Improved methods to transform frequency‐dependent complex stiffness to time domain

A method to transform the frequency‐dependent complex stiffness to the impulse response in the time domain was proposed in the previous paper. However, there is a problem in that the accuracy and the convergence of the transformed impulse response are not good in some cases. Moreover, the hysteretic damping was not considered in the previous study although it is essential for practical purposes. In this paper, transform method improvements are proposed. First, the accuracy and the convergence are improved by taking the concept of virtual mass into account. Then, a more improved method for transforming the complex stiffness with large hysteretic damping to the time domain is proposed using the least square method. It is well known that the rigorous transform of the hysteretic damping is impossible because it is non‐causal. So this method is thought to be an approximate causalization process. Copyright © 2006 John Wiley & Sons, Ltd.     

Temporal Variations of Seismic Wave Velocity Associated with 1998 M6.1 Shizukuishi Earthquake

—?We attempt to detect temporal variations of seismic wave velocity before and after 1998 M6.1 Shizukuishi, northeastern Japan, earthquake by using waveform data from explosions and earthquake doublets spanning the period immediately before and after the earthquake. Direct P waves of the second explosion are delayed by ～20 ms at observation stations with epicentral distances less than 15 km. This tendency does not change if the analysis frequency band is changed. Our result suggests that the P-wave velocity decreased by at least 1% in the extremely shallow region of the hanging wall of the M6.1 thrust event after its occurrence. On the other hand, there was the frequency dependence of the coda wave delays for both artificial sources and for natural events. At 5–10 Hz, immediate sharp increases by more than 20 ms in time delays and lower coherency were observed at several stations. We estimated the region in which P-wave velocity might have decreased after the M6.1 earthquake. Maximum depth of the region is 13 km. The region includes the aftershock area of the M6.1 earthquake, but is eccentric to the earthquake in the west. Considering the frequency band analyzed (5–10 Hz), the scale of the spatial inhomogeneity which led to the coda wave delay is several hundreds meters. We investigated candidates for the cause of the direct P-wave and coda wave delay. Observed direct P-wave delay can be partly explained by the stress changes caused by coseismic fault slip. However, the coda wave delay cannot be explained by the stress changes that are limited to the superficial area. Crustal heterogeneity should have changed at coseismic time in the deeper area where aftershocks of the M6.1 earthquake occurred.     

Observational tests of x-matter models

We study gravitational lensing statistics, matter power spectra and the angular power spectra of the cosmic microwave background (CMB) radiation in x-matter models. We adopt an equation of state of x-matter which can express a wide range of matter from pressureless dust to the cosmological constant. A new ingredient in this model is the sound speed of the x-component, in addition to the equation of state w ₀ =  p _x0/ρ_x0. Except for the cosmological constant case, the perturbations of x-matter itself are considered. Our primary interest is in the effect of non-zero sound speed on the structure formation and the CMB spectra. It is found that there exist parameter ranges where x-matter models are consistent with all current observations. The x-matter generally leaves imprints in the CMB anisotropy and the matter power spectrum, which should be detectable in future observations.     

The Holocene development of Kushu Lake on Rebun Island in Hokkaido, Japan

Diatom assemblages and sulfur content in sediments were analyzed to clarify changes in the sedimentary environment of Kushu Lake, a coastal lake on Rebun Island in Hokkaido, Japan. Salinity variations were assessed by means of a diatom-based index of paleosalinity and the sedimentary sulfur content. This paper discusses the Holocene development of the lake, in relation to Holocene relative sea-level change. For paleoenvironmental interpretation of the lake development, the rationale of the threshold method (Anundsen et al., 1994) was applied.At ca. 8000 yr BP, a coastal embayment (paleo-Kushu Bay) resulted from marine ingression. The threshold elevation at the mouth of the paleo-Kushu Bay kept pace with the rising sea-level, resulting in its enclosure at the culmination of Holocene marine transgression (ca. 6500–5000 yr BP). From predicted relative sea-level at ca. 6000 yr BP for Rebun Island (Nakada et al., 1991), the threshold may have been at least above –3 to –5 m altitude. A freshwater lake environment with strongly anoxic bottom conditions may have occurred from ca. 5500 to 5100 yr BP. After an important episode of marine ingression, the lake was isolated completely from the open sea at ca. 4900 yr BP. The diatom record suggests that the maximum lacustrine extent occurred at ca. 4900–3100 yr BP. Thereafter, water depth decreased at the lake margins.In Kushu Lake, the threshold elevation, due to a build-up of a coastal barrier, prevents us from determining the amplitude of sea-level changes, even though the age of isolation contacts corresponds to periods of regression and climatic deterioration. In spite of isostatic subsidence, the effective protection provided by the well-developed barrier did not allow registration of any relative sea-level fluctuations since its isolation.     

Non-destructive alpha-spectrometry and radiochemical studies of deep-sea manganese nodules

A deep-sea Mn nodule handpicked from top of a 20 cm diameter North Pacific tripod core was directly counted by an α-spectrometer with a silicon surface barrier detector. A distinct ²¹⁰Po peak was recorded for the nodule upper side in contact with bottom seawater. Based on the spectrum, the ²¹⁰Po (supported by ²¹⁰Pb with 22 year half life) is thought to be confined in the top few microns of the nodule. Since the ²¹⁰Po peak diminishes considerably for the bottom side resting on sediment, nondestructive α-spectrometry is useful for determining the sea floor orientation of nodules. This method was tested for a western North Pacific dredged Mn nodule, and was successful in identifying the orientation (and also surface loss of the nodule by dredging).Radiochemical analysis was also carried out for carefully scraped samples from the top and bottom of the two nodules. The growth rates of the hand-picked nodule based on excess ²³⁰Th and ²³¹Pa profiles were 1 to 4 mm/10⁶ y and apparently varied with time. The dredged sample has grown at 2 to 6 mm/ 10⁶ y and the growth has likely been episodic. The radionuclide composition was significantly different between the top and bottom. Evidence was found that the radionuclide fluxes of ²³⁰Th and ²³¹Pa into the nodules considerably changed during the growing histories. The ²³⁰Th/²³²Th activity ratios in the nodule tops are comparable with those in bottom seawater suggesting that the top is growing with metals of seawater origin. However it remains unresolved whether the nodule bottoms are growing by utilizing metals in the sediment pore water.     

Oblique and near collision subduction, Sagami and Suruga Troughs —preliminary results of the French-Japanese 1984 Kaiko cruise, Leg 2

Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the Suruga and the Sagami Troughs, which lie on both sides of the northwestward moving and colliding Izu-Bonin Ridge, the northernmost part of the Philippine Sea plate. The transition from the Nankai Trough to the Suruga Trough is characterized by northward decrease in width of the accretionary prism, in good agreement with the increasing obliquity between the through axis and the direction of the convergence, as the strike of the convergent boundary changes from ENE-NNE to south-north. South of the area, the southern margin of the Zenisu Ridge shows contractional deformations. This supports the interpretation made by the team of Leg 1 who studied the western extension of the area we studied, that it is an intra-oceanic thrusting of the ridge over the Shikoku Basin. In the Sagami Trough, where the relative motion is highly oblique to the plate boundary, active subduction is mostly confined in the east-west trending portions of the trough located south of the Boso Peninsula and along the lower Boso Canyon, near the TTT triple junction. In between, the present motion is mainly right-lateral along the northwest trending Boso escarpment. However, an inactive but recent (Pliocene to lower Pleistocene) accretionary prism exists south of the Boso escarpment, which suggests that the relative motion was more northerly than at present before about 1 Ma ago.     

Evolutionary computations of the early-type contact binary SV Centauri

Evolutionary models of the early-type contact binary SV Centauri are recalculated with contact condition taken into account. Two types of the contact condition are employed in the contact phase. With the initial masses of 13.4 and 7.0M for the component stars, the observed features such as the rate of mass transfer, the degree of contact, and the positions of both components in the H-R diagram can be reproduced. In agreement with the conclusion given in the previous paper (Nakamuraet al., 1978), this indicates that the binary system SV Cen is actually in the rapid phase of mass transfer preceding the reversal of the mass ratio.In contrast to the steadily increasing character of the rate of mass transfer shown in the previous paper, however, the rate of mass transfer suddenly turns to decrease as soon as the system evolves into the contact phase. This decreasing character continues until the rate drops to a minimum. In such contact phase the radius of the primary component remains almost unchanged, the secondary component increases its radius slowly, and the degree of contact increases in a definite way. Except a slight difference in the degree of contact evaluated, the use of different expressions for the contact condition does not produce practically any appreciable difference in the results.     

Determination of Chromium, Nickel, Copper and Zinc in Milligram Samples of Geological Materials Using Isotope Dilution High Resolution Inductively Coupled Plasma-Mass Spectrometry

A simple and accurate method for the determination of Cr, Ni, Cu and Zn at μg g^?1 levels in milligram‐sized bulk silicate materials is reported using isotope dilution high‐resolution inductively coupled plasma‐mass spectrometry (HR‐ICP‐MS) with a flow injection system. Silicate samples with Cr, Ni, Cu and Zn spikes were digested with HF‐HBr and Br₂, and subsequently decomposed at 518 K in a Teflon bomb. In this procedure, all sulfides and chromite, major hosts of these elements, were completely decomposed, thus allowing for isotope equilibration between the sample and spike. Magnesium and Al fluorides formed after the digestion of the sample were removed by centrifugation, and the supernatant was directly aspirated into a HR‐ICP‐MS at a mass resolution of 7500, where interfering oxide ions, ArO⁺, CaO⁺, TiO⁺, CrO⁺ and VO⁺, were separated from Cr⁺, Ni⁺, Cu⁺ and Zn⁺. No matrix effects were observed down to a dilution factor of 50. Detection limits for these elements in silicate samples were < 0.04 μg g^?1. The effectiveness of the technique was demonstrated by the analysis of 13 to 40 mg test portions of USGS and GSJ silicate reference materials with a major element composition ranging from andesite to peridotite, in addition to 8‐23 mg of the Smithsonian reference Allende. Both the reproducibility and the deviation from the reference value for most reference materials of various rock types were < 9%, and thus confirm that the method gives accurate analytical results for small sample sizes over a wide range of Cr, Ni, Cu and Zn contents. This method is, therefore, suitable for analysing small and/or precious bulk samples, such as meteorites, mantle peridotites and mineral separates, and for the characterisation of silicate and sulfide minerals for use as calibration samples in secondary ion mass spectrometry or laser ablation ICP‐MS.     

Deformable backstop as seaward end of coseismic slip in the Nankai Trough seismogenic zone

Wide-angle seismic surveys performed in the last decade have clarified the 3-D crustal structure along the Nankai Trough. The geometry and velocity structure of the southwestern Japan subduction zone are now well constrained. Comparing these observations with the rupture distribution of historic great thrust earthquakes, it appears that the coseismic rupture occurred along plate boundaries deeper than the wedge/backstop boundary (the boundary between the Neogene-Quaternary accretionary wedge and the crust forming the backstop). From the view of spatial relationship, both rupture distributions of the last two large events and the crust forming the backstop are considerably retreated from the trough axis in the west and east off the Kii Peninsula. In both areas, seamount or ridge subduction is apparent in seismic results, geomorphological data and geomagnetic data. The landward indentation of the deformable backstop, which corresponds to the crustal block of old accreted sediments, may be formed by seamount subduction according to published results of sandbox modeling. In particular, the subducted seamount may be a structural factor affecting the recession of the crustal block forming the backstop.