In nonlinear elastic equations, there exist two kinds of simple waves, i.e., Non-Coupled and Coupled Simple Waves which are alias named nonlinear P and S waves, respectively. Nonlinear equation used in this paper is of second order with respect to displacement and, as the result, used stress is also of second order. A used model is a two-dimensional one. The used stress condition is truely nonlinear instead of quasi-linear, where the latter is usually solved by use of perturbation procedure. In the model of a half-infinite elastic medium, the nonlinear stress-free surface condition is, as the first approximation, separated into three branches of stress conditions in the case of a direct-hit earthquake. For the incidence of Simple (nonlinear) Waves on the free surface, the reflected waves are evaluated for the above three branches of condition. The energy expression is then obtained in a weakly nonlinear case (the elastic constants associated with higher order terms are negligibly small). Among three branches of condition, the first one is a condition which causes ordinary reflected waves similar to P and S waves in a linear theory. The other two are conditions which produce retarded reflected waves with slow velocity near the free surface. As the result of the retardation, high energy flux occurs along the free surface. This high energy causes a large disaster on the occasion of large earthquake. When nonlinear P wave is retarded near the free surface, Rayleigh-type nonlinear wave appears along the free surface. The wave is then propagated at a velocity a little less than that of linear S wave. This behavior indicates the generation process of Rayleigh wave. These results are obtained by use of truely nonlinear stress condition at the free surface instead of quasi-linear one based on perturbation procedure.

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We calculated the stress field on and around an elliptical open crack located in an elastic medium under various confining pressures. The problems were treated as two-dimensional ones by using complex potentials, and the considerable differences between uniaxial and triaxial crack growth mechanisms were recognized. A uniaxial condition allows a certain elliptical open crack to develop by itself (without any crack-crack interactions) along its major semi-axis, whereas a triaxial condition does not. This is the principal mechanism of transition from uniaxial to triaxial crack growth. Our transition model, derived using a theoretical approach, explains the dependence of experimental results, such as crack distribution and internal friction angle, on confining pressure well. We conclude that uniaxial and triaxial crack growth mechanisms fundamentally differ from each other, and that attention must be paid to experimental conditions when applying experimental results to phenomena in the Earth.

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A creepmeter has been developed to monitor gradual displacements of near-surface movement in an active fault zone. This rod-type creepmeter is a robust, low-cost instrument that is simple to construct and install. This creepmeter consists of two 3-m invar rods attached to anchored steel piers at each end, straddling the surface traces of active fault. The invar rods are supported by a pair of U-shaped solid steel girders. A mechanical dial-gauge sensor in the middle of the creepmeter is adopted to record the displacement of fault creep, and has a precision of 0.01 mm. Because the creepmeter is installed on the surface, the temperature effect is important. To calibrate and correct for the temperature effect, we carried out hourly measurements over a period of 30 hours to calculate the thermal expansion coefficients for each creepmeter. Thermal corrections could thus be made when readings were taken. Five of these creepmeters have been installed in the Chihshang active fault zone of eastern Taiwan, in the present collision suture zone between the Philippine Sea plate and the Eurasian plate. Readings taken over one year have shown that this rod-type creepmeter is effective in providing a near-continuous record of active fault creep with a good precision.

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A rock magnetic study was performed on sediment cores from four sites in the South Atlantic off the western coast of Africa, which were taken during the Ocean Drilling Program Leg 175 (Sites 1078, 1082, 1084, and 1085). The sites are within the Angola-Namibia upwelling system, and the sediments have a high total-organic-carbon content. Concentration of ferrimagnetic minerals at these sites is very low, and the magnetic susceptibility is dominated by paramagnetic and diamagnetic minerals. Severe and rapid loss of remanent magnetization occurred during storage of the cores, with less than 10% of the initial intensity remaining a few months after core recovery. The loss of magnetization may prevail in organic-rich sediments. Changes of magnetic properties with time were examined using samples that were kept frozen before the experiment. Hysteresis parameters and the ratio of ARM (anhysteretic remanent magnetization) to SIRM (saturation isothermal remanent magnetization) indicate increases in the average magnetic grain size with the decay of magnetization, which suggests preferential dissolution of finer magnetic minerals. Loss of low-coercivity magnetic minerals with time was estimated from the decrease of S ratios. Low-temperature magnetometry revealed the presence of magnetite in the sediments even after the completion of sulfate reduction. Magnetization attributable to magnetite decreased with the loss of magnetization. This suggests the transformation of magnetite into non-magnetic phases, which is consistent with the decrease of S ratios.

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Sandstones and siltstones collected from the Siwalik molasse sequence (-16 to 5 Ma) of the Karnali river section have been studied for their magnetic properties. Behavior of the specimens during demagnetization (of theNRMand IRM) and magnetic susceptibility vs. temperature data suggest that goethite, maghemite/magnetite, and hematite are the main magnetic minerals in the section. Goethite, carrying a recent component, is the dominant magnetic mineral in the fine-grained lithologies from the lower part. Maghemite and magnetite, which also carry a secondary remanence, occur in the sandstones from the upper part. Hematite, mainly of detrital origin, is present in the whole sequence. The magnetic fabric is defined by mainly oblate AMS ellipsoids and a low degree of anisotropy (P′< 1.20). The magnetic lineations (declinations: 75°-130°or 245°-310°; peak orientation: 290°/2.8°) are subparallel to the fold axes/bedding strikes/thrust fronts (WNW-ESE). The initial sedimentary-compactional fabric has been overprinted by a secondary tectonic fabric, which was probably induced by mild deformation active in the compressive tectonic setting.

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Decomposition of the magnetotelluric (MT) tensor when the electric field alone is subject to local galvanic distortion has received much attention in recent years. Recently some authors have extended such procedures to include the effects of the associated magnetic field distortion as well. With the aid of a three-dimensional modelling program the validity of the assumptions made in these analyses have been examined by considering the response over a range of periods of a small conductive cube at the surface of the earth and near a two-dimensional fault. The study indicates that the inclusion of magnetic distortion is necessary and important at short periods when induction occurs in the anomaly itself, but that the elements defining the magnetic distortion matrix become complex at such periods so that the assumptions underlying the theory are no longer valid. At longer periods the magnetic distortion matrix does become real and frequency-independent and therefore determinate, but its effect on the response becomes insignificant compared with that due to electric distortion. At these longer periods the phases of the regional impedances and the ratios of the electric distortion parameters are correctly recovered whether magnetic distortion is present or not. Calculations were repeated for a resistive cube, and also for a highly resistive region beneath the surface layer, with no significant enhancement of magnetic distortion. It is concluded that, at least for such models, inclusion of magnetic distortion in decompositions of the MT tensor does not, in general, offer any improvement over the conventional decompositions in which only distortion of the electric field is taken into account.

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Magnetic induction arrow responses of elongated conductors embedded in a uniformly resistive host earth and underlain by a conductive substratum are studied with the aid of laboratory analogue model measurements. The dependence on period, the overburden depth, and the underlying conductive basement depth, are examined for three different conductor lengths that were chosen so as to include the transition in the responses from two dimensional (2D) to those of three dimensional (3D) structures. The characteristic period Tc (the period at which the in-phase anomalous response of the elongated conductor model is maximum and the quadrature response is minimum) is found to depend on the conductor length, the overburden depth, and the conductive basement depth. A decrease in conductor length or conductive basement depth, each have the effect of decreasing both the response amplitude and the characteristic period, while increasing the overburden depth has the effect of increasing the characteristic period and decreasing the response amplitude. At short periods, say below the characteristic period, the responses appear to be roughly 2D, and to become progressively more 3D-like with increasing period above the characteristic period.

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This is the second paper of a series of our studies, in which we have investigated the light element synthesis (Li, Be, B, and the CNO-elements) in the He-layer and the H-rich envelope of supernovae and the aim of this paper is to see the sensitivity of the element synthesis to the chemical compositions in the presupernova stage. First, we deduced the probable range of the chemical composition in the presupernova stage on the basis of the previous studies of stellar evolution as well as the observational studies of stellar chemical compositions. Secondly, we pursued the explosive nucleosynthesis by using artificially modeled chemical compositions composed of only four elements (¹H, ⁴He, ¹²C, and ¹⁶O). Combining these results, we constructed five kinds of diagrams between two isotopic/elemental ratios which are useful for distinguishing presolar grains of supernova origin: ⁶Li/¹⁶O and ⁹Be/¹⁶O to ⁷Li/¹⁶O, and ¹¹B/⁷Li, ¹⁴N/¹⁵N, and ¹⁶O/¹⁷O to ¹²C/¹³C. In conclusion, the uncertainties in the chemical compositions in the presupernova stage brought smaller varieties in the isotopic/elemental ratios of the light elements compared with the varieties investigated in our first paper about the neutrino emission models.

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As part of seismotectonic investigations in Northwestern Taiwan, eight magnetotelluric (MT) soundings were conducted across the Sanyi-Puli seismic zone, a distinct NW-SE trending linear seismic zone in the fold-thrust belt of Northwestern Taiwan. A preliminary one-dimensional resistivity model according to the determinant MT response was computed, and the resistivity model includes a high-conductivity anomaly beneath this seismic zone. This conductivity anomaly raises the possibility of metamorphic dehydration triggering the seismic events in the seismic zone. The correlation between the conductivity anomaly and active seismicity in this area is tentatively discussed in this study.

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