The Kabato Mountains in Hokkaido are located in the northern part of the Northeast Japan Arc. The Neogene System, which is distributed in the Aoyama area in the southern part of Kabato Mountains, consists of the Lower Miocene terrestrial basaltic rock, and Middle to Upper Miocene and Pliocene submarine sedimentary rock. In this area, the regional marine transgression occurred in early Middle Miocene, resulting in sedimentation of the Ponsubetsu Formation. After the interrupted sedimentation in the Middle Miocene, the Middle to Upper Miocene Subetsu and Ichibangawa Formations onlapped over the Ponsubetsu Formation. In Late Miocene, the regional subsidence resulted in deposition of the Morai Formation consisting mainly of siliceous shale. These submarine Miocene formations interbedded with tuff beds which consist of scoria fall deposits, silicic ash flow turbidites, and thier resedimented deposits. Based on mineral assemblages, it is inferred that these tuff beds have been derived from the Miocene volcanic activity in the Atsuta-Hamamasu area in the western part of the Kabato Mountains. In the Kabato Mountains, the Neogene stratigraphy and the volcanic activity take on the both features of the Central Hokkaido and the Western Hokkaido, suggesting that the Neogene system of the Kabato Mountains was subject to the influence of the two different tectonics.

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The powerful earthquake registering 7.8 on the Richter scale, which had its epicenter to the northwest 60km off western coast of southwest-Hokkaido, took place at 10:17pm July12, 1993. Tsunami broke out directly after the quake occured, attacked the coast along southwest-Hokkaido and 229 people died and missing. Hirahama, Miyano and Ota blocks in Taisei town are located facing Japan Sea are heavily damaged by the earthquake and tsunami. We tried to research the damage of the tsunami in the three blocks on the basis of the following three points; 1) surged direction and height of the tsunami, broken condition of buildings, 2) refuged course and escaped method for people, 3) topographic and geological obstruction for taking refuge in the safe place. The result of our research indicates that the tsunami with 4m〜7m waves struck Taisei town from west and/or northwest direction within 5〜7 minutes after the quake left off. The significant difference regarding to the damage of buildings and injury of people can be observed among the three blocks. The following points must be taken into consideration for the safety of people lives; 1) various understanding of people on earthquake and tsunami, 2) to be ensured enough time for people escaping from tsunami attack, 3) security of refuge course and shelter, 4) construction of effective breakwaters against tsunami and flood tide.

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Since 1990 we have been studying the geology of the Plio-Pleistocene series exposed in the western hills of the Kanto Plain to clarify the stratigraphic relations, geological structures, geological age, and paleoenvironment of this area. In this paper, we first describe the morphology of the basement Chichibu and Shimanto Supergroups, and discuss the sedimentation of the Kasumi Gravel Bed in relation to the basement. Next, we discuss the age of deposition of the lowest stratum of the Kasumi Gravel Bed, which is correlative with the Hanno Gravel Bed, and is considered to be of latest Pliocene age, based on fission track measurements of volcanic ash in the lower part of the gravel bed, and fossils identified in this area, such as plant macrofossils, fossil pollen, and fossil elephant bones.

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A change from pyroclastic to reworked deposits in the Lower Pliocene Ohta Tephra Bed is preliminary clarified, based on facies analysis. The Ohta Tephra Bed is a wide spread marker-tephra, distributed over a distance of more than 150km. Eight sedimentary facies are recognized in this tephra bed. The facies are interpreted as pyroclastic fall, pyroclastic flow, and reworked (hyperconcentrated flood flow, normal flood flow, channel fill, swamp or backswamp, and lacustrine) deposits. In underlying and overlain non-volcaniclastic beds, four sedimentary facies are recognized and indicate a fluvio-lacustrine depositional system. According to the successions of the volcaniclastic sedimentary facies, the deposition of the Ohta Tephra Bed can be divided into 5 stages. The depositional process is reconstructed using these 5 stages as follows: Stage I (first eruption stage); the north part of the study area was covered with air fall ash. Stage II (second eruption stage); air fall ash was distributed over all of the study area. Stage III (quiescent stage); development of a widespread lacustrine environment. Stage IV (pyroclastic flow stage); pyroclastic flow deposits reached at the north of the study area. Stage V (reworking stage); volcaniclastic sediments were deposited from a fluvio-lacustrine system including hyperconcentrated flood flow.

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The Neogene strata distributed in Toyama Prefecture are divided into the Nirehara, Iwaine, Kurosedani, Higashibessho, Otogawa and Himi Formations in ascending order. The Otogawa Formation assigned Late Miocene in age, yields abundant molluscan fossils which is called the Otogawa molluscan fauna. There are several problems to be solved for the Otogawa molluscan fauna. One of the problems is whether the Otogawa molluscan fauna is warm water fauna as shown by the presence of Nanaochlamys notoensis or cold water fauna as suggested by Megangulus zyonoensis, Mya, Serripes etc. Another problem is detailed age assignment of the fauna. Our conclusion for these problems are as follows; 1. The Otogawa fauna can be divided into two types on the basis of species composition. The Otogawa fauna (Type I) occurs in the lower part of the Otogawa Formation and is composed of Glycymeris matumoriensis, Dosinia ettyuensis, D. kaneharai, Mya cuneiformis etc. The Otogawa fauna (Type II) consists of Anadara amicula, Mizuhopecten tribilium and Dosinia kaneharai, found at Takakuma and Fukujima of Yatuo area. 2. The Otogawa fauna is not the warm water but shallow cold (mild to cold temperate) water type. 3. The Otogawa fauna (Type I) is correlated with the Shiobara type fauna in the Kanto-Tohoku Region. 4. The Otogawa fauna (Type II) is correlated with the Shigarami fauna in Nagano based on the presence of Anadara amicula and Mizuhopecten tribilium. 5. From Late Miocene to Pliocene, the Otogawa fauna (Type I) transformed to the Otogawa fauna (Type II) and also transformed to the Omma type fauna. The Otogawa fauna (Type II) may be an ancester of the Omma type fauna.

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