Seasonal variations of water system distribution and flow patterns in the southern sea area of Hokkaido,Japan

Hydrographic data and composite current velocity data (ADCP and GEK) were used to examine the seasonal variations of upper-ocean flow in the southern sea area of Hokkaido, which includes the “off-Doto” and “Hidaka Bay” areas separated by Cape Erimo. During the heating season (April–September), the outflow of the Tsugaru Warm Current (TWC) from the Tsugaru Strait first extends north-eastward, and then one branch of TWC turns to the west along the shelf slope after it approaches the Hidaka Shelf. The main flow of TWC evolves continuously, extending eastward as far as the area off Cape Erimo. In the late cooling season (January–March), part of the Oyashio enters Hidaka Bay along the shallower part of the shelf slope through the area off Cape Erimo, replacing almost all of the TWC water, and hence the TWC devolves. It is suggested that the bottom-controlled barotropic flow of the Oyashio, which may be caused by the small density difference between the Oyashio and the TWC waters and the southward migration of main front of TWC, permits the Oyashio water to intrude along the Hidaka shelf slope.     

Identification of shear strength and seismic coefficient by back analyzing surficial slides in the 2004 Mid-Niigata prefecture earthquake

Sliding of natural and artificial slopes generally occurs during or following strong earthquakes. Such sliding is greatly affected by a combination of geological conditions and earthquake loading. Earthquake-induced landslides often cause more damage to infrastructure and human lives than the earthquake itself. Pseudo-static analysis is widely implemented as one of several design methods used in engineering practice to assess the seismic stability of natural and artificial slopes. However, the most important issue of pseudo-static analysis is to select the most appropriate method for measuring seismic coefficient. In order to investigate this, back analysis was conducted for surficial slides subjected to strong ground motion during the 2004 Mid-Niigata prefecture earthquake in Japan. This paper surveyed the stochastic properties of earthquake-induced surficial slides and clearly showed that the obtained results were applicable to back analysis of shear strength and seismic coefficient. In back analysis, soil properties such as soil strength and density and sliding depth were assumed as random variables owing to their uncertainties. Seismic coefficient is also assumed to be a random variable and varies with distance from the epicenter fault line. The analysis of 4504 recorded surficial slides clearly shows a unique relationship of landslide occurrence ratio with slope angle and distance from the epicenter fault line. This study’s results enhance the calculation of the shear strength of weathered soil covering slopes and the horizontal seismic coefficient through back analysis procedure. By considering possible stochastic properties of variables, some case studies were implemented in the back analysis.     

Transition from frontal accretion to underplating in a part of the Nankai Trough Accretionary Complex off Shikoku (SW Japan) and extensional features on the lower trench slope

Migrated multichannel seismic reflection profiles and bathymetry from a 200 × 120 km area of the Nankai Trough inner slope reveal three physiographic-tectonic domains on the lowermost slope. Linear ridges demarking laterally-continuous hangingwall anticlines above ramps in a relatively simple imbricate stack of trench turbidites characterize the western domain. An imbricate fan underlies a relatively flat structural terrace in the east. Between these two domains lies a compound knoll (Minami Muroto Knoll) some 40 km long, opposite which the thrust front pushes some 10 km further seaward than is the case in the domains to east and west. In the western ‘linear-ridge’ domain previous DSDP drilling penetrated turbiditic trench fill uplifted in the lowermost thrust-fold terrace above a decollement within the underthrusting Shikoku Basin (oceanic plate) sequence. The Shikoku Basin sequence in the western domain is divided into an upper, poorly reflective, hemipelagic claystone unit and a lower, strongly reflective, unit comprising Pliocene turbidites. The lower unit is traceable intact up to c.20 km landward below the lower trench slope and in the better resolved profiles the decollement lies along the base of the claystone unit. A similar decollement within the Shikoku Basin sequence in the eastern domain is traceable up to c.22 km landward. A critical seismic record crossing the western part of Minami-Muroto Knoll shows that the decollement is traceable only 8 km landward to a point, under the steep slope at the front of the knoll, landward of which the subducting Shikoku basin sequence is apparently thickened by as much as twice. This thickening, occuring as it does immediately along-strike from a simple imbricate fan to the east of the knoll and a relatively simple imbricate stack to the west (both evidently involving no strata from the lower Shikoku Basin unit) we ascribe to underplating by formation of duplexes of Shikoku Basin strata. Strike-parallel extension, akin to that postulated for high structural levels in certain thrust belts, is caused by uplift of the knoll as a result either of the underplating, or segmentation of the subducting oceanic crust, or both: a normal fault throws to the west off the west flank of the knoll. It bounds a transverse, trough-like, slope-basin with at least 900 m of fill. Upslope from the knoll broadly slope-parallel normal faults cut, and pond, recent slope sediments. The most impressive is a listric growth fault which dips trenchward. Alternative explanations for these involve extensional collapse of this part of the prism resulting from the subduction of a topographic high, or a zone of selective underplating below the trenchward portion of Minami Murato Knoll.     

Climate control on Venus: Comparison of the carbonate and pyrite models

We review two models describing the Venus climate system: the carbonate and pyrite models. It has been argued carbonate and pyrite are potentially important minerals controlling the climate of Venus, though existence of either minerals has not been confirmed. Although it used to be proposed that carbonation reaction might explain the Venus’ atmospheric CO₂ abundance, it is unlikely Venus’ surface is reactive enough to control the Venus’ massive CO₂ atmosphere. Venus’ surface carbonate is also able to affect the climate through the reaction with atmospheric SO₂ to form anhydrite. Under the carbonate model the climate state is not in equilibrium and would be unstable due to the reaction between carbonate and SO₂. On the other hand, pyrite-magnetite reaction is proposed to explain the Venus’ atmospheric SO₂ abundance. Under pyrite-magnetite reaction, however, the climate would be stabilized such that the existing climate state is maintained over a geological timescale, while some observational facts such as atmospheric abundance of SO₂ and surface temperature could also be reasonably explained.     

Warm eddy movements in the eastern Japan Sea

Warm eddy movements and their areal extent in the eastern Japan Sea were described by presenting space-time diagrams for the warm eddy locations and magnitudes. The analyzed data were compiled from Japan Maritime Safety Agency thermal maps at 200 m depth from 1985 to 1992. Two to four warm eddies always existed in the eastern Japan Sea and exhibited both internnual and annual signals. We found that warm eddies were generated in spring around Oki Spur at least three times during the analyzed period of eight years, moved eastward, and interacted with neighboring warm eddies, which were involved in coalescences or separations. The warm eddy distributions off Noto Peninsula have clear seasonal preference. Warm eddies moved eastward from Noto Peninsula in winter-spring to North Japan in the next winter, with mean translation speeds of 0.5–2 cm s^–1. Warm eddies reaching North Japan typically decayed during a few month after splitting into two or three mesoscale warm eddies.     

Southward intrusion of the intermediate Oyashio water along the east coast of the Boso Peninsula I. Coastal salinity-minimum-layer water off the Boso Peninsula

Index species of zooplankton of the Oyashio water are found in and beneath the salinity minimum layer in Sagami Bay. In order to clarify the intrusion path of the intermediate Oyashio Water (or the water of the Mixed Water Region), the oceanographic conditions off the Boso Peninsula are studied by using available hydrographic data obtained mainly by Japan Meteorological Agency. The cross-sectional salinity distribution along KJ line which extends southeastward from off the tip of the peninsula always indicates the existence of a low salinity patch just off the coast in the salinity minimum layer. This water is well separated from the offshore low salinity water which is considered as the water in the western margin of the so-called North Pacific Intermediate Water. We refer to the former water as the coastal salinity-minimum-layer (SML) water and to the latter as the offshore SML water. The coastal SML water is usually bounded by the current zone of the Kuroshio. The existence of the coastal SML water seems to indicate the possible pathway of the intermediate Oyashio water along the Boso Peninsula into Sagami Bay. The detailed water type analysis is made in T-S plane, S-_st plane, and O₂-_st plane. There is no significant difference in distribution ranges of the water types between the coastal SML water and the offshore SML water. However, the water types of the coastal SML water is not uniformly distributed, and the water can be classified into two groups: group A with relatively high oxygen content and relatively low salinity value and group B with relatively low oxygen content and relatively high salinity value. Group A is thought to be associated with strong event-like intrusions, the details of which will be discussed in Part II.     

Autocovariance function and power spectrum of the vertical temperature gradient in the thermocline

The nature of the autocovariance function and power spectrum of time series of spike-array type is discussed. As the spacing of spikes is not easily seen directly in power spectrum, prudence should be exercised in interpreting the gradient spectrum of oceanic fine-structure. We calculated the autocovariance function and lag joint probability density of the record of the vertical profile of the vertical temperature gradient measured in the San Diego Trough. The results obtained support the results of the bispectral analysis in the previous paper (Nagata, 1978) that the predominant length scale of about 5 m has two meanings: spacing of the spikes and wavelength of a sinusoidal wave. The results seem to show the existence of nernal waves having a vertical wavelength the same as the spacing of the spikes or the thickness of the homogeneous layer in the oceanic fine-structure.     

Stokes' expansion of internal deep water waves to the fifth order

Stokes' expansion is applied to the internal waves of finite amplitude, which propagate on the interface between two layers of infinite thickness. Stream function, wave profile, phase velocity and mass transport velocity are given in the fifth order approximation. It is shown that (a) phase velocity increases with increase of wave steepness, (b) mass transport appears in the direction of the wave propagation in both layers as in the case of the surface waves, and (c) when the density difference is very small, the wave profile is flattened not only at the troughs but also at the crests.     

Evolutionary tracks of the terrestrial planets

On the basis of the model proposed by Matsui and Abe, we will show that two major factors — distance from the Sun and the efficiency of retention of accretional energy — control the early evolution of the terrestrial planets. A diagram of accretional energy versus the optical depth of a proto-atmosphere provides a means to follow the evolutionary track of surface temperature of the terrestrial planets and an explanation for why the third planet in our solar system is an aqua-planet.     

Electron density distribution of FeTiO<Subscript>3</Subscript> ilmenite under high pressure analyzed by MEM using single crystal diffraction intensities

Many of ilmenites ABO₃ compounds bearing transition elements have semiconductive, ferroelectric and antiferromagnetic properties. The high-pressure diffraction studies of FeTiO₃ have been conducted up to 8.2 GPa using synchrotron radiation in KEK at Tsukuba with diamond anvil cell. The compression mechanism of FeTiO₃ ilmenite has been investigated by the structure refinements converged to the reliable factors R = 0.05. The deformations of the FeO₆ and TiO₆ octahedra were reduced with increasing pressure. In order to elucidate the electric conductivity change with pressure, electron density distribution of ilmenite have been executed by maximum entropy method (MEM) using single-crystal diffraction intensity data. MEM based on F _obs(hkl) of FeTiO₃ clearly shows electron density in comparison with the difference Fourier synthesis based on F _obs(hkl) − F _calc(hkl). The radial distribution of the electron density indicates electron localization around the cation positions. The bonding electron density found in bond Fe–O and Ti–O is lowered with pressure. The isotropic temperature factors B _iso become smaller with increasing pressure. Nevertheless the thermal vibration is considerably restrained by the compression, the electric conductivity is enhanced with pressure. Neither charge transfer nor electron hopping between Fe and Ti along the c axis in FeTiO₃ is plausible under high pressure. But the electric conductivity due to electron super-exchange in Fe–Fe and Ti–Ti has been clarified by the MEM electron density distribution. The anisotropy in the electric conductivity has been clarified.