The tectonic setting of arc-arc collision and arc accretion in the Izu-collision zone is similar to that of the Archean orogenic belts (e.g., Taira et al., 1992). Understanding the petrological processes of granite formation in the Izu-collision zone, where geodynamic information is not modified by polyphase deformation and metamorphism, may contribute to an understanding of ancient orogenic belts, especially those related to collisional settings. The Pacific plate began subducting the Philippine Sea plate about 50 million years ago to produce the currently active Izu–Bonin–Mariana (IBM) arc. The collision between the northern IBM arc system and the Honshu arc of the Eurasia plate has been occurring since the middle Miocene (ca. 15 Ma) as a consequence of the northwestward migration of the Philippine Sea plate (e.g., Yamazaki et al., 2010). Neogene granite plutons are widely exhumed by tectonic uplifts associated with arc collision. Seismic imaging suggests that most of the present Izu-Bonin arc crust was created in the Eo-Oligocene (Kodaira et al., 2008; Kodaira et al., 2010). However, remnants of this older crust have not been found in the Izu collision zone. Tamura et al. (2010) integrated new geochemical results with recent geophysical imaging of the arc and concluded that Miocene plutonic rocks in the Izu collision zone are from the Eocene–Oligocene middle crust, which was partially melted, remobilized, and rejuvenated during the collision. Moreover, (1) the mafic arc lower crust is missing at the collision zone (Kitamura et al., 2003) and (2) the aseismic Philippine Sea plate, which is subducted at depths of 130-140 km without evidence of a tear or other gap, has been detected even beneath areas 120 km NW of the collision zone (Nakajima et al., 2009). These lines of evidence suggest that the down-dragged middle crust would partially melt and coalesce in the upper plate, but the mafic (high in iron and magnesium) lower crust would not melt and subduct into the deep mantle, resulting in delamination and separation of the middle crust from the lower crust. Both processes are inevitable at the collision and are necessary to yield continental crust. Thus, it is suggested that collisional orogeny plays an important role in the genesis of continental crust.
Many Rivers are concentrated in the central part of the Kanto Plain, central Japan. The Tone River, the largest river, flowed south-southeast before it was artificially diverted eastward at the end of the 16th century. A large meandering paleochannel in the present Arakawa Lowland, which is situated about 30 km south of the present Tone River's course, is thought to have been formed by the former Tone River. However, the upriver part of this paleochannel is indistinct in the Menuma Lowland lying north of the Arakawa Basin. Detailed aerial photo interpretation and observations of sediment samples obtained with hand augers reveal that a paleochannel with a north-south trend in the eastern Menuma Lowland contains volcaniclastic sediments derived from the Tone River catchment. The age of the buried paleochannel, i.e., the former Tone River, was estimated to be around 1,300 years ago based on many archeological survey reports of ancient tombs and other sites located around the paleochannel. The subsequent eastward natural diversion of the Tone River into the Kazo Lowland seems to have been caused by aggradation due to a rapid supply of volcanic sediments and tectonic subsidence in the Kazo Lowland situated in the central part of the Kanto Tectonic Basin.
This study examined isotope ratios of acid-soluble lead in boring cores taken from sediments in the Osaka City area. The sediments are more than 2,500 years old and contain only naturally occurring lead. The isotope ratios of the acid-soluble lead were similar to those obtained with a whole rock analysis, which indicated that the method of measuring acid-soluble lead instead of whole rock was simple and accurate enough to analyze isotope ratios of lead in sediments. The isotope ratios of lead from the boring cores had similar values and were independent of deposition age and environment during deposition. The average of the isotope ratios of lead from one boring core differed slightly from the average of those from another boring core. Isotope ratios of 207Pb/206Pb from the boring cores were between 0.83 and 0.85, and the isotope ratios of 208Pb/206Pb were between 2.06 and 2.13, which were the same as the isotope ratios of lead from ores and soils measured so far in Japan. The range was narrow compared to the range of isotope ratios of lead from various ores obtained around the world; 207Pb/206Pb from ores obtained around the world are between 0.7 and 1.1 and 208Pb/206Pb are between 1.8 and 2.5. Because Japan imports most of its lead, isotope ratios of anthropogenic lead in soils can differ from isotope ratios of lead occurring naturally in soils in Japan. Therefore, the isotope ratios of lead can be used to determine whether lead is anthropogenic or natural.
This study investigated the spatial distribution of heavy rainfall that enhanced the occurrence of potential landslide hazards throughout Japan during the period 2001-2008, using the Soil Water Index (SWI). We calculated SWI using a tank model, with Radar/Raingauge-Analyzed Precipitation for the period 1991-2008 as input data provided by the Japan Meteorological Agency. The SWI can represent and elucidate the conceptual soil water contents during a rainfall-event associated with the shallow landslide initiation. Comparing the SWI of the present rainfall event with the upper level record of SWI during the past decade, we can evaluate the occurrence of potential landslide hazards at a location during rainfall. For this research, by comparing the SWI for the past decade, we defined the top 3 heavy rainfall events that raised the SWI, enhancing the occurrence of potential landslide hazards. We then calculated the frequency of such rainfall events. The results showed regional differences of heavy rainfall frequency that raised the SWI during the last 8 years. The high-frequency regions were eastern Hokkaido, central Tohoku District, the Oga Peninsula, the Izu Peninsula, Shikoku Island, southeastern Kyushu Island, Amami-Oshima Island, and others. In these high-frequency regions, the maximum hourly rainfall event that raised the SWI became larger. On the other hand, the low-frequency regions were western Hokkaido, eastern Kanto District, central Kinki District, Setouchi District of Hiroshima Prefecture, northwestern Kyushu Island, and others. In these low-frequency regions, the maximum hourly rainfall event that raised the frequency became progressively smaller. Therefore, we conclude that these regional differences are brought about by interannual variation of the maximum hourly precipitation, which enhanced the potential landslides to occur. Moreover, the heavy rainfall frequency increased in eastern Hokkaido, the Shimokita Peninsula, the Oga Peninsula, and central Tohoku District. In contrast, heavy rainfall decreased at parts of the Kii Peninsula, and part of northeastern Kyushu Island. The results suggest that the frequency of heavy rainfall events has changed during the last 8 years in some regions.
This study examines vegetation succession and soil physico-chemical properties in the upper reaches of the O'Donell River, which have been affected by the seasonal lahar since the 1991 eruption of Mt. Pinatubo, central Luzon, the Philippines. Field surveys and physico-chemical analyses of lahar deposits and soil were carried out in areas including the living zone of the Aeta to understand the land recovery process along with land management practiced by the Aeta. Seedlings of Ararong (Trema orientalis) associated with root nodule bacteria are widely distributed on the lower lahar terraces and it was pointed out that Ararong is a pioneer plant. It is also the dominant species of tree and shrub layers on higher lahar terraces, and has a tendency to reduce its height in response to increases in coverage of herbs such as Talahib, Magkakamote, and Bureok. Vegetation succession was divided into the following stages (I–IV) based on an interpretation of the development of the geomorphological surface and a vegetation survey. The former surface soil of the 1991 pre-eruption was tentatively defined as the goal of land recovery and was classified as Stage V. A grain-size analysis of lahar deposits and soils in φ scale was carried out by the dry sieving method. Chemical properties of lahar deposits and soils were examined by analyzing total carbon (T-C), total nitrogen (T-N), total phosphorus content (T-P), C/N ratio, pH (H2O), cation exchange capacity (CEC), base saturation (BS), and total elemental composition. Lower terrace of post-eruption lahar deposits with no vegetation (Stage I) and lower terrace of lahar with Ararong seedlings or herbs (Talahib, Bureok) had low T-C contents 0.2–0.58 g kg-1, and no significant differences were recognized between the two stages. The higher terrace of lahars with Ararong of 2–5 m height and herb coverage (Stage III) and ridge area affected by the 1991 pyroclastic flow and/or fall deposits, currently used as fields with shifting cultivation (Stage IV), had an increased T-C content of from 0.37 to 8.9 g kg-1 in response to the increased density of the vegetation canopy and number of vegetation species, which showed an exponential increase towards Stage V (21 g kg-1). The T-N content showed a slight tendency of nitrogen accumulation occupied by Ararong and rhizobial nitrogen fixation. As far as we examined soil pH (H2O), T-N, CEC, and BS, the soil fertility of the shifting cultivated field and the secondary forest (stage III, IV) in the upper reaches of the O'Donell River is equivalent to that of the fields on plains of the Bambang and Pasig-Potrero rivers. Nitrogen fixation by root nodule bacteria of Ararong, and available phosphorus supply originating from apatite in the lahar deposits are both accelerated by the elution of organic acids from herbs such as Talahib. Potassium supply from the weathering of plagioclase and biotite, the major minerals of lahar deposits, is also the basic process responsible for land recovery on the upper slope of Mt. Pinatubo. Based on analytical data from this study, it is found that the activity of the Aeta people of burning down secondary forests of Ararong to open small areas of arable land is not a regulating factor against land restoration. The soil-forming process, which is closely related to vegetation management from the activities of local people, may help us to understand the ongoing sustainable development in the upper reaches of the O'Donell River.
It is demonstrated that the eastern side of the Tanna fault has subsided relative to the western side using digital mesh data of geographic altitudes. The estimated subsidence is concordant with accumulated displacements of the Tanna fault and the fault slip at the 1930 Kita-Izu earthquake. Having noted that the eastern flank of Hakone volcano is steeper than the western flank, Suzuki (1971) concluded that Hakone volcano inclined to the east. This inclination of Hakone volcano is understandable if we note that it may be a manifestation of subsidence at the eastern side of the Tanna fault and its northern extension. We consider that subsidence at the eastern side of the Tanna fault represents a tectonic movement whereby the Manazuru block squared by the Tanna fault, Hirayama fault, Kannawa fault, and Kozu-Matsuda fault performs buoyant subduction. Based on the characteristics of crustal deformation at the Kita-Izu earthquake and aftershock distribution, it has been suggested that the western side of the Tanna fault moved south. Tectonically, this can be interpreted from the thesis that the western side of the Tanna fault, which had been dragged north by the subducting Manazuru block, rebounded at the Kita-Izu earthquake.
The Kamo area is situated in the eastern margin of the Niigata sedimentary basin, Northeast Japan Arc. The Lower to Middle Miocene successions in the Kamo area are divided into the Tonoiri, Otani, and Nanatani Formations in ascending order. In this study, we show newly determined fission-track ages, chronostratigraphy, and regional stratigraphic correlations of the Lower to Middle Miocene successions, and discuss basin development and tectonics in the Kamo area. The age of each formation is as follows—approximately 18-17 Ma, 17-15.2 Ma and 15.2-12.3 Ma for the Tonoiri, Otani, and Nanatani Formations, respectively. On the basis of the age framework of the successions studied, we can correlate the successions with stratigraphic successions in other areas of the northern Niigata sedimentary basin. In particular, a glauconite sandstone bed in the uppermost part of the Nanatani Formation can be correlated to that intercalated in the boundary horizon between the Nanatani and Teradomari Stages in the Niigata sedimentary basin and that in the boundary horizon between the Nishikurosawa and Onnagawa Stages in the Akita region. Lithofacies and paleobathymetrical data show rapid subsidence and transgressions during the deposition of the Tonoiri and Otani Formations. Furthermore, the Tonoiri and Otani Formations show abrupt lateral changes in lithologies and thickness, which suggests that differential subsidence occurred in the basin, probably in response to rifting of the Eastern Japan Sea Rift System during the Early Miocene between 18 and 15.2 Ma. In contrast, the overlying Nanatani Formation has almost a uniform lithology with minor lateral changes in thickness. This suggests that the rifting may have ceased at around 15.2 Ma and subsequent uniform subsidence seems to have occurred between 15.2 and 12.3 Ma. Consequently, the syn-rift stage sediments can be assigned to the Tonoiri and Otani Formations and the post-rift stage sediments can be assigned to the Nanatani Formation.
This paper discusses locational conflicts surrounding a new waste disposal facility in Koganei City from the viewpoint of “the politics of scale”. Since 1957, Koganei, Chofu, and Fuchu cities have disposed of their waste at Nimaibashi Waste Disposal Facility. In 2007, Koganei City officials proposed the construction of a new facility by 2017 at the site of the Nimaibashi Waste Disposal Facility, which was located at the periphery of Koganei City, or at the site of a former Janome sewing machine factory, which was located at the center of the city. Although people residing near both sites protested against the construction of the new facility, the Koganei City officials decided to construct the new facility at the site of the Nimaibashi Waste Disposal Facility. The reason for this decision is summarized as follows: (1) Most of the citizens of Koganei City appeared to be indifferent to this locational conflict; (2) The Koganei City officials were adamant about constructing an incineration facility; (3) The actions of the people residing near the former Janome sewing factory, unlike those of the people residing near Nimaibashi Waste Disposal Facility, were successful in protesting given the scale of Koganei City.
Edmund Naumann (1854–1927) came from Germany and stayed in Japan for ten years from 1875. He performed a lot of pioneering research on the geology and geography of Japan, which were written mostly in German and recently translated into Japanese by N. Yamashita (1996). However, a paper titled “Skizze der Orographie von Japan” published in 1893 on “Petermanns Geographische Mitteilungen” as the third article of Naumann's “Neue Beiträge zur Geologie und Geographie Japans” has not yet been translated. The paper, which we have now translated into Japanese as “Nihon Sangaku Shi Taiyo”, consists of two parts. The first part deals with the relationship between the topography of mountainous lands and geology, in terms of volcanoes, old mountainlands, granitic mountains, coastlines, depressions, longitudinal and transverse valleys, watersheds, etc. The second part deals with the topographic character of each geotectonic zone of Japan: the northern and the southern wings of the Japan Arc and the Fossa Magna intervening between both wings. One figure and one table are included in this paper. Fig. 1 is an altitude layer map of the main three islands of Japan with a scale of 1:2,600,000, the earliest one drawn with contour lines. Table 1 is a list of the main mountaintops of Japan (eighty-seven mountains), including location, altitude, and geologic nature. Furthermore, a supplementary figure, which shows the distribution of the main mountaintops, watersheds, and geotectonic divisions of Japan, was made by the translators.