Apparent velocity measurements on an oceanic lithosphere     

Hideki Shimamura  Toshi Asada 《Physics of the Earth and Planetary Interiors》1976,13(1):P15-P22

A direct measurement of apparent velocities for oceanic paths was made with an array of sensitive ocean bottom seismographs. The measurement was performed by recording waves from shallow earthquakes which occurred in the area close to trench axes and which were accurately located by the land seismological network in Japan. The range of epicentral distances is from 500 to 1,800 km.The observed P travel times are less than those in the Jeffreys-Bullen tables by 6–10 s for the range of distances.Since the dimension of the OBS array is about 400 km, the apparent velocities are determined quite precisely and show little dependence on the epicentral distances. The average value of the apparent velocities for the range 500–1,700 km is 8.64 ± 0.13 km/s.An offset of travel times, which is thought to be associated with a low-velocity layer underneath the oceanic lithosphere, has been observed.These results indicate that a high-velocity layer with a velocity of 8.6 km/s exists in the lower part of the oceanic lithosphere. Beneath the 8.6-km/s layer there is a thin low-velocity layer under which the velocity of the P wave is again 8.6 km/s.  相似文献   

V_pIV_s ratio structure of the Lofoten continental margin, northern Norway, and its geological implications     

Shuichi Kodaira  Martin Bellenberg    Takaya Iwasaki    Toshihiko Kanazawa    Hans B. Hirschleber  Hideki Shimamura 《Geophysical Journal International》1996,124(3):724-740

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Effect of Mine Water on the Stability of Underground Coal Mine Roadways     

Takashi?SasaokaEmail author  Tri?Karian  Akihiro?Hamanaka  Hideki?Shimada  Kikuo?Matsui  Masatomo?Ichinose 《Geotechnical and Geological Engineering》2016,34(2):671-678

The basic objectives of mine roadways are to provide sufficient cross sections to accommodate equipment, transport, personnel travel and ventilation. However, many roadways become damaged to the extent of needing maintenance, generally dinting, and in some cases requiring re-ripping. Strata conditions play important effects on stability of roadways and other mining activities. Weak rocks cause excessive roadway closures, and water softens some rocks and worsens the closure problem. Therefore, study on effect of mine water on the stability of underground coal mine roadways is important for underground coal mine development. A scientific discussion on the effect of water on the stability of roadways is given on the basis of results obtained by means of field investigation and laboratory tests. Based on analysis, rock in saturated condition has only between 0.19 to 0.49 of its compressive strength and 0.17 to 0.59 of its tensile strength in dry condition. Among the coal measures rock in research area, water has the most obvious effect on the strength reduction of shale. The slaking behavior of shale has also been the worst among the other. Field investigation of roadway driven in shale shown vertical closure in the wet and dry condition area reached 40–60 and 5–15 cm respectively, in 30 days after the drivage. Among the measures, drainage is considered to be the most economical and simplest method to reduce the water content in the rocks or the rock masses.  相似文献   

Crustal structure and transform margin development south of Svalbard based on ocean bottom seismometer data     

Asbjrn Johan Breivik  Rolf Mjelde  Paul Grogan  Hideki Shimamura  Yoshio Murai  Yuichi Nishimura 《Tectonophysics》2003,369(1-2):37-70

The Barents Sea is located in the northwestern corner of the Eurasian continent, where the crustal terrain was assembled in the Caledonian orogeny during Late Ordovician and Silurian times. The western Barents Sea margin developed primarily as a transform margin during the early Tertiary. In the northwestern part south of Svalbard, multichannel reflection seismic lines have poor resolution below the Permian sequence, and the early post-orogenic development is not well known here. In 1998, an ocean bottom seismometer (OBS) survey was collected southwest to southeast of the Svalbard archipelago. One profile was shot across the continental transform margin south of Svalbard, which is presented here. P-wave modeling of the OBS profile indicates a Caledonian suture in the continental basement south of Svalbard, also proposed previously based on a deep seismic reflection line coincident with the OBS profile. The suture zone is associated with a small crustal root and westward dipping mantle reflectivity, and it marks a boundary between two different crystalline basement terrains. The western terrain has low (6.2–6.45 km s⁻¹) P-wave velocities, while the eastern has higher (6.3–6.9 km s⁻¹) velocities. Gravity modeling agrees with this, as an increased density is needed in the eastern block. The S-wave data predict a quartz-rich lithology compatible with felsic gneiss to granite within and west of the suture zone, and an intermediate lithological composition to the east. A geological model assuming westward dipping Caledonian subduction and collision can explain the missing lower crust in the western block by subduction erosion of the lower crust, as well as the observed structuring. Due to the transform margin setting, the tectonic thinning of the continental block during opening of the Norwegian-Greenland Sea is restricted to the outer 35 km of the continental block, and the continent–ocean boundary (COB) can be located to within 5 km in our data. Distinct from the outer high commonly observed on transform margins, the upper part of the continental crust at the margin is dominated by two large, rotated down-faulted blocks with throws of 2–3 km on each fault, apparently formed during the transform margin development. Analysis of the gravity field shows that these faults probably merge to one single fault to the south of our profile, and that the downfaulting dominates the whole margin segment from Spitsbergen to Bjørnøya. South of Bjørnøya, the faulting leaves the continental margin to terminate as a graben 75 km south of the island. Adjacent to the continental margin, there is no clear oceanic layer 2 seismic signature. However, the top basement velocity of 6.55 km s⁻¹ is significantly lower than the high (7 km s⁻¹) velocity reported earlier from expanding spread profiles (ESPs), and we interpret the velocity structure of the oceanic crust to be a result of a development induced by the 7–8-km-thick sedimentary overburden.  相似文献   

A tidal front in winter influenced by river discharge     

Ichiro Yuasa  Hideki Ueshima 《Journal of Oceanography》1992,48(3):239-255

Water quality of Osaka Bay is greatly influenced by freshwater discharge from rivers, to the effect of salinity playing a major role in forming the stratification. The tidal front is expected to appear in such an area even in winter considering the theory of the formation of a tidal front. From the field observation, we recognized a tidal front in winter for the first time in Osaka Bay. The critical value of a parameter log(H/U ³) for generation of a tidal front is obtained as a function of river discharge and cooling effect through sea surface. Differences of nutrients (NH₄–N, PO₄–P) concentration across the front are not clear, probably because they are utilized by phytoplankton easily. But in (NO₂–N)+(NO₃–N), DIN, PP and T–P, there is a discontinuity structure across the front, even if the absolute value of concentration difference is small.  相似文献   

Numerical Simulation of Crown Pillar Behaviour in Transition from Open Pit to Underground Mining     

Dintwe  Tumelo K. M.  Sasaoka  Takashi  Shimada  Hideki  Hamanaka  Akihiro  Moses  Dyson N.  Peng  Ma  Fanfei  Meng  Liu  Seifei  Ssebadduka  Ronald  Onyango  Joan A. 《Geotechnical and Geological Engineering》2022,40(4):2213-2229

Geotechnical and Geological Engineering - Crown pillars provide regional and local support by isolating the ground surface from underground mine workings. Topography above the underground mine may...  相似文献   

Variations in the thermal conductivities of surface sediments in the Nankai subduction zone off Tokai, central Japan     

Shusaku Goto  Tomonobu Mizoguchi  Ryo Kimura  Masataka Kinoshita  Makoto Yamano  Hideki Hamamoto 《Marine Geophysical Researches》2012,33(3):269-283

We investigated the relationship between variations in the thermal conductivity of surface sediments and the topography in the Nankai subduction zone off Tokai, central Japan, the easternmost part of the Nankai subduction zone, which has an accretionary prism with varied topography. We analyzed sediment thermal conductivity data obtained from the trough floor and accretionary prism. Variations in the thermal conductivity of sediments were related to the topographic features formed by accretionary prism development. Thermal conductivities of 1.1?W/m?K were measured on the trough floor where thick terrigenous turbidites have been deposited. The thermal conductivity of Nankai Trough floor sediments decreases from northeast to southwest along the trough, probably because of the decreased grain size and/or changes in sediment mineral composition. High thermal conductivities (??1.0?W/m?K) were measured in fault scarps on the accretionary prism. A landward increase in these values on the prism may be explained by decreased porosity of the sediments attributable to tectonic deformation during accretionary prism development. At the base of the fault scarp of the frontal thrust, low thermal conductivities (<0.9?W/m?K) were measured, likely reflecting the high porosity of the talus deposits. Low thermal conductivity (0.9?W/m?K) was also measured in slope basins on the accretionary prism, likely also related to the high porosity of the sediments. Our results demonstrate that, for accurate heat flow measurement in an area of varied topography, the geothermal gradient and the thermal conductivity of the sediments must be measured within regions with similar topographic features.  相似文献   

Calibration shifts in Scintrex CG‐3M gravimeters with an application to detection of microgravity changes at Iwo‐tou caldera,Japan     

Motoo Ukawa  Kyozo Nozaki  Hideki Ueda  Eisuke Fujita 《Geophysical Prospecting》2010,58(6):1123-1132

Calibration shift seriously influences gravity values measured using Scintrex CG‐3M gravimeters. We calibrated three Scintrex CG‐3M gravimeters three times (1999, 2003 and 2006) over eight years, using a calibration line with a gravity difference of 1.38 Gal. The scale factor correction coefficients (calibration factors) obtained here range from 0.9998–1.0005. The calibration factors vary with time by 89 ppm, ?102 ppm and ?126 ppm between the 1999–2003 surveys. The calibration shifts of two of the three gravimeters decreased to about 20 ppm or less in the second interval, the other remained about the same (142 ppm). The results indicate that they shifted at rates on the order of 10 ppm/year even several years after manufacturing. The large shift in calibration factors indicates that they must be corrected using calibrations done before and after the measurements to perform microgravity measurements when gravity differences between a reference gravity site and survey sites are on the order of a hundred milligals (mGal) or more. The results also indicate that the calibration factors change gradually with time, so their interpolation provides a good practical approximation for a specific survey time. We applied the time‐dependent calibration factors to microgravity monitoring at the Iwo‐tou caldera, Japan, where the gravity difference between the base site on the island and the reference site on Honshu (the mainland of Japan) is about 870 mGal mainly due to the 11° latitude difference. Gravity surveys were conducted every two years from 1998–2006. The correction of scale factors estimated from the repeated calibration surveys leads to satisfactory measurements, in which the average of the absolute differences between two Scintrex CG‐3M instruments in five surveys is reduced from 207μGal to 19μGal; for three of those surveys, it is less than 10 μGal. This result demonstrates the importance of repeated calibration surveys.  相似文献   

Geology and age model of the Lower Pleistocene Nojima,Ofuna, and Koshiba Formations of the middle Kazusa Group,a forearc basin‐fill sequence on the Miura Peninsula,the Pacific side of central Japan     

Atsushi Nozaki  Ryuichi Majima  Koji Kameo  Saburo Sakai  Atsuro Kouda  Shungo Kawagata  Hideki Wada  Hiroshi Kitazato 《Island Arc》2014,23(2):157-179

We present field and core observations, nannofossil biostratigraphy, and stable oxygen isotope fluctuations in foraminiferal tests to describe the geology and to construct an age model of the Lower Pleistocene Nojima, Ofuna, and Koshiba Formations (in ascending order) of the middle Kazusa Group, a forearc basin‐fill succession, exposed on the northern Miura Peninsula on the Pacific side of central Japan. In the study area, the Nojima Formation is composed of sandy mudstone and alternating sandy mudstone and mudstone, the Ofuna Formation of massive mudstone, and the Koshiba Formation of sandy mudstone, muddy sandstone, and sandstone. The Kazusa Group contains many tuff beds that are characteristic of forearc deposits. Thirty‐six of those tuff beds have characteristic lithologies and stratigraphic positions that allow them to be traced over considerable distances. Examination of calcareous nannofossils revealed three nannofossil datum planes in the sequences: datum 10 (first appearance of large Gephyrocapsa), datum 11 (first appearance of Gephyrocapsa oceanica), and datum 12 (first appearance of Gephyrocapsa caribbeanica). Stable oxygen isotope data from the tests of the planktonic foraminifer Globorotalia inflata extracted from cores were measured to identify the stratigraphic fluctuations of oxygen isotope ratios that are controlled by glacial–interglacial cycles. The observed fluctuations were assigned to marine isotope stages (MISs) 49–61 on the basis of correlations of the fluctuations with nannofossil datum planes. Using the age model obtained, we estimated the ages of 24 tuff beds. Among these, the SKT‐11 and SKT‐12 tuff beds have been correlated with the Kd25 and Kd24 tuff beds, respectively, of the Kiwada Formation on the Boso Peninsula. The Kd25 and Kd24 tuff beds are widely recognized in Pleistocene strata in Japan. We used our age model to date SKT‐11 at 1573 ka and SKT‐12 at 1543 ka.  相似文献   

Timescale of material circulation in subduction zone: U–Pb zircon and K–Ar phengite double‐dating of the Sanbagawa metamorphic complex in the Ikeda district,central Shikoku,southwest Japan     

Mitsuhiro Nagata  Kazuhiro Miyazaki  Hideki Iwano  Tohru Danhara  Hideyuki Obayashi  Takafumi Hirata  Koshi Yagi  Yoshikazu Kouchi  Koshi Yamamoto  Shigeru Otoh 《Island Arc》2019,28(4)

We have estimated the timescale of material circulation in the Sanbagawa subduction zone based on U–Pb zircon and K–Ar phengite dating in the Ikeda district, central Shikoku. The Minawa and Koboke units are major constituents of the high‐P Sanbagawa metamorphic complex in Shikoku, southwest Japan. For the Minawa unit, ages of 92–81 Ma for the trench‐fill sediments, are indicated, whereas the age of ductile deformation and metamorphism of garnet and chlorite zones are 74–72 Ma and 65 Ma, respectively. Our results and occurrence of c. 150 Ma Besshi‐type deposits formed at mid‐ocean ridge suggest that the 60‐Myr‐old Izanagi Plate was subducted beneath the Eurasian Plate at c. 90 Ma, and this observation is consistent with recent plate reconstructions. For the Koboke unit, the depositional ages of the trench‐fill sediments and the dates for the termination of ductile deformation and metamorphism are estimated at c. 76–74 and 64–62 Ma, respectively. In the Ikeda district, the depositional ages generally become younger towards lower structural levels in the Sanbagawa metamorphic complex. Our results of U–Pb and K–Ar dating show that the circulation of material from the deposition of the Minawa and Koboke units at the trench through an active high‐P metamorphic domain to the final exhumation from the domain occurred continuously throughout c. 30 Myr (from c. 90 to 60 Ma).  相似文献