The territory of Vietnam is divided into five structural blocks : Northeast (NE), Northwest (NW), Truongson, Kontum and Nambo. The NE block is a part of the South China plate, in which strata and igneous rocks have been found dating from the Early Paleozoic to the Quaternary. The NW and Truongson blocks are regarded as NW-SE trending Paleozoic folded systems filled with thick (>12000 m) Paleozoic formations. Precambrian strata are widespread in the Red River fault zone and Fansipan range in the NW block, and in the Kontum block. Archean rocks are found only in the Kontum block, which is regarded as a stable massif without Paleozoic sedimentary rocks. The Nambo block is covered with a very thick (>6000 m) sequence of Cenozoic formations deposited in a continental rift. During the Mesozoic many such graben structures were formed and become basins for sedimentation. Igneous activity in Vietnam is divided into five episodes from the Archean to Quaternary. The most important impediments to the description and interpretation of geology in Vietnam are lack of reliable radiometric and structural data, and scarcity of good outcrops.

抄録全体を表示

The Crustal Activity Observatory of Shizuoka University has been carrying out seismological observations since 1979. This report describes the results of the seismological observations since 1990, especially on the earthquakes in Shizuoka on 7-11 August 1993, which had a maximum magnitude of 4.2, and their relation to the tectonics of central Japan.
The differences in magnitude (ΔM) between Shizuoka University and the Japan Meteorological Agency for 858 earthquakes with an epicenter less than 200 km from the observatory and with a focal depth shallower than 200 km have been examined, and the systematic differences with the focal azimuths are found. The relation between the magnitude and duration of the earthquakes is determined, based on seismological observations from June to August 1993.
The foci of the earthquakes in Shizuoka in August 1993 were located on the Kusanagi Fault, and the changes in the ratios of the maximum amplitudes of P and S waves were observed during the earthquakes.
The dislocation along the Kusanagi Fault is tilting the Udo Hills about 10° northwestward, and the rate of uplift of the Udo Hills is 4 mm/yr. The Kusanagi Fault is characterized by the landward boundary of a fore-arc basin along the subductive boundary of the Philippine Sea Plate and gravitational collapse.
The seismic activities in central Japan are related to the bending of the zonal stucture and collision of the Izu Arc, caused by the motion of the Philippine Sea Plate. Historically, large earthquakes in Shizuoka, Southwest Japan, the northern Fossa Magna, Niigata, Misaka, the Matsuzaki Uplift Zones, and the Zenisu Ridge have happened closely in a time series.

抄録全体を表示

Only about half of all the CO₂ that has been produced by the burning of fossil fuels now remains in the atmosphere. The CO₂ “missing” from the atmosphere is the subject of an important debate. It was thought that the great majority of the missing CO₂ has invaded the ocean, for this system naturally acts as a giant chemical regulator of the atmosphere. Although it is clear that ocean processes have a major role in the regulation of the carbon dioxide content of the atmosphere through air-sea exchange processes, recent studies of the oceanic carbon cycle and air-sea interaction indicate that oceanic carbon is in a quasi-steady state via the system of biological and physical processes in the ocean interior. It is difficult to determine whether the ocean has the capacity to take up the increasing air-born CO₂ released by human activities over the past five or six decades. To understand this enigma, we need a better understanding of the natural variability of the oceanic carbon cycle.

抄録全体を表示

The δ¹³C fluctuation of tree rings in two Japanese cypress (Chamaecyparis obtusa) samples, one from the last glacial period and another from the present time, is examined. Each annual growth ring was divided into four or five segments, consisting of three earlywood segments and two latewood segments or no division of latewood, in order to measure seasonal variations within a year.
The last glacial sample has about 400 annual growth rings and the mean ring width is 0.71 mm. Pith and sapwood were not present. Seasonal variations were measured for about 10 successive annual tree rings. A secular trend analysis is presented covering 100 years. The isotopic composition for three years was measured for a modern sample from Mt. Amagi, Izu Peninsula. The tree grew over a period of about 200 years, from 1794 A.D. to 1986 A.D., and the mean ring width is 1.27 mm.
The carbon isotope composition of glacial period samples shows that the range is from -23.77 to -21.08‰, the mean value is -22.16 ± 0.40‰ and the mean seasonal fluctuation in each tree ring is 1.20 ± 0.32‰. That of modern samples shows that the range is from -23.00 to -21.41‰, the mean value is -22.29 ± 0.45‰ and the mean seasonal fluctuation is 0.73 ± 0.38‰.
Assuming the δ¹³C values of cellulose in the Hinoki tree rings directly reflected those of atmospheric CO₂ and the secular trend in depletion of δ¹³C values via the Suess effect, these results give good agreement with the trend of depletion in atmospheric δ¹³C values of CO₂ ascertained by ice core analysis and C4 plant analysis. There are poor correlations between seasonal fluctuations of isotope data and dendrochronological data influenced by climatic factors.

抄録全体を表示

In this study, we improved the benzene synthetic system for radiocarbon dating constructed by Inomata (1992 MS) at Shizuoka University and describe the method in detail. A decarbonation system for carbonate samples such as shells and corals was additionally installed into the benzene synthetic system in order to measure carbonate ¹⁴C-ages.
We used 10 benzene samples, such as buried woods, charcoal, commercial cokes and fossil shells to measure ¹⁴C using a liquid scintillation counter. The yields of these 10 samples during sample combustion were more than 99% after correction for the charcoal carbon content, and during decarbonation were 69~94% with a mean value of 84%. The yields from carbon dioxide into acetylene were 80~96% with a mean value of 90%, and from acetylene into benzene were 76~95% with a mean value of 86%.
Carbon isotopic fractionation during combution into CO₂ from charcoal samples was less than 1‰ and that through the whole procedure of benzene synthesis was about ±3‰.
The purity of benzene synthesized was 98.7~99.9% by NMR and there were no impurities such as acetone, or ethyl alcohol, but 0.06~1.34% toluene was detected.
Using this system, a buried wood sample collected from the Kawagodaira pyroclastic flow was synthesized into benzene. The measured ¹⁴C-age was 3,400 ± 80 y B.P., which was slightly older than before reported.

抄録全体を表示