Seismic waves suffer strong attenuation when propagating beneath the Hida Mountains in Central Honshu, Japan. In order to study this region in detail, we conducted three kinds of dense seismic array observations in and around the Hida Mountains in the summer of 1996. Picking P- and S-wave arrival time data from 54 events at 101 stations, 3175 P- and 2335 S-wave arrival time data were obtained for our tomographic study. Hypocenter locations and velocity structure were determined simultaneously. We assessed ray coverage and resolution of the velocity structure with checkerboard resolution tests. Ray paths for the model of the obtained velocity structure were examined in detail. There are two zones of low P-wave velocity, one at a depth of 4 km and the other at 15 km. The resolution is good at depths of 0-20 km for P-wave velocities and at depths of 0-15 km for S-wave velocities. A high V_P/V_S ratio (2.7) indicates that a partially melting rock exists beneath the Hida Mountains. The deep low velocity zone is located just above the upper/lower crustal boundary. These observations indicate that a magma reservoir exists in the upper crust beneath the Hida Mountains.

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Autoregressive moving average (ARMA) method is applied to modeling the time series of position changes of GPS sites, obtained by the Geographical Survey Institute (GSI) of Japan during the period from April 1996 to March 1998. The present application is focused on denoising of the GPS time series data where only white noise is considered, and detection of data discontinuities and outliers in order to obtain time-averaged velocity and strain fields in central Japan. The data discontinuities are detected by a typical Kalman filter algorithm. The outliers are eliminated by using robust estimation techniques during the ARMA process. The averaged strain field, calculated by the least-squares collocation method from the improved two-year time series data, distinguishes clearly between the tectonically active and inactive regions. Higher maximum shear strain rates were detected in the southern area of the Kanto district. In the areas with very high seismicities, however, the difference between the maximum shear strain rates, that were estimated from the raw time series data and the ARMA-analyzed data, amounted to about 0.2 microstrain/yr. This indicates that the existence of noise and discontinuities can lead to an over-prediction of the strain field.

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The knowledge of the geomagnetic field and its secular variation allows us to compute the fluid flow at the core surface. The poloidal and toroidal components of the fluid flow at the core-mantle boundary (CMB) have been calculated every year from the Bloxham and Jackson model (1992) and plotted at 50 year intervals over the last three centuries. The flow patterns conserve some broad features over this whole time-span. The time constant of the degree 1 component of the motion is larger than the time constant of the rest of the flow. The average motion over 300 years appears to be in large part symmetrical with respect to the equator. This average flow can be represented by the sum of a few geostrophic vectors. The acceleration fields corresponding to the well documented jerks of 1969, 1979, 1992 have also been computed. The geometry of these acceleration fields is the same, within a change of sign, for the three events. Moreover, this geometry has close connections with the geometry of the flow itself. The spatial and temporal variations of the flow field can be simply described, in a first approximation; it is possible to give an analytical schematic representation of the flow field during the last three decades. Some characteristics of the decadal length-of-day variations follow if the coupling torque between core and mantle is topographic.

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Sedimentary rocks in the Late Permian Gobangsan Formation were collected at 7 sites for paleomagnetic study in the southeastern periphery of the Bagjisan Syncline, Korean Peninsula. The Gobangsan Formation revealed a stable secondary magnetization component with unblocking temperatures of 500-580°C and 650°C from two sites, while the other sites possessed only a present day viscous remanence. The secondary component resides in magnetite and hematite. The site-mean directions of the two sites before tilt correction (D = 355.9°, I = 50.2° with _α95 = 4.3°and D = 355.7°, I = 53.3° with _α95 = 6.1°) suggest that the remagnetization occurred after Early Cretaceous. The most plausible mechanism of the remagnetization is considered to be a chemical authigenesis because the other possible mechanisms of the remagnetization such as thermoviscous process and Recent weathering can be ruled out by rock magnetic experiments. The timing of the remagnetization is constrained during Tertiary time, because the observed directions are distinguishable from the Cretaceous directions and because Recent remagnetization is unlikely. This is ascertained by good agreement between the observed and the Tertiary directions.

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Polynomial surfaces are a very useful tool in order to represent the geomagnetic field over small area of the Earth's surface. Nevertheless, the method has not always been applied with enough precision, nor its limitations are considered in detail. This work analyses the conditions of applicability of this method to data from the aeromagnetic survey of the Spanish mainland accomplished in 1986, and the fulfilment of the necessary conditions to enhance a merely descriptive model to produce another of inferential character. The best statistical adjustment with only descriptive character, valid for the whole Spanish mainland, is a third degree model. The study proves that the conditions of validity for inferential models are met using polynomials of second and third degree in restrictive areas, but not over the whole Iberian Peninsula. In this case, the model of second degree is proposed as the best option because of its simplicity and the extension of its region of validity. To a great extent, the restrictions to enhance the descriptive models are due to the own nature, specially to their geographical dependence, and can also affect the analysis of other kinds of geophysical data.

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It has been suggested that the atmospheric concentration of SO₂ observed on Venus coincides with the equilibrium concentration over pyrite-magnetite assemblage (pyrite-magnetite buffer). If the atmospheric SO₂ abundance is controlled by the chemical reaction at the planetary surface, we expect coupling between the atmospheric SO₂ abundance and the surface temperature. Here, we propose that the pyrite-magnetite buffer combined with cloud albedo feedback controls the surface temperature on Venus. We show that this mechanism keeps the surface temperature in a rather narrow range around the presently observed value against large variations of solar luminosity and total atmospheric mass.

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In order to establish a suitable manner for finding presolar grains of supernova origin, we simulated the explosive nucleosynthesis of light elements, i.e., CNO-elements and X-elements (Li, Be, and B), in the He-layer and the Hrich envelope of a 16.2 M_??_ supernova and calculated their final abundances and abundance ratios using the nuclear reaction network. We also investigated the response of the synthesized abundances of light elements to the change of strength and duration of the neutrino emission, about which we have not a precise knowledge. The obtained results are as follows. The amounts of ⁶Li and ⁹Be produced during the supernova explosion are quite small. The ratios of ⁶Li/⁷Li and 9Be/7Li are less than 2 × 10^-4, which are much smaller than the corresponding solar-system values. The other X-elements and CNO-elements (except ¹²C and ¹⁶O) are synthesized, more or less, and their abundances depend strongly on the internal mass coordinate as well as the adopted neutrino emission model. However, ¹¹B/⁷Li ratio and isotopic ratios of CNO-elements are confined within one order of magnitude or so. On the basis of the above results, we finally proposed useful diagrams between two isotopic (elemental) ratios, which would help us to find presolar grains of supernova origin.

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