A brief history of my research over the past 50 years is presented in this paper. The first and most basic concept behind my research is the exploration of recent events for the purpose of understanding the present and near future environments of the earth surface. After studying recent developments in mountain slopes and valleys based on correlations between geomorphology and historical documents for each event, I had the chance to observe a conspicuous tephra derived from Fuji volcano and some artifacts of prehistoric age below the tephra. This experience gave me the idea that tephra shows the strict datum plane for a wide occurrence area and has a high capability of recovering many events in the past, if we have excellent techniques to promote fingerprinting and dating of tephra. Tephra stratigraphy and chronology of Fuji and Hakone volcanoes supplied excellent data for recovering the history of explosive activity during the middle to late Pleistocene and Holocene.
I have worked on several topics, as follows, with all of my work based on tephra stratigraphy and chronology in the Japan area : a) Identification of very widespread tephra layers ; b) Establishment of a tephra-based Quaternary chronology of Japan ; c) Interglacial-glacial sea-level sequences in the Quaternary ; d) Tectonic evolution of mountains and basins of central Honshu using Quaternary and Pliocene tephrochronology ; e) Paleoenvironments during the last deglaciation stage in Japan ; f) Outreach from the earth sciences to society.

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Three major treads of fluvial terraces (lower, middle, and higher terraces) are well developed along the Kabura-gawa river, tributary of the Tone-gawa river, in the southern part of Gunma prefecture, northern Kanto region. We detected cryptotephras in overbank and aeolian deposits covering the middle terrace gravels, and examined refractive indices and chemical compositions of some of amphiboles in the cryptotephras. We also investigated refractive indices and chemical compositions of amphiboles in the Iizuna-Kamitaru tephra (Iz-KT) and the Iizuna-Nishiyama (Iz-NY) tephra extracted from outcrops near the Iizuna volcano, northern Nagano prefecture. Based on these data, we identified possibly Iz-KT and Iz-NY tephra at the bottom of overbank and aeolian deposits of the middle terrace of the Kabura-gawa river. This indicates that the middle terrace was formed through aggradation at the glacial age of marine oxygen isotope stage (MIS) 6 and subsequent degradation at interglacial of MIS 5e. The Kabura-gawa river basin is located out of the previously reported distribution area of Iz-KT. This fact implies that Iz-KT is distributed more widely in the northern Kanto and southern Tohoku regions, and that Iz-KT is useful as marker tephra of the middle Pleistocene in these regions.

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We newly found six crystal-rich and vitric tephra layers below the horizons of the AiraTn (AT) and Daisen Namatake (DNP) tephra layers in a 70-m long boring core from the Kamiyoshi Basin. These tephra layers were named BY116, BY172, BY186, BY189, BY440, and BY478, in descending stratigraphic order. We analyzed such petrographic characteristics as refractive indeices of hornblende, cummingtonite, and orthopyroxene phenocrysts, and the major element compositions of hornblende and orthopyroxene crystals and glass shards. On the basis of the analyses, these are correlated to tephra layers in the Daisen Lowermost Volcanic Ash Member and in the Higashi-Nada 1700-m core sediments from Osaka Bay (K1-175=BT72 tephra in the Takashima-Oki core from Lake Biwa). BY186, BY189, and BY478 are correlated to hpm1, gpm, and cpm of the member, respectively. BY440 is correlative to K1-175 and BT72 because the major element compositions of glass shards are very similar in these tephra layers. The eruption age of BY478 and cpm is inferred to be between 370 and 400 ka, based on the assumption of a constant sedimentation rate of the Kamiyoshi Basin fill and on the ages of AT, DNP, hpm1, and BT72. The peaty clay bed immediately beneath BY478 yielded a pollen fossil assemblage dominated by Quercus Cycrobalanopsis. This suggests that the horizon is the warmest period in MIS 11. Therefore, we consider that the eruption age of cpm is close to 400 ka. Furthermore, the middle Pleistocene sediments in the Kamiyoshi Basin are estimated to have started at from 470 to 550 ka by using the sedimentation rate. The age is almost contemporaneous with the period when active faulting and westward tilting of faulted blocks was more intense in the central and northern parts of the Kinki District, around 500 ka. The beginning of deposition in the basin thus reflects such regional crustal movement.

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Shirakawa pyroclastic flow (Yoshida and Takahashi, 1991) deposits erupted from several calderas located in the south part of Northeast Japan arc during the early Pleistocene. Yamamoto et al. (1999) re-examined a part of the stratigraphy identified by Yoshida and Takahashi (1991), and classified Shirakawa pyroclastic flow deposits into Ten-ei pyroclastic flow deposit, Nishigo pyroclastic flow deposit, Nagurasawa pyroclastic flow deposit, Ashino pyroclastic flow deposit, and Kumado pyroclastic flow deposit. In this study, in order to re-examine the stratigraphy of the Shirakawa pyroclastic flow deposits, we noticed tephras in the Obama Formation of the Inubo Group in the Choshi area.
Ashino pyroclastic flow deposit correlates to Ob5c (Kd8B) tephra in the Obama Formation of the Inubo Group, based on chemical composition of volcanic glass and orthopyroxene. Eruption age of Ashino pyroclastic flow deposit is estimated to be 1.27-1.45 Ma on the basis of calcareous nannofossil biostratigraphic position of the Obama Formation (Fujioka and Kameo, 2004).
Akai pyroclastic flow deposit, located at Akai in Aizuwakamatsu City, was correlated to Ob5aU3 (Kd18) tephra, 4.8 m above Ob5c in the Obama Formation. Eruption age of Akai pyroclastic flow is estimated to be 1.45-1.51 Ma on the basis of its calcareous nannofossil biostratigraphic position in the Obama Formation (Fujioka and Kameo, 2004). Akai pyroclastic flow deposit exists between Kumado pyroclastic flow deposit and Ashino pyroclastic flow deposit, because Kumado pyroclastic flow deposits correlate to Sr-Kmd tephra (1.51 Ma : Suzuki and Murata, 2008) 10.6 m above Ob5c in the Obama Formation.

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Peat deposits were found under the sand beach on the Desaki Coast (Tamano City, Okayama Prefecture) in the central part of the Seto Inland Sea. In this paper, we re-examined the paleoenvironmental data on the deposits reported by Matsushita et al. (2004). The results of analyses of sulfur contents, diatom fossils, and plant macrofossils in the deposits were used along with ¹⁴C ages and the eruption age (ca. 7,300 cal BP) of Kikai-Akahoya tephra to derive the depositional environments. Peat depositions occurred in a salt marsh developing in a tidal range between mean high water level and the highest water level (HHWL), closely related to mid-Holocene relative sea-level (RSL) variations. The peat layer at this site was formed during a period ranging mainly from ca. 7,000 to 6,600 cal BP, and its age corresponds to the period of mid-Holocene RSL highstand when a rate of RSL rise slowed down after ca. 7,000 cal BP. RSLs have risen with an amplitude more than 0.27-1.27 m during the period from ca. 7,000 to 6,600 cal BP (at a rate more than 0.7-3.2 mm/yr), and reached the highest position at ca. 6,700-6,600 cal BP. Using a RSL (HHWL) index point of transgressive overlap with an altitude of +0.22 m at ca. 7,000 cal BP, half of the present highest tidal range (1.40 m) near the Desaki Coast, and tectonically corrected sea-level position with an altitude of +1.0 m at ca. 7,000 cal BP along the coast of Harimanada in the eastern part of the Seto Inland Sea (Sato, 2007), tectonic subsidence is calculated as 0.2-0.3 mm/yr at this site. Thus, sea-level rise over the period concerned depends mostly on the eustatic factor. This study provides valuable information for mid-Holocene RSL variation along the coast of the Seto Inland Sea.

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