Potential of in situ-produced cosmogenic nuclides for quantifying strength reduction of bedrock in soil-mantled hillslopes

This study presents a semi-empirical model for quantifying the reduction in the mechanical strength of bedrock beneath actively eroding soil-mantled hillslopes. The strength reduction of bedrock controls the rate of physical disintegration of saprolite, which supplies fresh minerals that are then exposed to intense chemical weathering in soil sections. To determine the values of parameters employed in the model requires knowledge of the denudation rate of the hillslope, the thickness of the soil and saprolite layers, the strength of fresh bedrock, and the threshold strength for physical erosion at the uppermost face of the saprolite. These parameters can be obtained from cosmogenic nuclide analyses for quartz samples from the soil–saprolite boundary and basic field- and laboratory-based investigations. Further testing of the model within a diverse range of climatic, tectonic, and lithologic environments is likely to provide clues to the mechanisms responsible for local and regional variations in the rates of soil production and chemical weathering upon hillslopes.     

Bioaccumulation of nitroarenes in bivalves at Osaka Bay, Japan

This paper reports for the first time the detection and occurrence of nitroarenes (NPAHs) in marine organisms. Mussels and oysters collected from Osaka Bay, Japan, had total NPAHs concentrations that ranged from 2380 to 24,688 pg/g dry and 2672 to 25,961 pg/g dry, respectively. Relatively higher concentrations were detected in sampling sites located near the central district and suburbs of Osaka City implying that the most probable sources of NPAHs in the two bivalves are exhaust gases and smokes emitted by automobiles and industrial plants. Bivalves had relatively higher residues of 1-nitronaphthalene, 2-nitronaphthalene, 3-nitrophenanthrene, and 9-nitrophenanthrenes. Residues of 2-nitrofluorene, 1-nitropyrene, 4-nitropyrenes, and 6-nitrochrysene were much lower compared to nitronaphthalenes and nitrophenanthrenes. Inter-species differences was only observed for 2-nitronaphthalene with oysters exhibiting significantly higher residues than mussels.     

Strong motion uncertainty determined from observed records by dense network in Japan

The variation of ground motions at specific stations from events in six narrow areas was inspected by using K-NET and KiK-net records. A source-area factor for individual observation stations was calculated by averaging ratios between observed values for horizontal peak acceleration and velocity, as well as acceleration response spectra for 5% damping, and predicted values using a ground-motion model (usually known as an attenuation relation) by Kanno et al. (Bull Seismol Soc Am, 96:879–897, 2006). Standard deviations between observed and predicted amplitudes after the correction factor are less than 0.2 on the logarithmic scale and decrease down to around 0.15 in the short-period range. Intra-event standard deviation clearly increases with decreasing distance due to differing paths around near source area. Standard deviations may increase with amplitude or decrease with magnitude; however, both amplitude and magnitude of the data are strongly correlated with distance. The standard deviation calculated in this study is obviously much smaller than that of the original ground-motion model, as epistemic uncertainties are minimized by grouping ground motions at specific stations. This result indicates that the accuracy of strong ground motion prediction could be improved if ground-motion models for specified region are determined individually. For this to be possible, it is necessary to have dense strong-motion networks in high-seismicity regions, such as K-NET and KiK-net.     

Source process of very-long-period events accompanying long-period signals at Cotopaxi Volcano,Ecuador

Renewed seismic activity of Cotopaxi, Ecuador, began in January 2001 with the increased number of long-period (LP) events, followed by a swarm of volcano-tectonic (VT) earthquakes in November 2001. In late June 2002, the activity of very-long-period (VLP) (2 s) events accompanying LP (0.5–1 s) signals began beneath the volcano. The VLP waveform was characterized by an impulsive signature, which was accompanied by the LP signal showing non-harmonic oscillations. We observed temporal changes of both the VLP and LP signals from the beginning until September 2003: The VLP signal gradually disappeared and the LP signal characterized by decaying harmonic oscillations became dominant. Assuming possible source geometries, we applied a waveform inversion method to the observed waveforms of the largest VLP event. Our inversion and particle motion analyses point to volumetric changes of a sub-vertical crack as the VLP source, which is located at a depth of 2–3 km beneath the northeastern flank. The spectral analysis of the decaying harmonic oscillations of LP events shows frequencies between 2.0 and 3.5 Hz, with quality factors significantly above 100. The increased VT activity and deformation data suggest an intrusion of magma beneath the volcano. A release of gases with small magma particles may have repetitively occurred due to the pressurization, which was caused by sustained bubble growth at the magma ceiling. The released particle-laden gases opened a crack above the magma system and triggered the resonance of the crack. We interpret the VLP and LP events as the gas-release process and the resonance of the crack, respectively.     

Sedimentary and geochemical characterization of Middle–Late Miocene formations in the Neogene Tsugaru Basin,Japan by means of DTH27‐1 well sediment analysis

Middle–Late Miocene age siliceous formations outcropping along the northwestern side of Honshu Island are considered prospective source rocks for hydrocarbons. An analysis of geophysical, sedimentological, and geochemical properties is essential to evaluate the formations' source potential, and to understand the factors that determined the accumulation and preservation of organic matter. This study investigates the Middle–Late Miocene geological record of the Tsugaru back‐arc basin, located in the western part of Aomori prefecture, through an analysis of a 200 m long portion of a core from the DTH27‐1 well; this core is composed of the diatomaceous siltstones of the Akaishi Formation and the siliceous mudstones of the Odoji Formation. Sedimentological and geophysical characterization showed that the Akaishi Formation's diatomaceous siltstones are mostly massive and bioturbated, have low magnetic susceptibility, and demonstrate moderate natural radioactivity. Although the Odoji Formation's siliceous mudstones are massive, they have exceedingly low magnetic susceptibility and high natural radioactivity. Geochemical data from a Rock‐Eval Pyrolysis such as total organic carbon and generative potential (S1 + S2) revealed that, in the Tsugaru area, only the Odoji Formation is a likely prospective source rock for hydrocarbons. On the other hand, T_max values indicate that both the formations are thermally immature for generating hydrocarbons. The difference between the Akaishi and Odoji Formation in the sedimentological facies, in terms of the degree of bioturbation and the organic carbon content, indicates variations in lithological properties, such as porosity and grain size; moreover, this difference indicates a variation in the paleo‐oxygenation of bottom waters, with the transition from oxygen‐deficient conditions in the Middle Miocene to the more oxygenated conditions in the Late Miocene. Both the lithological and paleo‐environmental factors possibly influenced the organic richness in the two formations.     

Attenuation of compressional waves in peridotite measured as a function of temperature at 200 MPa

A technique has been developed to determine attenuation in rocks at high temperature using a gas-media, high-pressure apparatus. A pulse transmission technique and a spectral ratio method are used to study compressional seismic properties of rocks. Seismic waves are transmitted to and from the sample through buffer rods of mullite. The effect of seismic wave reflections within the sample assembly are cancelled out by taking ratios of the spectra measured at different temperatures. In order to obtain good signal-to-noise ratio for resolving the attenuation at high pressure and temperature, special care is taken in the sample assembly and the ultrasonic coupling between the sample, buffer rods and transducers. A very tight connection of the sample-buffer rod-transducer is essential for obtaining high frequency signals (>300 kHz) at high temperature. A small mass is attached to each outside end of the transducer to drive low frequency signals (<250 kHz) into the sample. Before attenuation measurements, the sample and the buffer rods are tightly compacted in a platinum tube at high pressure and room temperature to ensure pressure seal of the sample assembly. The frequency range of measurement covers 50 to 450 kHz for the sample. Attenuation is very small in the buffer rod compared to the sample for the entire temperature range of the study. Because of the small attenuation, a wide frequency band of 50 kHz to 3.2 MHz can be covered for investigating the attenuation in the buffer rod. The technique has been used to measure attenuation at high confining pressure, and temperatures including sub- and hyper-solidus of upper mantle rocks. Therefore, effects of partial melting on attenuation can be studied.The method is applied to the attenuation measurement in a peridotite as a function of temperature to 1225°C at 200 MPa confining pressure. At high temperature, signal amplitude decays more rapidly at high frequency than at low frequency, from which attenuation (andQ) can be determined using a spectral ratio method. No frequency dependence ofQ is resolved for both the sample and the buffer rod over the entire temperature and frequency ranges of the measurement. The results show thatQ decreases rapidly with increasing temperature even in the temperature range below the solidus of peridotites. Such temperature sensitivity ofQ is probably more useful to probe thermal structure in the upper mantle than that of conductivity at temperatures below the solidus. The results in this study are compared with available seismic velocity, electrical conductivity and solidus data for peridotites, suggesting that there is no discontinuous change in both mechanical and electrical properties of peridotites at the solidus temperature. Even at hypersolidus temperatures, it appears that velocity drops and conductivity increases continuously (not abruptly) with increasing melt fraction. This implies that mechanical and electrical properties of the upper mantle will gradually change at the boundary where the geotherm crosses the solidus.     

Bottom pressure change associated with the 2004–2005 large meander of the Kuroshio south of Japan

A large meander of the Kuroshio was generated in the region off the southern coast of Japan in August 2004 and continued until approximately July 2005. The formation and decay of the large-meander (LM) path was observed by bottom pressure (BP) sensors installed on inverted echo sounders (PIESs) and a seismic observing system off Shikoku. The variation in BP was examined focusing on the development, persistence, and decay of the LM path. The BP was found to be depressed associated with a Kuroshio path disturbance, called a small meander, and this BP depression led the sea surface height (SSH) depression by up to approximately two months. The temporal phase shift between the sea surface and deep disturbances was significantly greater than those of other small meanders that did not develop into large meanders. After the formation of the LM path, the BP beneath the Kuroshio increased with a lag of approximately two months behind the SSH elevation along with the upward displacement of the main thermocline. The increase in BP is associated with that of the positive southward BP gradient anomaly, i.e., the eastward deep Kuroshio current anomaly, which suggests an enhancement of the topographic steering and stability of the LM path. This is consistent with the fact that no small meanders occurred in the early LM period from late July 2004 to late January 2005.     

Finite-element Simulation of Seismic Ground Motion with a Voxel Mesh

— Accurate simulation of seismic ground motion for three-dimensionally complex topography and structures is one of the most important goals of strong motion seismology. The finite-element method (FEM) is well suited for this kind of simulation, since traction-free conditions are already included in the formulation, and the Courant condition is less strict than for the finite-difference method (FDM). However, the FEM usually requires both large memory and computation time. These limitations can be overcome by using a mesh consisting of voxels (rectangular prisms) with isotropy built into the explicit formulation of the dynamic matrix equation. Since operators in the voxel FEM are the combinations of ordinary FDM operators and additional terms, the method keeps accuracy of the same order as FDM and the terms relax the Courant condition. The voxel FEM requires a similar amount of memory and only takes 1.2～1.4 times longer computation time. The voxel mesh can be generated considerably faster than the popular tetrahedral mesh. Both ground motions and static displacements due to a point or line source can be calculated using the voxel FEM approach. Comparisons with the reflectivity method and theoretical solutions demonstrate the successful implementation of the method, which is then applied to more complex problems.     

Distributions of polycyclic aromatic hydrocarbons and alkylated polycyclic aromatic hydrocarbons in Osaka Bay,Japan

Contaminations in sediments by polycyclic aromatic hydrocarbons (PAHs) and alkylated PAHs were investigated at 44 sites in Osaka Bay, Japan. Concentrations of total PAHs and alkylated PAHs were in the range 6.40–7800 ng/g dry weights and 13.7–1700 ng/g dry weights, respectively. The PAH concentrations tended to be higher along the shoreline in the vicinities of big ports, industrialized areas, and densely populated regions such as the cities of Osaka and Kobe. The major sources appeared to be pyrogenic or both pyrogenic and petrogenic at most of the sites. PAH concentrations were remarkably high at a site near Kobe, where the concentrations of dibenzo(a,h)anthracene and benzo(g,h,i)perylene exceeded the effects-range-medium concentration and eight PAHs were above the corresponding effects-range-low concentrations. Those PAHs may have been derived from the great fire associated with the large earthquake in 1995.