Gravity changes at the Esashi Gravity Station observed by the absolute gravimeter with a rotating vacuum pipe

Summary We have developed an absolute gravimeter with a rotating vacuum pipe. The rotating vacuum pipe has an angular velocity high enough to keep a falling object to the end of it, where the falling object begins to drop by stopping the pipe vertical. We put a Michelson interferometer with a stabilized He-Ne laser under the vacuum pipe to measure the position of the falling object at every 1 ms synchronized with a rubidium frequency standard. This absolute gravimeter has succeeded in the measurements with a drop-to-drop scatter of 1.9 × 10^–7ms^–2 (19µGal) at the Esashi Gravity Station in the end of 1989 and also has succeeded in the continuous measurements for a week at the same place in December 1991. During the three-year experiments, the measured gravity values have gradually increased until the end of 1991 and then gained in the rate of increase, although we cannot deny the possibility of instrumental origin. The comparisons with other types of absolute gravimeters showed that the values obtained by the absolute gravimeter with a rotating vacuum pipe are close to those obtained by the absolute gravimeters ILOM#1 and NAOM#2 and are lower than those obtained by JILA#4, though the times of comparisons are different.     

Upper limit of heaving pressure derived by pore-water pressure measurements of partially frozen soil

Experiments were conducted to estimate heaving pressures of saturated soil partially frozen in a closed system. Temperatures at both ends of a specimen were kept constant, i.e., positive at the top and negative at the bottom. When the overburden pressure P was maintained at a constant value, the pore-water pressure P_w, which showed a certain value before freezing, decreased gradually as freezing progressed, finally attaining a specific value, whereafter the specimen ceased taking water into it. The pressure difference between P and P_w, at this stage was defined as the upper limit of heaving pressure σ_u, which evidently depended on the temperature θ_c of the cooling end, in accordance with the relation: σ_u = −11.4 θ_c (kg/cm²)

It corresponds to the modified Clausius-Clapeyron's formula, which gives the freezingpoint depression of an ice—water system, where the pressure acting on the ice differs from that on the water. This is the same as the value obtained by Radd and Oertle (1973). It is considered, however, that, when θ_c lowers, the value of θ_u reaches finally a constant value smaller than the one obtained by the above equation. Denoted by σ_{u max}, it was defined as a maximum heaving pressure. The value of σ_{u max} depended on soil type.     

Damage to coastal villages due to the 1992 Flores Island earthquake tsunami

A field survey of the 1992 Flores Island earthquake tsunami was conducted during December 29, 1992 to January 5, 1993 along the north coast of the eastern part of Flores Island. We visited over 40 villages, measured tsunami heights, and interviewed the inhabitants. It was clarified that the first wave attacked the coast within five minutes at most of the surveyed villages. The crust was uplifted west of the Cape of Batumanuk, and subsided east of it. In the residential area of Wuring, which is located on a sand spit with ground height of 2 meters, most wooden houses built on stilts collapsed and 87 people were killed even though the tsunami height reached only 3.2 meters. In the two villages on Babi Island, the tsunami swept away all wooden houses and killed 263 of 1,093 inhabitants. Tsunami height at Riang-Kroko village on the northeastern end of Flores Island reached 26.2 meters and 137 of the 406 inhabitants were killed by the tsumani. Evidence of landslides was detected at a few points on the coast of Hading Bay, and the huge tsunami was probably formed by earthquake-induced landslides. The relationship between tsunami height and mortality was checked for seven villages. The efficiencies of trees arranged in front of coastal villages, and coral reefs in dissipating the tsunami energy are discussed.     

Effect of divergence,stretching and nonnormal distribution on apparent diffusion of surface drifters

Kawai (1976, 1979) derived two simple relations among the apparent diffusivity (based on the geometric-average variance for principal axes of spreading) for many surface drifters in a patch and horizontal divergence and isotropic turbulent diffusivity for ambient water. By a new term medley diffusivity is meant an average of the products of the position coordinates relative to the centroid of drifters and the residual velocity, that is, the velocity left by subtracting the mean velocity and the velocity due to the linear velocity gradient from the velocity of a drifter. Regarding the medley diffusivity as the isotropic turbulent diffusivity that is weighted anisotropically according to drifters spreading along each of the principal axes, this paper derives a new relation with a form intermediate between the above two relations. Using the relation, this paper defines the new critical period within which the effect of horizontal divergence on the apparent diffusion surpasses that of isotropic turbulent diffusion, and discusses the time-scales on which the space-scale dependence of the squared divergence (Kawai, 1985b) is based. Stretching of a patch of drifters and deviation of drifters' position from the bivariate normal distribution enhance the effect of isotropic turbulent diffusion on the apparent diffusion. This stretching effect is equivalent to the shear effect on a horizontal plane. Remarks on reducing errors in the estimation of isotropic turbulent diffusivity are made.     

Structure of the strong tidal jet in the naruto strait

In order to clarify the structure of the strong tidal current at the Naruto Strait in the Seto Inland Sea of Japan, the sea-level values were observed in the strait and the current measurements were made with an Acoustic Doppler Current Profiler (ADCP).The tidal volume transports for M₂ and S₂ tides were about 74×10³ and 26×10³ m³ sec^–1, respectively. The horizontal profile of the velocity at the phase of the strong tidal current compares favorably with a theoretical profile of the two-dimensional steady turbulent jet except for the side parts of the profile. Moreover, the entrainment rate of the surrounding water into the strong tidal jet was estimated from the difference of mass flux between two cross-sections at the strait, the entrainment rate and entrainment constant for both the northward and southward flows being about 1.3–2.5×10^–4m^–1 and about 0.03–0.05, respectively.     

Neogene geological history of the Tohoku Island Arc system

The Pacific-type orogeny in the Tohoku Island Arc is discussed using marine geological and geophysical data from both Pacific and Japan Sea along the Tohoku region. The Tohoku Arc is divided into three belts; inner volcanic and sedimentary belt, intermediate uplifted belt and outer sedimentary trench belt. Thick Neogene sediments which are distinguished in several layers by continuous seismic reflection profiling occur on both sides of the intermediate belt. The dominant structural trend of the Neogene layers is approximately parallel to the coast line and to the axis of the Japan Trench and has a extension of approximately 100 km in each unit on the Pacific side. The trench slope break is an uplifted zone of Neogene layers. The structural trend of the upper continental slope and outer shelf is relative uplift of the landward side. Tilted block movement toward the west is the dominant structural trend on the Japan Sea side. Structural trends which can be seen in both the inner and outer belts may suggest horizontal compressional stress of east to west. Orogenesis and tectogenesis in the Tohoku Arc has been active since early Miocene or latest Oligocene. It may be implied that the Japan Trench was not present during Late Cretaceous to Paleogene, as is suggested by the volcanism of the Tohoku Arc. The basic framework of the present structure was formed during late Miocene to early Pliocene in both the inner and outer belts. Structural movements were reactivated during late Pleistocene.     

Current variation in the sea near the mouth of Suruga Bay

In order to investigate the circulation pattern and the characteristics of tidal currents in the sea near the mouth of Suruga Bay, current meausrements were conducted at two stations at near-surface levels during the warm months of the year (late May to early October). The mean currents at the eastern and western stations are inversely correlated with each other: when inflow occurs at the eastern station outflow occurs at the western one andvice versa. The circulation pattern, therefore, can be determined from the mean current at either station,e.g., inflow or outflow at the eastern station corresponds to counterclockwise and clockwise circulation, respectively. The predominant tidal constituents of the measured currents are diurnal while those of the sea level along the bay are semidiurnal. The amplitudes of the diurnal constituents of the current show clear long-term variations besides the semimonthly variation corresponding to the phase of the moon.Variation in the path of the Kuroshio off the bay mouth seems to influence not only the circulation pattern but also the tidal currents in the sea near the bay mouth. When the Kuroshio axis is to the north of Zenisu, a shoal off the eastern side of the bay, the circulation pattern is counterclockwise and the amplitude of the tidal current is small. On the other hand, when the Kuroshio is to the south ofZenisu, the circulation pattern is weakly clockwise or stagnant and the amplitude of the tidal current is large.     

Diurnal changes of pH distribution and the cascading of shore water in aSargassum forest

This paper examines the influence of aSargassum forest on the pH distribution in a small cove facing Wakasa Bay. The diurnal changes in spatial distribution of density and pH were observed in May 1977 during the season of luxuriant seaweed growth, and in August 1977 during the season of little growth. The observations were made at three-hour intervals for 24 hr in May, and 9 hr in August. During the period of strong density stratification, the observed pH distributions were divided into two types: (1) lower pH inshore and higher offshore in the upper layer in the evening and early morning, and (2) higher pH inshore and lower offshore in the daytime. During the period of weak stratification, a homogeneous pH distribution was observed. The pH distribution influenced bySargassum forest during the season of luxuriant growth was different from that during the season of little growth. The process that brought these spatial distributions is discussed. From early dawn to forenoon during the season of luxuriant growth, the heavier water, produced inshore by cooling through nocturnal radiation, cascaded as a stable laminar flow under the lighter water remaining under canopies of the forest and detached floating seaweeds. Reaching the offshore lower layer, and co-acting with algal respiration, this cascade influenced pH distribution. Some dynamical parameters for the process of cascading are calculated and examined.     

Variation of the Kuroshio south of Japan

The variations of the velocity and path of the Kuroshio are investigated by using the data obtained after the World War II. The time scales of these variations are classified into three categories,i.e. long-, medium- and short-terms. Period of the long-term variations seems to be about 7 to 9 years. Large meanders of the Kuroshio off Enshu-nada in 1953–1955 and 1959–1962 are accompanied with the low mean velocity of the Kuroshio. These large meanders are explained as a stationary Rossby wave by applying the equation for the phase velocity of the barotropic Rossby wave with the disturbance of finite width. To obtain the above conclusion, it is assumed that the Kuroshio extends down to the depth of 2,300 db and that the east component of the over-all mean velocity of the meandering Kuroshio should be substituted for the velocity of the eastward basic current in the above equation. As for the medium-term variation of the Kuroshio, there seems to exist variations in the velocity with the periods of 4, 6, 8 and 12 months and those in the position of the Kuroshio axis with the periods of 8 and 12 months. These meanders of the Kuroshio progress towards east with the mean phase velocity of about 5 miles a day, which is nearly the same as the calculated mean phase velocity of a progressive Rossby wave.     

Chemical forms of chromium in natural water

The interaction of chromium (III) with humic substances obtained from pond sediment was experimentally studied using electrophoresis combined with ultrafiltration. The results show that within the neutral pH range chromium (III) in the presence of humic substances and some organic acids forms uncharged and/or negatively charged organic complexes of various molecular weights. A part of the chromium(III)-humic or -fulvic acid complexes having a negative charge was of lower molecular weight. Chromium (III) spiked in fresh water exists as various soluble anionic and/or uncharged species, and the molecular weights of these anionic complexes correspond to those of chromium(III)-humic and -fulvic complexes. These complexes may remain as stable dissolved forms for 10 days. The significance of the occurrence of chromium(III)-organic complexes in natural water in the geochemical cycle of chromium in the hydrosphere is discussed.