Northeast Japan Research Articles

Spatiotemporal distributions of afterslip and locking on the plate interface associated with the 2011 Tohoku-Oki earthquake using a 3-D temperature and strain rate-dependent heterogeneous viscosity model

We introduced a novel approach to produce a 3D heterogeneous viscosity distribution model of the upper mantle below Northeast Japan. The viscoelastic relaxation constitutes a significant portion of the postseismic deformation associated with a megathrust earthquake. Our viscosity distribution was based on a 3D simulation of thermomechanical modeling with realistic 3D oceanic plate geometry and subduction velocity of the Pacific plate in the upper mantle. After obtaining the thermal and strain-rate distributions obtained from the thermomechanical modeling, 3D viscosity distribution was calculated. We used the resulting viscosity distribution in a viscoelastic relaxation model to calculate the viscoelastic response associated with the MW 9.0 2011 Tohoku-Oki earthquake, using a finite element method. We found it necessary to include an elastic cold nose in the mantle wedge and a low-viscosity layer at the subducted lithosphere-asthenosphere boundary to reproduce the observed onshore uplifts and landward horizontal displacements of the seafloor, respectively. We then divided the calculated viscoelastic displacements into 7 one-year intervals and subtracted them from the corresponding observed postseismic displacements to obtain yearly residual displacements. Assuming the residual displacements were the result of afterslip and subduction fault locking on the plate interface, we employed a time-dependent inversion to estimate the spatiotemporal distributions of afterslip and subduction fault locking. Our results showed decrease in the afterslip amount and extent over time, which primarily occurred during the first two years following the 2011 event, which is followed by gradual extension of the subduction fault locking region. We concluded that the plate interface off Northeast Japan is gradually entering an interseismic stage of interplate coupling and elastic strain accumulation within several years after the occurrence of the Tohoku-Oki earthquake.

Pliocene zircon U-Pb ages from tuff in the Hotokegaura area of Shimokita Peninsula, northeast Japan

Pliocene zircon U-Pb ages from tuff in the Hotokegaura area of Shimokita Peninsula, northeast Japan

Magmatic evolution and metal systematics of back-arc volcanic rocks of north–east Japan and implications for deposition of massive sulphide Kuroko ores

The Hokuroku region of north-eastern Japan is endowed with important volcanic-hosted massive sulphide Zn–Pb–Cu deposits, which are considered the archetype of Kuroko (black ore) deposits worldwide. The bimodal, felsic-dominated volcanic succession that hosts the ore was deposited in a continental rift formed during continental extension in the final stages of the Miocene back-arc opening that led to the formation of the Japan Sea. In this study, we define some of the fundamental intensive parameters of this volcanism (temperature, pressure of crystallisation, fluid saturation, fO2) based on rock textures, and analyses of whole-rock samples, minerals and melt inclusions. Based on the melt inclusion analyses, we assess the behaviour of metals during magma evolution and degassing, and evaluate the possible implications for ore deposition. Plagioclase-melt geothermometry in felsic tuff and lava samples collected from both the units underlying and overlying the Kuroko indicates temperatures of 880–940 °C, and Fe–Ti oxide equilibrium indicates oxygen fugacity of ca. FMQ + 1.5. Melt inclusions have high-SiO2 rhyolite compositions (> 75 wt%, on an anhydrous basis), and the plot of normative mineral compositions in the granitic triplot indicates low pressure of magma stalling and crystallisation (< 1 kbar) at cotectic compositions. Melt inclusion metal contents plotted vs incompatible element Y suggest contrasting behaviour of different metals during fractionation and degassing. Zinc was mostly retained in the melt during crystallisation, whereas other metals, such as Pb, Cu, Sn and Mo, were released to an exsolving fluid phase. The latter may have thus been transferred to the hydrothermal system from a degassing magma. Shallow storage of relatively hot magma would have induced vigorous hydrothermal circulation on the seafloor, a precondition for ore deposition.

Tectonic stress fields inferred from long-term CMT data ranging over different periods

SUMMARYWe have developed an inversion method for estimating 3-D tectonic stress fields from centroid moment tensor (CMT) data of seismic events. To obtain reliable inversion analysis results, we need to use as much data as possible, ranging over different periods. In this study, we improved the previously developed CMT data inversion method to incorporate preceding inversion analysis results into the present analysis as direct prior information about the stress field. The spatial distribution of seismic events and/or their focal mechanisms in a region would change from one period to another, reflecting temporal stress changes there. In the improved inversion method, in addition to the hyperparameter controlling the roughness of stress fields, we introduced another hyperparameter that controls the relative weight of direct prior information to the observed data to be analysed. The relative weights of direct and indirect prior information to the observed data are appropriately determined by using Akaike's Bayesian Information Criterion. We applied the improved inversion method to the observed CMT data of seismic events of 24 yr (1997−2020) in northeast Japan, where the Tohoku-oki megathrust event (Mw 9.0) occurred on 11 March 2011 at the North American–Pacific Plate boundary. To investigate temporal changes in tectonic stress orientation, we divided the target period into the inter-, pre- and post-seismic periods of the megathrust event: P1 (January 1997−January 2007), P2 (February 2007−February 2011), and P3 (March 2011−December 2020). According to successive inversion analysis results for P1 and P2, we confirmed that the tectonic stress orientation was stable for 14 yr before the megathrust event. On the other hand, successive inversion analysis results for P2 and P3 indicated a possibility that the 2011 megathrust event caused significant changes in tectonic stress orientation in three regions. However, two of them were negative in the quantitative evaluation of the reliability of detected temporal changes. In conclusion, the stress orientation change in the region off Fukushima and Ibaraki, the southern margin of the main rupture area of the megathrust event, seems to be real.

Deterioration of an annual kelp Saccharina japonica forest and its effects on dominant herbivores, sea urchin Mesocentrotus nudus and abalone Haliotis discus hannai, in northeast Japan

A marine forest of the annual kelp Saccharina japonica was monitored at three depth sites (6, 9 and 12 m) along a 200-meter-long transect in the Pacific coast of northeastern Japan from 2014 to 2021, during which the abundance of the kelp plummeted, and the forest shifted to a crustose coralline bed. Colder seawater temperature in the winter/spring (February and March), which is regulated by the extension of the cold Oyashio Current, plays an important role in establishing the kelp forest through reducing the grazing of herbivores during the critical period of kelp germination and early growth. The Oyashio Current reached the region in 2014 and 2015 but not from 2016 to 2021, making a fundamental change in the kelp forest. Macroalgal abundance was strikingly different between these two periods. Biomass of S. japonica reached up to 38.6–45.4 kg-ww m−2 in the first two years, whereas macroalgal biomass was less than 3.2 kg-ww m−2 in the latter 6 years except for in 2016 when the biomass reached 9.0 kg-ww m−2 at the 6 m site. Gonad and somatic growth of sea urchin Mesocentrotus nudus declined during the kelp deforestation, while the density of the sea urchin moderately increased in 2017. Similarly, the density of abalone Haliotis discus hannai plummeted by about 90% and the mean shell length decreased from 83 mm in 2014 to 31 mm in 2020. Under the ocean warming scheme, ecosystems depending on the primary production of S. japonica are expected to be altered in the future, and thus urging the need of environmental monitoring.

Population dynamics of Capitella aff. teleta (Polychaeta, Capitellidae) in Gamo Lagoon, northeastern Japan, during a series of restoration works following the 2011 Great East Japan Earthquake and tsunami

The population dynamics of the capitellid polychaete Capitella aff. teleta were studied in Gamo Lagoon, located in northeast Japan, for the subsequent 2 years from 2016, when a series of restoration works was conducted following the 2011 Great East Japan Earthquake and tsunami. Capitella aff. teleta was found to be widely distributed from the estuary side, where the levee was located, to the innermost part but was more abundant in the innermost part, which is rich in organic matter. In the lagoon, the daily maximum water level dropped from 2017 to 2018 during the reconstruction of a flow-conducting levee, which blocked water flow and isolated the inner part of the lagoon. Although the density decreased drastically for approximately 11 months under diurnal hypoxia and strongly reducing conditions, small-sized new recruits were observed and the population recovered quickly after the daily maximum water level increased. In Gamo Lagoon, C. aff. teleta inhabiting the innermost part and estuary side of the lagoon contributed to maintaining the population by dispersing planktonic larvae between them. Thus, the maximum water level had a significant effect on the maintenance of the C. aff. teleta population in the lagoon, and sufficiently high water levels enable the dispersion of planktonic larvae to help recover the population quickly, suggesting that it is important to keep the water area connected.

ANALYSIS OF THE FORMATION OF THE SUBSYSTEM OF INTERNATIONAL RELATIONS IN NORTHEAST ASIA FROM THE POINT OF VIEW OF THE CIVILIZATIONAL APPROACH PART 2

The purpose and objectives. The purpose of the article is to analyze the formation of the subsystem of international relations in Northeast Asia from the point of view of the civilizational approach, and the role of such countries as China, Japan, Mongolia and Korea in this process. Methodological approach. The methodology of L.N. Gumilev is the main one in the work, as well as the systematic approach developed in Oriental political science by A.D. Voskresensky, general scientific methods of cognition and the comparative historical method are also used. Results and conclusions. The article offers an examination of the ideology and model of conducting foreign policy of individual countries of Northeast Asia, primarily China, Japan, Mongolia and Korea, from the point of view of a civilizational approach in the context of the formation of a new order and a new subsystem of international relations in Northeast Asia. Originality and value. The study is valuable in view of its relevance and originality of the work, which focuses on obtaining new knowledge about the ideology and model of conducting foreign policy of individual countries of Northeast Asia, primarily China, Japan, Mongolia and Korea, the study of contradictions between ethnic and non-ethnic systems of these states, determining their role in the emergence of a new, the change of the Mongol-Chinese-centric order, and a new stage in the history of international relations in Northeast Asia.

Multitemporal UAV surveys of geomorphological changes caused by postfire heavy rain in Kamaishi city, northeast Japan

Unmanned aerial vehicles (UAVs) are widely utilized for surveying various sediment disasters, and the high-resolution imaging they provide reveals detailed topographic characteristics. However, the availability of multitemporal UAV observations, which reveal geomorphological changes before and after the disaster, is still limited. In this research, we performed multitemporal UAV observations for geomorphological changes before and after extreme postfire heavy rain in the Ozaki Peninsula wildfire area in Japan. Significant geomorphological changes were observed associated with eight landslides and stream shifting, with a spatial resolution of 0.30 cm obtained by structure from motion (SfM) processing. To extract the surface changes based on multitemporal UAV observations, in addition to elevation displacement, vertex normal vectors were calculated for each mesh. We found that the elevation displacement showed comprehensive alterations in the entire area and that the changes in vertex normal vectors effectively captured the shape of these alterations. Deterioration, deposition, and scarping were shown for landslides, and the dynamics of erosion and deposition in the cross-section were for stream shifting. In addition, the detected geomorphological changes were compared with the predisaster topographic characteristics. Landslides occurred with a catchment area of 500 to 2,000 m2 and a slope inclination of 30 to 40°, and stream shifting occurred with a catchment area of 10,000 m2 or more and a slope inclination of less than 20°. In both regions affected by landslides and stream shifting, the logarithmic stream power index (logSPI), a natural logarithmic representation of the conventional stream power index (SPI), could describe the characteristics roughly. A logSPI value of 4 to 5 represented landslides, and 5 and greater represented stream shifting, with the probability of landslides and stream shifting occurring in areas with these values exceeding 99.5% and 99.9%, respectively. Therefore, the logSPI was utilized to roughly classify the terrain and geomorphological condition and the points among the unburnt, unlogged-burnt, and logged-burnt areas were compared. Even when the logSPI was similar, the damage was much more severe in the burnt areas. On slopes with a logSPI of approximately 4, there were signs of erosion in the logged-burnt area, and the increase in fallen trees was significant in the unlogged-burnt area, while there were no substantial changes in the unburnt area. In valleys with a logSPI of 8 or greater, sediment and woody debris were significantly deposited in the burnt areas following the postfire rain, while there were few changes in the unburnt area.

Development of Open Transport of Aqueous Fluid from Pegmatite Revealed by Trace Elements in Garnet

We investigated the fluid flow and elemental transport from a granitic body to the middle crust by determining the trace element compositions of garnet in pegmatites related to a quartz diorite intrusion and metamorphic rocks on Kinkasan Island, northeast Japan. Garnet in the pegmatites and biotite schists is characterized by spessartine– (Sps–) almandine- (Alm-) rich compositions of Sps14–69Alm22–70Prp2–14Grs1–13 and Sps16–30Alm54–66Prp9–16Grs3–6, respectively. A garnetite pod in the metamorphic unit has grossular- (Grs-) rich compositions (Sps1–4Alm8–11Prp0.1–0.4Grs80–87Adr3–4). The peak temperature ( T ) and pressure ( P ) conditions of the biotite schist during contact metamorphism were 600–650°C and 0.27–0.41 GPa, respectively. The primary fluid inclusions in quartz crystals within the pegmatites hosted by the quartz diorite and hosted by the metamorphic rocks have a wide range of homogenization temperatures (200–380°C). These correspond to the trapping temperature of 500–700°C, assuming a salinity of 4 wt.% NaClequivalent at pressure of the crystallization of the quartz diorite. Chondrite-normalized rare earth element (REE) patterns of garnets in the pegmatites in the quartz diorite and metamorphic unit are generally characterized by enrichment of heavy REEs and negative Eu anomalies with the REE contents in the schists which are systematically lower than in the pegmatites. However, garnet in the biotite schists close to the pegmatites has similar REE contents to garnet in the adjacent pegmatites. These geochemical features suggest that garnet in the biotite schists grew in response to fluid infiltration from the pegmatites. Besides, the Grs-rich garnet in the garnetite pod and its host quartz schist have flat heavy REE patterns and no Eu anomalies, which probably reflect a metasomatic process related to plagioclase replacement that produced Ca-Al-rich fluids. Our results suggest that the infiltration of pegmatitic fluids enhances elemental transport and metamorphic reactions in the middle crust.

Disaster, traces of displacement, and mizuaoi seeds

Curated by socio-cultural anthropologist Fuyubi Nakamura, the exhibition entitled A Future for Memory: Art and Life after the Great Japan Earthquake at the Museum of Anthropology at UBC in British Columbia addresses the sociocultural role of art produced in situ in the aftermath of the triple disaster which occurred in the Tōhoku region of northeast Japan in 2011. The exhibition’s curatorial project was born in the affected regions through anthropological research, and the selections of works brought to British Columbia are by The center for remembering 3.11; Lost &amp; Found Project; Lost Homes Scale Model Restoration Project; Chihiro Minato; Atsunobu Katagiri; Masao Okabe; Rias Ark Museum of Art; Tsunami Ladies film project team. This article engages with the conversations that the curator, artists, and collaborators wove through the exhibition. The construction of social memory building on the experiences of a drastically changing environment is its main theme.

Fine Structure of the Subducting Slab and the 2022 M 7.4 Fukushima–Oki Intraslab Earthquake

Abstract The 16 March 2022 M 7.4 Fukushima–Oki earthquake is the largest one among forearc intraslab earthquakes in Japan since 2000. These subcoast events can cause severe damage to the local society because of their proximity to inhabited areas. However, their generating mechanism is still not clear. Here, we present 3D high-resolution seismic tomography of the source zone of four large intraslab events (M ≥7.0) during 2003–2022 in northeast Japan, which is obtained by inverting high-quality arrival-time data recorded at both onshore and offshore seismic stations. Aftershocks of the subcoast intraslab earthquakes are mainly distributed in gaps of high-velocity bodies with high Poisson’s ratio and at the upper ∼20 km depth of the subducting Pacific slab. Our results indicate that the four large intraslab events were caused by rupturing of buried hydrated faults that formed at the outer rise and dehydration embrittlement on the fault planes.

Iron deposition during recovery from Late Devonian oceanic anoxia: Implications of the geochemistry of the Kawame ferromanganese deposit, Nedamo Belt, Northeast Japan

The Late Devonian, during which one of the “Big Five” Phanerozoic mass extinction events occurred, was one of the most important time intervals in Earth history. Nevertheless, the paucity of deep-sea records due to subduction has hampered elucidation of the pelagic environment during the Late Devonian in Panthalassa. However, ancient hydrothermal ferromanganese sediments, which were deposited on the abyssal seafloor and then accreted onto continental margins, are preserved as umber deposits and exposed in accretionary prisms. These sediments can provide key information to characterize the paleo-ocean. An Upper Devonian (Famennian) umber deposit is exposed in Kawame Quarry, Nedamo Belt, northeastern Japan. We investigated the bulk chemical composition and osmium isotopic ratio ( 187 Os/ 188 Os) of the Kawame umber to discuss the pelagic environment of the Panthalassa Ocean during the Late Devonian. The Kawame umber shows positive correlations between Fe content and the levels of P, V, Os, and rare earth elements, suggesting that hydrothermal Fe-oxyhydroxide particles adsorbed these elements from ambient seawater in the same way as hydrothermal ferromanganese sediments in the present-day ocean. Newly obtained initial 187 Os/ 188 Os values of Late Devonian seawater are 0.49–0.57, lower than that of present-day seawater (~1.06) and comparable to Paleogene and Late Cretaceous values. One of the most remarkable characteristics of the Kawame umber is its markedly high Fe levels (up to 90 wt%) compared to present-day seafloor hydrothermal ferromanganese sediments and younger umber deposits in Japanese accretionary complexes. Because seawater 187 Os/ 188 Os values were not notably different from those during the Paleogene and Late Cretaceous, anomalously vigorous hydrothermal activity in the Late Devonian is unlikely to have caused the increased Fe deposition. The rare earth element patterns of the Kawame umber (normalized to post-Archean average Australian shale) show weaker negative Ce anomalies than those of younger umbers and present-day hydrothermal ferromanganese sediments, suggesting that the Kawame umbers were deposited under less oxidizing conditions. Considering that many red beds were deposited at ~370 Ma, during recovery from the oceanic anoxic events (OAEs) of the Late Devonian (early Famennian), the anomalous Fe enrichment in the Kawame umber likely reflects enhanced oxidization and precipitation of abundant dissolved Fe during the post-OAE transition from anoxic to oxic conditions. • Geochemistry of Late Devonian Panthalassa was reconstructed from NE Japan. • Weakly negative Ce anomaly may reflect recovery from oceanic anoxia. • Reoxygenation of seawater caused Fe-enrichment in the Kawame umber. • Late Devonian seawater 187 Os/ 188 Os ratio was 0.47–0.57.

Revision of Early Spathian (Late Olenekian, Early Triassic) Ammonoids from the Osawa Formation at Akaushi in the Motoyoshi Area, South Kitakami Belt, Northeast Japan

A taxonomic revision of the ammonoid assemblage previously reported from the Osawa Formation at Akaushi in the Motoyoshi area, i.e., Columbites parisianus, Subcolumbites perrinismithi and Eophyllites cf. dieneri, leads to the conclusion that the fauna should be attributed to Hellenites tchernyschewiensis, H. inopinatus, Neocolumbites grammi, N. insignis, Procolumbites ussuriensis and P. subquadratus. In addition, this fauna also includes Deweveria kovalenkoi. Because these ammonoids are characteristic of the N. insignis Zone of South Primorye, Russian Far East, the lowest part of the Osawa Formation clearly correlates with the upper part of the lower Spathian (upper Olenekian, Lower Triassic), and the faunal similarity suggests that the South Kitakami Belt may have been located near South Primorye on the western side of the Panthalassa.

87Sr/86Sr Isotopic Ratio of Ferromanganese Crusts as a Record of Detrital Influx to the Western North Pacific Ocean

In this study, the Sr isotope ratios (IRs; 87Sr/86Sr) of ferromanganese (Fe–Mn) crusts are analyzed through laser ablation inductively coupled plasma multiple-collector mass spectrometry. A sample collected from off Minamitorishima Island showed uniform Sr IRs (0.70906–0.70927) similar to that of present-day seawater with more than 36 mm thickness. Meanwhile, a detritus-rich sample collected from off northeast (NE) Japan showed a wide variation in Sr IRs (0.707761–0.709963). The Sr IR variation in the Fe–Mn crust from off NE Japan suggests detrital influx contributions from both the NE Japan arc (&lt;0.708) and aeolian dust from China (&gt;0.718). Detrital flux from the NE Japan arc increases from the bottom to middle layers, possibly due to the uplift of the Ou backbone range that occurred after ~2 Ma. The increased influx of the aeolian dust in the outer layer is attributable to global cooling in the Quaternary that increased the loess dust transportation from China to the western North Pacific Ocean. Meanwhile, the influence of the detrital influx on the sample from off Minamitorishima Island appeared to be negligible. The Sr IR analysis with high spatial resolution proposed in this study possibly improves the burial history of Fe–Mn nodules.

Spatiotemporal distributions of interplate coupling in Tohoku, northeast Japan, for 14 years prior to the 2011 Tohoku-oki earthquake inverted from GNSS data

We extracted tectonic crustal deformation caused by the interplate coupling between the Pacific and North American plates from GNSS time series data at land-based stations in the Tohoku district, northeastern Japan, during the period from 21 March 1996 to 7 March 2011. Next, we estimated the spatiotemporal distributions of the slip deficit rates between the two plates prior to the 2011 Tohoku-oki earthquake for the 1997–2010 period by performing time-dependent inversion analysis. As a result, two strongly coupled regions were identified. One is located off the coast of Iwate Prefecture, whose reliability may not be high, where coseismic slip of the Tohoku-oki earthquake was small, indicating that this area may be a source region for a future megathrust earthquake. The other is a region with strong and continuous interplate coupling of more than 8 cm/yr from 1997 to 2010 in and around the epicenter of the Tohoku-oki earthquake. This area coincides with the area where coseismic slip greater than 20 m was estimated. Decrease of interplate coupling by approximately 0.4 cm/yr was identified at the downdip side of the region during the period from 2003 to 2008, reaching a coupling of 1.5 cm/yr in 2008, which would yield an increase in shear stress at its shallower strongly coupled region. We also identified that in 2010, the strongly coupled region rapidly expanded in southwestern and downdip directions. These results suggest that the shear stressing rate in the strongly coupled region might be accelerated, reaching rupture strength because of these two effects, thereby causing the main shock.

Umbilicaria esculenta Extract Exhibits Antiwrinkle Activity by Suppressing ErbB2 Phosphorylation.

Umbilicaria esculenta (UE), an edible lichen, is widespread in northeast Asian countries, including China, Japan, and Korea. In the present study, we examined the antiwrinkle activity of UE. We observed that the UE extract (UEE) suppressed ultraviolet (UV)-induced matrix metalloprotein-1 (MMP-1) expression and reactive oxygen species (ROS) generation in a human keratinocyte cell line (HaCaT) and human skin tissue. In addition, UEE reversed the UV-induced decrease in collagen in the human skin tissue. Excessive and chronic UV exposure is a key factor underlying skin wrinkle formation via MMP-1 expression. As treatment with UEE disrupted the UV-activated mitogen-activated protein kinase (MAPK) signaling pathway, we applied an antibody array to unveil the underlying mechanism of UEE. Interestingly, UEE treatment inhibited ErbB2 phosphorylation, but not epidermal growth factor receptor phosphorylation, a heterodimerization partner with ErbB2. Furthermore, UEE treatment enhanced UV-suppressed phosphatase activity via ROS suppression. Collectively, our findings indicate that UEE enhances ErbB2 dephosphorylation to suppress UV-induced MMP-1 expression.

Reexamination of a Permian Tentaculites-Like Fossil Iwakiella ichiroi Hatai, Kotaka and Noda, 1972, as an Orthocerid Cephalopod

The holotype of Iwakiella ichiroi Hatai, Kotaka and Noda, 1972, from the Permian Kashiwadaira Member of Northeast Japan is reexamined. Although previously placed within the Tentaculitoidea or retained in the Problematica, the possession of the septum and the spherical shape of the initial chamber suggest that this specimen belongs to sphaerorthoceratid orthocerid of the Cephalopoda. The present result confirms that the Tentaculitoidea probably became extinct near the Carboniferous – Permian boundary with Hidagaienites arcuatus Niko, 2000, as the latest representative.

Attenuation Characteristics of High-Frequency Ground Motions from Local Sources Caused by Great Subduction Zone Earthquakes in Northeast Japan

AbstractGround-motion attenuation characteristics are examined for the peak ground accelerations (PGAs) and peak ground velocities (PGVs) from strong-motion generation areas (SMGAs), which emit strong high-frequency waves during great subduction zone earthquakes. A conventional ground-motion prediction equation (GMPE) for earthquakes is designed based on the magnitude and distance from the source fault to predict the peak ground motion amplitude. For great subduction zone earthquakes, significant wavetrains of high-frequency ground motions are often separately observed in seismograms, and the corresponding rupture areas are estimated as SMGAs along the plate interface. In this case, although the advantages of using the shortest distances measured from the closest SMGAs rather than the shortest fault distance have been confirmed in previous studies, it is more physically reasonable to examine the ground-motion attenuation characteristics of individual SMGAs based on their magnitudes and locations. Therefore, we examine the attenuation characteristics of the PGAs and PGVs of individual SMGAs of the 2003 Mw 8.2 Tokachi–Oki earthquake and 2011 Mw 9.1 Tohoku earthquake in the Japan subduction zone considering data at stations within an SMGA distance of 300 km and the SMGA moment magnitude. The model, exhibiting the same functional form as a conventional GMPE, is fitted to the PGA and PGV data pertaining to each SMGA that are normalized to a bedrock site with VS30=760 m/s at any distance. The uncertainties in the obtained PGAs and PGVs are 3.6% and 13.5% lower, respectively, in terms of the residual in logarithmic units than those in the results of previous approaches considering only the SMGA distance. This result could help develop GMPEs for SMGAs to more appropriately predict the strong motions generated during great subduction zone earthquakes.

Ejecta stratigraphy and eruptive sequence of the Eruptive Episode M in Towada Volcano, Northeast Japan

Ejecta stratigraphy and eruptive sequence of the Eruptive Episode M in Towada Volcano, Northeast Japan

Geothermal geology and comprehensive temperature model based on surface and borehole geology in Sengan, Northeast Japan

Geological data were used to explore the geological structure and hydrothermal alteration distribution of the Sengan area, Northeast Japan. Acidic alteration and high-temperature hot springs surround several Quaternary volcanoes within the Sengan area. Quaternary intrusive granitic-dioritic rocks are distributed beneath the Kakkonda, Matsukawa, and Matsuo-Hachimantai geothermal fields. In particular, the Kakkonda granite, a heat source in the Kakkonda geothermal field, has a high temperature exceeding 500 °C inside the body. The temperature structure of the Sengan area was investigated using temperature logging data (WD-1a and 53 wells) collected as part of the temperature recovery test conducted in the geothermal wells. The temperature structure was estimated based on temperature extrapolation, a 2-D temperature distribution created with the thin-plate spline method, and a 3-D temperature distribution obtained by combining the horizontal 2-D temperature data. High-temperature zones above 200 °C are distributed at sea level in Nyuto (eastern Lake Tazawa), Kakkonda, Matsukawa, Matsuo-Hachimantai (Hachimantai), and Mt. Akita-Yakeyama, as predicted using the temperature structure estimation method. The granitic-dioritic magma penetrates the pre-Tertiary basement units and the Neogene beneath several fields to form a high-temperature zone in the shallow depth, contributing to the formation of geothermal systems in the Sengan area. However, the linear extrapolation of the subsurface temperature tended to over-estimate the temperature in the greater depth and was valid only at a depth shallower than 3 or 4 km below sea level. This study delineated several high-temperature zones exceeding 200 °C above sea level in the Sengan area.