Blocks of garnet-glaucophane-quartz schist and gneissic garnet-actinolite schsit were found as float adjacent to a serpentinite exposure on the Saganoseki Peninsula, Oita Prefecture. These metamorphic blocks are inferred to have been derived from a serpentinite-matrix mélange exposed in the area. Phengite K-Ar dating of the garnet-glaucophane-quartz schist yielded an age of 94.1±2.1 Ma, indicating that this rock belongs to the Sanbagawa metamorphic rocks or to another Cretaceous metamorphic unit. Pressure–temperature conditions estimated by phase-equilibrium modeling for the garnet-glaucophane-quartz schist are approximately 490 °C and 2.1 GPa. These data suggest that the distribution of blueschist–eclogite transitional facies in the Cretaceous metamorphic unit extends to the eastern margin of Kyushu.

This paper presents a comprehensive investigation of obsidian sources on the Oki Islands, Iki Island, and northwestern Kyushu. The study focuses on the geological occurrences and lithological characteristics of the obsidian, and the results are integrated with those of previous studies. Obsidian samples were analyzed quantitatively using WD-XRF, and a database was constructed, incorporating previously published data. Using this dataset, we identified 24 obsidian geochemical provenances across the study area: Oki Kumi 1, Oki Kumi 2, Oki Shirashima 1, Oki Kamo 1, Oki Kamo 2, Oki Sai 1, Oki Sai 2, Iki Hakozaki 1, Iki Kuki 1, Iki Hase 1, Kameura 1, Kameura 2, Shiiba-Kawa 1, Shiiba-Kawa 2, Koshidake 1, Koshidake 2, Kawatana-Osaki 1, Ushinodake 1, Ushinodake 2, Ushinodake 3, Yodohime-shrine 1, Hario-Nakamachi 1, Hario-Nakamachi 2, and Hario-Nakamachi 3. Notably, Oki Kumi 2, Hario-Nakamachi 2, and Hario-Nakamachi 3 are identified as new provenances in this study.

The Niigata Basin is a Neogene back-arc rift basin in the Northeast Japan Arc, situated offshore and onshore in Niigata Prefecture. Extensive seismic and well data have been acquired through hydrocarbon exploration since the 1890s; however, these industry data have not been synthesized at the basin scale, thereby hindering the establishment of a comprehensive tectonic model. We combined subsurface data, including recent offshore high-resolution 3D seismic surveys and onshore well logs, to investigate the structural architecture of the basin. The Niigata Basin began forming as a large (~60 km wide) graben between the Kosado Ridge and the Echigo Mountains before 15 Ma. Small (2–3 km wide) rift-related structures also formed within the Niigata Basin, although most of these were buried by syn-rift volcanic rocks. During the transitional stage (15.0–6.5 Ma), differential subsidence and sediment accumulation produced a thick sedimentary succession in the basin interior but thin to absent deposits on the basin-margin highs, defining the overall basin outline. Minor relief within the basin was infilled progressively by this sedimentation, yielding a relatively simple geometry that deepened toward the basin center. During the compressional stage (6.5 Ma–present, particularly after 3 Ma), the inversion of pre-existing normal faults led to the development of NNE–SSW-trending anticlines within the basin.

This model of the tectonic evolution necessitates a reinterpretation of the Minami–Nagaoka–Katakai gas field as an anticline rather than a fault-bounded horst. This revised understanding reduces the uncertainty in the prediction of closure geometry, particularly in the onshore region where seismic imaging remains limited. It also indicates that large-displacement reverse faults are essential for the Green Tuff petroleum system, as they provide the source–reservoir juxtaposition required for hydrocarbon migration from stratigraphically overlying source rocks into the reservoir.

To clarify the stratigraphy and age of the Plio–Pleistocene Osaka Group around the Seidan Mountains in southwestern Awajishima Island, we investigated the lithostratigraphy and tephrostratigraphy of the Osaka Group, reexamined the tephrostratigraphy of central Awajishima Island based on the chemical composition of volcanic glass, and constructed a tephrostratigraphic framework applicable to both the Seidan region and central Awajishima Island. A total of five volcanic ash layers were identified around the Seidan Mountains. We described the tephra layers exposed in central Awajishima and partially revised the tephrostratigraphy established in previous studies. The Shitoori, Kenjogaoka1, Maruyama, Kenjogaoka2, and Tsunokawa volcanic ash layers are distributed across the Seidan area, and the Tsunokawa volcanic ash layer is correlated with the SD187–188 volcanic ash layer in the Seidan Observation Well core. Based on lithofacies characteristics and correlations of volcanic ash layers, we assign the Atago Formation, which contains no crystalline schist gravel, to the Osaka Group around the Seidan Mountains. The age of the Atago Formation in the Seidan Mountains is constrained to the late Pliocene to early Pleistocene based on its stratigraphic relationship with widespread tephra layers.

We describe the lithology and petrography of sedimentary rocks and the geochemistry of detrital chromian spinel from the Sanjissenzawa Fossiliferous Sandstone Member of the Chikubetsu Formation in the Soeushinai area, Horokanai Town, northern Hokkaido, Japan. The sedimentary rocks consist of granule to boulder conglomerate and medium to very coarse sandstone with abundant macrofossils. They are composed of dominant serpentinite rock fragments and detrital chromian spinel, with rare detrital blue amphibole. All the clasts are considered to have been derived from the Kamuikotan belt. The conglomerate also contains pebbles to cobbles of microdiorite and microgabbro that possibly originated from the lower part of the Horokanai ophiolite or blocks in serpentinite melange. Detrital chromian spinels exhibit very high Cr^#, moderate Mg^#, and low TiO₂ contents. These features are similar to those of serpentinites in the Kamuikotan metamorphic rocks and the Horokanai ophiolite.

We established a high-resolution chronostratigraphy in order to reconstruct the depositional paleoenvironment of the Miocene Onnagawa Formation in Akita Prefecture, NE Japan, based on a laser ablation-ICP-mass spectrometry (LA–ICP–MS) zircon U–Pb dating of two tuff beds that occur within the formation in the Yashima Area. The lower and middle parts of the Onnagawa Formation yielded ages of 12.7±0.2 Ma and 10.8±0.3 Ma (2σ), respectively. Accordingly, the stratigraphic interval that was previously defined as the Onnagawa Formation in this area was re-classified into the Sugota and Onnagawa formations in ascending stratigraphic order. The boundary between these formations has been dated to 14.2–13.4 Ma. The updated chronology indicates a high sedimentation rate at 69–378 m/m.y. during the period 11.5–10.8 Ma.

The Middle Pleistocene Tanabu Formation crops out along the sea cliffs in the northern part of the Shimokita Peninsula, northern Japan. The formation consists of the Shiosaki and Ishimochinaya members, (in ascending stratigraphic order) and the present study investigated its depositional environments, formation processes, and sequence stratigraphy. Six major depositional facies were identified in the formation, which comprise deposits formed in coastal to shallow-marine environments. On the basis of lateral and vertical variations in the properties of the six depositional facies, along with identification of some distinct erosional surfaces, three depositional sequences (DS1–3; in ascending order) were identified. The depositional sequences are characterized primarily by lowstand incised valley fill and transgressive three-dimensional dune and gravelly spit deposits formed in coastal and shallow-marine environments. On the basis of the age framework of the formation and its correlation with marine O isotopic stages (MISs), DS1 was deposited in MIS12–11, and DS2 and DS3 were deposited in MIS10–9 and MIS8–7, respectively. The maximum water depths for DS1, DS2, and DS3 during their highstand stages are estimated to have been 40–130, 20–70, and <10 m, respectively, based on the set thickness of the trough cross-stratified beds of the three-dimensional dune deposits. Paleocurrent directions obtained from the three-dimensional dune and gravelly spit deposits, along with the correlation of DS1–3 with MISs 12–7, suggest that a strait may have developed in an area between the present-day Osorezan Mountains and Shiriyazaki. During highstand stages, oceanic currents flowed northwestward to northward through this strait. These currents are thought to have originated from the Tsugaru Warm Current and likely flowed from the present-day Mutsu Bay area to the Tsugaru Strait region.

Dikes and mineral veins are formed by the injection of crustal fluids into fractures. Repeated influx of fluid with varying pressure results in the formation of those dilational fractures. Their orientations reflect the prevailing stress condition at the time of their formation. The orientations can be used to infer the stress regime and the maximum fluid pressure during fracturing. In this paper, we explain the driving pressure index (DPI) defined by Faye et al. (2018). The DPI represents the driving pressure ratios of the overpressured geofluids that formed a swarm of dilational fractures, and is a conservative and approximate estimate of the highest driving fluid pressure ratio observed amongst multiple high-pressure fluid influx events. Several applications of the DPI in the interpretation of crustal fluid activity are also introduced.

Three debris avalanche deposits (DADs) distributed on the southwest foot of Bandai volcano (from bottom to top: the Okinajima, Iwane, and Furukan-non) were correlated with hummocks and volcanic edifices based on geological, geomorphological, and petrological analyses. The longitudinal distribution of hummocks changes distinctly at a distance of about 7 km from the source, indicating formation by multiple debris avalanches. Hummocks within 7 km are subdivided into Groups A and B according to their distribution and size. The petrological characteristics of the Okinajima DAD (46 ka), Iwane DAD, Group A, and Group B samples closely resemble those of the younger edifice of Bandai volcano. Stratigraphic and petrological correlations indicate that hummocks distributed beyond 7 km correspond to the Okinajima DAD, and Group A within 7 km corresponds to the Iwane DAD. Group B could not be correlated with any known debris avalanche deposit. The Furukan-non DAD (18~17 ka) contains rocks with higher FeO*/MgO ratios and abundant plagioclase phenocrysts, which are consistent with those of the younger edifice of Nekoma volcano that had already ceased volcanic activity. However, no corresponding hummocks are observed. These findings demonstrate that petrological characterization of lava fragments within debris avalanche deposits is an effective tool for correlating deposits with their source volcanoes. The results also provide important insights into the collapse history of the Bandai and Nekoma volcanoes.

Stick–slip is a frictional slip that causes earthquakes on faults. It occurs when the friction on the fault decreases with slip velocity, and the elasticity of the system is sufficiently small. Although there have been many studies of these characteristics of fault rocks, there is still uncertainty about the elastic properties of subduction zones. The hanging wall of a subduction zone consists of sediment that has been lithified by the pressure and temperature at their depth of burial. The implication is that the elastic properties may depend on temperature, which would complicate mechanical models of seismogenic zones. The goal of this study is to examine exhumed paleo-seismogenic rocks of the Shimanto accretionary complex to elucidate the elastic properties of the hanging wall of the Nankai Trough subduction zone. The investigation focused on the physical properties and microstructure of sandstones and mudstones in the Shimanto complex, in an area with paleotemperatures of 150–320 °C, corresponding to the seismogenic zone in the Nankai Trough. The average porosity is 1.48% and the average modulus of elasticity is 55.12 GPa. These values are similar at all temperatures within the study area. The rocks are highly lithified. The grains of sandstone and mudstone are closely packed, and the grain boundaries have been deformed by dissolution–precipitation creep. The high degree of lithification probably resulted in the homogenous elastic properties of the hanging wall in this subduction zone.

Monazite crystals from the Hikami Granites associated with the Tsubonosawa Metamorphic Rock in the South Kitakami Belt were analyzed for U, Th, Pb, and rare earth element contents by electron microprobe analysis. Back-scattered electron imaging revealed that the crystals have cores with a high Si/(Th + U) ratio, which are surrounded and partially replaced by mantles with a low Si/(Th + U) ratio. The empirical formulae of the average core and mantle compositions are (Ce_0.445La_0.234Nd_0.156Y_0.027Pr_0.046Sm_0.021Gd_0.013Dy_0.005Er_0.001Th_0.034U_0.001Ca_0.018)_Σ1.001(P_0.981Si_0.019)_Σ1.000O₄ and (Ce_0.417La_0.218Nd_0.152Y_0.044Pr_0.043Sm_0.023Gd_0.017Dy_0.008Er_0.002Th_0.036U_0.003Ca_0.034)_Σ0.997(P_0.995Si_0.007)_Σ1.002O₄, respectively. The distinct chemical trends observed in the core and mantle suggest they formed under different conditions. Analytical data for the core and mantle show linear arrays on a PbO–ThO₂* (ThO₂* = ThO₂ plus the equivalent of UO₂) diagram and yield a single isochron corresponding to a CHIME age of 441 ± 33 Ma (2σ). The core and mantle are interpreted to have formed during Silurian magmatic and hydrothermal stages, respectively.

We reassessed the lithostratigraphy and spatial distribution of the Miocene Narisawa Formation and its associated extrusive and intrusive igneous rocks in the Sakazukiyama–Atagoyama area along the middle reaches of the Mamigasakigawa River, Yamagata City, northeast Japan. The Narisawa Formation consists mainly of gray to pale-gray pumiceous lapilli tuff that locally exhibits hyaloclastite facies. Field mapping, petrographic observations, and whole-rock X-ray fluorescence (XRF) analyses allowed us to redefine three volcanic rocks and several intrusive rock units in the Narisawa Formation: Sakazukiyama Lava, Atagoyama Lava, Atagoyama East Lava, and a suite of late-stage dark-gray dikes. The Atagoyama Lava occurs as an isolated rhyolitic lava dome, whereas the other two lava units form isolated rhyolitic domes (lava domes or cryptodomes) that occur within the outcrop area of the Narisawa Formation. The dark-gray dikes are andesitic to dacitic in composition and intrude both the basal pyroclastic facies of the Miocene Narisawa Formation and the base of the Atagoyama Lava, indicating the youngest magmatic activity in the study area. The presence of hyaloclastite facies in the pyroclastic rocks indicates that volcanic activity occurred in a submarine and sea-floor setting, and a subaqueous environment persisted from deposition of the Narisawa Formation through to emplacement of the coherent volcanic bodies. The volcanic sequence began with accumulation of the basal pyroclastic facies, followed by formation of the Sakazukiyama Lava, Atagoyama Lava, and Atagoyama East Lava, and then intrusion of the dark-gray dikes. K-Ar ages for all four of the coherent volcanic rocks cluster at ca. 12 Ma, suggesting they are nearly coeval with respect to the errors on the K-Ar dating (±0.3 Myr). These results provide a refined chronology and depositional model for the Sakazukiyama–Atagoyama area and advance our understanding of the Miocene subaqueous (submarine) volcanism in northern Japan.

Zircon U–Pb dating was carried out on the Asagara Formation (felsic volcaniclastic rocks and a biotite tonalite) along the Median Tectonic Line in the Asagara area of Mie Prefecture, eastern Kii Peninsula, Japan. The stratigraphy of the Asagara Formation was reexamined and the following stratigraphic units were identified (from the base upward): Matsubadani Rhyolitic Tuff Member, Kamide Alternating Beds of the Rhyolitic Tuffaceous Sandstone and Mudstone Member, Fudodani Rhyolitic Tuff Breccia Member, and Nakade Rhyolitic Tuff Member. The lower Matsubadani Rhyolitic Tuff Member yielded a major age cluster at 96.1 ± 0.9 Ma and the upper Nakade Rhyolitic Tuff Member yielded a weighted-mean age of 91.7 ± 1.3 Ma. Based on these ages, the Asagara Formation is correlated to the Sennan Rhyolites that are distributed from the western Kii Peninsula to Awaji Island. However, the associated biotite tonalite yielded a weighted-mean age of 79.9 ± 8.9 Ma, which is similar to a minor age cluster of ca. 78 Ma for the Matsubadani Rhyolitic Tuff Member. We propose that weak contact metamorphism and hydrothermal alteration affected the Matsubadani Rhyolitic Tuff Member at ca. 78 Ma.

We present a detailed three-dimensional geological model of the shallow structure (depth < 30 m) in Hokkaido, Japan. To construct the model, we used borehole data and geological descriptions, along with the N-value for the ground strength. We enhanced the model using geological and geophysical constraints, such as those from exploration geophysics. Our approach yielded geologically plausible data for the average shear-wave velocity in the upper 30 m (AVS30) that reflect the actual geological structure, which are essential for detailed strong ground motion predictions. The model and the predicted strong ground motion data will enable government and local stakeholders to undertake accurate disaster mitigation.