Long-period waves in the vicinity of Onagawa Bay (I)

To investigate the response characteristics of a bay to tsunamis, field measurements of long-period waves have been carried out at Onagawa and Okachi Bays, both of which face the Pacific Ocean in northern Japan. In Onagawa Bay, the observed transfer function is in good agreement with the prediction based on the one-dimensional numerical model, in the period range larger than about 15 minutes. The response of shorter periods seems to be influenced by the two-dimensionality of the bay. The oscillations within Onagawa Harbor are also discussed with respect to the relative amplitude and phase at two stations inside the harbor and it is estimated that the reflection coefficient at the waterfront is about 0.7. In Okachi Bay, the oscillations in the period range larger than about 10 minutes could be explained by a Y-shaped model of the bay. The dominant oscillations offshore of these two-bays are found to be the mode with the motion predominantly in the direction normal to the shelf orientation, and the estimated power spectral density of incoming waves in deep water varies asf ^–2,f being the frequency. The waves of lateral modes, such as edge waves on the shelf, are small and of minor importance to generate bay oscillations of longer periods.     

ASCA observations of colliding stellar winds in gamma velorum

We present new high spectral resolution X-ray observations of the colliding wind binary Vel taken with the ASCA satellite. We find two spectral components, one of which is post-shock emission from the colliding winds. Spectral variability is also seen, consistent with current notions of colliding wind phenomena.     

Limits on isocurvature perturbations from non-Gaussianity in WMAP temperature anisotropy

We study the effect of primordial isocurvature perturbations on non-Gaussian properties of cosmic microwave background (CMB) temperature anisotropies. We consider generic forms of the non-linearity of isocurvature perturbations which can be applied to a wide range of theoretical models. We derive analytical expressions for the bispectrum and the Minkowski Functionals for CMB temperature fluctuations to describe the non-Gaussianity from isocurvature perturbations. We find that the isocurvature non-Gaussianity in the quadratic isocurvature model, where the isocurvature perturbation S is written as a quadratic function of the Gaussian variable  σ,  S =σ²−〈σ²〉  , can give the same signal-to-noise ratio as   f _NL= 30  even if we impose the current observational limit on the fraction of isocurvature perturbations contained in the primordial power spectrum α. We give constraints on isocurvature non-Gaussianity from Minkowski Functionals using the Wilkinson Microwave Anisotropy Probe ( WMAP ) 5-year data. We do not find a significant signal of isocurvature non-Gaussianity. For the quadratic isocurvature model, we obtain a stringent upper limit on the isocurvature fraction  α < 0.070  (95 per cent CL) for a scale-invariant spectrum which is comparable to the limit obtained from the power spectrum.     

A multiple-code simulation study of the long-term EDZ evolution of geological nuclear waste repositories

This simulation study shows how widely different model approaches can be adapted to model the evolution of the excavation disturbed zone (EDZ) around a heated nuclear waste emplacement drift in fractured rock. The study includes modeling of coupled thermal-hydrological-mechanical (THM) processes, with simplified consideration of chemical coupling in terms of time-dependent strength degradation or subcritical crack growth. The different model approaches applied in this study include boundary element, finite element, finite difference, particle mechanics, and elasto-plastic cellular automata methods. The simulation results indicate that thermally induced differential stresses near the top of the emplacement drift may cause progressive failure and permeability changes during the first 100 years (i.e., after emplacement and drift closure). Moreover, the results indicate that time-dependent mechanical changes may play only a small role during the first 100 years of increasing temperature and thermal stress, whereas such time-dependency is insignificant after peak temperature, because of decreasing thermal stress.     

X‐Ray observations of the Galactic Centre with Suzaku

We report on the diffuse X‐ray emission from the Galactic Centre (GCDX) observed with the X‐ray Imaging Spectrometer (XIS) on board the Suzaku satellite. The highly accurate energy calibration and extremely low background of the XIS provide many new facts on the GCDX. These are (1) the origin of the lines at 6.7 and 7.0 keV is collisional excitation in a hot plasma, (2) the discovery of new SNR and super‐bubble candidates, (3) most of the 6.4 keV line is X‐ray fluorescence, and (4) time variability of the 6.4 keV line is found from the Sgr B2 complex. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Seismic intensity distribution of shallow earthquakes due to rupture velocities and faulting modes

Isoseismals of seismic intensity distributions are represented by earthquake source size, faulting mode, and rupture velocity of fault propagation. Unilateral faulting forms egg—shaped isoseimals, while bilateral faulting forms elliptical ones. It is found that the ratio of major to minor axes of isoseismals is sensitive to rupture velocity. Rupture velocity, faulting mode, and fault trend have been determined from the seismic intensity maps of the 1964 Niigata and the 1983 Japan Sea earthquakes in China by matching theoretical isoseismals. Rupture velocities thus estimated are mostly 70% to 90% of shear wave velocity. The difference would be considered as follows: short—waves which determine the seismic intensity are strongly dependent on the rupture of small—scale fault heterogeneities and on the jerky—onsets and abrupt terminations of local rupture propagations. On the other hand, rupture velocity from long—waves represents an average rupture propagation along the whole fault length. Faulting mode and fault trend estimated from seismic intensity maps match with each earthquake faulting independently determined. This suggests that the present method would be applicable to some historical earthquakes with known seismic intensity distribution to obtain detailed information on the faulting process.     

The occurrence of stanols in various living organisms and the behavior of sterols in contemporary sediments

Unsaturated sterols (stenols) and saturated sterols (stanols) in phytoplankton and Zooplankton from Lake Suwa and from higher plants around the lake were analyzed by combined GLC and MS. In all the organisms investigated, 5α-cholestanol, 24-methylcholestan-3β-ol and 24-ethyl-5α-cholestanol were found, although in low concentrations, together with large quantities of stenols. This strongly suggests the contribution of stanols from living organisms to recent sediments.Findings from incubation experiments of cholesterol and 5α-cholestanol in the surface sediment from Lake Suwa extending through 450 days suggest the following: (1) stanols are slowly degraded and tend to survive unaltered in sediments in comparison with stenols, (2) in a relatively oxidative depositional environment such as Lake Suwa, the greater part of the stanols in the surface sediment originates from living organisms, (3) the reduction zone in which the degradation of sterols is suppressed and the rapid hydrogenation of stenols takes place may be in the microbiologically active sediment from 1 cm to about 10 cm in Lake Suwa, and (4) the increase in the ratio of stanols to stenols with depth below the zone may be caused by the simultaneous progress of preferential degradation and slow hydrogenation of stenols during long-term preservation in sediments.     

A scaling model for quantification of earthquakes in and near Japan

A simple method is developed to determine seismic moments of earthquakes. The method is qualified through criteria such as simplicity of calculations, coverage of wide magnitude range, and insensitivity to detailed instrumental response. The method is applied to 163 major earthquakes which occurred underneath Japan and the Japan Sea in the time from 1926 to 1977. Magnitudes of these earthquakes, which have been determined by the Japan Meteorological Agency, (M_JMA) cover the range from 4.3 to 7.5. At first, source spectra are analyzed through a very simple way introducing two new parameters: characteristic period T_c and seismic-moment factor M_c. The former is defined as an average value of apparent periods of seismic waves with the maximum trace amplitude at many stations. The latter is an average of products of maximum trace amplitude and its apparent period multiplied by epicentral distance. It is shown that T_c corresponds to the period of the corner frequency of an earthquake and M_c to the seismic-moment density at the period of T_c. A scaling model of earthquake source spectra is presented which satisfies the empirical relations between the surface-wave magnitude M_s and M_JMA, and M_JMA and the body-wave magnitude m_b. Those relations are independent of the Gutenberg and Richter relation between M_s and m_b, because M_JMA is determined from maximum amplitudes of seismic waves with a period of about 4 sec. The static seismic moment of each earthquake can be estimated from calculated M_c using the source spectra of the scaling model. Seismic moments of 18 earthquakes determined by conventional analyses from near- and/or far-field observations are consistent with static seismic moments thus estimated over the range from 2 × 10²³ to 3 × 10²⁷ dyne cm. This shows the potential in practice of the present method, especially in the routine processing of seismic data.     

Physical basis of earthquake magnitudes: an extreme value of seismic amplitudes from incoherent fracture of random fault patches

A relation between surface-wave magnitude M_s and fault area S for great earthquakes has been proposed

log S 2M_{s − 11.5 (}M_{s > 7.5)}

A similar formula has been also derived for body-wave magnitude m_b^* redetermined from maximum amplitudes of short-period P-wavetrains

log S ∝ 1.7 m_b^*

These are quite different from a theoretical relation expected on the basis of long-wave approximation. Because wavelengths of seismic waves used for the above magnitudes are very short compared to the size of earthquake sources, these relations represent the short-period nature of the earthquake process. The statistical theory of extreme values has been applied to understand the relations considering that the component waves which constitute the wavetrains for m_b^* and M_s determinations originate from the random fracture of fault heterogeneities.     

On the importance of accurately ray-traced troposphere corrections for Interferometric SAR data

Numerical weather models offer the possibility to compute corrections for a variety of space geodetic applications, including remote sensing techniques like interferometric SAR. Due to the computational complexity, exact ray-tracing is avoided in many cases and mapping approaches are applied to transform vertically integrated delay corrections into slant direction. Such an approach works well as long as lateral atmospheric gradients are small enough to be neglected. But since such an approximation holds only for very rare cases it is investigated how horizontal gradients of different atmospheric constituents can evoke errors caused by the mapping strategy. Moreover, it is discussed how sudden changes of wet refractivity can easily lead to millimeter order biases when simplified methods are applied instead of ray-tracing. By an example, based on real InSAR data, the differences of the various troposphere correction schemes are evaluated and it is shown how the interpretation of the geophysical signals can be affected. In addition, it is studied to which extend troposphere noise can be reduced by applying the exact ray-tracing solution.