The coastal lowland area of Kawazu, Izu Peninsula, central Japan, have been inundated by 1923 Kanto earthquake with wave heights of 3 m, although there have been no investigations of the frequencies or magnitudes of tsunami occurrences over long-term geological timescales in this area. The present study therefore conducted stratigraphic and paleoenvironmental researches on Holocene deposits based on sediment cores (8 m long) from two sites in the coastal lowland. We also examined grain distribution and grain compositions of recent deposits collected from beach, mouth and floodplain of Kawazu River. These results show that no evidence of Level 2 tsunamis, which are considered to be caused by the largest conceivable earthquakes (Mw 9.1) along the Nankai Trough, was identified from the deposits.

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We report AMS (accelerator mass spectrometry) ¹⁴C ages of molluscan shells and coral skeletons off Tokai region, central Japan. Ten samples are selected from the three deep-sea localities of the GH97 and GA97 cruises (St. GH97-71 on the Izu Spur, St. GH97-74 on the bank in the northern Izu Islands, and St. GA97-235 on the slope off Enshu-nada), where shell remains of deep-sea limopsid bivalves are dominant. The latter two localities (GH97-74 and GH97-235) are also characterized by abundant occurrence of coral skeletons Goniocorella dumosa, which is known to be a representative constructer of deep-water coral reefs. The AMS ¹⁴C ages of two molluscan samples from GH97-71 show large variation (703–881 cal AD, 47911–46066 cal BC), whreas the ages from GH97-74 and GA97-235 (two limopsid shells and two Goniocorella skeletons, respectively) fall into certain durations in the last glacial period: 43893–42787 cal BC to 40637–39983 cal BC (GH97-74) and 25946–25626 cal BC to 17997–17592 cal BC (GA97-235). It is noteworthy that the ages of GA-235 Goniocorella samples (19945–19543 cal BC. to 19339–18896 cal BC) corresponds to the Last Glacial Maximum. These suggest that the mass accumulation of Goniocorella skeletons has been controlled by climatic changes.

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Based on the precise geological mappings in the Komoro Basin and in the Kirigamine area, Miyasaka and Kano (2015, 2017) discussed the Early-Middle Pleistocene tectonics and volcanisms in the central Shinshu district, central Japan. The stratigraphic correlation between the Lower Pleistocene Enrei Formation in the Kirigamine area (Kirigamine-Enrei) and the Komoro Group in the Komoro Basin played an important role for the discussions. We have re-evaluated here the stratigraphy and geologic structures of the Enrei Formation on the southwestern area of the Suwa Basin (SW Suwa-Enrei) based on the seismic profiling data. We also have checked the previous studies of the Lower Pleistocene Ina Formation to the south of the Suwa Basin in the southern Shinshu district. The lithological similarities of volcani-clastic rocks, widespread marker tephras and paleomagnetic data from the SW Suwa-Enrei and the Ina Formation suggest that they are well correlatable to the Kirigamine-Enrei. As the result of these analyses, the paleogeography, volcanic activity and tectonics of the central and southern Shinshu districts during the Early to Middle Pleistocene are summarized as followings.
The central and southern Shinshu districts during the Pliocene kept a widespread low-relief landform mostly under lacustrine and fluvial environments. A large collapse basin of several tens of kilometers in horizontal scale was formed in the Kirigamine and Komoro areas during the Early Pleistocene before 2.2 Ma. The basin was filled with the mafic to andesitic volcanic rocks of the Older Enrei Formation caused by violent fissure eruptions. The NW-SE trending upheaval of the pre-Pliocene basement rocks and the mafic to felsic volcanisms to form the Younger Enrei Formation occurred in the central part of the Kirigamine area during the late Early Pleistocene (1.3–0.8 Ma). The volcani-clastic rocks supplied from the Older and Younger Enrei Formations filled with the Komoro Basin and to the southwest of the Suwa Basin. During the Middle Pleistocene, the near-horizontal structures of the lower Pleistocene strata in these areas were modified by faultings, foldings and flexures. Subsidence of the Suwa Basin in association with the sinistral faulting of the Itoigawa-Shizuoka Tectonic Line was the most conspicuous tectonic event during this period. The Daimon-Oiwake Graven and the westward tilting of the eastern side of the Ina Basin, were also formed almost simultaneously. These tectonic and volcanic events during the Early-Middle Pleistocene might have been controlled by the complicatedly changing local stress caused by the interaction between the westward subduction of the Pacific Plate, and the orthogonal collision of the Izu-Bonin volcanic arc on the northwestward advancing Philippine Sea Plate with the central part of Honshu Arc.

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Numerical simulations of isothermal decompression-driven crystallization were performed for the basaltic andesite melt of the 1986 eruption at Izu-Oshima volcano (SiO₂ = 54.4 wt.%) by “rhyolite-MELTS” program under the conditions of temperatures of 1132–1079°C, initial melt H₂O contents of 1–4 wt.%, initial pressure of 200 MPa and fO₂ at Ni-NiO buffer, respectively. The starting melt composition is similar to a groundmass glass composition of microlite-poor strombolian scoria from the A vent and also a whole rock composition of phenocryst-poor sub-plinian scoria from the B vents of the 1986 eruption. The results show that starting pressure of crystallization, and increasing rate and final value of crystallinity increase as temperature decreases. In addition, assemblage, abundance and order of crystallization of mineral phases change with temperature. The temperature-dependent changes of crystallization behaviors induce change of melt SiO₂ content-crystallinity-pressure paths. As a result, increasing rates and final values of both melt viscosity and relative viscosity, and hence those of bulk viscosity of magma increase as temperature decreases. Increases of crystallinity and magma viscosity inhibit outgassing and also facilitate magma fragmentation during magma ascent, resulting in violently explosive eruption. Present results suggest that temperature difference between magmas from the A and B vents is a key factor to induce the contrasting eruption styles observed during the 1986 eruption at Izu-Oshima volcano.

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Thermodynamic simulations of crystallization differentiation using “Rhyolite-MELTS” program are performed for a relatively undifferentiated basaltic melt from Fuji Volcano to examine the relationship between viscosity and SiO₂ content of melt. The relations between viscosities and SiO₂ contents of initially H₂O-rich melts, with initial melt H₂O contents >~2wt.%, are consistent with the melt viscosity scale (MVS) proposed by Takeuchi (2015). However, viscosities of initially H₂O-poor melts, with initial melt H₂O content <~2wt.%, are larger than those predicted by MVS. The deviation from MVS occurs during the early stage of crystallization characterized by increases of FeO* and TiO₂ contents without significant increase of SiO₂ content in melt, due to absence of Fe-Ti oxide minerals in crystallized phases. In the middle stage of crystallization characterized by increase of SiO₂ content and decreases of FeO* and TiO₂ contents, chiefly due to crystallization of Fe-Ti oxide minerals, the melt viscosity-SiO₂ content relations are almost parallel with MVS. Therefore, once melt viscosity deviates from MVS, it remains larger than MVS even the melt H₂O content becomes >2wt.%, suggesting that melt H₂O content is not necessarily an adequate criterion for judging whether MVS is applicable or not. Alternatively, we propose new criteria for MVS application based on SiO₂-Al₂O₃-FeO*-TiO₂ relation of melt, which is useful for a melt sample of unknown H₂O content.

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