A submersible study of the western intersection of the Mid-Atlantic ridge and Kane fracture zone (WMARK)     

Tivey  Maurice  Takeuchi  Akira  Scientific Party  WMARK 《Marine Geophysical Researches》1998,20(3):195-218

In 1994, a joint Japanese-American dive program utilizing the worlds deepest diving active research submersible (SHINKAI 6500) was carried out at the western ridge-transform intersection (RTI) of the Mid-Atlantic Ridge and Kane transform in the central North Atlantic Ocean. A total of 15 dives were completed along with surface-ship geophysical mapping of bathymetry, magnetic and gravity fields. Dives at the RTI traced the neovolcanic zone up to, and for a short distance (2.5 km) along, the Kane transform. At the RTI, the active trace of the transform is marked by a narrow valley (<50 m wide) that separates the recent lavas of the neovolcanic zone from the south wall of the transform. The south wall of the transform at the western RTI consists of a diabase section near its base between 5000 and 4600 m depth overlain by basaltic lavas, with no evidence of gabbro or deeper crustal rocks. The south wall is undergoing normal faulting with considerable strike-slip component. The lavas of the neovolcanic zone at the RTI are highly magnetized (17 A m^–1) compared to the lavas of the south wall (4 A m^–1), consistent with their age difference. The trace of the active transform changes eastwards into a prominent median ridge, which is composed of heavily sedimented and highly serpentinized peridotites. Submersible observations made from SHINKAI find that the western RTI of the Kane transform has a very different seafloor morphology and lithology compared to the eastern RTI. Large rounded massifs exposing lower crustal rocks are found on the inside corner of the eastern RTI whereas volcanic ridge and valley terrain with hooked ridges are found on the outside corner of the eastern RTI. The western RTI is much less asymmetric with both inside and outside corner crust showing a preponderance of volcanic terrain. The dominance of low-angle detachment faulting at the eastern RTI has resulted in a seafloor morphology and architecture that is diagnostic of the process whereas crust formed at the WMARK RTI must clearly be operating under a different set of conditions that suppresses the initiation of such faulting.  相似文献   

A complete introduction to the SCJ proposal and its commentary on the development of seismically resilient cities     

Akira Wada  Ikuo Towhata  Kazuo Tamura  Qu Zhe 《地震工程与工程振动(英文版)》2018,17(4):677-691

In modern society, the population, wealth and social functions are increasingly concentrated in a few large cities. Such concentration brings about efficiency while at the same time it increases the vulnerability of the society. To address this issue, the Science Council of Japan (SCJ) published a proposal entitled “Shape Cities and Societies Safer against Severe Earthquakes”. It makes a variety of suggestions for enhancing the seismic resilience of large cities, including: (1) comprehensive thinking based on the latest scientific knowledge and rich imagination; (2) selection of sites suitable for residence and social activities; (3) introducing the urban seismic coefficient; (4) promoting the enhancement of the seismic performance of buildings and civil structures; (5) easing the concentration of population and functions; (6) building communities that enable shelter and escape; (7) resilient technology of information and communication system and its effective utilization; (8) preparation and implementation of emergency response after earthquakes; (9) development and application of new structural seismic technology; (10) learning from domestic and foreign earthquake disasters and launching of international cooperation and knowledge sharing and (11) taking actions from a multidisciplinary perspective. The proposal is introduced in this paper.  相似文献   

Crustal structure and origin of the northeast Japan arc   总被引：1，自引：0，他引：1  

Koji  Minoura Akira  Hasegawa 《Island Arc》1992,1(1):2-15

Abstract Northeast Japan is a typical island arc region and its topographic arrangement reflects the geophysical characteristics of the island arc system. However, the structural style of the arc is very complicated and varied due to the repeated superposing of faults and folds on to earlier structures.
Geotectonic events that involved creation of the fundamental framework of the island arc crust occurred in east Asia in the Late Jurassic to Early Cretaceous and were probably induced by accretion and collision tectonics. The fragmentation and subsequent displacement of the crust took place during the Early Neogene in response to the terrane collision and the change in oceanic plate motion, leading to the opening of the Japan Sea. Huge amounts of volcano-sedimentary rocks buried the tilted fault blocks of pre-Tertiary basement with the development of the island arc.  相似文献   

Late Permian (Lopingian) foraminifers from the Doi Chiang Dao Limestone in the Inthanon Zone of Northern Thailand     

Akira MIYAHIGASHI  Katsumi UENO  Thasinee CHAROENTITIRAT 《地球学报》2009,30(Z1):40-43

Late Permian foraminifers were found from six localities belonging to the Doi Chiang Dao Limestone, which is considered as having a Paleo-Tethyan seamount-capping carbonates origin, distributed in the Inthanon Zone of Northern Thailand. Among them, three age-diagnostic assemblages are recognized. They are represented by the occurrences of Codonofusiella kwangsiana, Palaeofusulina cf. minima, and P. prisca, and are referable to the Wuchiapingian, early Changhsingian, and late Changhsingian, respectively. In particular, the discovery of an assemblage characterized by advanced Palaeofusulina (P. prisca) is significant because it clearly indicates that the deposition of the Doi Chiang Dao Limestone had continued until the very end of the Permian. These three Late Permian assemblages can be compared with those recently reported from the Shifodong Formation of Pa-leo-Tethyan mid-oceanic carbonates in the Changning-Menglian Belt of West Yunnan, Southwest China.  相似文献   

Themetic Issue: Collection of Achievements on Earth Resources from AUN/SEED-Net, Part III     

Akira Imai  Carla B. Dimalanta  Arifudin Idrus  I. Wayan Warmada    Agung Harijoko 《Resource Geology》2009,59(3):213-214

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High‐pressure polymorphs in Yamato‐790729 L6 chondrite and their significance for collisional conditions          下载免费PDF全文

Yukako Kato  Toshimori Sekine  Masahiko Kayama  Masaaki Miyahara  Akira Yamaguchi 《Meteoritics & planetary science》2017,52(12):2570-2585

Shock pressure recorded in Yamato (Y)‐790729, classified as L6 type ordinary chondrite, was evaluated based on high‐pressure polymorph assemblages and cathodoluminescence (CL) spectra of maskelynite. The host‐rock of Y‐790729 consists mainly of olivine, low‐Ca pyroxene, plagioclase, metallic Fe‐Ni, and iron‐sulfide with minor amounts of phosphate and chromite. A shock‐melt vein was observed in the hostrock. Ringwoodite, majorite, akimotoite, lingunite, tuite, and xieite occurred in and around the shock‐melt vein. The shock pressure in the shock‐melt vein is about 14–23 GPa based on the phase equilibrium diagrams of high‐pressure polymorphs. Some plagioclase portions in the host‐rock occurred as maskelynite. Sixteen different CL spectra of maskelynite portions were deconvolved using three assigned emission components (centered at 2.95, 3.26, and 3.88 eV). The intensity of emission component at 2.95 eV was selected as a calibrated barometer to estimate shock pressure, and the results indicate pressures of about 11–19 GPa. The difference in pressure between the shock‐melt vein and host‐rock might suggest heterogeneous shock conditions. Assuming an average shock pressure of 18 GPa, the impact velocity of the parent‐body of Y‐790729 is calculated to be ~1.90 km s^?1. The parent‐body would be at least ~10 km in size based on the incoherent formation mechanism of ringwoodite in Y‐790729.  相似文献   

Ancient Os isotope signatures from the Ontong Java Plateau lithosphere: Tracing lithospheric accretion history     

Akira Ishikawa  D. Graham Pearson  Christopher W. Dale 《Earth and Planetary Science Letters》2011,301(1-2):159-170

In order to better understand the nature and formation of oceanic lithosphere beneath the Early Cretaceous Ontong Java Plateau, Re–Os isotopes have been analysed in a suite of peridotite xenoliths from Malaita, Solomon Islands. Geological, thermobarometric and petrological evidence from previous studies reveal that the xenoliths represent virtually the entire thickness of the southern part of subplateau lithospheric mantle (< 120 km). This study demonstrates that vertical Os isotopic variations correlate with compositional variations in a stratified lithosphere. The shallowest plateau lithosphere (< 85 km) is dominated by fertile lherzolites showing a restricted range of ¹⁸⁷Os/¹⁸⁸Os (0.1222 to 0.1288), consistent with an origin from ~ 160 Ma Pacific lithosphere. In contrast, the basal section of subplateau lithospheric mantle (~ 95–120 km) is enriched in refractory harzburgites with highly unradiogenic ¹⁸⁷Os/¹⁸⁸Os ratios ranging from 0.1152 to 0.1196, which yield Proterozoic model ages of 0.9–1.7 Ga. Although the whole range of Os isotope compositions of Malaita peridotites is within the variations seen in modern abyssal peridotites, the contrasting isotopic compositions of shallow and deep plateau lithosphere suggest their derivation from different mantle reservoirs. We propose that the subplateau lithosphere forms a genetically unrelated two-layered structure, comprising shallower, typical oceanic lithosphere underpinned by deeper impinged material, which included a component of recycled Proterozoic lithosphere. The impingement of residual but chemically heterogeneous mantle, mechanically coupled to the recently formed, thin lithosphere, may have a bearing on the anomalous initial uplift and late subsidence history of the seismically anomalous plateau root.  相似文献   

Shear-wave polarization anisotropy beneath the north-eastern part of Honshu, Japan     

Tomomi Okada  Toru Matsuzawa  Akira Hasegawa 《Geophysical Journal International》1995,123(3):781-797

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The Influence Of Urban Canopy Configuration On Urban Albedo   总被引：6，自引：0，他引：6  

Akira Kondo  Megumi Ueno  Akikazu Kaga  Katsuhito Yamaguchi 《Boundary-Layer Meteorology》2001,100(2):225-242

We propose a calculation method for shortwave radiation flux and longwave radiation flux within the urban canopy and investigate the influence of urban canopy configuration on net radiation flux. In the assumed urban configuration, buildings of equal size are arranged in a regular lattice within the urban canopy. The net shortwave radiation flux and longwave radiation flux within the urban canopy were calculated by the photon tracking method based on the Monte Carlo method. The albedo value obtained by this method shows close agreement with experimental data, and the average sky view factor shows almost perfect agreement with the theoretical value. Moreover, we calculated the urban albedo for the urban canopy configuration including roads and building height distribution.%Moreover, we calculated net radiation within the urban canopy in %consideration of roads and building height distribution.We found that the sky view factor of the ground surface is high when building coverage is low, building height is low, open space by roads exists, and building height is non-uniform. Moreover, we found that the albedo value is high when building height is small, open space by roads is wide, and building height is uniform. The albedo value was found to vary in a complicated manner with change in building coverage.  相似文献   

Verification of two-dimensional non-linear analysis of sand-structure system by examining the results of the shaking-table test     

Akira Ohtsuki  Masanori Hirota  Kikuo Ishimura  Kazutomo Yokoyama  Kiyoshi Fukutake 《地震工程与结构动力学》1992,21(7):591-607

The verification of a two-dimensional non-linear analysis based on a hybrid constitutive model, which consists of the Ramberg-Osgood model extended to two-dimensional problems and a dilatancy model, is discussed through a comparison of the shaking-table test results for a one-storey structure standing on a dry sandy deposit with those for the same on a saturated sandy deposit. Since the relationships G vs. γ and h vs. γ for shear strains of 10^?5?10^?3 can be obtained accurately by an inversion analysis of the resonance curve of the sandy deposit the proposed non-linear method can represent well the observed non-linear response of the dry or the saturated sandy deposit including the structure. This method, however, should be applied carefully to assess the response of a soil-structure system when the three-dimensional interaction affects significantly the response of the model. An equivalent linear analysis using a sway-rocking model is applied to simulate the non-linear ground motion including the three-dimensional interaction, and it is found that the sway-rocking model can also represent well the non-linear response of the system.  相似文献