This paper examines changes to the river system and faulting in the Ikachi Basin and surrounding area in the southwestern part of the Chugoku Mountains since the Middle Pleistocene, based of an investigation of the fluvial terrace and tectonic landforms. Fluvial terrace surfaces in the study area are classified into five levels: H, M1, M2, L1, and L2, in descending order. The M1 terrace surface is widely observed in the Ikachi Basin, and there is a narrow band of Sanbe-Kisuki tephra on the top layer of the terrace deposit, suggesting that the surface was formed around 110-115 ka. Aira-Tn tephra is observed in the L2 terrace deposit, indicating that it was formed around 30 ka. The distribution of terrace and deposit indicates the existence of the Paleo-Shiwari River, which differed from the river system existing today. The Paleo-Shiwari River flowed northwestward from the southeastern margin of the Ikachi Basin, and from near Hizumi, westward through the basin. There is a possibility that the upper reaches of the Paleo-Shiwari River reached Yashiro Island. The Paleo-Shiwari River lost its upper reaches as a result of river capture around the current Obatake-Seto in Middle Pleistocene. Furthermore, as a result of continued large-scale uplifting in the downstream area of the Paleo-Shiwari River basin, accompanied by activities of the Hizumi and Oguni faults since the Middle Pleistocene, the height of the riverbed of the Paleo-Shiwari River increased and its riverbed slope became gentle. At the same time, continued large-scale subsidence with faulting from the downstream basin of the Yuu River to Aki-Nada led to a gradual steepening of the riverbed of the Yuu River, and the valley head of the Yuu River along the fracture zone expanded due to erosion. Subsequently, the Paleo-Shiwari River was captured by the Yuu River at the Hizumi depression around 110-115 ka during the formation period of the M1 surface. It is concluded that river capture between the Yuu River and the Shiwari River occurred due to the influence of crustal movements.
Tephras interbedded with Holocene sediments in coastal lowlands along the Sanriku Coast, northeast Japan are described, and well-known widespread and local tephras are correlated based on morphology, refractive index, and chemical composition of volcanic glass shards, stratigraphy, and radiocarbon age. As a result, tephras that are correlated with Towada-a (To-a), Towada-Chuseri (To-Cu), Kikai-Akahoya (K-Ah), Oguni pumice, Towada-Nambu (To-Nb), and Hijiori-Obanazawa (Hj-O) are identified. In particular, To-Cu is distributed throughout the study area and is recognized to be a useful key tephra. In addition, To-a and Oguni pumice were discovered for the first time on the Sanriku Coast. The results suggest that the morphologies and the refractive indexes of volcanic glasses are useful and important for distinguishing tephras and correlating with widespread and local tephras on the Sanriku Coast.
The process of terrace formation in Japan is discussed in the context of advances in tephrochronology. In particular, the ages of fluvial and marine terraces correlate with climate and glacio-eustatic sea-level changes. However, previous studies do not distinguish between the influences of climate changes and base-level changes (glacio-eustatic sea-level changes) for terrace formations, and mainly target northeast Japan due to advantages related to tephrochronology and terrace development. Therefore, this study focuses on the Ohmi Basin, Shiga Prefecture, southwest Japan, to reveal the process of terrace formation under conditions with no base-level changes, and compares them to those in northeast Japan. To achieve this aim, a drilling survey at terrace surfaces and a cryptotephra analysis are conducted. Widespread tephras, Kikai-Akahoya (K-Ah) tephra, Aira-Tn (AT) tephra, and Kikai-Tozurahara (K-Tz) tephra, are used to correlate fluvial terraces and establish the chronology of fluvial terraces in the eastern and western parts (Koto and Takashima regions, respectively) of the Ohmi Basin. Terrace correlation shows that terraces formed during Marine Isotope Stage (MIS) 2 are distributed in both regions under different tectonic settings. This indicates that climate change is the main factor of terrace formation in the Ohmi Basin. Therefore, river conditions during MIS 1, 2, and 5 are compared, and influences of climate changes and crustal movements for terrace formation are estimated. As a result, terrace formation in the Takashima region is explained by climate changes and fault movements. On the other hand, terrace formation in the Koto region is explained by climate changes and tectonic tilting. Consequently, these results suggest that the fluvial terraces in the Ohmi Basin are climatic terraces and that older to younger terrace steps could be the result of a combination of climate changes and crustal movements without base-level changes.
Small-scale tectonic landforms are identified from detailed aerial photograph interpretations, in order to clarify evidence of faulting since the late Pleistocene period along the marginal fault zone at the western foot of the Suzuka Mountains, central Japan. Surface fault traces at least 9-km long are recognized in the central part of this fault zone. Along the Uso River, small tectonic scarps are recognized on young fan terraces where assumed faults were identified previously by seismic reflection profiling. These scarps suggest that the most recent movement at the fault occurred during or after the late Pleistocene period. Progressive vertical displacement is recognized in the subsurface structure of Kobiwako Group, indicating that the fault zone has been active since the Plio-Pleistocene period. This fault may extend further to the north and south in the western foot area of the Suzuka Mountains.
ブルサン火山複合体（BVC）を構成するイロシンカルデラとブルサン火山は，フィリピン共和国のルソン島南東端に位置する。この論文は，小特集「フィリピン・ルソン島のイロシンカルデラとブルサン火山の地質と最近の噴火活動」（その1）と（その2）に掲載された論文について，その概要を述べたものである。Moriya（2014）は，フィリピン諸島の84火山の地形発達史を予察的にまとめており，フィリピンの火山の概要を理解することができる。Kobayashi et al. （2014a, b）は，カルデラ形成噴火の推移を明らかにしており，Kobayashi（2014）では，姶良カルデラと比較して両者の共通性を明確にしている。Danhara et al. （2014）は，イロシン火砕流および関連する堆積物の岩石記載的特徴を報告している。Komazawa et al. （2014）は，カルデラ壁に対応する急勾配が地下にも存在することをブーゲー異常図から示している。Takashima and Kobayashi （2014）は，イロシン火砕流とそれに伴う降下火山灰（co-ignimbrite ash-falls）について，36±8 ka, 38±10 ka, 33±8 ka, 45±10 kaの熱ルミネッセンス年代を報告している。Mirabueno et al. （2014）は，カルデラ内のボーリングコア試料から火砕流堆積物1層と降下テフラ12層をみいだしている。Kinoshita and Laguerta （2014）は，ブルサン火山やマヨン火山で実施されている噴煙映像観測の方法・結果を紹介している。Delos Reyes et al. （2014）は，ブルサン火山2006-2007年噴火による降下テフラの分布と岩石記載的特徴を報告している。Taguchi et al. （2014a）は，ブルサン火山周辺の温泉と冷泉を紹介している。
The eruption sequences of two caldera volcanoes, Aira in Japan and Irosin in the Philippines, were examined, and an eruption sequence for caldera volcanoes is constructed as follows: first, eruptions of felsic magma as a precursory event; second, a plinian eruption associated with intra-plinian flows followed; third, a fine-grained ignimbrite eruption occurred, which is followed by a catastrophic caldera-forming eruption; fourth, voluminous co-ignimbrite ash was generated and dispersed over a wide area; and, finally, post-caldera volcanoes were formed. In addition, intense earthquakes occurred either at the late stage of the plinian phase of the Irosin caldera or shortly before the final caldera-forming eruption of the Aira caldera. A similar eruption sequence was observed for the 7.3 cal ka BP caldera-forming eruption at Kikai caldera in Japan. Therefore, the eruption sequences observed at caldera volcanoes in Japan and the Philippines are considered to form the most fundamental processes of a caldera-forming eruption.