Organic substances in sediment and settling matter during spring in a meromictic Lake Suigetsu

Vertical distribution of organic substances of settling matter and sediments was measured in a typically meromictic lake, Lake Suigetsu. Proteinous materials were rapidly eliminated in the early stage of settling in the lake water. No major difference was found in amino acid composition of the hydrolysates of all samples, indicating that there had occurred no selective decomposition of amino acids. In contrast, the decomposition of carbohydrates was sluggish. Water-extractable carbohydrates were abundant in the sediments. Dissolved carbohydrates and amino acids, formed in the sediments, were partly metabolized by photosynthetic sulfur bacteria,Chromatum sp. populated in the chemocline.     

Distribution of Overturn Induced by Internal Tides and Thorpe Scale in Uchiura Bay

The vertical mixing process induced by internal tides was investigated by repeated conductivity, temperature, and depth (CTD) measurements and bottom-mounted acoustic Doppler current profiler (ADCP) in Uchiura Bay from July 24 to 25, 2001. Internal tides were observed with a wave height of 40 m and a horizontal current of 0.3 ms⁻¹. Density inversions were found in the CTD data, and the method of Galbraith and Kelley (1996) was applied to the data to identify overturns and to calculate Thorpe scale. Most of the overturns distributed in the region of low Richardson number, so that they were considered to be caused by shear instability associated with the internal tides. Thorpe scale was calculated to be 0.48 m. From the Thorpe scale, the vertical eddy diffusivity due to internal tides in Uchiura Bay was estimated as K _ρ ∼ 10⁻⁴ m²s⁻¹. This revised version was published online in July 2006 with corrections to the Cover Date.     

中国渤海地区海底地震观测的初步研究

本文是在日本造船振兴财团的经费赞助下在中国渤海地区发展海底地震观测的初步技术报告。本报告主要包括以下内容: (1) 渤海及其邻近地区的地震活动性与地震危险性估计; (2) 渤海地区海底地震观测的条件; (3) 渤海地区海底地震观测仪器与技术的某些初步研究结果; (4) 浅海情况下海底地震观测噪声的模拟实验研究结果。文中研究了该地区的历史与现代地震活动性、长期地震危险性以及发生在华北与日本的大地震之间的关系。还讨论了渤海强震和海啸对渤海及其邻近海域航海的影响问题。在这些研究结果的基础上对在渤海地区开展海底地震观测的必要性进行了解释。本文探讨了在渤海地区进行海底地震观测的某些仪器和技术问题。天津市地震局曾在渤海地区石油平台的油井中作过初步地震观测试验。日本气象厅气象研究所发展的海底地震观测中有关降低噪声与信号传输等方面的新技术与经验可用于渤海地区的海底地震观测。在这些研究的基础上,提出了一个在中国渤海地区发展渤海地震观测的初步设想。指出渤海海底地震观测试验应从观测和研究渤海海底海洋噪声谱开始。为了研究浅海情况下海底地震观测的噪声,本文完成了一项新的模拟试验工作。分别对海面源及海内源产生的噪声进行了实验研究。文中给出了一些重要实验结果。本文所得的初步结果,对发展渤海及邻近地区海底地震观测有一定的参考价值。     

美国南加州地壳、上地幔三维P波速度成像-莫霍面起伏的影响

应用南加州地震台网记录的6 347次区域地震的146 422个P波到时数据，并结合其它地震学方法所得到的莫霍面深度资料，在考虑莫霍面起伏变化下，确定了该区0～35 km深度内的精细三维P波速度模型．检测板分辨率实验结果说明了反演结果的可靠性．该模型提供了有关区域结构的重要信息，浅层P波速度结构与地表的地质特征相关，非常准确地反映了山脉盆地、地形地貌的差异；各深度层上的速度结构表现为分块特征，反映了以大型活动断裂为界勾画出的分区，如洛杉矶盆地、文图拉盆地、莫哈韦沙漠、半岛山脉、圣华金山谷、内华达山区及思尔顿凹槽区域整体变化的特征．圣安德列斯断层成为研究区内明显的边界，几个深度层上可以看到两侧的速度存在明显的差异，而横跨该断裂的几个剖面图上则显示了位于断层西南侧的太平洋板块区域速度较高、不均匀性较大，地震活动较深；位于断层东北部的北美板块速度偏低，非均匀性稍弱，地震活动性则较弱．比较平缓莫霍界面和起伏莫霍界面下反演得到的结果可以说明:莫霍界面的起伏对中下地壳层位速度结构的影响是明显的，尤其是在莫霍面深度变化较大的部位，这种影响更为强烈．带莫霍面形状的模型能更精确地计算射线路径和理论到时，从而使反演后的残差更小、结果更精确．      

Autumn intensification of the Ryukyu Current during 2003-2007

Inverse calculations using data from 16 repeat hydrographic transects collected from April 2003 to June 2007 have yielded velocity structures and volume transports(VTs) of the Ryukyu Current in the region east of the northern Ryukyu Islands.The inverse calculation results show that the Ryukyu Current is dominated by a subsurface velocity core with maximum velocities from 15.1 to 80.0 cm/s,whose positions vary between 110 and 600 dbar and 27.2°-28.2°N along the transect.The mean velocity exhibits a subsurface velocity core with a maximum value of 24.6 cm/s at 326 dbar depth,a VT of 14.0 Sv(1 Sv≡106 m3/s),a vertical dimension of 800 m,and a horizontal dimension of 60 km.The seasonal mean velocities show that the Ryukyu Current is stronger in autumn than in other seasons.It is suggested that this seasonal variation is coincident with the intensification of the anticyclonic eddy south of Shikoku,Japan.     

Flood monitoring, mapping and assessing capabilities using RADARSAT remote sensing, GIS and ground data for Bangladesh

Remote sensing is the most practical method available to managers of flood-prone areas for quantifying and mapping flood impacts. This study explored large inundation areas in the Maghna River Basin, around the northeastern Bangladesh, as determined from passive sensor LANDSAT data and the cloud-penetrating capabilities of the active sensors of the remote imaging microwave RADARSAT. This study also used passive sensor LANDSAT wet and dry images for the year 2000. Spatial resolution was 30 m by 30 m for comparisons of the inundation area with RADARSAT images. RADARSAT images with spatial resolution of 50 m by 50 m were used for frequency analysis of floods from 2000 to 2004. Time series images for 2004 were also used. RADARSAT remote sensing data, GIS data, and ground data were used for the purpose of flood monitoring, mapping and assessing. A supervised classification technique was used for this processing. They were processed for creating a maximum water extent map and for estimating inundation areas. The results of this study indicated that the maximum extent of the inundation area as estimated using RADARSAT satellite imaging was about 29, 900.72 km² in 2004, which corresponded well with the heavy rainfall around northeast region, as seen at the Bhairab Bazar station and with the highest water level of the Ganges–Brahmaputra–Meghna (GBM) Rivers. A composite of 5 years of RADARSAT inundation maps from 2000 to 2004, GIS data, and damage data, was used to create unique flood hazard maps. Using the damage data for 2004 and the GIS data, a set of damage maps was also created. These maps are expected to be useful for future planning and flood disaster management. Thus, it has been demonstrated that RADARSAT imaging data acquired over the Bangladesh have the ability to precisely assess and clarify inundation areas allowing for successful flood monitoring, mapping and disaster management.     

Surface circulation in Sagami Bay: the response to variations of the Kuroshio axis

In order to study the characteristics of the surface circulation in Sagami Bay, long-term current measurements were carried out at five moored stations during the period from October 1982 to January 1984. The majority of current patterns show the existence of a cyclonic eddy in the bay, while at times the direction of the circulation is reversed. When the Kuroshio current flows over the Izu-Ogasawara Ridge and approaches Sagami Bay, the current that passes through the Oshima-West Channel north of Oshima Island (COWC), has a strong clockwise flow, while the counterclockwise circulation in the bay becomes intensified. When the Kuroshio shifts southward off the shore, the COWC and the flow in the bay are weak or at times reverse their directions.     

Changes in the limnological features of a meromictic Lake Suigetsu during the years, 1926–1967

Changes in the limnological features of a typical meromictic lake, Lake Suigetsu, from 1926 to 1967 are summarized. Until 1934, the lake was stratified due to the balance between the flushing of fresh water and the intrusion of salt water through a canal by which the lake was connected to a coastal polyhaline lake. Total chloride content of the lake had been within the range of 100–230×10³ tons and the thermal stratification had been well developed. In 1934, another channel was constructed, by which the lake was connected to another polyhaline coastal lake. This resulted in the influx of large quantities of salt water (maximum total chloride content of the lake: 790×10³tons), and characteristics of stratification were altered. In the data after 1951, a two-layered system was re-established (total chloride content of the lake: 470–620×10³tons), and distinct stratification began to appear.     

Alkaline phosphatase activity and phosphatase hydrolyzable phosphorus for phytoplankton in hiroshima bay, Japan

We investigated the seasonal variability of free alkaline phosphatase activity in seawater and alkaline phosphatase hydrolysable phosphorus (APHP) at 3 stations in Hiroshima Bay using alkaline phosphatase extracted from the dinoflagellates Alexandrium tamarense and Gymnodinium catenatum. The dissolved inorganic phosphorus (DIP) was lower than 1 μM in all samples; the lowest values were in May. The amount of APHP was high at the surface and bottom waters of all stations in May, showing DIP-depleted conditions. In August and November, the amount of APHP was much less than the amount of APHP in May, indicating that the availability of dissolved organic phosphorus (DOP) for these species was low and/or uptake during the dinoflagellate blooming might have occurred in the area. The results obtained from short-term variations of AP activity might suggest that the growth of dinoflagellates in this season may be partly supported by the AP produced by other diatoms.