In light of newly-acquired geophysical information about earthquake generation in the Tokai area, Central Japan, where occurrence of a great earthquake of magnitude 8 or so has recently been feared, probabilities of earthquake occurrence in the near future are reevaluated. Much of the data used for evaluation here relies on recently-developed paleoseismology, tsunami study and GPS geodesy. The new Weibull distribution analysis of recurrence tendency of great earthquakes in the Tokai-Nankai zone indicates that the mean return period of great earthquakes there is estimated as 109 yr with a standard deviation amounting to 33 yr. These values do not differ much from those of previous studies (Rikitake, 1976, 1986; Utsu, 1984). Taking the newly-determined velocities of the motion of Philippine Sea plate at various portions of the Tokai-Nankai zone into account, the ultimate displacements to rupture at the plate boundary are obtained. A Weibull distribution analysis results in the mean ultimate displacement amounting to 4.70 m with a standard deviation estimated as 0.86 m. A return period amounting to 117 yr is obtained at the Suruga Bay portion by dividing the mean ultimate displacement by the relative plate velocity. With the aid of the fault models as determined from the tsunami studies, the increases in the cumulative seismic slips associated with the great earthquakes are examined at various portions of the zone. It appears that a slip-predictable model can better be applied to the occurrence mode of great earthquakes in the zone than a time-predictable model. The crustal strain accumulating over the Tokai area as estimated from the newly-developed geodetic work including the GPS observations is compared to the ultimate strain presumed by the above two models. The probabilities for a great earthquake to recur in the Tokai district are then estimated with the aid of theWeibull analysis parameters obtained for the four cases discussed in the above. All the probabilities evaluated for the four cases take on values ranging 35-45 percent for a ten-year period following the year 2000.

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From the International Terrestrial Reference Frame 1994 (ITRF94) coordinates of the Doppler Orbitography Radiopositioning Integrated by Satellites (DORIS) beacon marker at Syowa Station (69.0°S, 39.6°E), Antarctica, we derived the ellipsoidal height of the Scientific Committee on Antarctic Research (SCAR) Global Positioning System (GPS) point as 42.240 m on the World Geodetic System 1984 (WGS84) associated ellipsoid of a tide free Earth at the epoch of 1993.0. Because the SCAR GPS reference mark was 21.165 m above the local mean sea level at the epoch of 1993.0, and because the sea surface topography is estimated as -1.29 m, the ground data of geoid height can be estimated as 22.37 m on the WGS84 ellipsoid. As for error estimate of the above value, 20-30 cm formal error can be assigned including 3 cm error from the DORIS determination, 1 cm error from the local geodetic tie, and the dominant 20-30 cm error from uncertain modeling of sea surface topography, etc. The EGM96 geoid model gives the synthetic geoid height of 22.10 m at Syowa Station; the discrepancy of 27 cm from the observed value is within the 36 cm cumulative (to the degree 360) rms (root-mean-square) error of the model. We retried similar determination at Breid Bay (70.2°S, 23.8°E) and made a tie to the inland outcropped Seal Rock; the obtained value of 21.4 m has an overall error of 1.8 m. These ground data can be used as test data for generating higherorder (n, m > 360) geopotential models. With the establishment of the International Absolute Gravity Basestation Network (IAGBN) standard value at Syowa Station and gravimetric connection to the Seal Rock, ground data of free-air gravity anomalies of 0.05 mgal accuracy at Syowa Station and 1 mgal accuracy at Seal Rock were obtained. These gravity ground data will also serve as test data for adjusting the satellite- and/or shipborne-derived gravity anomaly maps in the region concerned.

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The Matuyama-Brunhes geomagnetic reversal was continuously recorded in the massive siltstones of the Boso marine sediments in Japan with the time resolution better than 9 years. However the Boso sediments probably have the post-depositional detrital remanent magnetization (pDRM) depressing short-period variations due to the convolution of the geomagnetic field. The previous application of the deconvolution method of pDRM to the 900 year records gave the half fixing depth of 21 cm. Further application of this fixing depth to the 4100 year record clearly shows nearly 100 year variations in both of the inclination and declination.

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Palaeomagnetic studies were carried out in several archaeological and palaeontological sites across Argentina and Chile. Remanence directions far removed from the present magnetic field, probably corresponding to a field excursion were recorded in several stratigraphic sections spaning -11 to 2 ky bp. Detailed palaeomagnetic data from 235 cores obtained in six sections from archaeological sites in Argentina are reported. Stability of the NRM was analyzed by progressive thermal and alternating field demagnetization and ChRM directions determined in most cases. Computed VGPs from those directions tend to be concentrated over North America, Europe, Eastern Asia and Africa. Using 275 VGPs a palaeomagnetic pole that is -20°apart from the Earth's rotation's axis was obtained.

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The Network MT method was used in the eastern part of the Shikoku district, southwestern Japan, and a total of thirty-nine MT impedances (64 to 2560 sec) were obtained. These MT impedances had their values averaged over a triangular element, whose sides were a few kilometers long with geomagnetic observatory data from the Kakioka Geomagnetic Observatory. Well-determinedMTimpedances varied from north to south with the greatest differences being at the Median Tectonic Line, which is consistent with the surface geology in the area. In addition, very large or very small values of apparent resistivity were obtained in some triangular elements. These triangles were located on a cape or near an estuary, with effects of three-dimensionality clearly apparent. Stable MT impedances were not obtained for three groups of triangular elements: (1) those where one or two sides of the triangular element cross the coast; (2) those where the electric field was contaminated by severe artificial noise, these were mainly in the northwestern part of the survey area; (3) those where the triangles had an extremely acute- or obtuse-angle. A resistivity cross section was derived from the TM-mode data for a profile crossing the eastern part of the area. The shallower layer, which approximately corresponds to the crust, was divided into three blocks. Two resistive boundaries coincide with the geological tectonic lines and the strong horizontal contrast found at the Median Tectonic Line. The northernmost block is the most resistive, and the block to the south is the most conductive. Beneath these blocks, the subducting Philippine Sea plate was represented by a thick and highly resistive north-dipping layer. A highly conductive thin layer was found above the resistive layer on the southern side of the Median Tectonic Line. This layer is only found beneath the southern side of the Median Tectonic Line and is probably caused by pore water and/or sediment at the upper plane of the subducting Philippine Sea plate. Another conductive layer was found under the highly resistive north-dipping layer. The resistivity structure from the lower crust to the upper mantle is firstly obtained using the Network-MT method. However, further developments are needed in methods of data analysis, which are robust to artificial electric noise, in order to clarify the spatial distribution of MT impedances in the complete study area.

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The object of this series of studies is to develop a highly accurate statistical code for describing the planetary accumulation process. In the present paper, as a first step, we check the validity of the method proposed by Wetherill and Stewart (1989) by comparing the results obtained by their method with the analytical solution to the stochastic coagulation equation (or to a well-evaluated numerical solution). As the collisional probability Aij between bodies with masses of im1 and jm1 (m1 being the unit mass), we consider the two cases: one is Aij α i × j and another is Aij α min (i, j) (i1/3+j1/3) (i+j). In both cases, it is known that runaway growth occurs. The latter case corresponds to a simplified model of the planetesimal accumulation. We assumed that a collision of two bodies leads to their coalescence. Wetherill and Stewart's method contains some parameters controlling the practical numerical computation. Among these, two parameters are important: the mass division parameter δ, which determines the mass ratio of the adjacent mass batches, and the time division parameter ∈, which controls the size of a time step in numerical integration. Through a number of numerical simulations for the case of Ai j = i × j, we find that when δ < 1.6 and ∈ < 0.03 the numerical simulation can reproduce the analytical solution within a certain level of accuracy independently of the size of the body system. For the case of the planetesimal accumulation, it is shown that the simulation with δ < 1.3 and ∈ < 0.04 can describe precisely runaway growth. Because the accumulation process is stochastic, in order to obtain reliable mean values it is necessary to take the ensemble mean of the numerical results obtained with different random number generators. It is also found that the number of simulations, Nc, demanded to obtain the reliable mean value is about 500 and does not strongly depend on the functional form of Aij. From the viewpoint of the numerical handling, the above value of δ (< 1.3) and Nc (-500) are reasonable and, hence, we conclude that the numerical method proposed by Wetherill and Stewart is a valid and useful method for describing the planetary accumulation process. The real planetary accumulation process is more complex since it is coupled with the velocity evolution of the planetesimals. In the subsequent paper, we will complete the high-accuracy statistical code which simulate the accumulation process coupled with the velocity evolution and test the accuracy of the code by comparing with the results of N-body simulation.

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Anewapproach to predict the relativistic electron flux at geosynchronous orbit has been developed. Our prediction technique is based on a linear prediction filter with the input consisting of magnetic field variations at geosynchronous orbit for 6 consecutive days. The output is a prediction, made one day ahead, of the average daily flux of >2 MeV electrons. This technique was developed with data from GOES 7 for the period from January 1994 through April 1994. The technique's performance was tested by comparing the output results with the fluxes observed for the period from May 1994 through May 1995. The results showed that the empirical technique using in situ magnetic field measurements provides reasonably accurate forecasts of the geosynchronous electron environment one day in advance.

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In this study, baroclinic-barotropic instability of the Martian atmosphere is studied for a zonal basic state based on Mariner 9 observations, using a spherical linear primitive equation model derived from a method of 3-D normal mode expansion. As a result of solving a matrix eigenvalue problem, a distinctly unstable mode at synoptic to planetary scales was found with a peak growth rate of 2.3 (1/sol) at zonal wavenumbers n = 5 to 6 with an eastward phase speed of 50 (°/sol). The unstable mode has a period of 2.4 sol for n = 3, which agrees well with Viking observations. The geopotential amplitude maximum is located at 50°N near the surface, and the phase tilts westward with height. The structure is similar to baroclinic instability of a Charney mode associated with the subtropical jet on Earth. It is found, however, that the mode on Mars, where the subtropical jet is absent, is not the ordinary Charney mode, but a different one that is referred to as a monopole Charney mode in previous study which is characterized by its overall northward eddy momentum flux.

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