GPS-measured land subsidence in Ojiya City, Niigata Prefecture, Japan   总被引：1，自引：0，他引：1  

Hiroshi P. Sato  Kaoru Abe  Osamu Ootaki 《Engineering Geology》2003,67(3-4):379-390

Land subsidence caused by compression of clay layers in Ojiya City, Japan was measured by global positioning system (GPS) between 1 April 1996 and 31 December 1998.

Three baselines were selected in and around the city, and height difference on a WGS-84 ellipsoid was measured by GPS on each baseline. The ground at the GPS station in the city subsides and rebounds 7 cm every winter and spring, respectively. Measurement accuracy was 9.5 mm standard deviation. Ground water level was observed at a well near the GPS station. Regression analysis between total strain, calculated as ratio of the height difference displacement to the total thickness of the clay layers, and the layers' effective stress change with ground water level change gave good correlation. The slope of regression line 7.0×10⁻¹¹ m²/N was obtained as an average apparent coefficient of volume compressibility of the layers.  相似文献   

Chemical, Isotopic, and Fluid Inclusion Evidence for the Hydrothermal Alteration of the Footwall Rocks of the BIF-Hosted Iron Ore Deposits in the Hamersley District, Western Australia     

Makoto Haruna    Takahiro Hanamuro    Kaoru Uyeda    Hirokazu Fujimaki  Hiroshi Ohmoto 《Resource Geology》2003,53(2):75-88

Abstract. The petrography, chemical, fluid inclusion and isotope analyses (O, Rb-Sr) were conducted for the shale samples of the Mount McRae Shale collected from the Tom Price, Newman, and Paraburdoo mines in the Hamersley Basin, Western Australia. The Mount McRae Shale at these mines occurs as a footwall unit of the secondary, hematite-rich iron ores derived from the Brockman Iron Formation, one of the largest banded iron formations (BIFs) in the world. Unusually low contents of Na, Ca, and Sr in the shales suggest that these elements were leached away from the shale after deposition. The δ¹⁸O (SMOW) values fall in the range of + 15.0 to +17.9 per mil and show the positive correlation with calculated quartz/sericite ratios of the shale samples. This suggests that the oxygen isotopic compositions of shale samples were homogenized and equilibrated by postdepositional event. The pyrite nodules hosted by shales are often rimmed by thin layers of silica of varying crystallinity. Fluid inclusions in quartz crystals rimming a pyrite nodule show homogenization temperatures ranging from 100 to 240C for 47 inclusions and salinities ranging from 0.4 to 12.3 wt% NaCl equivalent for 18 inclusions. These fluid inclusion data give direct evidence for the hydrothermal activity and are comparable to those of the vein quartz collected from the BIF-derived secondary iron ores (Taylor et al, 2001). The Rb-Sr age for the Mount McRae Shale is 1,952 ± 289 Ma and at least 200 million years younger than the depositional age of the Brockman Iron Formation of ∼ 2.5 Ga in age. All the data obtained in this study are consistent with the suggestion that high temperature hydrothermal fluids were responsible for both the secondary iron ore formation and the alteration of the Mount McRae Shale.  相似文献   

Microearthquakes and faulting in the southern Okinawa Trough     

Mamoru Nakamura  Hiroshi Katao 《Tectonophysics》2003,372(3-4):167-177

Southern Okinawa Trough represents an early stage of back-arc rifting and is characterized by normal faulting and microearthquakes. Earthquake distribution and deep structure of fault was investigated to clarify active rifting in the southern Okinawa Trough, where two parallel grabens are located. A network of ocean bottom seismometers (OBSs) that displayed the hypocenters of 105 earthquakes were observed for a period of 4 days in southern-graben (SG). Most of the microearthquakes occurred in a cluster about 7 km wide, which on a cross-section striking N45°E dips 48° to the southwest. Relocated hypocenters, which are recorded by a local seismic network, show scattered distribution around the southern-graben. There are no remarkable surface faults in the southern-graben. On the other hand, the recalculation of hypocenter locations of 1996 earthquakes swarm recorded by a local seismic network suggests that the swarm is associated with normal faulting on the southern side of northern-graben (NG). Thus, the undeveloped southern-graben is located to the south of the developed northern-graben. Southward migration of rifting, which may be caused by migration of volcanism, could thus be occurring in the southern Okinawa Trough. The extension rate computed for the southern Okinawa Trough from the fault model of the northern-graben is 4.6 cm/year, which is 59–102% of the extension rate (GPS measurements). This result indicates that the majority of extensional deformation is concentrated within the center of the northern-graben in the Okinawa Trough.  相似文献   

Candidate precursors: pulse-like geoelectric signals possibly related to recent seismic activity in Japan   总被引：1，自引：0，他引：1  

Yuji Enomoto  Akito Tsutsumi  Yukio Fujinawa  Minoru Kasahara  Hiroshi Hashimoto 《Geophysical Journal International》1997,131(3):485-494

Telemetric network observations of pulse-like geoelectric charge signals using a vertical dipole buried under the ground were undertaken at various observation sites over a wide area in Japan from April 1996. From continuous records of the signals during the six months following that, we attempted to select anomalous signals, possibly related to seismic electric activity. Special attention was paid to the elimination of noise resulting from industrial and meteorological electric activity, comparison with other electromagnetic signals in the VLF band and the relation between the precursor period and the distance from the eventual main shock that occurred in Japan. Four candidate precursor electric signals, which were not contaminated by industrial and meteorological electric activity, were then selected, of which the second appeared before the Akita-ken Nairiku-nanbu earthquake swarm, for which the maximum M = 5.9 occurred on 1996 August 11, and the third and fourth before the Chiba-ken Toho-oki earthquake, M = 6.6, on 1996 September 11. A tentative qualitative model explaining why the candidate precursory signal is related to stress building up before an earthquake is presented in terms of the electrification of gases released from fracturing rocks immediately prior to the main shock.  相似文献   

A 2.5-D time-domain elastodynamic equation for a general anisotropic medium     

Hiroshi Takenaka  Brian L. N. Kennett 《Geophysical Journal International》1996,127(2):F1-F4

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Interactive behavior of soil–pile-superstructure system in transient state to liquefaction by means of large shake table tests     

Shintaro Yao  Koichi Kobayashi  Nozomu Yoshida  Hiroshi Matsuo 《Soil Dynamics and Earthquake Engineering》2004,24(5):572-409

Shaking table tests were conducted by means of a large-scale laminar box with 4 m in length, 2 m in width and 2 m in height in order to investigate behavior of a soil-pile-superstructure system in liquefiable ground. A model two-storey structure, supported by a pile group, was set in a saturated sand deposit, and subjected to a sinusoidal base motion with increasing amplitude. Discussions are focused on the transient behavior until soil liquefaction occurs. Main interests are characteristics of springs used in a sway-rocking model and a multi-freedom lumped mass (MFLM) model that are frequently used in soil–pile interaction analysis. The spring constant in the sway-rocking model is represented by restoring force characteristics at the pile head, and that in the MFLM system is represented by an interaction spring connecting the pile to the free field. The transient state prior to soil liquefaction is shown to be important in the design of a pile because dynamic earth pressure shows peak response in this state. The reduction of the stiffness due to excess porewater generation and strain dependent nonlinear behavior is evaluated.  相似文献   

MHD Dynamo Simulation Using the GeoFEM Platform: Comparison with a Spectral Method     

Hiroaki Matsui  Hiroshi Okuda 《Pure and Applied Geophysics》2004,161(11-12):2199-2212

— We investigate differences of results of a MHD dynamo simulation using the finite-element method (FEM) with that by the spherical harmonics expansion. The simulation was performed with a total of 7.8×10⁴ elements in the simulation domain. The results show that the magnetic energy is generated approximately four times of the kinetic energy in both cases. The outline of present results has consistence with that by the spectral method. However, small-scale patterns can be observed in the spectral method case. These discrepancies are caused by limitation of the spatial resolution in both of the present simulations.  相似文献   

Exploration Indices and Mineral Potential Map of the Kuroko Deposits in Northeast Japan     

Hiroshi Kubota    Tetsuro Urabe  Ryoichi Yamada  Shojiro Tanimura 《Resource Geology》2004,54(4):387-397

Abstract. Recent discoveries of seafloor hydrothermal mineralization in submarine volcanic centers of felsic magma in western Pacific island arcs are regarded as modern analogues of Kuroko type deposits. Studies of these deposits and their surrounding geology raised question whether the exploration activity for the Kuroko deposits on land which peaked in the 1960's was adequate or not. However, such an evaluation is not easy because the exploration data are about to be lost as a result of the closure of all the Kuroko mines in the area since 1994.
The Metal Mining Agency of Japan (MMAJ), therefore, decided to compile existing data on about 180 Kuroko deposits and related mineral occurrences in northeast Japan as a new Kuroko database.
This study extends a concept called "exploration indices" which was developed based on a case study of the thoroughly surveyed Hokuroku district to draw a potential map of the Kuroko occurrences for the entire northeast Japan quantitatively with a Geographical Information System (GIS). Effective exploration indices include: 1) distribution of dacitic-rhy-olitic submarine volcanic rocks of the Nishikurosawa and Onnagawa stages, 2) distribution of intrusive rocks of pre- and post-Kuroko horizon, 3) low aeromagnetic anomaly caused by hydrothermal alteration of magnetite, 4) low gravity anomaly which suggests depressions in the basement rocks such as a tectonic basin and/or caldera, and 5) nearby existence of vein type deposits. It is concluded that about 33 % of known Kuroko deposits fall within the high potential zone (score=4 and 5) that occupies only 4 % of the entire northeast Japan arc. The Kuroko potential map is, therefore, useful for limiting the target area for Kuroko type deposits in an island arc setting.  相似文献   

Neodymium isotopic variations in Northwest Pacific waters     

Hiroshi Amakawa  Yoshiyuki Nozaki  Jing Zhang  Hisao Nagai 《Geochimica et cosmochimica acta》2004,68(4):715-727

Four vertical profiles of the concentration and isotopic composition of Nd in seawater were obtained in the western North Pacific. Two profiles from the Kuroshio Current regime showed congruently that although the Nd concentration increases gradually with depth, its isotopic composition varies significantly with depth depending upon the water mass occupying the water column. The high-salinity Kuroshio waters originating from the North Pacific Tropical Water (NPTW) carry the least radiogenic Nd (?_Nd = −7.4 to −8.7) to this region at ∼250 m from the western margin continental shelves, most likely from the East China Sea. The Nd isotopic compositions in the North Pacific Intermediate Water (NPIW) that occurs at 600 to 1000 m in the subtropical region are fairly uniform at ?_Nd = −3.7. The profile data from the ∼38° to 40°N Kuroshio/Oyashio mixed water region off Sanriku of Honshu, Japan, also suggest that the newest NPIW with ?_Nd = −3.2 is formed there by the mixing of various source waters, and the radiogenic component of Nd is derived mainly from the Oyashio waters.In the Pacific Deep Water (PDW) below ∼1000 m, the Nd isotopic composition is neither vertically nor horizontally homogeneous, suggesting that it serves as a useful tracer for sluggish deep water circulation as well. Two profiles from the Izu-Ogasawara Trench showed a minimum ?_Nd value at ∼2000 m, suggesting that there exists a horizontal advective flow in the vicinity of Honshu, Japan. There is some evidence from other chemical properties to support this observation. The waters below 4000 m including those within the trench in the subtropical region have ?_Nd values of around −5, suggesting that the deep waters are fed from the south along the western boundary, ultimately from the Antarctic Bottom Water (AABW) in the South Pacific. This extends up to ∼40°N along the Japanese Islands. In the subarctic region (>∼42°N), the waters have more radiogenic Nd with ?_Nd > −4.0 throughout the water column, presumably due to the supply of Nd by weathering in such igneous provinces as the Kuril-Kamchatska-Aleutian Island chain. The lateral inhomogeneity of the Nd isotopic composition in PDW suggests that there may be different circulation and mixing regimes in the North Pacific Basin.  相似文献   

Formation and shortening deformation of a back-arc rift basin revealed by deep seismic profiling, central Japan   总被引：1，自引：0，他引：1  

Hiroshi Sato  Takaya Iwasaki  Shinji Kawasaki  Yasutaka Ikeda  Nobuhisa Matsuta  Tetsuya Takeda  Naoshi Hirata  Taku Kawanaka   《Tectonophysics》2004,388(1-4):47

The northern Fossa Magna (NFM) basin is a Miocene rift system produced in the final stages of the opening of the Sea of Japan. It divides the major structure of Japan into two regions, with north-trending geological structures to the NE of the basin and EW trending structures to the west of the basin. The Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the northern Fossa Magna and forms an active fault system that displays one of the largest slip rates (4–9 mm/year) in the Japanese islands. Deep seismic reflection and refraction/wide-angle reflection profiling were undertaken in 2002 across the northern part of ISTL in order to delineate structures in the crust, and the deep geometry of the active fault systems. The seismic images are interpreted based on the pattern of reflectors, the surface geology and velocities derived from refraction analysis. The 68-km-long seismic section suggests that the Miocene NFM basin was formed by an east dipping normal fault with a shallow flat segment to 6 km depth and a deeper ramp penetrating to 15 km depth. This low-angle normal fault originated as a comparatively shallow brittle/ductile detachment in a high thermal regime present in the Miocene. The NFM basin was filled by a thick (>6 km) accumulation of sediments. Shortening since the late Neogene is accommodated along NS to NE–SE trending thrust faults that previously accommodated extension and produce fault-related folds on their hanging wall. Based on our balanced geologic cross-section, the total amount of Miocene extension is ca. 42 km and the total amount of late Neogene to Quaternary shortening is ca. 23 km.  相似文献