Non-hypersingular boundary integral equations for two-dimensional non-planar crack analysis

We derive a set of non-hypersingular boundary integral equations, both elastodynamic and elastostatic, for the analysis of arbitrarily shaped 2-D anti-plane and in-plane cracks located in an infinite homogeneous isotropic medium, rendered in a unified nomenclature for all cases. The hypersingularities that appear in the usual formulations for the dynamic cases, existent both at the source point and at the wavefront, are removed by way of a regularization technique based on integration by parts. The equations for the in-plane cases are presented in terms of a local Cartesian coordinate system, one of the axes of which is always held locally tangential to the crack trace. The expressions for the elastic field at any point on the model plane are also given.
Our formulations are shown to yield accurate numerical results, as long as appropriate stabilization measures are taken in the numerical scheme. The numerical applicability of our method to non-planar crack problems is illustrated by simulations of dynamic growth of a hackly crack which has small off-plane side-branches. The results imply that the branching of a crack brings about a significant decrease in the crack-tip stress concentration level and consequently may play an essential role in the arrest of earthquake rupturing.     

Comparison of Chlorophyll a Concentrations Obtained with 90% Acetone and N,N-dimethylformamide Extraction in Coastal Seawater

Determinations of Chlorophyll a (Chl a) in eutrophic coastal marine waters were compared using N, N-dimethylformamide (DMF) and 90% acetone techniques. Measured Chl a concentrations ranged from 0.89 to 10.65 μg l₋₁ for 90% acetone extracts and from 0.97 to 12.92 μg l⁻¹ for DMF extracts, respectively, for 24 surface water samples from the Harima-nada, Seto Inland Sea, Japan. Chl a concentrations using DMF as a solvent were consistently higher than those found using 90% acetone (p < 0.001, n= 24). Chl a is thus likely to be underestimated (by 13%) when 90% acetone is used for eutrophic waters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Crustal deformations associated with the 1986 fissure eruption of Izu-Oshima volcano, Japan, and their tectonic significance

We have investigated crustal deformations associated with the 1986 eruption of Izu-Oshima volcano, Japan, which was accompanied by an intensive fissure eruption. Two fissure crater chains, with NW-SE trend were created in the northern part of the caldera and on its northwestern flank. Their trend is consistent with the direction of compressive stress in this region. Depression of > 30 cm in the central zone including the caldera, and in the northwestern and southeastern parts in the island, was detected by precise leveling. On the other hand, uplifts up to 20 cm in the northeastern and southwestern parts were observed. Tide observations revealed that the Okada tide station, the leveling datum in Izu-Oshima, may have subsided by 5 cm after the eruption. An 1 m opening of fissure craters was detected by distance measurements of the baselines which cross fissure craters. Horizontal displacements obtained by reoccupation of control points showed a symmetrical pattern which was consistent with the opening of fissure craters. Anomalous strain changes were also observed in the surrounding regions—contractions were observed in the Boso and the Miura peninsula, northeast of Izu-Oshima, and extensions in the Izu peninsula.

To interpret these crustal deformations, a model which consists of a nearly vertical tensile fault and a deflation source is presented. The tensile fault lies parallel to the fissures and is divided into two parts according to depth. The deeper part of the tensile fault is 12 km long, 10 km wide, and has 2 km burial depth and 2.7 m opening displacement. The shallower part, which may represent the fissure craters, is 4 km long, 2 km wide, and the amount of opening is estimated to be 1 m. However, the deflation source may be located at a depth of 10 km beneath the northwestern flank of the caldera and depression just above the source is estimated to be 30 cm. A deflation source is required to explain the subsidence at the Okada tide station and the extension in the Izu peninsula. This model suggests that the eruption might have released tensile stresses in and around the Izu region which result from bending of the subducting Philippine Sea plate.     

Elastic stress and strain in jointed rock masses by means of crack tensor analysis

Summary An elastic stress-strain relation is formulated in terms of crack tensors which makes it possible to take into account explicitly the effect of joints on elastic behavior of rock masses. The present study is to discuss some related topics which may be encountered in its practical application. Two problems are solved by incorporating the elastic stress-strain relation into a program for three-dimensional finite element analyses; i. e., stress concentration by surface loading and displacement by excavation of an intersecting tunnel. Validity of the results is checked by comparing them with a laboratory model test and a field test, with the following conclusions: The overall distribution of stress definitely depends on a joint stiffness ratio (i. e., normal stiffness to shear stiffness). If the ratio is chosen as unity, the stress concentration occurs mainly in the direction parallel to major joints. If the ratio is high, say 10, then the stress concentrates along the perpendicular as well as the parallel directions to major joints. It can be said, on the basis of the fairly good agreement of the calculations using the high stiffness ratio with the field and laboratory measurements, that the elastic solution by crack tensors provides a practical tool for estimating the stress and strain in strongly jointed rock masses.     

Distribution of glycerol dialkyl glycerol tetraethers, alkenones and polyunsaturated fatty acids in suspended particulate organic matter in the East China Sea

We investigated the spatial distribution of glycerol dialkyl glycerol tetraethers (GDGTs), alkenones, and polyunsaturated fatty acids in particulate organic matter collected at four sites along a depth transect from the continental shelf to the Okinawa Trough in the East China Sea during the spring bloom in 2008. The maximum alkenone concentration appeared in the top 25?m at all sites and the $ U_{37}^{{{\text{K}}'}} $ values were consistent with in situ water temperatures in the depth interval, suggesting that the alkenones were produced mainly in surface water. At the slope and shelf sites, GDGTs in the water column showed a concentration maximum at 74?C99?m depth, and the $ {\text{TEX}}_{86}^{\text{H}} $ agreed with in situ water temperatures, suggesting the in situ production of GDGTs in the depth interval. The low-salinity surface water above 20?m depth was characterized by low GDGT concentrations and low $ {\text{TEX}}_{86}^{\text{L}} $ -based temperatures, suggesting either the production of GDGTs in winter season or the lateral advection of GDGTs by an eastward current. At the slope and Okinawa Trough sites, TEX₈₆-based temperatures were nearly constant in the water column deeper than 300?m and corresponded to temperatures at the surface and near-surface waters rather than in situ temperatures. This observation is consistent with a hypothesis that Thaumarchaeota cells produced in surface waters are delivered to deeper water and also indicates that the residence time of suspended GDGTs in the deep-water column is large enough to mix the GDGTs produced in different seasons.     

Redistribution and export of contaminated sediment within eastern Fukushima Prefecture due to typhoon flooding

Tropical cyclones expose river basins to heavy rainfall and flooding, and cause substantial soil erosion and sediment transport. There is heightened interest in the effects of typhoon floods on river basins in northeast Japan, as the migration of radiocaesium‐bearing soils contaminated by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident will affect future radiation levels. The five main catchments surrounding FDNPP are the Odaka, Ukedo, Maeda, Kuma and Tomioka basins, but little quantitative modelling has been undertaken to identify the sediment redistribution patterns and controlling processes across these basins. Here we address this issue and report catchment‐scale modelling of the five basins using the GETFLOWS simulation code. The three‐dimensional (3D) models of the basins incorporated details of the geology, soil type, land cover, and used data from meteorological records as inputs. The simulation results were checked against field monitoring data for water flow rates, suspended sediment concentrations and accumulated sediment erosion and deposition. The results show that the majority of annual sediment migration in the basins occurs over storm periods, thus making typhoons the main vectors for redistribution. The Ukedo and Tomioka basins are the most important basins in the region in terms of overall sediment transport, followed by the other three basins each with similar discharge amounts. Erosion is strongly correlated with the underlying geology and the surface topography in the study area. A low permeability Pliocene Dainenji formation in the coastal area causes high surface water flow rates and soil erosion. Conversely, erosion is lower in an area with high permeability granite basement rocks between the Hatagawa and Futaba faults in the centre of the study area. Land cover is also a factor controlling differences in erosion and transport rates between forested areas in the west of the study area and predominantly agricultural areas towards the east. The largest sediment depositions occur in the Ogaki and Takigawa Dams, at the confluence of the Takase and Ukedo Rivers, and at the Ukedo River mouth. Having clarified the sediment redistribution patterns and controlling processes, these results can assist the ongoing task of monitoring radioactive caesium redistribution within Fukushima Prefecture, and contribute to the design and implementation of measures to protect health and the environment. Copyright © 2016 John Wiley & Sons, Ltd.     

A role for a superficial sediment layer in upward nutrient fluxes across the overlying water–sediment interface

A superficial sediment layer (SL) is the top 2–3 mm layer of surface sediment that may contribute to high upward nutrient flux. To study the characteristics and the biogeochemical processes in the superficial layer, the seasonal variation in the total phytopigments (chlorophyll a and pheo-pigments), total organic carbon (TOC), and total nitrogen (TN) of the surface sediments in a shallow coastal area, Shido Bay, were measured, and the influence of the superficial sediment layer on nutrient flux at the sediment–water interface was investigated. TOC and TN content were relatively constant for the SL and subsurface layers (0–1 and 1–2 cm) during the study period. In contrast, total phytopigments content was higher in the SL layer than in the subsurface layers. The results of upward nutrient flux experiments showed higher nutrient release within the whole sediment core (SL remaining) than the SL-less (SL removed) core. Moreover, high nutrient fluxes were observed during the high temperature season, indicating that seasonal variation in nutrient flux was regulated by temperature. Moreover, in the low temperature season, the SL seemed to absorb nutrients, probably because of microphytobenthos photosynthesis that took up the nutrients under the sufficient light penetration to the sea floor.     

Effect of solid to solution ratio on biogenic silica determination in coastal sediments

The method of DeMaster (1981) for measuring biogenic silica (BSi) in marine sediments was modified. We found a considerable effect of the solid to solution (S/S) ratio on BSi determination in coastal sediments. The BSi contents determined by DeMaster’s method were overestimated due to the contribution of clay mineral-derived extractable Si at a low S/S ratio (<1 g l⁻¹), and incomplete recovery of BSi was observed due to incomplete extraction when using a high S/S ratio (>2 g l⁻¹). For BSi analysis of coastal sediment samples, the BSi content of representative samples must be measured using various S/S ratios to determine the appropriate S/S ratio in order to minimize the contribution of extractable Si derived from clay minerals and to obtain sufficiently efficient extraction with 1% Na₂CO₃ before actual sample measurement. One way to determine the appropriate S/S ratio is to use artificial sediment which has a theoretical Si and clay mineral content similar to the sediment sample composition. Using artificial sediment, the S/S ratio of 2 g l⁻¹ seems appropriate for measuring the BSi content of the coastal sediments investigated in this study, because 99.6% recovery can be achieved.     

Determination of the freshwater origin of Coastal Oyashio Water using humic-like fluorescence in dissolved organic matter

Coastal Oyashio Water (COW), defined as a water mass with a temperature lower than 2 °C and a salinity lower than 33.0, is distributed in the North Pacific Ocean off southeastern Hokkaido, Japan, from winter to spring. COW is rich in macronutrients and dissolved iron and is thus considered to affect the spring phytoplankton blooms in the Oyashio region. Although river water and sea-ice melt water have been considered freshwater end-members of COW, the contributions of these freshwater sources to COW have not been well described. In this study, the humic-like components in dissolved organic matter were first applied as a parameter to evaluate the freshwater end-members of COW in March 2015. Linear regressions with negative slopes were determined between the humic-like components and the salinity of COW. The intercepts of the regressions against the humic-like components were within the ranges of those observed for the local rivers of Hokkaido but were very different from those of sea ice. These findings suggest that river water contributed to the COW observed here as a freshwater end-member, although the contribution of sea-ice melt water to COW could not be evaluated. This novel approach also highlighted two different less-saline water masses in COW. The first was characterized by a lower temperature and relatively high levels of humic-like components, while the second was higher in temperature and had higher levels of humic-like components. It is suggested that these different characteristics are due to the contributions of water from different rivers and/or different effects of sea-ice melt water.