慢地震与深部不连续颤动和慢滑事件同时发生

我们对日本西南部南海俯冲板块界面过渡带发生的超低频地震进行报道。由矩震级为3.1～3.5的超低频地震引起的地震波显示其卓越长周期约为20s。超低频地震活动伴随深源低频颤动和慢滑事件同时发生,并随之移动。慢地震一直被认为会增加向上倾斜的大型逆冲地震断裂带上的应力,而这3种现象的同时发生提高了人们对慢地震的探测和特性鉴定水平。     

Decomposition of Phytoplankton in Seawater. Part I: Kinetic Analysis of the Effect of Organic Matter Concentration

Decomposition experiments were conducted on cultured phytoplankton (Skeletonema costatum) in seawater containing decomposer and consumer of size less than 500 μm. We determined the decomposition rates of bulk particulate organic matter (POM), the ratio of labile to semi-refractory fractions in the POM, and the POM carbon/ nitrogen (C/N) ratio during decomposition. To identify the kinetic mechanisms involved in the reactions of different order (e.g., first- and second-order), we studied the sensitivity of reaction rates to the initial concentration of POM, ranging from 2.4 to 71 mg-C L⁻¹. The results showed that decomposition consists of two first-order reactions: decomposition of labile and of semi-refractory particulate organic carbon (POC). The decomposition rate constants found for labile (0.13 day⁻¹ at 20°C), and semi-refractory POC (0.008 day⁻¹ at 20°C), and the carbon weight ratio of semi-refractory POC (13% at 20°C), were insensitive to the initial organic matter concentration. The time-dependence of the C/N ratio was also independent of this initial concentration. The decomposition rate constants and the content of semi-refractory POC did not change, regardless of the absence or presence of 25–500 μm organisms in natural seawater. This revised version was published online in July 2006 with corrections to the Cover Date.     

Laboratory Study of O(1S) Formation Process in the Photolysis of O3 and its Atmospheric Implications

A high-sensitive technique to detect O(¹S) atoms using vacuum ultraviolet laser-induced fluorescence (VUV-LIF) spectroscopy has been applied to study the O(¹S) production process from the UV photodissociation of O₃, N₂O, and H₂O₂. The quantum yields for O(¹S) formation from O₃ photolysis at 215 and 220 nm are determined to be (1.4 ± 0.4) × 10⁻⁴ and (5 ± 3) × 10⁻⁵, respectively. Based on thermochemical considerations, the O(¹S) formation from O₃ photolysis at 215 and 220 nm is attributed to a spin-forbidden process of O(¹S)+O₂(X³Σ_g ⁻). Analysis of the Doppler profile of O(¹S) produced from O₃ photolysis at 193 nm also indicates that the O(¹S) atoms are produced from the spin-forbidden process. In the photolysis of N₂O and H₂O₂ at 193 nm, no discernible signal of O(¹S) atoms has been detected. The upper limit values of the quantum yields for O(¹S) production from N₂O and H₂O₂ photolysis at 193 nm are estimated to be 8 × 10⁻⁵ and 3 × 10⁻⁵, respectively. Using the experimental results, the impact of the O(¹S) formation from O₃ photolysis on the atmospheric OH radical formation through the reaction of O(¹S)+H₂O has been estimated. The calculated results show that the contribution of the O(¹S)+H₂O reaction to the OH production rate is ∼2% of that of the O(¹D)+H₂O reaction at 30 km altitude in mid-latitude. Implications of the present laboratory experimental results for the terrestrial airglow of O(¹S) at 557.7 nm have also been discussed.     

Modification of the damping function in the k–ε model to analyse oscillatory boundary layers

A simple relationship has been developed between the wall coordinate y⁺ and Kolmogorov's length scale using direct numerical simulation (DNS) data for a steady boundary layer. This relationship is then utilized to modify two popular versions of low Reynolds number k–ε model. The modified models are used to analyse a transitional oscillatory boundary layer. A detailed comparison has been made by virtue of velocity profile, turbulent kinetic energy, Reynolds stress and wall shear stress with the available DNS data. It is observed that the low Reynolds number models used in the present study can predict the boundary layer properties in an excellent manner.     

Free-surface implementation in a mesh-free finite-difference method for elastic wave propagation in the frequency domain

We present an original implementation of the free-surface boundary condition in a mesh-free finite-difference method for simulating elastic wave propagation in the frequency domain. For elastic wave modelling in the frequency domain, the treatment of free surfaces is a key issue which requires special consideration. In the present study, the free-surface boundary condition is directly implemented at node positions located on the free-surface. Flexible nature of the mesh-free method for nodal distribution enables us to introduce topography into numerical models in an efficient manner. We investigate the accuracy of the proposed implementation by comparing numerical results with an analytical solution. The results show that the proposed method can calculate surface wave propagation even for an inclined free surface with substantial accuracy. Next, we calculate surface wave propagation in a model with a topographic surface using our method, and compare the numerical result with that using the finite-element method. The comparison shows the excellent agreement with each other. Finally, we apply our method to the SEG foothill model to investigate the effectiveness of the proposed method. Since the mesh-free method has high flexibility of nodal distribution, the proposed implementation would deal with models of topographic surface with sufficient accuracy and efficiency.     

Process of the Tsushima Current formation revealed by ADCP measurements in summer

In order to clarify detailed current structures over the continental shelf margin in the East China Sea, ADCP measurements were carried out in summers in 1991 and 1994 by the quadrireciprocal method (Katoh, 1988) for removing diurnal and semidiurnal tidal flows from observed flows, together with CTD measurements. We discussed the process of the Tsushima Current formation in the East China Sea. The Tsushima Current with a volume transport of 2 Sv (1 Sv=10⁶ m³s^–1) was found north of 31°N. A current with a volume transport of 0.4 Sv was clearly found along the 100 m isobath. Between the Kuroshio and the current along the 100 m isobath, southeastward component of velocity was dominant compared to northwestward one. Four eastward to southeastward currents were found over the sea bed shallower than 90 m depth. Total volume transport of these four currents was 1 Sv, and they seemed to be originated from the Taiwan Strait. Intrusion of offshore water into the inner shelf northwest of Amami Oshima was estimated to have a volume transport of 0.6 Sv. It is concluded that the Tsushima Current is the confluence of these currents over the continental shelf margin with the offshore water intruding northwest of Amami Oshima.     

Summary of taxa and distribution of Sirenia in the North Pacific Ocean

Abstract North Pacific fossil sirenians comprise representatives of three subfamilies of the Dugongidae: Halitheriinae ( Metaxytherium arctodites , Middle Miocene, North America), Hy-drodamalinae ( Dusisiren spp., Early-Late Miocene, and Hydrodamalis spp., Late Miocene-Pleistocene, North America and Japan), and Dugonginae ( Dioplotherium allisoni , Early-Middle Miocene, North America). Indeterminate dugongid remains are also known from the Late Oligocene of Japan, and the discovery of additional taxa in the western Pacific, especially in Paleogene rocks, can be anticipated. The known North Pacific Neogene taxa apparently dispersed into the Pacific from the Caribbean. Metaxytherium gave rise in the Pacific to Dusisiren ; a series of chronospecies of the latter genus eventually culminated in Hydrodamalis , which was exterminated by humans circa AD 1768. Dioplotherium left no known descendants in the Pacific. The Recent Dugong probably entered the Pacific from the Indian Ocean. The presence in the North Pacific Miocene of at least three sympatric dugongid lineages, together with desmostylians, is evidence for a diversity of marine plants that was reduced by subsequent climatic cooling.     

Diverse mineralogical and oxygen isotopic signatures recorded in CV3 carbonaceous chondrites

We describe the petrography and mineralogy of six CV3 carbonaceous chondrites. LAP02206, LAP02228, LAP04843, and GRA06101 are classified as oxidized Allende-like chondrites (CV3_oxA). RBT04143 and QUE97186 are classified as members of the reduced subtype (CV3_red). Chondrules in the CV3_oxA chondrites show extensive Fe–Mg zoning. Fe-rich olivine in the rims of the CV3_oxA chondrules are ¹⁶O-poor relative to Mg-rich olivine in the cores, suggesting that in addition to Fe and Mg, oxygen was exchanged between chondrules and matrix during weak thermal metamorphism. The CV3_red chondrites appear to have formed through various processes. QUE97186 shows chondrule flattening with a preferred orientation, which is interpreted to have resulted from shock impact at a pressure of ～20 GPa. The post-shock residual heat (～1000 °C) is likely to be responsible for the restricted Fe/Mg ratios of matrix olivine. Based on the degree of Fe–Mg homogenization of matrix olivines, we estimate the spatial scale of the shock-heated region to be ～1 m. RBT04143 is a breccia containing many clasts of two types of lithologies: reduced-type material and very weakly altered material.     

Upwelling of oxygen-depleted water (Sumishio) in Omura Bay,Japan

Wind-induced circulation and the distribution of hypoxia corresponding to the upwelling of oxygen-depleted water (called “Aoshio” in Japan and “Sumishio” locally in Omura Bay) in Omura Bay, Japan, was examined field observations and by three-dimensional modeling. During the calm weather in summer, well-mixed strait water, in rich oxygen at the mouth of the bay intruded into the middle layer of the bay, southward and northward along the west and east coast, respectively, forming basin-scale cyclonic circulation. A stagnant water mass was formed below the center of this cyclonic circulation, and it became hypoxic water. As a result of the prevailing strong southeast (northeast) wind, the bottom hypoxia moved in a southeasterly (northeasterly) direction. This induces the upwelling of hypoxic water, accompanied by mass mortality of marine organisms.     

Analyzing rainfall-induced mass movements in Taiwan using the soil water index

This study applied the soil water index (SWI), which can represent the conceptual soil water contents as influenced by present and antecedent rainfall, for analyzing rainfall-induced mass movements in Taiwan. The SWI has been used in Japan for nationwide mass movement warnings. This study examined whether the SWI can be also applied to Taiwan, which has a climatic condition and high-relief topography similar to Japan. We used data for mass movements for 2006–2012 (n = 263) for the main analyses and those for 2013 (n = 19) for verification. The SWI values before the rainfall events that triggered mass movements were used as the indicator of the antecedent rainfall condition. We found that when SWI values before rainfall events increased from <17.5 to >35, the upper threshold of rainfall conditions needed for triggering mass movements significantly decreased. The mass movements in 2013 support this finding. We classified rainfall conditions for triggering mass movements into two types, short duration–high intensity (SH) and long duration–low intensity (LL), based on a principal component analysis (PCA). The SH type is associated with a rapid increase in SWI, and the LL type is associated with a gradual rise and subsequent constancy of SWI except in some extremely long rainfall events. Based on this result, we modeled the general trend of the time series changes in SWI for the two types, which was verified using the mass movements in 2013.