Branching and joining of the Tsushima Current around the Oki Islands

We discussed the branching and joining of the Tsushima Current around the Oki Islands, based on ADCP and CTD measurements carried out in June 1990 by the quadrireciprocal method (Katoh, 1988). The volume transport of the northeastward current northwest of the Izumo Coast was about 2 Sv. The triple-branch structure of the Tsushima Current was obscure there. This northeastward current divided into the eastward and northward currents, with volume transports of 0.5 Sv and 1.5 Sv, respectively, at the west entrance of the Oki Strait. Most of the first branch of the Tsushima Current seemed to be separated again from the other confluent branches and to pass through the Oki Strait as this eastward current. The northward current was composed of the second and the third branches of the Tsushima Current. It detoured the Oki Islands, and almost all of it returned south to the Tajima Coast. In the vicinity of the Tajima Coast, the eastward current was abruptly strengthened through the confluence of the southward one which was originated from the northward current west of the Oki Islands. This showed that the first branch finally joined the compound of the second and the third branches detouring the Oki Islands. Between the Oki Strait and the Tajima Coast, the two-layer structure of currents was clearly found.     

Distributions and upward fluxes of particulate matter near bottom in the southeastern Bering Sea Shelf

Time-series measurements of temperature, salinity, suspended matter and beam attenuation coefficient () were measured at four hour intervals for about two days in June/ July 1982 in the middle shelf region and the coastal region of the southeastern Bering Sea. Current meters were also moored at the same locations.Depth-time distributions of indicated that profiles of suspended matter resulted from a combined process of resuspension of underlying sediments and sinking of suspended particles. Average-values for all measurements for particles revealed that the upward transport of particles due to resuspension formed a boundary layer, with a thickness apparently related to scalar speed. The average-profiles of the particle volume concentration were assumed to result from a balance between the sinking and diffusive flux of particles under a steady state, and the upward fluxes were calculated. Within the boundary layer, values of the upward fluxes of particulate organic matter linearly decreased with the logarithm of distance from the bottom. Fluxes of organic carbon at the upper edge of the boundary layer were 0.375 gC·m^–2·day^–1 in the middle shelf region (18 m above the bottom, bottom depth=78m) and 0.484gC·m^–2·day^–1 in the coastal region (25 m above the bottom, bottom depth=33m), and fluxes of nitrogen in both regions were 0.067 gN·m^–2·day^–1. The flux of organic carbon obtained in the middle shelf region (18 m above the bottom) agreed approximately with the flux (0.416 gC·m^–2·day^–1) calculated by substituting primary production data into the empirical equation of Suess (1980).     

Scaling wave impact pressures on vertical walls

It is widely recognized that the use of Froude similarity for scaling up wave impact pressures recorded during physical model tests may lead to over-estimation of impact maxima. Based on reviewing historical work dating back to the 30s and further developments in the 60s and 80s, a general method is presented that is suitable for scaling up impact pressures and rise times measured during small scale physical model tests. The method accounts for the effect of air leakage and is applicable to most wave impact loads. The model is applied to scale wave impact pressures on vertical walls and similar structures, and consistent correction factors for the Froude scaling law are derived.     

Summary of the fossil record of pinnipeds of Japan, and comparisons with that from the eastern North Pacific

Abstract The fossil pinniped record of the North Pacific Ocean includes both Phocidae and Otariidae ( sensu lato ), extends from the Late Oligocene to the Late Pleistocene, is taxonomically diverse, and is constantly becoming more complete owing to additional important discoveries. The earliest and most diverse fossil pinnipeds in the North Pacific are otariids, the phocids not appearing until the latest Pliocene. The theoretical center of otariid pinniped evolutionary history has been considered by some to be in the eastern North Pacific. New materials from the western North Pacific, however, including representatives of the subfamilies Enaliarctinae, Imagotariinae, Odobeninae and Otariinae, indicate that pinniped evolutionary patterns were basin-wide phenomena, and that a more complete record undoubtedly would reveal numerous trans-Pacific distributions. This would be expected considering the distributions of living species. The paucity of fossil Phocidae and their absence from pre-Pliocene deposits are consistent with theories that the family primarily evolved outside the North Pacific.     

Strain and tilt changes measured during a water injection experiment at the Nojima Fault zone, Japan

Abstract In order to make geophysical and geological investigations of the Nojima Fault on Awaji Island, Japan, three boreholes measuring 1800 m, 800 m and 500 m deep were drilled into the fault zone. The fault is one of the seismic source faults of the 1995 Hyogo-ken Nanbu earthquake of M 7.2. A new multicomponent borehole instrument was installed at the bottom of the 800 m borehole and continuous observations of crustal strain and tilt have been made using this instrument since May 1996. A high-pressure water injection experiment within the 1800 m borehole was done in February and March 1997 to study the geophysical response, behavior, permeability, and other aspects of the fault zone. The injection site was located approximately 140 m horizontally and 800 m vertically from the instrument. Associated with the water injection, contraction of approximately 0.7 × 10⁻⁷ str (almost parallel to the fault) and tilt of approximately 1 × 10^-7 rad in the sense of upheaval toward the injection site were observed. In addition to these controlled experiments, the strainmeter and tiltmeter also recorded daily variations. We interpret strain and tilt changes to be related to groundwater discharge and increased ultra-micro seismicity induced by the injected water.     

Relationship between chemical composition of Japanese Island-Arc volcanic rocks and gravimetric data

1670 published chemical analyses from 137 Quaternary eruption centers of Kurile, Japan and Izu—Mariana arcs are collected and K₅₅ and K₆₀ values of individual centers are calculated. Along with the well-established trend of increase of K₅₅ and K₆₀ values across the arc towards the continent, longitudinal change is also recognizable. Interrelation between K₅₅ or K₆₀ value pattern and a map of Bouguer anomaly distribution along the volcanic front is characteristic.To develop one possible simplified model to explain the facts, we assume first that the asthenosphere has a homogeneous thermal state beneath the volcanic zone, second that the thickness of the crust is quite variable areally, and third that the mean density of the crust is constant throughout the area. Thus, the composition of the magma generated along a zone corresponding to equal depths along the Benioff zone should be identical throughout the volcanic arc, and lower values of the Bouguer anomaly should be observed where the crust is thicker. We then assume that magma generated in the asthenosphere ascends into the crust, then cools as fractional crystallization proceeds. Depths of fractional crystallization would be deeper where the crust is thicker. The ratio of the clinopyroxene phase separated from the magma to the plagioclase phase should be larger where the level of fractionation is deeper. Thus, magmatic liquid should be enriched in K where K-poor crystal fractions are subtracted at the deeper level.     

Water saturated phase relations of some andesites with application to the estimation of the initial temperature and water pressure at the time of eruption

Near-liquidus phase relations in one-atmosphere dry and water-saturated high pressure conditions were experimentally determined on products of three historic andesitic eruptions. Run conditions ranged from 900 to 1100°C, at pressures up to 1500 bars with fO₂ controlled close to the nickel-nickel oxide (NNO) buffer. In order to represent the compositions of the true liquid parts of the erupting magmas, groundmass portions were mechanically separated from the porphyritic andesites. Such groundmass materials should lie exactly on the liquidus field boundary between the phases precipitating from the magma just prior to eruption under the prevailing P-T condition.All the samples showed a crossing of the plagioclase and orthopyroxene liquidi in the pressure-temperature range from 1 to 800 bar and 950–1090°C. The crossing condition approximates the magmatic condition immediately prior to eruption. In the case of the 1970 eruption of Akita-komagatake, the crossing point is at 150 bar and 1090°C, matching closely the observed explosive gas pressure and the temperature. In both cases of the 1783 eruption of Asama and the 1914–1915 eruption of Sakurajima volcanoes, the crossing point shift from higher water pressure and lower temperature for the earlier erupting magmas to lower pressure and higher temperature for the later magmas. This regularity may be explained by a vertical gradient of the temperature and water content within the magma column prior to eruption.     

10Be dating of North Pacific sediment cores up to 2.5 million years B.P.

The¹⁰Be method of dating of marine sediment cores is applied to five North Pacific cores. Assuming a constant¹⁰Be precipitation rate and varying sedimentation rates with time during the past 2.5 m.y. dating confirms to that obtained from paleomagnetic stratigraphy. The¹⁰Be concentration variations with depth in the cores are primarily due to changes in sediment dilution and do not reflect cosmic ray intensity or global climate variations. The limits of¹⁰Be deposition rate variation in the investigated cores are less than ± 10% for periods of (2–7) × 10⁵ years and less than ±30% for periods of 1 × 10⁵ years. The data set gives a half-life of¹⁰Be is 1.50 × 10⁶ years. The latitudinal effect of¹⁰Be concentrations and¹⁰Be/⁹Be ratios relates to a frequency of particulate matter occurrence (detrital and biological particles) in the oceans and to oceanic circulation.     

10Be in marine sediments

The depth profile of the long-lived radionuclide¹⁰Be in a marine sediment recovered in the vicinity of the Samoan Islands has been precisely assayed with a highly sensitive needle-type gas counter. The obtained irregular pattern of¹⁰Be concentrations with depths ranging from 4.7 to 0.3 dpm/kg dry sediment is interpreted as being due to dilution of¹⁰Be by volcanic eruptions in the past.     

Solar vector magnetograms of the Okayama Astrophysical Observatory

The title instrument is mounted on the 65 cm solar Coudé telescope at the Okayama observatory. Observation is usually of the Fei 5250 Å line. The data obtained are briefly described.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.