Zircons in Cretaceous sandstones have recently been the focus of a new provenance analysis technique. Microfossil-bearing clasts in conglomerate are also used in this analysis; however, there are few reports on clasts, including microfossils, which are mainly found in the vicinity of Shikoku and Hokuriku districts. Although the Ishido Formation, Sanchu Group, in the Kanto Mountains, is considered to be Barremian in age based on the occurrences of ammonoids, interbedded with conglomeratic beds, there have been no reports of microfossil-bearing clasts from this bed. To confirm whether each fore-arc basin in the whole of southwest Japan had a common hinterland during the Late Mesozoic, microfossils are extracted from gravels in the Ishido Formation to examine their ages. As a result of acidic treatment, Permian and Triassic radiolarians and Triassic conodonts are obtained from eight chert and siliceous mudstone pebbles of the Ishido Formation. On the basis of their lithofacies and ages, these pebbles of this formation are likely to derive from the Chichibu Belt, which constitutes a large part of the Kanto Mountains. These results indicate that, in the Early Cretaceous period, not only granitic rocks but also Jurassic accretionary complexes were exposed as hinterlands of each fore-arc basin in the Shikoku and Kanto districts. Moreover, based on previous reports on the zircon spectra in Cretaceous sandstones and microfossil-bearing clasts in the conglomerates, hinterlands, such as granitic rocks and Jurassic accretionary complexes, of each fore-arc basin in the whole of southwest Japan including the Kanto district, as well as Kyushu and Shikoku districts, indicate common exposure and denudation histories.
Vitric tephra layers named Ushinosawa tephra from the Moroyama Hills in the western Kanto Plain, and KJT-380.5 tephra and KJT-384.15 tephra from the 600 m-long Kawajima core in the central part of Kanto Plain are described. Magnetic polarities suggest these tephra layers are intercalated at the lowest part of the Matuyama choronozone. Ushinosawa and KJT-380.5 tephra layers are characterized by low K2O concentration in volcanic glass, and can be correlated with Reiho tephra of the Tokai Group in Suzuka City, Mie Prefecture. KJT-384.15 tephra is characterized by a high K2O concentration in volcanic glass and can be correlated with newly described Onbebashi 1 and 2 tephras of the Tokai Group. These tephra layers can be considered to be wide spread marker tephras. This is the first report on widespread marker tephras just above the Gauss–Matuyama boundary in Japan. These marker tephra layers are expected to be a valuable key bed for exploring the Neogene–Quaternary boundary in various parts of Japan in the future.
Precipitation on the Ogasawara (Bonin) Islands from summer to autumn depends the intensity, frequency and tracks of tropical cyclones (TCs), which are affected by El Niño/La Niña events (EN/LN). This study is the first to investigate the seasonal variability of the ratio of TCs approaching the Ogasawara Islands to the total number of TCs generated, and to calculate the ratio of TC-induced precipitation to total precipitation during EN/LN TCs extracted from within 300 km of Chichi-jima by QGIS, when they are defined as “TCs approaching Chichi-jima”. Using precipitation at Chichi-jima Observation Station, TC-induced precipitation is calculated when a TC is within 500 km of Chichi-jima. From August to November, the ratio of TCs approaching the Ogasawara Islands to the total number of TCs generated over the Western North Pacific is highest in October. For the same period, the number of TCs approaching the Ogasawara Islands per year during EN is more than that during LN. Reflecting the anomaly of sea surface temperature, the genesis position of TCs during LN shifts westward. TC-induced precipitation on Chichi-jima from August to October during EN is larger than that during LN. in particular, in September, TC-induced precipitation during EN is 40 mm more than that during LN. The former accounts for 61% of the total precipitation in September. These phenomena are explained by the fact that the genesis position of TCs shifts eastward or south-eastward during EN, keeping the central pressure of TCs approaching Chichi-jima lower than that during LN. Also, the presence time of a typhoon from its genesis until it enters the 500 km range of Chichi-jima is longer in EN than in LN. Within the 500 km range of Chichi-jima, the central pressure of a TC is lower in EN than in LN. All these contribute to a large volume of TC-induced precipitation on Chichi-jima during EN.
Kozushima is a volcanic island, located in the northern part of the Izu Islands, approximately 170 km SSW of central Tokyo. The volcanism of Kozushima Volcano started at 80-60 ka and formed monogenetic volcanoes. The latest eruption occurred in AD 838, and was associated with the formation of the lava dome and pyroclastic cone of Tenjo-san. In order to contribute to the long-term forecasting of volcanic eruptions on the Izu Islands off Tokyo, the tephrostratigraphy and eruption history of Kozushima Volcano during the last 30,000 years is reconstructed. According to the geological surveys, the widespread Kikai-Akahoya (K-Ah; 7.3 cal ka) and Aira-Tanzawa (AT; 30.0 cal ka) tephras from southern Kyushu, and Niijima-Mukaiyama (Nj-My; AD 886), Niijima-Shikinejima (Nj-Sk; 8 cal ka) and Niijima-Miyatsukayama (Nj-Mt; 12.8 cal ka) tephras from the Niijima Volcano 20 km NNE of Kozushima are recognized. Tephras that also erupted from Kozushima Volcano are Kozushima-Tenjosan (Kz-Tj; AD 838), Kozushima-Ananoyama (Kz-An; 7-9 c), Kozushima-Chichibuyama-A′ (Kz-CbA′; 14-12.8 cal ka), Kozushima-Chichibuyama-A (Kz-CbA; 30-22 cal ka), and Kozushima-Chichibuyama-B (Kz-CbB; ca. 30 ka). Kz-An is formed by the activity of the northern volcanic chains (Kobe-yama–Anano-yama–Hanatate) just before the Tenjo-san eruption. Kz-CbA′ is distributed in southern Kozushima. Source vent and distribution of Kz-CbA′ have not yet been identified. The eruption history of Kozushima volcano over the last 30,000 years is as follows. At ca. 30 ka, Kz-CbB erupted in the central Kozushima, and the Nachi-san dome and Takodo-yama dome formed. At 30-22 ka, the Kz-CbA eruption in the southern Kozushima and the formation of the southern volcanic chain occurred. After the eruption of Kz-CbA′, the formation of the northern volcanic chain was followed by the eruption of Tenjo-san volcano. In addition, the eruption rate of Kozushima volcano is estimated during the last 30,000 years to be approximately 0.06 km3/1000 y in DRE.
The Lower-Middle Miocene Futamata Group, which is exposed at the southwestern-most part of the N–S trending Akaishi Tectonic Zone (ATZ), central Japan, is believed to record important information concerning the several tens of kilometers scale left-lateral motion of the ATZ. This motion was associated with the clockwise rotation of the SW Japan Arc at the eastern margin of the Asian continent, in association with the opening of the Sea of Japan and the northward bending of its eastern margin. Outcrop-scale structures of the Futamata Group in the “Futamata Graven” of the southwestern-most part of the Akaishi Tectonic Line, the western boundary fault zone of ATZ, are described. The Futamata Group has N–S to NW–SE trending, moderately to steeply westward-dipping overturned structures including some outcrop- and map-scale tight/closed folds due to horizontal shortening. Gentle strike-swings about moderately-plunging rotation axes are overprinted on them. In association with the above deformations, the strata are disrupted to varying degrees, forming broken and dismembered units in outcrop-scale. The coeval Ieta Group in the “Manze Graven” of the southeastern-most part of the Komyo Fault, the eastern boundary fault zone of ATZ, also deformed to form folds of a few meters to several hundred meters in wavelength. However, the stratal disturbances in outcrop-scale are not strong as compared to those of the Futamata Group. These sequence of deformations recorded on the Futamata and Ieta Groups might have been intimately related to the left-lateral motion of the ATZ during the latest Early to early Middle Miocene.
Localized significant subsidence, associated with the 2011 megathrust Tohoku-Oki earthquake (MW 9.0), was revealed with Interferometric Synthetic Aperture Radar (InSAR) in five volcanic regions in northeastern Japan. Focusing on Mt. Zao, Sato (2017) attempted to quantitatively interpret the observed localized subsidence with 2-D finite element modeling of a vertical plane in the E–W direction. A horizontally elongated elliptic body representing hot-and-weak rock (magmatic complex including water) was assumed beneath the volcano. Sato (2017) compared calculated and observed vertical displacements approximately derived with the assumption that the direction of surface displacement almost coincides with that of line-of-sight (LOS) in InSAR observations, while this study compares calculated and observed surface displacements in the LOS direction. The observed surface displacements are interpreted with sufficient confidence. Hence, it seems possible to conclude that surface displacements, such as localized subsidence, in volcanic regions associated with large earthquakes are due to the existence of hot-and-weak rock bodies beneath the volcanoes. In the case of Mt. Zao, an appropriate combination of its size (length of the major axis) and Poisson's ratio is found to be 11 km and 0.49. Young's modulus is not absolutely determined; it would be in the range between approximately 1 and 8 GPa with a plausible value of ∼1 GPa (bulk modulus of ∼17 GPa) or slightly larger.