely on temporal correlation. By means of simple instrumentation and data processing software, our method by and large enables us to isolate VLF signals whose direction of arrival is well focussed at the epicenter direction. In this preliminary report we present several examples that positively demonstrate the existence of a class of VLF signals having a peak frequency of occurrence 1-4 days prior to earthquakes of Magnitude 4-6. With an accumulation of experience the technique described in this paper would seem to offer a promising approach towards earthquake prediction.

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We estimated the seismic moment tensor for the initial phase of explosion earthquakes associated with Vulcanian explosions at the andesitic volcano Sakurajima. We applied a waveform inversion method to 24 events observed with borehole seismometers at 6 stations assuming a single triangular source time function. The three diagonal components representing volumetric change were dominant in the estimated moment tensors. The three diagonal components were positive and had almost the same values with the difference among them less than 10% for most of the explosion earthquakes, while non-diagonal components were negligibly small compared with the diagonal ones. Durations of source time functions had a range of 0.2-0.5 s and seismic moments were 10¹¹-10¹² Nm. We conclude that the explosion earthquakes are initiated by an isotropic expansion.

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A spatial-temporal model of the daily mean values of the geomagnetic total force field was constructed for Japan for the time interval from 1997 to 1999. The daily mean data of six geomagnetic observatories and nine continuous geomagnetic stations in Japan were used for the model. Temporal functions of the model were constructed using the Natural Orthogonal Components method, and the spatial functions are polynomial functions of the horizontal positions. The accuracy of the model is within approximately a few nT, and it can be used for eliminating external fields for geomagnetic surveys or detecting local geomagnetic changes for monitoring crustal activities. Each function, which is calculated using the mathematical (statistical) method, has physical meaning, such as the intensity of the globally symmetrical equatorial ring current and secular changes in the main field. The temporal and spatial functions can be then used for geophysical study of the Earth.

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