西南日本から連続する中央構造線（MTL）は，群馬県西部下仁田地域を通過し，北側に領家帯の白亜紀花崗岩類や美濃・丹波帯（＝足尾帯）のジュラ紀付加体が，南側に三波川帯の白亜紀高圧型片岩が広く露出する．MTLのすぐ南側には，低角度断層を介して三波川変成岩の上に特異な岩石・地層群がクリッペとして累重する．「跡倉クリッペ」はペルム紀花崗岩やホルンフェルス，非変成の白亜系–古第三系礫岩・砂岩など，どれも少量だが多様な岩石から構成される．1950年代から研究されはじめたが，その起源は長らく謎であった．写真は，下仁田南方の南牧川沿いに露出する逆転した跡倉層の下部白亜系砂岩・泥岩交互層である．近年，同層を含む関東山地の白亜紀砂岩中の砕屑性ジルコンのU–Pb年代が測定され（中畑ほか, 2015, 2016），跡倉層には原生代（25-15億年前）粒子が大量に含まれることが判明した．同じ関東山地でも約10 km南方の秩父帯の同時代砂岩とは大きく異なる後背地をもつこと，また西南日本では飛騨帯と舞鶴帯にのみ知られるペルム紀花崗岩を随伴することから，跡倉クリッペは関東でできた岩石群ではなく，もともと中生代日本の大陸側に起源をもち，二次的に長距離（最大100 km程度）移動してきた岩体と推定される．こんにゃくとネギで有名な下仁田地域の地質には，日本列島のルーツに関する秘密が隠されている．地元を走る上信電鉄の車体に大きくペイントされた「跡倉クリッペ」の文字をみて嬉しくなるのは「撮り鉄」ファンだけではないだろう．

（写真・解説：磯﨑行雄）

 Low-frequency earthquakes (LFEs) occurring in the continental plate are reviewed. Most LFEs in the continental plate occur at depths of ∼15-45 km in the uppermost mantle to the lower crust beneath volcanoes, but they also occur within the same depth range beneath non-volcanic areas. Because they occur at greater depths than the typical depth limit for shallow regular earthquakes, they are called “deep low-frequency earthquakes (deep LFE).” However, a recent study reveals that LFEs also occur at depths shallower than 15 km in the upper crust where many regular earthquakes occur. This indicates that LFEs occur over the entire depth range from the uppermost mantle to the upper crust. In the upper crust, LFEs and regular earthquakes coexist and occur in close proximity. Focal mechanisms and activity patterns of LFEs show that tensile-shear crack is the dominant mechanism generating LFEs. In addition, the long duration of waveforms is probably caused by resonance in the fluid-filled crack. Distributions of peak frequency (fp) and frequency index (FI) values of waveforms, both of which are expected to be significantly small for LFEs and large for regular earthquakes, show that there is no clear boundary for fp and FI values between LFEs and regular earthquakes; rather, they are distributed continuously. It is presumed that the distribution of high and low pore fluid pressures in source faults creates such distributions of small and large fp and FI values, respectively, and a LFE occurs when the pore pressure is extremely high. This indicates that pore pressure is directly related also to the genesis of regular earthquakes. In source areas of recent large inland earthquakes, LFEs are activated by the mainshock, and FI and fp values and stress drop synchronously decrease immediately after the mainshock, gradually recovering thereafter. The activation of LFEs by the mainshock and such temporal changes of FI, fp, and stress drop after the mainshock can be explained within the framework of fault-valve behavior. Furthermore, fp and FI values tend to be small along prominent tectonic lines, such as the Itoigawa-Shizuoka tectonic line. It is inferred that pore fluid pressure is locally high along those tectonic lines, thereby facilitating the current crustal deformation.

 Ata welded ignimbrite (110 ka) lies beneath Ito non-welded ignimbrite and Osumi pumice fall deposit (30 ka) in the Kimotsuki Plain, southern Kyushu. Previous geomorphological studies of the Kimotsuki Plain focused on landform development after deposition of Ito ignimbrite. Landform development of Kimotsuki Plain since the last interglacial, especially until just before deposition of the Osumi pumice fall, is reconstructed using geological data collected from outcrop observations and borehole records. The basal-surface of the Osumi pumice fall deposit obtained shows that Ata welded ignimbrite had been dissected by the Kimotsuki River and its tributaries in response to the last glacial sea-level drop before the Osumi pumice fall was deposited. Longitudinal profiles along the Kushira River in 110 ka and 30 ka indicate recession of the Tanida waterfall, which formed at the edge of the Ata welded ignimbrite plateau. These profile changes imply that the Tanida waterfall retreated 2.4-6.0 km upstream between 110 ka and 30 ka. The Kushira formation, Marine oxygen Isotope Stages (MIS) 5e marine deposits under Ata welded ignimbrite, was found below the present sea-level at multiple locations in the Kimotsuki Plain. This vertical distribution of the Kushira formation indicates that the Kimotsuki Plain has been in a tectonically stable or subsidence area since the MIS 5e, in contrast with the Onejime and Natsui areas, which have been tectonically uplifting. The depositions of the two ignimbrites had significant impacts on filling the Paleo-Shibushi Bay (Sea) and the development of the Kimotsuki Plain under sea-level lowering during the last glacial period. The top-surface of basement rocks is more than −120 m below sea level at the floors of paleo valleys, even though it is adjacent to mountains composed of the basement. Further investigation of lower alluvium and its basal-surface is required for an understanding of valley incision and delta evolution of the Kimotsuki Lowland after deposition of the Ito pyroclastic flow.

 Communities and residents in Fukushima Prefecture have been adversely affected and jeopardized by radioactive contamination of the environment and associated socioeconomic reputational damage due to the TEPCO Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, which occurred immediately after the extremely large earthquake and tsunami on March 11, 2011. Therefore, how long radionuclides will persist in the regional environment is a major concern. The vertical profiles of radiocesium concentrations in sediments of Lake Inawashiro-ko in Fukushima Prefecture, 85 km west of the nuclear power plant are clarified, and decadal changes in future concentrations are predicted. Sedimentary cores, 17.0 to 40.5 cm in length, were obtained from 27 sites in the lake at water depths greater than 60 m. Lacustrine sediments consist of a black upper part (unit 1) and an olive-gray lower part (unit 2). These units, providing a stratigraphic record that covers the past 130 years, are mainly composed of clayey silt as background lake floor deposits with intercalations of the 2011 and 1888 event deposits. Radiocesium (¹³⁴Cs+¹³⁷Cs) inventory, derived from the FDNPP accident, in lacustrine sediment cores at 12 sites has a range between 39,000 and 93,000 Bq/m². These values are larger than that of the initial deposition (ca. 30,000 Bq/m²) of radiocesium on the ground around Lake Inawashiro-ko. The excess radiocesium was supplied from the river catchments feeding the lake. Together with sedimentation rates at the individual sites, the ¹³⁷Cs concentration for the 50 years after 2011 is predicted based on the exponential decay patterns of global fallouts of nuclear weapons tests in the 1960s, which were also recorded in the lacustrine sediments. Using this assumption, the concentration of ¹³⁷Cs in lake floor sediments in the 2060s is estimated to be from 159 to 815 Bq/kg at a site where ¹³⁷Cs of 4,000 to 10,866 Bq/kg was initially deposited. However, the predicted radiocesium concentration may be more persistent if the flux of radiocesium from the upper catchments increases, because the deposition of radionuclides and its impacts at the 2011 accident are larger than those in the 1960s.

 The effects of drought on vegetation activity at Chichi-jima, Ogasawara (Bonin) Islands from 2015 to 2018 were investigated based on remote sensing data. Chichi-jima suffered from severe periods of drought in 2016-2017 and 2018-2019. The Normalized Difference Vegetation Index (NDVI) was calculated using data from Band 4 (Red) and Band 5 (Near Infrared) of the Landsat-8 Operational Land Imager. Even at Chichi-jima, which is located in the subtropics, the NDVI exhibited seasonal variations. The range of the NDVI (maximum minus minimum) is approximately 0.11 when averaged at the watersheds of reservoirs supplying drinking water. During drought periods in September 2016 and 2018, the NDVI values decreased compared to those of September in the other years, which were less affected by drought. The decrease in the NDVI is approximately 0.04, which corresponds to approximately one-third of the seasonal variations in the NDVI. Although the absolute value of this decrease is subtle, it is relatively large considering seasonal variations. From a difference map of the NDVI between September 2016 (during the drought) and September 2015 (before the drought), it is observed that the decrease in the NDVI occurred across all of Chichi-jima. When comparing the vegetation map of Chichi-jima, the decrease in the NDVI did not occur in a specific species or region. This implies that the effects of drought appeared in the vegetation of Chichi-jima as a whole.

 On September 1, 1923, the Great Kanto earthquake struck Japan. Recently found testimony and documents are discussed that shed new light on the effects of the initial earthquake and tsunami along the Zushi-Kotsubo coastline at Sagami Bay, Kanagawa Prefecture. Mrs. Fuji Takashima (née Hirai), a resident of the Zushi-Kotsubo area when the earthquake and tsunami struck, provides first-hand evidence in her testimony. In addition, the artist Shiun provides a first-hand account of the earthquake in his artwork “Shin go tsunami shuurai (after the earthquake and tsunami struck).” According to testimony and documents, it is noted that the Zushi-Kotsubo coastline, with its small, quaint fishing villages, was changed greatly by the Great Kanto Earthquake. The first wave of the tsunami struck the southwest Kotsubo coastline five to six minutes after the earthquake occurred. The third wave was the largest. The tsunami traveled up the Kotsubo river channel, washing away many houses on its banks. A field survey indicates the tsunami was up to 12 m in height at the northwest beach and 5 m at the south beach. The tsunami that traveled up the Kotsubo River was more than 5.0 m in height. The earthquake also caused the land to uplift in the area by an average of 0.4 m, before gradually subsiding. Large-scale landslides occurred at northwest cape Iijima and south cape Oosaki.