Double seismic zones and stresses of intermediate depth earthquakes     

Kazuya Fujita  Hiroo Kanamori 《Geophysical Journal International》1981,66(1):131-156

Summary. Data from Japanese local seismograph networks suggest that the stresses in double seismic zones are in-plate compression for the upper zone and in-plate tension for the lower zone; the stresses do not necessarily appear to be down-dip. It may therefore be possible to identify other double seismic zones on the basis of data which indicate that events with differing orientations of in-plate stresses occur in a given segment of slab.
A global survey of published focal mechanisms for intermediate depth earthquakes suggests that the stress in the slab is controlled, at least in part, by the age of the slab and the rate of convergence. Old and slow slabs are under in-plate tensile stresses and the amount of in-plate compression in the slab increases with increasing convergence rate or decreasing slab age. Young and fast slabs are an exception to this trend; all such slabs are down-dip tensile. Since these slabs all subduct under continents, they may be bent by continental loading. Double seismic zones are not a feature common to all subduction zones and are only observed in slabs which are not dominated by tensile or compressive stresses.
Unbending of the lithosphere and upper mantle phase changes are unlikely to be the causes of the major features of double zones, although they may contribute to producing some of their characteristics. Sagging or thermal effects, possibly aided by asthenospheric relative motion, may produce the local deviatoric stresses that cause double zones.  相似文献   

Silicate darkening in the Kobe CK chondrite: Evidence for shock metamorphism at high temperature     

Kazushige Tomeoka  Ichiro Ohnishi  Noboru Nakamura 《Meteoritics & planetary science》2001,36(11):1535-1545

Abstract— The Kobe CK4 chondrite, like most metamorphosed CK chondrites, exhibits pronounced silicate darkening of matrix and chondrule mesostases. Our petrographic and scanning electron microscopic study reveals that the matrix of Kobe consists mostly of intermixtures of two types of fine‐grained olivine. One forms subhedral to anhedral normal crystals. The other fills interstices of the subhedral to anhedral olivine crystals, exhibiting a complex network of veinlets. The latter type of olivine contains high densities of small spherical vesicles (<0.05‐3 μm in diameter) and grains (<0.05‐5 μm) of magnetite and pentlandite as well as round to anhedral grains (1–10 μm) of plagioclase, low‐Ca pyroxene, diopside and chlorapatite. The vesicular olivine is particularly abundant in regions of matrix that exhibit a relatively high degree of darkening and commonly fills chondrule mesostases. The vesicular olivine is clearly the principal cause of the silicate darkening in Kobe. The internal texture of the vesicular olivine closely resembles those of local melts produced from the matrices of experimentally and naturally shocked carbonaceous chondrites. The occurrence and texture of the vesicular olivine suggest that it resulted from recrystallization of partially melted matrix olivine by shock. Kobe exhibits light shock effects in olivine that are consistent with shock stage S2 that is too low to explain the occurrence of olivine melting. We suggest that the vesicular olivine in Kobe was produced by shock metamorphism at a relatively mild shock pressure (<25 GPa) and a high temperature (>600 °C). Thus, it is probable that the shock effects in olivine, manifest as fracturing and deformation, were relatively minor, but heating was strong enough to cause partial melting of matrix olivine.  相似文献   

Nitrogen and phosphorus distributions across the thermohaline front in Kii Channel in winter     

Ichiro Yuasa  Eisuke Hashimoto  Hideki Ueshima 《Journal of Oceanography》1993,49(4):407-424

We have studied nitrogen and phosphorus distributions across the thermohaline front in Kii Channel in winter by using engine-cooling sea water of a ferry boat. On Dec. 1986 and Jan. 1987, differences of PO₄–P and DIN across the front are recognized. Especially in the latter case, differences of nutrients concentrations across the front are very obvious. But differences of nutrients across the front on Feb. 1986, Feb. and Mar. 1987 are not obvious. Inspite of winter,Akashiwo had happened in Osaka Bay, nutrients mostly have already been utilized by phytoplankton in inner part of Osaka Bay. Consequently, differences of nutrients concentrations across the front are nearly zero.  相似文献   

Errata: Structure of Subsurface Intrusion of the Oyashio Water into the Kuroshio Extension and Formation Process of the North Pacific Intermediate Water     

Kuniaki Okuda  Ichiro Yasuda  Yutaka Hiroe  Yugo Shimizu 《Journal of Oceanography》2001,57(4):495-497

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Photographic observations of deep-sea infaunal ophiuroids in Suruga Bay,central Japan: an application of a free-fall system of time-lapse cameras and current meters     

Toshihiko Fujita  Suguru Ohta 《Journal of Oceanography》1990,46(5):230-236

Underwater observations of infaunal amphiurid ophiuroids were made at a depth of about 480 m in Suruga Bay, central Japan, using a free-fall system which consists of time-lapse stereo-photography units and current meters. The megabenthos fauna was characterized by the dominance of infaunal echinoderms; in particular, amphiurid ophiuroids were numerically dominant. The density and biomass of the amphiurids were 170 m^–2 and 37 g m^–2, respectively. They buried their discs in the sediment and extended their arm tips out of the sediment surface. They protruded 2.2 arms per individual on the average. Strong bottom currents were observed, and the average velocity was 12 cm sec^–1 at 4 m above the sea floor. No arm tip was observed to be raised vertically into the water column for suspension feeding utilizing the bottom current, and amphiurids were considered to be primarily a surface deposit feeder at the present site.  相似文献   

Absolute Volume Transports of the Oyashio Referred to Moored Current Meter Data Crossing the OICE     

Kazuyuki Uehara  Shin-Ichi Ito  Hideo Miyake  Ichiro Yasuda  Yugo Shimizu  Tomowo Watanabe 《Journal of Oceanography》2004,60(2):397-409

During November 2000–June 2002, both direct current measurements from deployment of a line of five moorings and repeated CTD observations were conducted along the Oyashio Intensive observation line off Cape Erimo (OICE). All the moorings were installed above the inshore-side slope of the Kuril-Kamchatka Trench. Before calculating the absolute volume transports, we compared vertical velocity differences of relative geostrophic velocities with those of the measured velocities. Since both the vertical velocity differences concerned with the middle three moorings were in good agreement, the flows above the continental slope are considered to be in thermal wind balance. We therefore used the current meter data of these three moorings, selected among all five moorings, to estimate the absolute volume transports of the Oyashio referred to the current meter data. As a result, we estimated that the southwestward absolute volume transports in 0–1000 db are 0.5–12.8 × 10⁶ m³/sec and the largest transport is obtained in winter, January 2001. The Oyashio absolute transports in January 2001, crossing the OICE between 42°N and 41°15′ N from the surface to near the bottom above the continental slope, is estimated to be at least 31 × 10⁶ m³/sec. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Two-dimensional asymmetric vortex merger: Contour dynamics experiment   总被引：1，自引：0，他引：1  

Ichiro Yasuda  Glenn R. Flierl 《Journal of Oceanography》1995,51(2):145-170

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Characteristics of SSH Anomaly Based on TOPEX/POSEIDON Altimetry and in situ Measured Velocity and Transport of Oyashio on OICE     

Shin-Ichi Ito  Kazuyuki Uehara  Takashi Miyao  Hideo Miyake  Ichiro Yasuda  Tomowo Watanabe  Yugo Shimizu 《Journal of Oceanography》2004,60(2):425-437

An observation line along the TOPEX/POSEIDON (T/P) ground track 060 was set to estimate the Oyashio transport. We call this line the OICE (Oyashio Intensive observation line off-Cape Erimo) along which we have been conducting repeated hydrographic observations and maintaining mooring systems. T/P derived sea surface height anomaly (SSHA) was compared with velocity and transport on OICE. Although the decorrelation scale of SSHA was estimated at about 80–110 km in the Oyashio region, the SSHA also contains horizontal, small-scale noise, which was eliminated using a Gaussian filter. In the comparison between the SSHA difference across two selected points and the subsurface velocity measured by a moored Acoustic Doppler Current Profiler (ADCP), the highest correlation (0.92) appeared when the smoothing scale was set at 30 km with the two points as near as possible. For the transport in the Oyashio region, the geostrophic transport between 39°30′ N and 42°N was compared with the SSHA difference across the same two points. In this case the highest correlations (0.79, 0.88 and 0.93) occurred when the smoothing scale was set at 38, 6 and 9 km for reference levels of 1000, 2000 and 3000 db, respectively. The annual mean transport was estimated as 9.46 Sv in the 3000 db reference case. The Oyashio transport time series was derived from the T/P SSHA data, and the transports are smaller than that estimated from the Sverdrup balance in 1994–1996 and larger than that in 1997–2000. This difference is consistent with baroclinic response to wind stress field. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Simulation of Formation and Spreading of Salinity Minimum Associated with NPIW Using a High-Resolution Model     

Hiroshi Ishizaki  Ichiro Ishikawa 《Journal of Oceanography》2002,58(2):463-485

A series of numerical experiments were conducted with a high-resolution (eddy-permitting) North Pacific model to simulate the formation and spreading of the salinity minimum associated with the North Pacific Intermediate Water (NPIW). It was found that two factors are required to simulate a realistic configuration of the salinity minimum: a realistic wind stress field and small-scale disturbances. The NCEP reanalyzed wind stress data lead to better results than the Hellerman and Rosenstein wind stress data, due to the closer location of the simulated Oyashio and Kuroshio at the western boundary. Small-scale disturbances formed by relaxing computational diffusivity included in the advection scheme promote the large-scale isopycnal mixing between the Oyashio and Kuroshio waters, simulating a realistic configuration of the salinity minimum. A detailed analysis of the Oyashio water transport was carried out on the final three-year data of the experiment with reduced computational diffusivity. Simulated transport of the Kuroshio Extension in the intermediate layer is generally smaller than the observed value, while those of the Oyashio and the flow at the subarctic front are comparable to the observed levels. In the Oyashio-Kuroshio interfrontal zone the zonally integrated southward transport of the Oyashio water (140-155°E) is borne by the eddy activity, though the time-mean flow reveals the existence of a coastal Oyashio intrusion. In the eastern part (155°E-180°) the zonally integrated transport of the Oyashio water indicates a southward peak at the southern edge of the Kuroshio Extension, which corresponds to the branching of the recirculating flow from the Kuroshio Extension.  相似文献   

Dynamics of the Kuroshio large meander     

Ichiro Yasuda  Jong -Hwan Yoon  Nobuo Suginohara 《Journal of Oceanography》1985,41(4):259-273

The bimodality of the Kuroshio path is studied numerically with a barotropic inflow-outflow model. The dynamics that determines the path depends on the Rossby number,Ro (proportional to inlet velocity) and the Reynolds number (representing effects of viscosity). At lowRo (<Ro ₁) only a meander path occurs, while at highRo(Ro ₂) only a straight path is developed. Between these critical values (Ro ₁RoRo₂) either of the two paths can occur (multiple states), and the choice of path is determined by its history. Increase (decrease) inRo acrossRo ₂ (Ro ₁) leads to catastrophic transition from one path to the other. In the intermediate range (Ro ₁RoRo₂), the straight path is conditionally unstable to finite amplitude disturbances, and abrupt changes to the meander path take place. Absolute vorticity is almost conserved along the meander path, while along the straight path it is dissipated in large amount near the coast. At low Re, the flow tends to a viscous flow, and steady states are obtained. At highRe, time variations with different periods for the meander and straight paths become dominant. Intermittent transitions from one state to the other without any changes of external parameters are found at intermediateRo and at highRe.  相似文献