Current and Material Transport at Tomogashima Strait, Japan

Tomogashima Strait, which is an entrance of Osaka Bay, is a very important area for material transport because a large volume of pollutant-rich water in Osaka Bay is transported through this strait. We have investigated the cross sectional structures of current and material fluxes at Tomogashima Strait, Osaka Bay. Tidal current and residual flow patterns in cross section at Tomogashima Strait during summer and winter are almost the same, and residual flow in summer is stronger than that in winter. However, the net water exchange through the strait in summer was almost the same volume as that in winter. At Tomogashima Strait, total suspended matter (TSM) and particulate organic carbon (POC), and individual nitrogen and phosphorus compartments in Osaka Bay always flowed out except particulate organic nitrogen (PON). The average outflows of total nitrogen (TN) and total phosphorus (TP) at Tomogashima Strait over a year were calculated to be 145 ton/d and 30 ton/d, respectively. The average outflow of DIN and DIP at Tomogashima Strait over a year were 95 ton/d and 13 ton/d (DIN/DIP = 16), respectively. Residence times of both DIN and DIP in Osaka Bay were estimated to be about two months.     

Vertical distributions of nitrogen,phosphorus and iron in Beppu Bay

Beppu Bay is a shallow basin located at the western end of the Seto Inland Sea with a sill depth ofca. 40 m. The bottom water (belowca. 65 m in summer andca. 70 m in winter) was anoxic and contained high concentrations of hydrogen sulfide, phosphate and ammonium. Maximum concentrations of nitrate and nitrite appeared near the top of the thermocline, suggesting the occurrence of bacterial nitrification in this layer and of bacterial denitrification in the anoxic bottom water. Concentrations of particulate phosphorus and particulate iron were highest near the bottom of the thermocline. The distribution of phosphorus in this bay is probably controlled by a dissolution-diffusion-precipitation cycle of iron or its hydrous oxides.     

The three-dimensional shear wave structure in the mantle by overtone waveform inversion - I. Radial seismogram inversion

Summary. The three-dimensional (3-D) shear wave structure of the mantle, down to the depth of about 900 km, is obtained by inverting waveforms of radial component seismograms. Radial component seismograms contain large amplitude overtone signals which circle the Earth as wave packets and are sometimes called X1, X2, X3, … We use data which contain R1, X1 and X2 and filtered between 2 and 10mHz. It is shown that, unless each seismogram is weighted, all seismograms are not fitted uniformly. Only data from large earthquakes are fitted and the final velocity anomalies are biased by the small number of large earthquake data. Resolution is good at shallow depths, becomes worse in the intermediate depth range between about 400 and 500 km and then becomes better at greater depth ranges (600–900km). Even though we use only spheroidal mode data, velocity anomalies in the shallow structure show excellent correlation with the age of the surface rocks of the Earth. In the deeper regions, between about 600 and 900km, South America shows a fast velocity anomaly which may indicate the slab penetration beyond 700 km there. Another region which shows a fast velocity anomaly is the Mariana trench, but other subduction regions do not show such features.     

State of stress within a bending spherical shell and its implications for subducting lithosphere

The state of stress within a bending spherical shell has some special features that are caused by sphericity. While most lithospheres are more like spherical shells than flat plates, our ideas of the state of stress have been dominated by flat-plate models. As a consequence, we might be missing some important aspects of the state of stress within subducting lithospheres. In order to examine this problem, we analyse spherical-shell bending problems from basic equations. We present two approaches to solve spherical-shell bending problems: one by the variational approach, which is suitable for global-scale problems, and the other by the asymptotic equation, which is valid to first order in h/R , where h is the thickness of the lithosphere and R is its curvature radius (i.e. under the assumption of small curvature). The form of the equation for displacement shows that wavelengths of deformation are determined by the spherical (elastic) effect and the gravitational buoyancy effect, for which only the latter effect is included in the usual flat-plate formulations. In the case of the Earth, the buoyancy force is dominant and, consequently, spherical effects are suppressed to a large extent; this explains why flat-plate models have been successful for Earth's lithospheric problems. On the other hand, the state of stress shows interesting spherical effects: while bending (fibre) stress along the subduction zone is always important, bending stress along the trench-strike direction can also be important, in particular when the subduction zone arc is small. Numerical results also indicate that compressive normal stress along the trench-strike direction is important when a subduction zone arc is large. These two stresses, the bending stress and the compressive normal stress, both along the trench-strike direction, may have important implications for intraplate earthquakes at subduction zones.     

Temporal variation of resuspended matter for one tidal cycle in the Hiuchi-Nada area

The observed relation between the vertical distribution of suspended matter and the size distribution of bottom sediment in the Hiuchi-Nada area indicates that a turbid bottom water layer exists above the bottom when the bottom sediment is of silt size. The turbid bottom water is thought to be produced by resuspension of bottom sediment. Measurements of the temporal variation of resuspended matter and water movement near the bottom was carried out for one full tidal cycle at an anchored station. The concentration of resuspended matter reached a maximum at the time of strongest ebb and flood flow.     

Seasonal variation of bottom turbidity layer in Etauchi Bay

The beam attenuation coefficient, organic carbon (POC) and organic nitrogen (PON) contents of suspended materials in Etauchi Bay, which has little inflow of river water as well as very weak tidal current (maximum speed: 6.5cm·sec⁻¹), were measured as a function of depth for all seasons to understand a seasonal variation of bottom turbidity layer. In spring and summer, the beam attenuation coefficient in bottom layer and POC and PON contents of suspended materials in the surface water layer increased with time, which brought the occurrence of the bottom turbidity layer. From autumn to winter, however, their concentrations became low and constant over the whole depth almost independent of time. As a result, the bottom turbidity layer disappeared in winter and beam attenuation coefficient became constant over the whole depth. From these results, it may be considered that the bottom turbidity layer was produced by phytodetritus brought from surface water layer, rather than by resuspension of bottom sediment in Etauchi Bay.