GEK measurements in a shallow water region

Evidence for the applicability of GEK (Geomagnetic Electrokinetography) measurements to shallow water regions is provided from observations in the shelf region of the East China Sea. The reason for the effectiveness of GEK measurements in this case is investigated theoretically, and it is shown to be attributable to the existence of a thick conductive sedimentary layer. In addition, it is shown that low conductive basement rock can be regarded as a good conductor for GEK measurements if the current width is broad enough and if the ratio of current width to water depth is larger than the resistivity ratio of basement rock to sea water. This implies that barotropic tidal currents can be measured with GEK in any ocean on the earth if they have significant magnitudes.     

Scaling model experiments on propagation of electromagnetic waves using salt solution to simulate the Earth's crust: Loma Prieta and Kobe earthquakes

Propagation of electromagnetic (EM) waves from an earthquake focus in the conductive Earth has been investigated using 1/1,000,000 scaling models taking earth-ionosphere and ocean-Moho plane parallel-plate waveguides into account. Microwaves at a frequency, ω_m, a million times higher than that of seismic EM signal (SEMS), ω, were generated at the model focus. They are propagated in a salt solution modeling the earth's crust and reflected by ocean, fault planes, ionosphere and Moho plane all made by aluminum. Distribution of EM power was mapped by scanning a detector antenna over the model Earth's surface. The skin depth, δ, calculated by the exact skin depth equation, 1/δ=ω(μ/2)^1/2 [(1+(1/ωρ)²)^1/2 −1]^1/2 where dielectric constant, and permeability, μ are the same but resistivity, ρ, 10⁻⁶ times smaller than that of Earth, gave 10⁻⁶ times small skin depth validating the model scaling index. Images for evanescent and wave-ripple standing waves disturbed by normal, strike-slip and dip-slip conductive fault planes have been obtained using an aluminum plate. The co-circular contour map above the epicenter due to evanescence was pushed to the north east direction from the epicenter by the presence of ocean for the Loma Prieta earthquake, while to north direction for the Kobe earthquake. The intensity of EM ULF emissions for the Loma Prieta earthquake is discussed quantitatively.     

A note on the Japan Sea Proper Water

The water under the main thermocline in the Japan Sea is a single water mass referred to as the Japan Sea Proper Water. It can be defined as having temperature below 2.0°C, salinity above 34.00%, and dissolved oxygen below 7.0 ml 1⁻¹. In the north most of the water above the potential temperature 0.1°C depth (about 800–1000 m) is a mode water, with σ_θ of 27.32 to 27.34 kg m⁻³. North of 40°N it has high oxygen (more than 6.00 ml 1⁻¹) with a distinct discontinuity (oxygen-cline) at the bottom of the mode water. The most probable region for the formation of the water is the area north of 41°N between 132° and 134°E. The deeper water probably is formed in the norther area of 43°N, and directly fills the main part of the Japan Basin north of 41°N and east of 134°E.     

Introduction of accumulation-dispersal coefficient of marine organisms

Avoiding the subject for fish accumulation, the traditional view in fish population dynamics has ascribed immigration and emigration of fish to dispersal of fish. The main purpose of this paper is to find a quantity that represents the time rate of accumulation-dispersal of marine organisms, and also has some relation to the horizontal convergence of current velocity of the surrounding water. For this, the accumulation-dispersal coefficient is introduced not in the form of diffusion, but in the same form as the convergence. Since the accumulation-dispersal of organisms is a factor that changes its distribution density, all factors causing the change are first classified to locate the position occupied by the accumulation dispersal. Each factor corresponds to each coefficient appearing in a linearized equation describing the rate of change in the density, averaged over a region or a group. The immigration-emigration coefficient is divided into three coefficients of passage, accumulation dispersal and diffusion velocity. For the organisms ranging in a nearly horizontal layer, the accumulation-dispersal coefficient is shown to equal the area-averaged horizontal convergence of organismal velocity relative to land, which is the sum of the area-averaged horizontal convergences of swimming velocity relative to water and of current velocity. However, the area-averaged convergence of current velocity associated with the accumulation-dispersal coefficient for a region is shown to be somewhat different from the usual one.     

Abyssal bedforms and sediment drifts effected by deep-sea flows

The investigation of abyssal bedforms and sediment drifts as a tool for understanding the deep flow characteristics allows us to interprete that a benthic storm is primarily related to sediment distribution, development of longitudinal ripple marks, and concentration of suspended particulate matter. There explicitly exists a strong and periodical bottom flow which is called the benthic storm having a current speed of over 15 cm sec^?1 and duration of more than two days. Hydrodynamic regime has been thought to affect underlying sediment textural natures which can be used to distinguish between bottom currents with different velocities. Therefore, concentration of medium silt mode (0.010–0.017 mm in size) delineates a high-velocity core of the benthic storm in the deep sea bottom. Bottom current measurements in most of the North Pacific Ocean indicate that present bottom current speeds are generally less than 10 cm sec^?1. It appears likely, therefore, that significant erosion is not taking place today. However, at current passages, bases of sea mounts, and other topographic obstructions locally accelerated current flows are recognized to affect bottom configuration. While, it is concluded from bottom echo-characteristics and bottom current measurements that widespread occurrences of echo type 3 (sediment-drift deposit facies) recognized at 22°N and 42°N in the Northwest Pacific are associated with the North Equatorial current and the North Pacific current respectively, and can best be interpreted to be originated from benthic storms, the source of which were come from those surface currents.     

Japan-equator XBT sections in late November 1989 and in early December 1991

A comparison between Japan-equator XBT sections along 150°E in late November 1989 and along 140°E in early December 1991 is made. The warmest surface water above 29°C diminished to the south of 2–4°N and the surface mixed layer noticeably decreased in thickness in the equatorial region in December 1991; besides, the North Equatorial Countercurrent was intensified. This is considered to be a manifestation of changes in the surface layer of the western equatorial Pacific in the mature phase of El Niño.     

Curvature derived from LiDAR digital elevation models as simple indicators of debris-flow susceptibility

To mitigate the damage caused by debris flows resulting from heavy precipitation and to aid in evacuation plan preparation, areas at risk should be mapped on a scale appropriate for affected individuals and communities. We tested the effectiveness of simply identifying debris-flow hazards through automated derivation of surface curvatures using LiDAR digital elevation models. We achieved useful correspondence between plan curvatures and areas of existing debris-flow damage in two localities in Japan using the analysis of digital elevation models(DEMs). We found that plan curvatures derived from 10 m DEMs may be useful to indicate areas that are susceptible to debris flow in mountainous areas. In residential areas located on gentle sloping debris flow fans, the greatest damage to houses was found to be located in the elongated depressions that are connected to mountain stream valleys. Plan curvaturederived from 5 m DEM was the most sensitive indicators for susceptibility to debris flows.     

Climato-limnological signals during the past 260 000 years in physical properties of bottom sediments from Lake Baikal

The St.16 core obtained from the Academician Ridge of Lake Baikal in eastern Siberia may span about 260 000 years, and some physical properties of the core samples are closely related to aquatic paleoproductivity and climatic change. The median of grain size, grain density, and water content fluctuate synchronously. They also are connected with change in the abundance of biogenic silica (diatoms). The physical parameters indicate that there were high aquatic productivity periods around interglacial periods (MIS 5 and 7; 70 000-125 000 yr B.P. and 180 000-250 000 yr B.P.). Comparatively large clastics were transported from outside of the lake through various routes (ice rafting, etc.) in addition to fluvial routes during the glacials or 'stadials. There are ca. 20 000 yr, 40 000 yr and 100 000 yr periods in the variations of physical properties. These are related to the three Milankovitch parameters of solar insolation.     

Mixing of magmatic CO2 into volcano groundwater flow at Aso volcano assessed combining carbon and water stable isotopes

To understand deep groundwater flow systems and their interaction with CO₂ emanated from magma at depth in a volcanic edifice, deep groundwater samples were collected from hot spring wells in the Aso volcanic area for hydrogen, oxygen and carbon isotope analyses and measurements of the stable carbon isotope ratios and concentrations of dissolved inorganic carbon (DIC). Relations between the stable carbon isotope ratio (δ¹³C_DIC) and DIC concentrations of the sampled waters show that magma-derived CO₂ mixed into the deep groundwater. Furthermore, groundwaters of deeper areas, except samples from fumarolic areas, show higher δ¹³C_DIC values. The waters' stable hydrogen and oxygen isotope ratios (δD and δ¹⁸O) reflect the meteoric-water origin of that region's deep groundwater. A negative correlation was found between the altitude of the well bottom and the altitude of groundwater recharge as calculated using the equation of the recharge-water line and δD value. This applies especially in the Aso-dani area, where deeper groundwater correlates with higher recharge. Groundwater recharged at high altitude has higher δ¹³C_DIC of than groundwater recharged at low altitude, strongly suggesting that magmatic CO₂ is present to a much greater degree in deeper groundwater. These results indicate that magmatic CO₂ mixes into deeper groundwater flowing nearer the magma conduit or chamber.     

Spatio-temporal analysis of recent groundwater-level trends in the Red River Delta, Vietnam

A groundwater-monitoring network has been in operation in the Red River Delta, Vietnam, since 1995. Trends in groundwater level (1995?C2009) in 57 wells in the Holocene unconfined aquifer and 63 wells in the Pleistocene confined aquifer were determined by applying the non-parametric Mann-Kendall trend test and Sen??s slope estimator. At each well, 17 time series (e.g. annual, seasonal, monthly), computed from the original data, were analyzed. Analysis of the annual groundwater-level means revealed that 35?% of the wells in the unconfined aquifer showed downward trends, while about 21?% showed upward trends. On the other hand, confined-aquifer groundwater levels experienced downward trends in almost all locations. Spatial distributions of trends indicated that the strongly declining trends (>0.3?m/year) were mainly found in urban areas around Hanoi where there is intensive abstraction of groundwater. Although the trend results for most of the 17 time series at a given well were quite similar, different trend patterns were detected in several. The findings reflect unsustainable groundwater development and the importance of maintaining groundwater monitoring and a database in the Delta, particularly in urban areas.