We present a three-dimensional geological model that reveals the detailed geological structures beneath Tsukuba City and the surrounding area in Ibaraki Prefecture, Japan. This model was created using numerous borehole logs and is based on the stratigraphic framework of Sakata et al. (2018). It shows the distribution of a buried valley filled by the lower part of the Kioroshi Formation (MIS 5e). As the valley-fill deposits consist mainly of soft muddy sediments with low N-values, a detailed understanding of the deposits is required for geological risk assessment. The model enables an intuitive understanding of the subsurface geological structures and can help non-expert stakeholders, including citizens and government officials, understand geological and geotechnical issues. In addition, as the model is provided in a digital format, it is compatible with urban digital transformation including smart cities, and will promote the social implementation of geological and geotechnical data.

A drilling survey (GS-ND-2) was carried out at Yamazaki, Noda City, Chiba Prefecture, to investigate lateral changes in the sedimentary facies of the Kiyokawa Formation, and the different facies and physical properties of the Kiyokawa and lower Kioroshi formations in the central Kanto Plain. The sedimentary facies of the Kiyokawa Formation in core GS-ND-2 transition from terrestrial through marine and back to terrestrial facies, in ascending order. Similar facies changes were reported in core GS-KW-1 (Kashiwa City, south of Noda City); however, the facies and pollen zone boundaries of the two cores are offset, suggesting that marine retreat occurred later in the south of the study area. The N-values from standard penetration tests of the mud beds and the pollen assemblages in the Kiyokawa and lower Kioroshi formations are different, which allows them to be distinguished. It is important for sustainable groundwater use and the assessment of seismic hazard to better understand the changes in facies in the Kiyokawa Formation and the distribution of soft mud beds in the lower Kioroshi Formation.

Sedimentary rocks containing molluscan and cetacean fossils occur along the Narashi–Moto Coast in Iwado, Muroto City, Kochi Prefecture, southwestern part of Shikoku Island, Japan. Here we describe the Narashi Formation of the Tonohama Group and estimate its depositional age based on planktonic foraminifera and calcareous nannofossil assemblages. The depositional age indicated by planktonic foraminifera is 8.58–4.37 Ma, and the age inferred from the calcareous nannofossil assemblage is 5.53–3.82 Ma. Therefore, the Narashi Formation is estimated to have been deposited at 5.53–4.37 Ma. The formation contains a pectinid-dominated molluscan association (i.e., the Amussiopecten–Mimachlamys–Amusium association). Based on the extant species present in the association, we estimate that the formation was deposited at depths shallower than 20–30 m. This association includes several characteristic species of the Kakegawa fauna, a Pliocene to early Pleistocene molluscan fauna on the Pacific side of southwest Japan. In contrast, the assemblage does not contain characteristic species of the Zushi fauna, which is a warm-marine molluscan fauna of the late Miocene to early Pliocene. The results suggest the Kakegawa fauna became established in Shikoku before 4.37 Ma.

A fragmentary hind wing of a fossil Cicadidae was obtained from Lower Miocene lacustrine deposits of the Masaragawa Formation in Seki, Sado City, Niigata Prefecture. Based on unique characteristics, such as the quite short apical cell 1 and partial preservation of small wing, the specimen is identified as Cicadidae gen. et sp. indet. The fossil does not having close relatives in modern Japan or East Asia. This occurrence is the oldest fossil record of a cicada in Japan.

Geological mapping and paleostress analysis were carried out in the coastal area around the border between Hyogo and Tottori prefectures in southwest Japan. The study area comprises volcanic rocks of the lower Miocene Yoka Formation of the Hokutan Group and the Kawabara Volcanic Member of the Tottori Group. The distribution of these rocks indicates they were emplaced into a >10-km length, E–W-trending basin that formed in response to normal faulting. The orientations of 63 syn-volcanic dikes were measured. Paleostress analysis of the dyke orientations indicates that the study area was under an extensional stress regime at the time of intrusion, consistent with previously reported paleostress data from the Hokutan Basin.

A geological field survey and zircon U–Pb dating were conducted for the volcanic and pyroclastic rocks of the Middle Miocene Hinokigawa Formation, distributed widely in the western Shimokita Peninsula, to re-evaluate the chronostratigraphy of the formation in the Hinokigawa, Otokogawa, Takiyama, and Ushitaki basins. A U–Pb age of 13.4 Ma was obtained from obsidian in the Hinokigawa basin, which is consistent with the previously suggested Middle Miocene age. A fan-shaped depression structure was observed in the Ushitaki basin around Hotokegaura and is filled with the newly defined Hotokegaura Tuff and Fukuura Rhyolite Lava. A caldera-collapse breccia occurs at the boundary between the lava and basement rocks and is interpreted as syn-caldera-forming ejecta. The Hotokegaura Tuff and Fukuura Rhyolite Lava that infill the newly defined “Hotokegaura Caldera” yield zircon U–Pb ages of 4.5 and 4.4 Ma, respectively, and the Nuidoishiyama Intrusive Body in the east of the caldera yields an age of 4.7 Ma. The Maruyama Rhyolite Lava, which occurs near the southeastern boundary of the caldera, yields a U–Pb age of 4.0 Ma. Therefore, the main activity of the Hotokegaura Caldera occurred during the Early Pliocene. In contrast, a U–Pb age of 7.6 Ma (Late Miocene) was obtained for the Ushitaki Tuff in the south of the Hotokegaura Caldera.

We review the timing, mode, and topographic development of the Ryukyu Arc from the viewpoint of Quaternary crustal deformation. We also discuss the uplift and subsidence trends of the island region of the Ryukyu Arc, and the timing and mechanism of formation of depressions (Tokara Tectonic Strait, Kerama Gap, and Yonaguni Depression) that divide the island region of the Ryukyu Arc. The strikes of normal faults in the Tokara Tectonic Strait and Kerama Gap differ from those in the Yonaguni Depression. The timing of uplift and subsidence is spatially variable throughout the island region of the Ryukyu Arc. Future work will focus on neotectonics, geomorphic processes and landform development in the context of neotectonics, and the ongoing tectonic evolution of the island arc.

In recent years, surveys and studies have been conducted on active faults that have been active since the late Pleistocene and inactive faults that have not been active since at least the late Pleistocene, respectively, in order to develop fault activity evaluation methods for fault zones by using fault rocks. This study analyzed and compared microstructures in fault gouges and slip surfaces of an active fault and an inactive fault at Watarigawa, Ato-Ikumo-Higashibun, Yamaguchi City, Yamaguchi Prefecture. We also examined the constituent minerals in a hydrothermal clay vein by using polarized light microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM). The active fault analyzed in this study is called the Chomonkyo Fault by previous studies, by the way. The polarized light microscopy analysis revealed gouge fragments (i.e., reworked fault gouge) are found in the fault gouge of the active fault, but not in that of the inactive fault. The SEM and STEM analyses revealed that the latest slip surface of the inactive fault is covered with randomly oriented illite crystals and crossed by clusters of barite crystals. This indicates that the inactive fault has not moved since the illite and barite were formed. In contrast, the latest slip surface of the active fault does not exhibit these features.

The Ashizawa Formation of the Futaba Group in the Iwaki area, northeast Japan, is considered to be early to middle Coniacian in age. Three inoceramid species, Mytiloides incertus (Jimbo, 1894), Cremnoceramus waltersdorfensis waltersdorfensis (Andert, 1911), and “Cremnoceramus rotundatus” (sensu Matsumoto and Noda, 1985), have been newly discovered in this formation. These fossils are indicative of a late Turonian age, and thus deposition of the Futaba Group probably started earlier than previously thought.

Shear planes are formed in various orientations within fault fracture zones. The sense of shear on each shear plane can be determined from the composite planar fabrics developed around it. However, it is not possible to distinguish whether all the shear planes in a fault fracture zone were formed during the same stage of motion by observations alone, because of the scarcity of chronological data. As such, we attempted to determine the kinematic and stress histories of the Shionohira and Kuruma faults by using both observations of composite planar fabrics and stress inversion analysis. As a result, we identified five stages of motion on the Shionohira Fault and two stages of motion on the Kuruma Fault. The chronological data are not sufficient to constrain the timing of the stages, but the reconstructed histories are consistent with the paleostress fields and tectonic activity around these faults, as determined in previous studies. Although the reconstructed stress history depends on the density of fault-slip data from the measurement area, this method is effective for investigating the formation mechanisms of fault fracture zones.

Low-frequency earthquakes (LFEs) are anomalous earthquakes with a lower predominant frequency than that expected from the earthquake magnitude. LFEs are also unique with respect to their anomalously deep focal depths, where ordinary earthquakes do not occur. Due to these characteristics, the generation mechanisms of LFEs have attracted seismological, lithological, and geochemical attention. Here, we review the observation and their interpretation of LFEs. Most LFEs worldwide occur near active volcanoes, although some LFEs in Japan are detected far from such volcanoes on account of the high sensitivity of the seismic network. The focal mechanisms of LFEs include double-couple that represent fault slip, isotropic and compensated linear vector dipole (CLVD) that suggest a volume change in the source. Tomographic studies have shown that the source areas of LFEs are characterized by low velocity and high V_P/V_S ratios, suggesting the contribution of geofluids to LFE occurrence. Recent observations in Japan have revealed the occurrence of LFEs at shallow depths, even in the upper crust, sometimes located close to ordinary earthquakes. These observations and the variation in focal mechanisms support an LFEs’ source model of tensile-shear crack. LFEs can occur in the upper crust if the pore fluids are close to H₂O rather than magma. The focal mechanism is double-couple if shear motion is dominant but is characterized by isotropic or CLVD components if crack opening is dominant, with the different focal mechanisms probably reflecting the pressure of pore fluids.

The late Heian period stone statues at Anraku-ju-in Temple in Kyoto City are made of tuff that is believed to be sourced from Mt. Hiyama, Sanuki City, Kagawa Prefecture. The authors conducted lithological observations and magnetic susceptibility measurements at tuff quarry sites in the Sanuki Group to compare the stone with the stone of the statues at Anrakuju-in Temple. The results show that the stone statutes of Anrakuju-in Temple were not quarried from tuff at the known quarry sites. Therefore, the stone was sourced from unknown quarries in Kagawa Prefecture or quarries outside of Kagawa Prefecture.

Peculiar mudstones beds containing sandstone breccia or blocks of various size named the Tako brecciastone-bearing mudstone layer, called “Sarashikubi beds”, occur in the coastal area of southern Kii Peninsula, southwest Japan. The formation of the Tako brecciastone-bearing mudstone layer has been ascribed to a mud volcano or a submarine debris flow. The beds are subdivided into three units: “the Ozarashi”, “the Tomiyama conglomerates”, and “the Kozarashi” in stratigraphic ascending order. We collected samples four types of sandstone breccia, one gravelly sandstone block from the Ozarashi, and one sandstone from the Tomiyama conglomerates. Modal composition analyses and detrital zircon U–Pb ages dating were conducted by LA–ICP–MS. Sandstone breccias from “the Ozarashi” and sandstone from the Tomiyama conglomerates are feldspathic wacke to arenite, and their composition is consistent with that of the Muro Accretionary Sequence (AS) that underlies the Tako brecciastone-bearing mudstone layer. In contrast, the gravelly sandstone block is quartz-rich lithic arenite. The youngest cluster of detrital zircon U–Pb ages from the sandstone breccias, spans the range of 32–62 Ma. Based on the compositional data and U–Pb ages, the breccia clasts are inferred to have been derived from the Muro AS. The sample of gravelly sandstone, which is thought to be foreshore deposit, yielded a detrital zircon U–Pb age of 27 Ma (Late Oligocene), younger than the Muro AS (Early Oligocene). This result ruled out the possibility that the Tako brecciastone-bearing mudstone layer was formed via mud volcano. Characteristic features of bedded formation are observed in “the Ozarashi”. It is inferred that “the Ozarashi” was transported by a submarine debris flow. It is likely that the Tako brecciastone-bearing mudstone layer comprises rock-fall deposits and shallow marine sediments. These deposits are presumed to have originated from the activity of thrust faults cutting the Muro Accretionary Sequence in a forearc basin.

In this study, using borehole logs and microtremor data, we demonstrate that the relationship between the shallow-subsurface (to maximum depths of a few decameters) properties in a valley-bottom lowland and the surrounding upland varies in the along-stream direction. Borehole logs were collected to assess geological structures, and microtremor array observations were used to model S-wave velocity structures and obtain ground-motion characteristics along survey lines across each valley of the Kanda (Zenpukuji) and Furukawa (Shibuya) rivers in the Musashino Upland, Tokyo, central Japan. Microtremor data reveal that the average S-wave velocity in the valley-bottom lowlands of the downstream area is generally low. This low value is ascribed to the occurrence of thick, soft, muddy sediments with S-wave velocities of <150 m/s beneath the lowlands. Particularly, in areas where soft sediments are ~15 m thick, marked peaks occur at low frequencies of 1.5–1.6 Hz in the microtremor H/V spectra. These areas correspond to areas that sustained severe building/house damage during the 1923 Kanto Earthquake. In contrast, the average S-wave velocity in the valley-bottom lowlands of the middle to upper reaches of the rivers is generally high. The gravel beds of Pleistocene terrace deposits have suffered minimal erosion from small rivers, meaning that these rivers flow on gravel beds, and soft sediments are lacking or very thin beneath these reaches. Therefore, the valley-bottom lowlands in the middle and upper reaches are characterized by hard ground. In contrast, the surrounding uplands are composed of volcanic-ash soil of the Kanto Loam and the muddy Tokyo Formation, which form softer ground compared with the valley-bottom lowlands. Therefore, it is important to accurately document the change in thickness of soft deposits in the along-stream direction beneath the valley-bottom lowlands and the lithologic and physical properties of the strata in the surrounding upland area for robust seismic-hazard assessment.

The Neogene lithostratigraphy in the Memanbetsu area, eastern Hokkaido, Japan, has been studied since the 1950s. However, stratigraphic reconstructions are hampered by a lack of age data. In this study, the fission-track (FT) and uranium–lead (U–Pb) ages of zircon grains were obtained via LA-ICP-MS for three tuff or tuffaceous sandstone samples from the Memanbetsu and adjacent areas. Considering previous study of lithostratigraphic correlations, the samples were collected from the lower Miocene Tokoro Formation (ID No. 2089), the lower part of the middle Miocene Mito Formation (ID No. 19091812-1), and the Pliocene Misaki Formation (ID No. 19061601). The weighted mean U–Pb and pooled FT ages of the youngest zircon grains were 11.9±0.2 Ma (2σ) and 10.8±0.9 Ma (1σ) for ID No. 2089, 12.4±0.2 Ma (2σ) and 13.4±0.8 Ma (1σ) for ID No. 19091812-1, and 8.7±0.1 Ma (2σ) and 8.2±0.4 Ma (1σ) for ID No. 19061601, respectively. Therefore, the stratum formerly referred to as the Tokoro Formation is equivalent in age to the upper Miocene Notoro Formation. In addition, on the basis of its lithological characteristics, geological continuity with adjacent areas, and previously reported ages from dinoflagellate cysts, the stratum is attributed to the Toika Formation. The ages obtained from the lower part of the Mito Formation correspond to the middle–upper Miocene Abashiri Formation, which supports the existing stratigraphic correlations. The ages obtained from the Misaki Formation suggest deposition during the late Miocene. Based on the radiometric ages, lithological characteristics, and geological distribution, the stratum is attributed to the Abashiri Formation. The results enabled us to re-evaluate the Neogene stratigraphy in the Memanbetsu area.