In 1997, two earthquakes (M6.5 and M6.3) occurred in the northwestern part of Kagoshima Prefecture, Japan. We carried out temporary seismic observation to obtain the detailed aftershock distribution. We constructed a 3-D P wave velocity model by inverting the travel times of aftershocks observed at 14 seismic stations in and around the focal area and relocated more than 14, 000 aftershocks with the 3-D velocity model. The general features of the aftershock distribution are as follows: (1) Aftershocks of the first main shock (M6.5) are distributed with a strike of nearly E-W (N100°E) in a vertical plane with a horizontal length of 21 km and a depth range of 2 to 9 km; (2) The second main shock (M6.3) has an ‘L’-shaped aftershock distribution: one plane strikes nearly E-W, which is parallel to the aftershock zone of the first main shock, and the other is a conjugate plane; (3) An obvious seismicity gap of about 3 km wide is found between the aftershock zones striking E-W for the first and second main shocks; (4) The aftershock activities are generally low around the hypocenters of the two main shocks. Our results show that most of aftershocks occurred not in high or low velocity zones, but in intermediate velocity areas. Several vertical linear distributions of aftershocks are also confirmed in the two focal zones striking E-W. These peculiar distributions suggest that the aftershock activity is affected by the underground structural boundaries.

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When a large earthquake occurs in and around the Japanese Islands, we should immediately determine the size and the fault geometry of the earthquake to mitigate earthquake disasters. Broad-band, strong-motion data of near-distance large earthquakes are used to investigate the possibility of CMT inversion. We perform a moment tensor inversion using the data sets of the 1993 Hokkaido Nansei-oki earthquake (M_JMA 7.8), the 1994 Hokkaido Toho-oki earthquake (M_JMA 8.1) and the 1994 Sanriku Haruka-oki earthquake (M_JMA 7.5). The accuracy of the solutions is examined by comparison with the solutions derived from global data sets, which reveals that the mechanism and moment magnitude are quite adequate for the purpose. Numerical experiments are made to analyze the influence of the source finiteness on the solutions. The numerical experiments suggest that the influence of the source finiteness on the mechanism retrieval is smaller than that on the moment retrieval. We suggest the installation of a network system of broad-band, strong-motion seismographs in the Japanese Islands.

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This paper presents a simple, generalized parameter constraint using a priori information to obtain a stable inverse of geophysical data. In the constraint the a priori information can be expressed by two limits: lower and upper bounds. This is a kind of inequality constraint, which is usually employed in linear programming. In this paper, we have derived this parameter constraint as a generalized version of positiveness constraint of parameter, which is routinely used in the inversion of electrical and EM data. However, the two bounds are not restricted to positive values. The width of two bounds reflects the reliability of ground information, which is obtained through well logging and surface geology survey. The effectiveness and convenience of this inequality constraint is demonstrated through the smoothness-constrained inversion of synthetic magnetotelluric data.

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The direction and intensity of primary remanence has been measured in oriented specimens from 367 lava flows of Late Miocene age in Western Iceland. The lavas which were sampled in 8 overlapping profiles, were generally good material for paleomagnetic measurements. In a 2500-m composite section, at least 15 reversals of polarity and several excursions are recorded. The mean remanence direction and other overall paleomagnetic parameters for the present collection of lavas are similar to those found elsewhere in Iceland. The average rate of buildup of this lava pile was rather low and possibly episodic. Hence, correlations to the geomagnetic polarity time scale and to polarity patterns in other composite sections mapped in Iceland are not straightforward.

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The anisotropy of local turbulence in the Earth's core is examined. It is recognized that small-scale motions in the core are strongly influenced by the Earth's rotation and its magnetic field. A small region of the core is simulated (the computational box), across which the prevailing large-scale (toroidal) magnetic field is supposed to be uniform and in which the temperature or compositional gradient providing the buoyancy that powers the turbulence is parallel to the (uniform) gravitational field. The simulations are used to estimate the turbulent fluxes of mean fields and their dependence on the latitude at which the computational box is situated. It is found that the effect of local turbulence on the diffusion of large-scale fields is significant, and that turbulent transport is anisotropic. It is believed that the results of the present study will prove useful in determining geophysically realistic diffusivities for use in future global geodynamo simulations.

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Two spatial-temporal models of the geomagnetic field secular variations in a time interval from 1970.0 to 1995.0 were constructed for the Far East region using the technique of regional modeling. First model covers a rectangular area, which includes Kamchatka, Sakhalin, the Kuril and Japanese Islands, the Ohotsk and the Japan Seas. Data from Russian, Japanese, and partially, Chinese observatories, and repeat stations were used for its construction. For filling of no data areas (the seas) the global models were used. Second model covers the area of the Japanese Islands and, partially, surrounding seas. Data from the Japanese magnetic observatories and first class repeat station network were utilized for its construction. We used natural orthogonal components method for obtaining temporal and spatial functions of the models. The accuracy of obtained models is much better than that of global models.

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A numerical algorithm to solve the 2D forward problem in magnetotellurics is presented. The method solves Maxwell's equations as a first order system of partial differential equations employing an iterative hybridized mixed domain decomposed finite element procedure. Absorbing boundary conditions are used on the artificial boundaries, diminishing undesired reflection effects and allowing the use of substantially smaller computational domains. Although the algorithm presented can be implemented on both serial and parallel computers, its capabilities are fully utilized on the latters. Results obtained on an IBM SP/2 parallel supercomputer of Purdue University are shown. Also the accuracy of the numerical method is verified by comparison with both numerical and analytical solutions provided by well known methods.

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In order to enhance the reproduction of the recovery phase D_st index of a geomagnetic storm which has been shown by previous studies to be poorly reproduced when compared with the initial and main phases, an artificial neural network with one hidden layer and error back-propagation learning has been developed. Three hourly D_st values before the minimum D_st in the main phase in addition to solar wind data of IMF southward-component B_s, the total strength B_t and the square root of the dynamic pressure, √nV², for the minimum D_st, i.e., information on the main phase was used to train the network. Twenty carefully selected storms from 1972-1982 were used for the training, and the performance of the trained network was then tested with three storms of different D_st strengths outside the training data set. Extremely good agreement between the measured D_st and the modeled D_st has been obtained for the recovery phase. The correlation coefficient between the predicted and observed D_st is more than 0.95. The average relative variance is 0.1 or less, which means that more than 90% of the observed D_st variance is predictable in our model. Our neural network model suggests that the minimum D_st of a storm is significant in the storm recovery process.

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