Daito Islands, ca. 150 km east of the Ryukyu Trench, are located on the northwestern part of the subducting Philippine Sea Plate, and are composed of two main islands, Kita (north) -Daito and Minami (sourth) -Daito. Both islands are well-known raised atolls and tectonic movements are expected to be different from the Ryukyu Islands on the Eurasian Plate. We investigated raised coastal morphology and geology at eight localities of these islands with special reference to the occurrence of fossil corals, and detailed maps and cross sections are drawn using Electric Distance Meter. Thirty three autochthonous corals are dated by 230Th/234U method and some corals are dated by ESR and non-destructive 226Ra/238U methods. Some allochthonous corals are also dated. 230Th/234U dates on autochthonous corals cluster in 113±6 to 133±6 ka in Kita-Daito and in 113±6 to 159±10 ka in Minami-Daito. Therefore, the dated autochthonous corals, except for one relatively older date, represent the sea level of the last interglacial maximum, corresponding to isotope stage 5e. The upper limit of occurrence of autochthonous corals is 8.1 m in Kita-Daito and 11 m in Minami-Daito. Elevation of the last interglacial shoreline judged by emerged coastal morphology is 10 m in Kita-Daito and 12 m in Minami Daito. No autochthonous corals younger than the last interglacial maximum exist in these islands. However, the occurrence of allochthonous corals dated at ca. 80-100 ka suggests the possible presence of corals corresponding to isotope stage 5c and 5a, below the present sea level. Average uplift rate in both islands is nearly the same, that is 0.03 m/ka at Kita Daito and 0.05 m/ka at Minami-Daito, however, it is significantly lower than 1.8 m/ka in Kikai Island and even 0.3 m/ka in Hateruma Island, both are located on the Eurasian Plate, being subducted by the Philippine Sea Plate. Sequence of vertical tectonic movement of Daito Islands can be estimated as follows : Daito Islands had been subsided resulting in the formation of atolls and lagoons underlain by more than 400 m thick coralline limestone (Sugiyama, 1934 and 1936), and then tectonic uplift, time of which is unknown because of dolomitization of calcareous sediments, has started in association of the northwestward migration of the Philippine Sea Plate and has resulted in the formation of upbulge which is represented by the emerged atolls, fringed by a series of fringing reefs. Uplift rate has been very slow at least since the last interglacial maximum, possibly implying that these islands are tending towards the new tectonic phase (stable to subsidence).
Alpha-spectrometric 230Th/234U dating was applied to fifty Pleistocene hermatypic corals from Kita-and Minami-Daito Islands (KDI and MDI), both of which have been well-known to be one of noteworthy representatives of raised atoll since the study of Aoki (1934). 230Th/234U dates range from 113±6 to 133±6 ka (ay. 123±1 ka) for autochthonous corals from KDI, and 111±5 to 159±10 (av. 123±1 ka) from MDI, intimate that the fringing reef has been developed during the high sea level stand of the last interglacial maximum. These dates are correlative to the oxygen isotope stage 5e (OIS-5e Martinson et al., 1987). The upper limit of occurrence of dated autochthonous corals was 8.1 m on KDI, and 11 m on MDI. Besides them, somewhat younger dates corresponding to the OIS-5 a or-5 c, were picked up from some allochthonous corals in a dental limestone unit on KDI. Such a detrital limestone unit cannot be geochronologically defined, because of the allochthonous mode of occurrence of dated samples. It is, however, free of doubt that hermatypic corals were alive forming a small-scaled reef in shallow sea around KDI. The detrital limestone unit including such young corals is very likely to have been formed as a storm deposit and have been preserved in some depressions on the island. The former shoreline was proved by the presence of raised surf bench at some localities (one on KDI and three on MDI), where dated autochthonous corals were collected. The height of it was measured to be ca. 10 m on KDI and 12.2 to 12.7 m on MDI. The amount and the rate of uplift is calculated to be from ca. 4 m for KDI to 6.7 m for MDI, and approximately from 0.03 m/ka for KDI to 0.05 m/ka for MDI, respectively, assuming the sea level of +6 m during the last interglacial maximum and the constant rate of tectonic upheaval since then. Compared with the amount and the rate of uplift estimated by the same way for the last interglacial limestone on Kikai, Hateruma and Yonaguni Islands arranged along the Ryukyu Arc, the vertical displacement of both KDI and MDI is considered to have been extremely small and slow during the last 130 ka.
Pleistocene corals have been successfully dated by the electron spin resonance (ESR) method among various geologic materials, however, this method even for corals has not been established yet. The ESR dating is controlled by following factors : pretreatment of samples, dose rates of artificial gamma irradiation, uranium concentrations in the samples, regression methods of total doses, and methods for calculating ESR dates from total doses. These methods are discussed in this paper. The present study shows that the total dose of a fossil coral generated at the higher dose rate of artificial gamma irradiation is larger than at the lower one. The total dose at a dose rate of 10 kR/hr was about 4 times as large as the one at 1 kR/hr for the last interglacial corals. The gamma doses-ESR intensities data produced by the so-called additive dose method are regressed into a saturation curve or a straight line using the program made by M. Koba. This can choose the most adequate curve by the iteration method. ESR ages are calculated using the program of M. Ikeya revised by M. Koba which does not presume radioactive equilibria in the processes of decay. Two raised atolls, Kita-Daito and Minami-Daito Islands, are situated nearly at lat. 26°N and long. 131°E in the Philippine Sea. They are almost entirely composed of the Daito Dolostone originated from coral reefs, and have small raised fissure deposits and reefs of the late Pleistocene time at a few places. Both allochthonous and autochthonous corals were dated by the ESR, the 230Th/234U (Omura et al., this issue) and the 226Ra/238U methods (Kawana et al., this issue). 238U concentrations of the common coral samples were obtained by the radioactivation analysis which the present authors used, the alpha spectrometry (Omura et al., this issue), and the gamma non-destructive spectrometry (Kawana et al., this issue). The 12 paired results of the former two methods demonstrate a correlation of 0.831. The 8 paired data by the radioactivation analysis and the gamma non-destructive spectrometry show a correlation of 0.211. The ESR and 230Th/234U methods for the six powdered coral samples give us the concordant dates almost within one standard deviation. The average dates of ESR and 230Th/234U are 119, 700 and 115, 500 yr B. P., respectively. 11 corals were collected at a small (2 × 3 m2) outcrop of late Pleistocene limestone in the eastern coast of Kita-Daito Island, checking carefully their stratigraphic relations. The stratigraphically upper samples show the younger ESR dates comparing with the lower. The shoreline of the last interglacial time is estimated to have been situated at 11 m above sea level here. Finely recrystallized 100 % dolomitic corals were collected from the top surface, 45 m above sea level, of the atoll ring of Kita-Daito Island. Their corals are dated ca. 4.6 million years ago by the ESR method. The uplift rate of Kita-Daito Island since 4.6 million years ago was estimated less than 0.01 m/kyr assuming that the present sea level did not move. Although every calcitic coral converted from an aragonitic coral shows dull ESR spectra on a warping base level, these dolomitic corals showed clear ESR spectra on a straight base level. ESR spectra of a dolomitic coral resemble the ones of an aragonitic coral. It was introduced that signal C, whose g value was identical with one of an aragonitic coral, could be used for the dating of the dolomitic corals. The dip-depth of signal C from the base level, so-called signal C/2, was used as an ESR intensity for the dating of dolomitic corals, because it was found that signal C/2 was more difficult to saturate than signal C. Their uranium concentrations were very low owing to leaching, therefore the lower limit of the ESR method was extended considerably.
The 226Ra/238U method based on non-destructive gamma-ray spectrometry is discussed for dating 18 specimens of Pleistocene autochthonous corals in Minami-and Kita-Daito Islands. The 238U and 226Ra contents are indirectly determined by their daughter nuclides of 234Th and 214Pb, respectively, on account of the assumption that the radioactive equilibrium between 288Th and 234Th, and between 220Ra and 214Pb has been maintained. All specimens, initially aragonitic minerals, were powdered, compressed to a disc shape of 50 mm in diameter and made airtight. These specimens have been measured by the Hyper Pure Germanium Detector for 2 to 3 days. The 226Ra/238U dates of 15 specimens in Minami-and 3 specimens in Kita-Daito Island are obtained. The highest frequency of the dates is 120 to 130 ka, and the mean value of those is 128±27 ka, although these dates including counting error (1σ) range from 80 to 190 ka. The mean value of the 226Ra/238U dates is concordant with the weighted average of the 230Th/234U dates of the autochthonous corals, i.e. 123±1 ka, obtained from the same locations of the islands by the method of alpha-spectrometry, which is correlated with the oxygen isotope stage 5 e. In addition, the 226Ra/238U dates of 12 in 18 specimens of the islands correspond to the 230Th/234U dates of the same specimens. This implies that the 226Ra/238U dates by the present study are, as a whole, correlated with the 5 e. The non-destructive 226Ra/238U dating by the gamma spectrometry has advantages that the elaborate and time-consuming chemical procedures are unnecessary and no change in the chemical state occurs in the sample analyzed, hence, the same sample can be used for further investigation after the non-destructive measurement. If smaller counting errors (1 σ) can be made than those by the present study on condition that a detector of lower background is prepared, the detector is heavily shielded, and sample is measured about one week, the non-destructive gamma-ray method will be more valid for dating middle to late Pleistocene corals.
A zonal scheme of the Cenomanian and Turonian stages is established on the basis of bio-stratigraphic sequences of ammonids and inoceramids along the provincial standard and supplementary sections in the Ikushumbetsu, Oyubari and some other areas in Hokkaido. Owing to the recent advances in palaeontology, the biostratigraphic zones in Hokkaido are much refined and fairly well correlated with the current standard zones (Table 1) in Europe and North America. While the stratigraphic sections of shallow-sea sediments are observable in the Ikushumbetsu and contiguous areas, those of off-shore and muddy sediments predominate in the Oyubari and certain other areas. The foraminiferal samples have been collected mainly in the latter areas, where some of the ammonoid and inoceramid zonal indices may also occur. Thus the zonal scheme by means of planktonic and benthic foraminifera can be directly tied up with that of ammonoids and inoceramids. The results obtained are summarized in Table 2, which evidently shows much improvement on previous schemes. There remain, however, several unsettled problems which should be worked out further, especially as regards the stage boundaries.
Frequently innundated Nishikanbara Area in Niigata Prefecture changed into fertile rice fields as a result of many flood control works. The main works are Ohkohzu Floodway of the Shinano River, improvement works of other small rivers and national irrigation and drainage improvement works in Nishikanbara Area. In June, 1978 when these works were almost completed, a large-scale inundation caused by poor drainage occurred in Nishikanbara Area. At that time the drainage system consisting of many pumps controlled by Nishikanbara Land Reclamation Union was almost completed. Total amount of precipitation was over the maximum estimated by the drainage system. But damage of the flood that inundated only rice fields was much slighter than that caused by the past flood of the Shinano River. In addition to low and swampy parts that have been frequently inundated, the author found a few new types of irregular inundation spots. The first type is caused by land subsidence resulting from pumping up ground water including natural gas. This type of inundation was found on the left bank side of the Nakanokuchi River, and total amount of subsidence from 1951 to 1975 was from 100 centimeters to 220 centimeters. Annual amount of subsidence became much less but at present subsidence continues. The second type is found on natural levees and comparatively high altitude spots. This type of flooding spots are located at terminations of small-sized irrigation waterways. The third type is found at back swamps. This type of spots are located along small-sized drainage waterways or at terminations of them. These spots are also adjacent to collective tableware factories removed from central part of Tsubame City on account of pollution and enlargement of business. The reasons of the second and third type of floods are presumed as follows. 1. Maintenance works of small-sized irrigation or drainage waterways became slack because of increasing part-time farmers and decreasing full-time farmers. Most of part-time farmers work for factories or companies located in near cities where they can commute. Waterways covering large or medium-sized area are maintained directly by Nishikanbara Land Reclamation Union. However small-sized waterways which benefit the area below 20 hectare have to be maintained by landowners. Futhermore farmers comming from other areas to cultivate their farm land in Nishikanbara Area have increased in number. They sold their farm land near their residential site in rapidly urbanizing area for example in Niigata City, and purchased substitutive farm land in Nishikanbara where they can come to work easily by car in a short time. They tend to neglect maintenance of irrigation or drainage waterways, since they are not habitants. 2. Diversion of agricultural land have progressed. This diversion area from 1967 to 1977 was 1133 hectare which came under 5 percent of Nishikanbara Area. Agricultural lands have been diverted into residential or factory sites. Especially in southern part of Nishikanbara Area, many rice fields were diverted into factories, because it is adjacent to Tsubame and Sanjo Cities noted for tableware industry. Drainage from these factories have increased and flowed into drainage waterways managed by farmers or Nishikanbara Land Reclamation Union. Nishikanbara Land Reclamation Union began to allot the expense for drainage to factories and municipal corporations in 1973. It was one of the earliest attempt in Japan. Also all the member of Nishikanbara Land Reclamation Union must bear the running cost of drainage and irrigation pumps. The expense in 1978 increased by 6.91 times in comparison with that in 1968. Countermeasure for flood is determined by a scale and feature of past big floods. But flexible countermeasures corresponding with changes of the area are needed.
Geometric patterns of surface earthquake faults are closely related to rupture propagation processes especially in the case of strike-slip faults. In this paper, the author examined geometric features of four strike-slip earthquake faults in Japan, that is, the Kita-Izu earthquake fault system of 1930, Izu-Oshima Kinkai earthquake fault system of 1978, the Nobi earthquake fault system of 1891 and the Kita-Tango earthquake fault system of 1927. Bends or jogs along a fault can be called as geometric barriers, which are classified into two categories as extensional and compressional, from a view point of volumetric change (Fig. 1). Studies of earthquake surface faults of recent inland large earthquakes in Japan show that ruptures were initiated from compressional barriers and terminated at extensional barriers. A compressional barrier in the course of rupture propagation could be the position of next rupture nucleation. Compressional barriers can be commonly described as “fragmentation barriers” (King, 1987) because short faults of various orientation are very popular in and around compressional barriers. The location of severe damage area is also closely related to that of barrier. Study of geometric characteristics of active faults will become very important because they give us a key idea on rupture initiation and termination as in the case of surface earthquake faults. Recognition of barriers along an active fault system from geometrical investigation will provide us some locations of detailed observation for geodetical, geophysical and geochemical anomalies along precautious faults.
In succession of our biostratigraphic study of the Paleozoic and Mesozoic Groups in Central Andes, which was started in 1980, more detailed field survey and re-examinination were undertaken during late July to late August, 1990. The present article is a preliminary report on the field survey of the Tarma Group (Middle Carboniferous) developed near Tarma City in Peru and the Copacabana Group (Upper Carboniferous to Lower Permian) developed near and around Titicaca Lake in Bolivia. The journeys in these two countries are shown in Figures 1 and 2. The aims of the present research are mainly as follows : 1) To be completed our biostratigraphic study of the Upper Paleozoic (Carboniferous-Permian) in Central Andes with the conodonts as additional data, and 2) To make clear the boundary between the Carboniferous and Permian Systems in the Copacabana Group. In Peru, the rock samples for conodont analysis were collected from the Tarma Group which consists mainly of dark gray limestone of ca. 70 m thick. The lower half of the limestone is more or less massive but the upper one intercalates slate and shale beds. In the field work, many fusulines, bryozoans, brachiopods, corals, molluscs, etc. were found, especially the genus Fusulinella which indicates the lower Middle Carboniferous were common through the lower to upper horizons. Although no conodont datum has been hitherto known from not only the Tarma Group but also the Paleozoic of South American Continent except for the Carboniferous in Colombia by Stibane (1967), it will become possible the world-wide relationships with the other regions by the conodonts expecting to be found. In Bolivia, the Copacabana Group developed in Yaurichambi, Ancoraimes, Matilde, Cuyavi and Yampupata, all of the routes formerly measured by us in 1982 and 1984 were re-examined and collected some rock samples for conodont analysis, especially around the boundary between the Triticites Zone and Pseudoschwagerina Zone in the Copacabana Group. The Copacabana Group has been divided into three fusuline zones, namely, the Triticites, Pseudoschwagerina and Eoparafusulina Zones in ascending order, and all of which has been recognized to be the Lower Permian (Wolfcampian) in age by previous students since Dunbar and Newell (1946) and so on. Su$eacute;rez-Riglos et al. (1987), however, reported on the conodont biostratigraphy of the Copacabana Group and they divided it into four conodont assemblage zones, namely, Streptognathodus elongatus Ass. Z., Idiognathodus ellisoni Ass. Z., Neogondolella bisselli-Sweetognathus whitei Ass. Z. and Neostreptognathodus pequopensis-Sweetognathus behnkeni Ass. Z. in ascending order. They are of the opinion that, the lower two assemblage zones, which have been recognized as the lowermost Permian Triticites Zone or Triticites nitens Subzone by the previous students including us, should be the uppermost Carboniferous (Virgilian) by the conodont evidence. The first occurrences of Pseudoschwagerina are in the horizons at 25-40 m from the base of the Copacabana Group in Yaurichambi, Ancoraimes and Matilde routes, and at 240-280 m in Cuyavi and Yampupata routes. Accordingly, at present, the correlation of stratigraphic sections is revised and the boundary between Carboniferous and Permian is tentatively placed at the base of Pseudoschwagerina Zone as shown in Figure 4. Furthermore, four lithologic sequences distinguished by the field observation are briefly described.