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The distribution and characteristics of marine surface sediments are a basic marine geological information. Large river floods are a frequent natural hazard that transport substantial terrigenous sediments into the marine environment. In August 2003, TY ETAU (0310) caused heavy rainfall in the southern coast of Hokkaido, north Japan, where some mountainous rivers in the Hidaka region flooded. Two deposition modes for the 2003 flood sediments can be identified by comparing the pre- and post-flood surface sediment distribution. Shore-normal shallow depressions off the mouth of the Saru and Atsubetsu rivers served as channels for the discharged floodwater preventing dispersion and maintaining the necessary water density to transport the materials as density bottom currents. This action also promoted long-distance transport of flood materials across the continental shelf. Absence of depression on the inner shelf off the mouth of the Niikappu and Shizunai rivers may have dispersed floodwaters near the river mouth and deposited the flood materials close to the shore. Marine geological mapping suggests that the differences in submarine topography (the presence or absence of shallow depressions) are closely related to the regional geological structure. Thus, submarine geology is a controlling factor of the seafloor environments influenced by the river flood.

On the Pacific coast of Hokkaido, historical documents and seismic detections spanning the last 200 years show that large earthquakes (magnitude 7–8) have occurred every 50–100 years along the Kuril Trench. Geological reconstructions of past earthquakes in the region show extend records to the last 6000 years and show that unusually large (>M8) earthquakes occurred at intervals of several hundreds of years. However, the magnitude and extent of tsunamis induced by these unusually large earthquakes are not fully understood. In this study, we found nine anomalous sand layers embedded in peat sediments that dated between 5.5 and 1.2 ka on the southern Hidaka coast, Hokkaido. Paleoenvironments reconstructed from the peat sediments using diatom, chemical, and mineralogical analyses showed an evolution of the site from a saltmarsh, to a supratidal marsh, to a back-barrier wetland environment. Radiocarbon dating of peat sediments showed that these environmental changes correspond to a sea-level fall following the mid-Holocene highstand. The anomalous sand layers showed inland thinning and shape contacts with the underlying peat. Amongst the layers, the most recent layer (1.7–1.2 ka) was designated as a storm deposit due to its limited distribution of less than 100 m from the beach ridge. Eight additional sand layers from 5.5 to 2.2 ka were distributed more than 300 m away from the beach ridge, and displayed a normal grading of sand particles, a characteristic typical of tsunami deposits. Because there was no record of tsunamis striking the Hidaka coast from the observed large earthquakes (M7–8) that recur every 50–100 years, we interpret the sand layers as tsunamis deposited by unusually large earthquakes. Because the eight inferred tsunami sand layers were deposited during the highstand period, prior to 2.2 ka, we infer that the inundation area due to tsunamis in the coastal zone has changed with relative sea-level (RSL) in the past. We highlight the necessity of evaluating the influence of long-term RSL change.

An active barrier system is present in Akkeshi Bay and a lagoonal area along the Pacific coast of eastern Hokkaido, Japan. This rare feature consists of a so-called lagoon (the Akkeshi-ko lagoon), a flood tidal delta, a barrier, and a tidal inlet that leads into the outer sea of Akkeshi Bay and the Pacific Ocean. Large oyster reefs were living on the flood tidal delta until their sudden extinction in the early 1980s. The processes involved in the formation of this barrier system during a postglacial transgression period are not well understood. Here, we use sedimentological, paleontological, and radiocarbon dating methods to analyze Holocene drillcore obtained from the Akkeshi fishing port in 2009 and borehole log data previously obtained from public works around the Akkeshi lowlands. The data obtained from both drillcore and borehole logs indicates that: (1) postglacial transgression occurred in this area some 11,400 years ago and reached 50 m below the present sea water level, with the barrier system in the study area established 8800 years ago. (2) The preservation of this barrier system is due to the stable sea level in this area. The sea level has been constant for the past 5500 years, and has led to the development of oyster reefs on the tidal flood delta. (3) The current topographically distinctive barrier system has been affected by sudden uplifts associated with giant earthquakes. These events occur every few hundred years, with the barrier undergoing prolonged but relatively rapid post-earthquake subsidence at about 1 cm/year, as observed after a giant 17th century earthquake.

石狩平野の深度50 mのSSC-1，同44 mのH16B-7，同40 mのH16B-3，同18 mのMHR-1および同28.5 mのYUB-1の5掘削井の後期更新世～完新世地質試料について14C年代測定，珪藻分析，火山灰分析を行い，層序や古環境を検討した．珪藻分析ではH16B-7とH16B-3で「大量な海水の流入」が認められ，完新世の中期にバリア（砂州）を海水が乗り越えたためと推定した．海生種の最大割合を示す層準は約6,000年前の縄文海進高頂期と考えられ，この時期には紅葉山砂丘より東に大きさ東西30 km，南北20 km程度の汽水湖が形成されたと推定される．火山灰分析ではSSC-1の深度42.38～42.25 mに約11.3万年前降灰のToyaが確認され，最終間氷期の堆積面が石狩平野下に埋積していることが示唆された．

A combined approach of field geology and numerical simulation was conducted for evaluating the tsunami impacts on the shelf sediments. The 2003 Tokachi-oki earthquake, M 8.0, that occurred on 25 September 2003 off southeastern Hokkaido, northern Japan, generated a locally destructive tsunami. Maximum run-up height of the tsunami waves reached 4 m above sea level. In order to estimate the tsunami impacts on shallow marine sediments, we compared pre- and post-tsunami marine sediments in water depths of 38–112 m in terms of grain size, sedimentary structure, and microfossil content. Decreases of fine fractions, especially finer than very fine sand, which led to coarsen the mean grain size, were detected in the inner shelf of the northern part of the study area. Foraminiferal assemblages also changed in the coarsened sediments. On the other hand, the other shelf sediments largely unchanged or slightly fined. We also simulated the tsunami wave velocity and direction, and grain size entrained by the modeled tsunami. The numerical simulation resulted in that the 2003 tsunami could transport very fine sand in water depths shallower than 45–95 m at the northern part of the study area. This is comparable with the actual grain-size changes after the tsunami had passed. However, some storms and tidal currents might also be possible to stir the surface sediments after the pre-tsunami survey, so we could not conclude that the grain-size changes had been caused only by the tsunami. Nevertheless, a combined approach of sampling and modeling was powerful for estimating the tsunami impacts under the sea.

北海道胆振海岸東部において，津波災害の履歴を知るために津波堆積物の調査を行った．その結果，地層中に17世紀のイベント堆積物が挟在することが明らかになった．この砂質堆積物は泥炭層に上下を挟まれ，海岸線に沿って平行に分布し，層厚は数cmで海側から陸側へ次第に薄くなって消滅する．内陸側へ約1～2kmの範囲まで分布し，最高で現標高8mの地点まで分布する．粒度組成は，海側では砂質，陸側では泥質になる．層厚は，局所的な地形の起伏にコントロールされ，凹地で厚く堆積している．さらに，海～汽水生の珪藻化石が含まれており，珪藻化石全体に占める海～汽水生の割合は，海側で高く陸側で低くなる．これらの特徴は，このイベント堆積物が津波によって形成されたことを示している．この津波のトリガーは不明だが，今後さらに検討される必要がある．

札幌市市街域の北海道大学構内で，1981年の掘削で得られた150 mコアの第四系層序を明らかにするために，14C年代測定，堆積相解析，火山灰分析，花粉分析，珪藻分析，放散虫分析を行った．孔内地質はHU-1層～HU-9層に区分され,分析結果に基づく野幌丘陵との地層対比では，HU-1層～HU-3層（深度150～75 m）は前・中期更新世の下野幌層，HU-4層～HU-5層（深度75～35 m）は後期更新世のもみじ台層，HU-6層（深度35～34 m）は後期更新世の小野幌層，HU-7層～HU-8層（深度34～7.8 m）は後期更新世の札幌扇状地堆積物，HU-9層（深度7.8～0 m）は完新統に相当することが明らかになった．最終間氷期堆積物であるHU-4層～HU-5層からは，寒冷期や温暖期が確認され，今後の石狩低地帯南部の第四系との詳細な対比の可能性が生じた．既存井との地層対比では，150 mコアの深度130 m（標高－119 m）付近に挟在する下野幌層相当層中の泥炭は，隣接する2本の井戸の標高－110 m付近に認められる亜炭質層と連続し，更には石狩海岸平野下の下野幌層相当層の標高－102～－126 mに挟在する泥炭に対比される．

The stratigraphy of tsunami deposits along the Japan Sea, southwest Hokkaido, northern Japan, reveals tsunami recurrences in this particular area. Sandy tsunami deposits are preserved in small valley plains, whereas gravelly deposits of possible tsunami origin are identified in surficial soils covering a Holocene marine terrace and a slope talus. At least five horizons of tsunami events can be defined in the Okushiri Island, the youngest of which immediately overlies the Ko-d tephra layer (1640 AD) and was likely formed by the historical Oshima-Ohshima tsunami in 1741 AD. The four older tsunami deposits, dated using accelerator mass spectrometry 14C, were formed at around the 12th century, 1.5–1.6, 2.4–2.6, and 2.8–3.1 ka, respectively. Tsunami sand beds of the 1741 AD and circa 12th century events are recognized in the Hiyama District of Hokkaido Island, but the older tsunami deposits are missing. The deposits of these two tsunamis are found together at the same sites and distributed in regions where wave heights of the 1993 tsunami (Hokkaido Nansei-oki earthquake, Mw = 7.7) were less than 3 m. Thus, the 12th century tsunami waves were possibly generated near the south of Okushiri Island, whereas the 1993 tsunami was generated towards the north of the island. The estimated recurrence intervals of paleotsunamis, 200–1100 years with an average of 500 years, likely represents the recurrence interval of large earthquakes which would have occurred along several active faults offshore of southwest Hokkaido.

Pollen and diatom analysis were conducted on sediments in a fossil gully at Schmidt Peninsula, northwest Sakhalin and in a tectonic basin at a western area of central Sakhalin. Between 8, 500 and 7, 400 yBP a taiga dominated with Larix gmelinii and Pinus pumila developed at the Schmidt Peninsula under colder and drier climate than that at present. The paleoenvironment was marine and marine to blackish conditions. After the Betula forest period between 7, 400 and 6, 000 yBP, the present taiga, which was composed of dominant Picea jezoensis, coexisted with Larix and Pinus which developed since 6, 000 yBP under the same climate as that at present. At a western area of central Sakhalin, taiga composed of Picea, Larix and Pinus was distributed under colder and drier climatic conditions than that at present during the interstadial since 37, 000 yBP. In early Holocene, Larix-Pins taiga changed to Picea-dominated taiga due to a recovery of the climate. At around 6, 000 yBP the present taiga was established under the same climate as that at present. Larix, which is a good indicator of a cold and dry climate, formed a taiga together with Picea and Pinus in northern and eastern Hokkaido during LGM. During the migration north since Lateglacial, Larix formed taiga in Sakhalin. However, it decreased in the western area of central Sakhalin during early Holocene, and also decreased in the northwest at around 7, 400 yBP.