A graben is defined geologically as a topographic depression generated by dip-slip faulting. The Kamogawa Graben on the southern Boso Peninsula, central Japan, is considered to be a typical graben, although it has yet to be geomorphologically or geologically verified. In this study we use seismic reflection profiling and analyses of geomorphic and geologic data to reexamine the unsolved question of whether this structure is in fact a graben. We present an interpretation of the structure in terms of its structurally controlled geomorphic evolution. On the basis of our analysis, we conclude that the so-called Kamogawa Graben is not actually a graben, as graben structures are not observed in seismic images and there is no positive cumulative deformation recorded by active faults at the surface. The topographic lineaments that bound the depression between the Kazusa Hills and the Awa Hills constitute fault-line valleys and obsequent fault-line scarps. The northern lineament has developed along the geologic contact between fractured shale of the Hota Group and mudstone and sandstone of the Miura Group, while the southern lineament has developed between mudstone and sandstone formations within the Hota Group, along the Sorogawa Fault. These observations suggest that differential erosion of shale, mudstone, and sandstone strata produced the observed graben-like relief. Weathered shale is especially susceptible to erosion and is marked by areas of relatively low relief. The presence of paleo-river channels and obsequent fault-line scarps leads us to the proposal that an earlier river system developed on the pre-erosion mountainous topography that overlaid the present Mineoka Mountains. Such a geomorphic evolution of the southern Boso Peninsula is therefore structurally controlled, and the Kamogawa Graben is in fact a structural depression rather than a graben.

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In order to reconstruct the depositional environment and paleoclimate changes since the middle Pleistocene in central Japan, tephra analysis, pollen analysis, magnetic susceptibility measurement, grain size analysis and color index measurement were conducted for a 25.3m-long sediment core obtained from Okute basin in central Japan.
The sediment consists of mainly fine materials characterized by periodic alternation of peat layers and inorganic clay layers for the past 300ka. Paleo-temperature were quantitatively reconstructed from the pollen profile using standard modern analog technique. The curve showed a fluctuation pattern remarkably similar to that of the deep sea δ¹⁸O record. Mean annual temperature was 3 to 4°C in the full-glacial period and 12 to 13°C in the full-interglacial period. The difference between full-glacial and full-interglacial is 10°C.
Color measurement enables us to discriminate peat layer/humic silt layer and inorganic silt layer using an L^* value that describes lightness. This boundary value is 30. L^* value variations were closely connected to the June insolation variations at 30N that are depended on the precession. The beginning of peat deposition corresponds to the timing of insolation values over 500W/m².
As observed above, global climatic changes and regional environment changes have been continuously recorded in this core for the past 300ka.

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The Younger Fuji volcano has erupted repeatedly over the past 11 ka forming enormous deposits of pyroclastics and lava flows with steep slopes exceeding 30 degrees near the summit. Rainfall during and after the eruptions triggered lahar flows that moved the ejecta from the upper slopes to the lower slopes of the volcano. These lahars created many alluvial fans, such as the Kamiide, Yoshida, and Osawa alluvial fans. We studied the history of the Kamiide alluvial fan on the western foot of the volcano and compared it with the eruptive history of the Younger Fuji volcano.
Based on topographical and geological studies and ¹⁴C dating, we found that the deposit of the Kamiide alluvial fan began to form about cal BC 3,400 and ended at about cal AD 300. The depositional sequence of the Kamiide alluvial fan can be divided into three periods, as follows. 1) YFM-K1 period (cal BC 3,400 to 2,100) : Formation of the oldest and widest fan. The fan deposited during this period consists of lahar deposits, mainly derived from the middle stage lavas of the Younger Fuji volcano (YFML) and the Iwatoi pyroclastic flow deposits. These Iwatoi pyroclastic flows extended down to an elevation of 950m, close to populated areas. The total volume of deposits from this period is estimated to be 1×10⁸m³. 2) YFM-K2 period (cal BC 1,500-100) : Formation of the middle portion of the fan partially covering the YFM-K1 fan. The fan deposit from this period consists of lahar deposits, mainly derived from the YFML, the Osawa pyroclastic flow deposit-2 and -3, the Osawa scoria fall deposit, and the Inokubo lahar deposit-A. The Inokubo lahar deposit-A has a volume of 1×10⁷m³ and contains altered blocks similar to those within the Gotemba debris avalanche deposit that was formed in cal BC 900 on the eastern slope of the volcano. The total volume of the YFM-K2 deposits is estimated to be 4.2×10⁷m³. 3) YFM-K3 period (cal BC 800-cal AD 300) : Formation of the youngest fan covering the YFM-K1 and YFM-K2 fans. The fan deposits from this period consist of lahar deposits derived from the YFML, including agglutinate fragments originating from the summit eruptions. The total volume of these deposits is estimated to be 3.3×10⁷m³.
The Kamiide alluvial fan began forming concurrently with the outflow of the YFML, and the oldest portion (YFM-K1) consists entirely of the YFML. These facts suggest that the YFML deposited on the steep slopes of the volcano became the source of the lahars that created the alluvial fans on the lower slopes. Large lahar deposits contributed to the formation of the YFM-K2 fan, though lahars originating from pyroclastic flows did not have a large influence in the formation of the fan. The accumulation of welded pyroclastics near the summit influenced the generation of lahars in the YFM-K3 period. Moreover, the magma discharge rate appears to have controlled the formation of the alluvial fan during the eruptive period, since the rate of change in magma discharge corresponds to the change in volume of the Kamiide alluvial fan deposit after cal BC 3,400.

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The purpose of this study is to examine the basal age and the formative environments of humic soil in the southeast part of Shimabara Peninsula, Kyushu, Japan. Soil description, physical and chemical properties of the soil, phytolith composition, and AMS ¹⁴C age were investigated for samples in a tephra-soil sequence. The humic soil successively developed prior to the deposit of Aira-Tn tephra in 24-25ka BP. It was considered that the main parent materials of the humic soil originated in secondary deposits from bare land around the volcanic body and/or in fine-grained tephra deposits that erupted from Unzen volcano. Phytolith composition was mainly grass vegetation such as Sasa sect. Crassinodi, and arboreal phytoliths were hardly detected even at the maximum stage of the last glacial. It was concluded that the humic soil continuously developed even under cold climate conditions due to the superior supply of organic matter to that of volcanic products.

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