Fine structure beneath the Tohoku district, northeastern Japan arc, as derived by an inversion of P-wave arrival times from local earthquakes

Three dimensional P-wave velocity structure beneath the Tohoku district, northeastern Japan arc, is investigated by an inversion of arrival times from local earthquakes using the method originally due to Aki and Lee (1976).In the crust (0–32 km depth) a low-velocity region is found along the volcanic front and its vicinity. Velocities at depths of 32–65 km are low beneath the regions where many Quaternary volcanoes and geothermal areas are distributed. In the region deeper than 65 km, the subduction of the Pacific plate is clearly revealed, and the mantle structure above the descending plate is rather uniform. These features suggest that volcanic activities have relation to the upper mantle structure. The results obtained in this study will be helpful in investigating the mechanism of magma generation in a subduction zone.     

The difference in chemical composition of the 1962 lava from miyakezima magma reservoir in Japan

Many examples of mixed magmas in banded lavas have been studied. Another type of mixed magmas or inhomogeneity of magma reservoir found in the 1962 lava flows of Miyake-zima Volcano erupted from fissures is reported.     

Fracture strength of dry silicate rocks at high confining pressures and activity of acoustic emission

Three dry silicate rocks, gabbro, dunite and eclogite, were triaxially compressed up to a confining pressure of 3 GPa at room temperature. These rocks exhibited brittle fracture behavior up to the highest confining pressure. The change of the mechanism of fracture in the brittle region is suggested from the measurement of the compressive fracture strength and the activity of acoustic emission. The existence of the “high-pressure brittle-fracture” phase is proposed. The fracture strength increased with increase of confining pressure. The increasing rate of strength was lowered at a value of confining pressure: at about 0.8 GPa on gabbro; at about 1.0 GPa on dunite; and at about 1.5 GPa on eclogite. At lower confining pressures than the above value, the acoustic emission rate began to increase at the onset of dilatancy and increased rapidly followed by fracture as the axial stress was increased. At the higher confining pressures, however, the acoustic emission rate did not increase rapidly before final fracture, and stayed constant to the fracture. The similar behavior was shown on the granite studied previously. It is interesting that the frictional strength forms the boundary between “low- and high-pressure brittle-fracture” phases.     

A New Coastal Marine Ecosystem Model Study Coupled with Hydrodynamics and Tidal Flat Ecosystem Effect

A new coastal marine ecosystem model was developed, which was composed of pelagic and benthic ecosystems, and was applied to Mikawa Bay, Japan. This model deals with variations of biochemical and physical interactions among dissolved oxygen and C–N–P species (composition formed out of carbon, nitrogen and phosphorus elements) so that it resolves the flux dynamics of carbon, nitrogen, phosphorus and oxygen elements. The physical and biochemical mechanism figured in this model is constructed for the purpose of simulating the estuarine lower trophic ecosystem, in areas where the sea was too deep for light to reach the sea-bottom. As a result of coupling the benthic with pelagic system, the effect of process of sedimentation and nutrient diffusion back to the pelagic system could be indicated. In addition, by implementing the tidal flat ecosystem model's calculation result, the integrated model can include the effect of water purification in tidal flats where the light can reach the sea-bottom, and where sea-weed, sea grass and benthic algae exist. In this study, the model indicates that oxygen-depleted water exists at the sea-bottom especially in summer mainly caused by an increase of oxygen consumption in the benthic system and a decrease of the vertical mixing water process. Furthermore, by comparing the case – with the tidal flat ecosystem model and the case without it, the effect of water purification of tidal flat estuaries was indicated. From the viewpoint of a short time scale, the tidal flat has the potential to restrict red tide (rapid increase of phytoplankton), and from the viewpoint of a long time scale, it restricts the sedimentation of detritus. Restricting the sedimentation prevents oxygen-depleted water occurring in the coastal marine system of Mikawa Bay.     

Fluid-rock history of granulite facies humite-marbles from Ambasamudram, southern India.

An extensive humite‐bearing marble horizon within a supracrustal sequence at Ambasamudram, southern India, was studied using petrological and stable isotopic techniques to define its metamorphic history and fluid characteristics. At peak metamorphic temperatures of 775±73°C, based on calcite‐graphite carbon isotope thermometry, the mineral assemblages suggest layer‐by‐layer control of fluid compositions. Clinohumite + calcite‐bearing assemblages suggest X_CO2 < 0.4 (at 700°C and 5 kbar), calcite + forsterite + K‐feldspar‐bearing assemblages suggest X_CO2>0.9 (at 790°C); and local wollastonite + scapolite + grossular‐bearing zones formed at X_CO2 of c. 0.3. Retrograde reaction textures such as scapolite + quartz symplectites after feldspar and calcite and replacement of dolomite + diopside or tremolite+dolomite after calcite+forsterite or calcite+clinohumite are indicative of retrogression under high X_CO2 conditions. Calcite preserves late Proterozoic carbon and oxygen isotopic signatures and the marble lacks evidence for extensive retrograde fluid infiltration, while during prograde metamorphism the possible infiltration of aqueous fluids did not produce significant isotopic resetting. Isotopic zonation of calcite and graphite grains was likely produced by localized CO₂ fluid infiltration during retrogression. Contrary to the widespread occurrence of humite‐marbles related to retrograde aqueous fluid infiltration, the Ambasamudram humite‐marbles record a prograde‐to‐peak metamorphic humite formation and retrogression under conditions of low X_H2O.     

The distribution of manganese and iron between ilmenite and granitic magma in the Ôsumi Peninsula,Japan

Manganoan ilmenite with a variable manganese content occurs as an early accessory constituent of granitic rocks in the Ôsumi Peninsula, southern Kyushu. Electron probe micro-analysis of a grain containing highest manganese gives the structural formula (Fe _1.23 ²⁺ Mn _0.81 ²⁺ ) (Ti_1.97) O₆, if all of the manganese and iron are in the divalent state. The manganese content of manganoan ilmenite increases with an increase of the differentiation index of host rocks, however, the amount of ilmenite tends to decrease with the increase of the same index. The mode of occurrence of the ilmenite suggests that it is the first mafic mineral to precipitate from the magma. The average value of the distribution coefficient of manganese and ferrous iron between ilmenite and granitic magma is 5.5, if the Mn/Fe ratio of the granitic rocks represents that of granitic magma. The variation in the FeO and MnO contents against the differentiation index for granitic rocks of the Ôsumi Peninsula, and the value of the distribution coefficient, show that high manganoan ilmenite is stable in the most differentiated granitic rock of the Ôsumi Peninsula.     

Interferometer observation of pulsating sources associated with a type IV solar radio burst

An extremely complex outburst, part of which showed unsually rapid intensity fluctuations of a few second interval, was observed on 1970 November 5 with the 160 MHz interferometer of the Nobeyama Solar Radio Station. The pulsating source, which was stable in position and strongly circularly polarized ( 60 %), had an extension as large as 17 (7.5 × 10⁵ km) in the east-west direction. The structure of the source remained unchanged while the source darkened and brightened repeatedly. The change of the source brightness occurred in a time shorter than a second.Two alternative mechanisms responsible for the pulsating phenomenon are suggested; (1) gyroresonance absorption of continuum radiation by a fast particle beam injected in a quasi-periodic manner into a large region of weak magnetic field, or (2) magnetohydrodynamic oscillation of the continuum source itself, which is intrinsically much smaller than observed. It is observed as a large source as a consequence of scattering of the emitted radiation in a region situated above the source.     

ASCA Observation of the Quiescent X-Ray Counterpart to SGR 1627-41

We present a 2-10 keV ASCA observation of the field around the soft gamma repeater SGR 1627-41. A quiescent X-ray source, whose position is consistent both with that of a recently discovered BeppoSAX X-ray source and with the Interplanetary Network localization for this soft gamma repeater, was detected in this observation. In 2-10 keV X-rays, the spectrum of the X-ray source may be fit equally well by a power-law, blackbody, or bremsstrahlung function, with unabsorbed flux approximately 5x10-12 ergs cm-2 s-1. We do not confirm a continuation of a fading trend in the flux, and we find no evidence for periodicity, both of which were noted in the earlier BeppoSAX observations.