Sediment and carbon storages in the Yahagi River Delta during the Holocene,central Japan

We estimated stored sediment and carbon during the Holocene for each layer of the Yahagi River Delta, central Japan and discussed the provenance of stored carbon. To estimate the bulk density and the carbon content of each layer, we collected two 30 m deep undisturbed cores. The volume of each layer was calculated using ArcView 3D analyst. Although the volume ratio of each layer to the total volume was calculated to be 9.5% for the top mud layer, 34.9% for the upper sand layer, 32.8% for the middle mud layer and 22.9% for the lower sand layer, the mass ratio of each layer to the total mass was calculated to be 8.5, 40.9, 25.2 and 25.4%, respectively, and the stored carbon ratio in each layer to the total stored carbon was 20.4, 4.7, 55.9 and 18.9%, respectively. These results suggest that the top mud and middle mud layers have a significant role as a place for carbon sequestration during postglacial time. Total stored carbon in the study area of only 92.1 km² was estimated at 26 Tg C, which is equivalent to 0.003% of atmospheric carbon. This suggests that deltas on the globe have accumulated a massive amount of carbon during the evolution. The inorganic carbon ratio to total carbon reached more than 45% around the boundary between the middle mud and lower sand layers. The increasing trend in the C_org/N_total ratio accompanied with a decrease in δ¹³C from the bottom to the top horizon in the middle mud layer indicates a gradual increase in terrestrial organic matter contribution. The relative proportion of terrestrially derived materials decreases with increasing distance seaward.     

Modeling Local Tidal Constituents Using TOPEX/POSEIDON SSH for the Malaysian Marine Region

A new Local Ocean Tide Model, has been produced for the Exclusive Economic Zone (EEZ) of Malaysia, which incorporates some of the latest TOPEX/POSEIDON data for the years 1992 to 1998. Local tide gauge data are used as a comparison, along with another leading Global Ocean Tide Model, Ori96. The leading diurnal and semidiurnal constituents M₂, S₂, N₂, K₁, O₁, P₁ and Q₁ are reproduced using TOPEX/POSEIDON Sea Surface Heights (SSH) in a response analysis type least squares derivation following Munk and Cartwright (1966).     

Identification and possible recurrence of an oversized tsunami on the Pacific coast of northern Japan

Geological evidence of severe tsunami inundation has been discovered in northern Japan. In the dune fields of Shimokita, in northernmost Tohoku, we have found two distinctive sand layers that are tsunami deposits. The run-up height of >20 m and inland inundation of at least 1.4 km are notably larger than any known historical case in Japan. The tsunami-genic earthquake that resulted in these deposits is thought to have taken place in the Kuril Forearc-Trench system nearly 700 years ago. The recurrence interval of major tsunamis originating in the Kuril subduction zone is about 400 years. Given that the most recent unusually large earthquake took place in AD 1611 (corresponding to the Keicho earthquake tsunami), the findings presented here increase the potential and hazard for an outsized tsunami striking the Pacific coast of northern Japan.     

The effect of a thin weak layer covering a basalt block on the impact cratering process

To clarify the effect of a surface regolith layer on the formation of craters in bedrock, we conducted impact-cratering experiments on two-layered targets composed of a basalt block covered with a mortar layer. A nylon projectile was impacted on the targets at velocities of 2 and 4 km s^?1, and we investigated the crater size formed on the basalt. The crater size decreased with increased mortar thickness and decreased projectile mass and impact velocity. The normalized crater volume, π_V, of all the data was successfully scaled by the following exponential equation with a reduction length λ₀: ${\pi }_{\text{V}}=b_0{\pi }_{\text{Y}}^{-b_1}\exp (-\lambda /{\lambda }_0)$, where λ is the normalized thickness T/L_p, T and L_p are the mortar thickness and the projectile length, respectively, b₀ and b₁ are fitted parameters obtained for a homogeneous basalt target, 10^?2.7±0.7 and ?1.4 ± 0.3, respectively, and λ₀ is obtained to be 0.38 ± 0.03. This empirical equation showing the effect of the mortar layer was physically explained by an improved non-dimensional scaling parameter, ${\pi }_{\text{Y}}^1$, defined by ${\pi }_{\text{Y}}^1=Y/({\rho }_{\text{t}}{u}_{\text{p}}^2)$, where u_p was the particle velocity of the mortar layer at the boundary between the mortar and the basalt. We performed the impact experiments to obtain the attenuation rate of the particle velocity in the mortar layer and derived the empirical equation of $\frac{u_{\text{p}}}{v_{\text{i}}}=0.50\exp \left(-\frac{\lambda }{1.03}\right)$, where v_i is the impact velocity of the projectile. We propose a simple model for the crater formation on the basalt block that the surface mortar layer with the impact velocity of u_p collides on the surface of the basalt block, and we confirmed that this model could reproduce our empirical equation showing the effect of the surface layer on the crater volume of basalt.     

Extended probit mortality model for zooplankton against transient change of PCO(2)

The direct injection of CO(2) in the deep ocean is a promising way to mitigate global warming. One of the uncertainties in this method, however, is its impact on marine organisms in the near field. Since the concentration of CO(2), which organisms experience in the ocean, changes with time, it is required to develop a biological impact model for the organisms against the unsteady change of CO(2) concentration. In general, the LC(50) concept is widely applied for testing a toxic agent for the acute mortality. Here, we regard the probit-transformed mortality as a linear function not only of the concentration of CO(2) but also of exposure time. A simple mathematical transform of the function gives a damage-accumulation mortality model for zooplankton. In this article, this model was validated by the mortality test of Metamphiascopsis hirsutus against the transient change of CO(2) concentration.     

Influence of climate fluctuation on clay formation in the Baikal drainage basin

Sedimentary cores BDP 96 and 98 and VER 96-2 St. 3 from Academician Ridge in Lake Baikal were investigated to investigate the effect of climatic fluctuations on rock weathering and clay formation in the Baikal drainage basin. Illite, smectite, vermiculite, and kaolinite were identified as the major clay minerals in the sediments by X-ray diffraction analysis. Biotite in gravels in alluvial soils of the Baikal drainage area weathers through illite to vermiculite, smectite, and finally to kaolinite. To investigate the relationship between weathering and climate, we measured the clay content and the concentration of biogenic silica in the sediments. High surface productivity (increased biogenic silica) and high chemical weathering (decreased clay content) occurred simultaneously, showing that crustal weathering and soil formation were enhanced under warm climatic conditions.Clay formation was enhanced in the watershed from the Late Miocene to the Middle Pliocene, and mechanical weathering of rocks increased during glacial intervals after the climate began to cool in Late Pliocene time. This change in the weathering mode in the watershed reduced the nutrient flux and aquatic productivity of Lake Baikal.     

Ocean Plate Stratigraphy in East and Southeast Asia

Ancient accretionary wedges have been recognised by the presence of glaucophane schist, radiolarian chert and mélange. Recent techniques for the reconstruction of disrupted fragments of such wedges by means of radiolarian biostratigraphy, provide a more comprehensive history of ocean plate subduction and successive accretion of ocean floor materials from the oceanic plate through offscraping and underplating.Reconstructed ocean floor sequences found in ancient accretionary complexes in Japan comprise, from oldest to youngest, pillow basalt, limestone, radiolarian chert, siliceous shale, and shale and sandstone. Similar lithologies also occur in the mélange complexes of the Philippines, Indonesia, Thailand and other regions. This succession is called ‘Ocean Plate Stratigraphy’ (OPS), and it represents the following sequence of processes: birth of the oceanic plate at the oceanic ridge; formation of volcanic islands near the ridge, covered by calcareous reefs; sedimentation of calcilutite on the flanks of the volcanic islands where radiolarian chert is also deposited; deposition of radiolarian skeletons on the oceanic plate in a pelagic setting, and sedimentary mixing of radiolarian remains and detrital grains to form siliceous shale in a hemipelagic setting; and sedimentation of coarse-grained sandstone and shale at or near the trench of the convergent margin.Radiolarian biostratigraphy of detrital sedimentary rocks provides information on the time and duration of ocean plate subduction. The ages of detrital sediments becomes younger oceanward as younger packages of OPS are scraped off the downgoing plate.OPS reconstructed from ancient accretionary complexes give us the age of subduction and accretion, direction of subduction, and ancient tectonic environments and is an important key to understanding the paleoenvironment and history of the paleo-oceans now represented only in suture zones and orogenic belts.     

Paleoseismology of the Itoigawa-Shizuoka tectonic line in central Japan

The 150-km-long Itoigawa-Shizuoka tectonic line active fault system (ISTL)in central Japan is one of the most active Quaternary fault systems inJapan. Estimated slip-rates on the fault system are as large as 10 m/ka, butthe historic seismicity has been low since 841 A.D. with no largeearthquakes recorded. The high slip rates contrast with the long time sincethe last major earthquake on the ITSL and indicates the high potential of a largeearthquake from the ISTL. Based on slip-rate estimates, more than 10 mof potential slip may have accumulated on the fault system since the 841A.D. earthquake. Recent paleoseismolgical studies on the middle andnorthern parts of the ISTL have determined that the average recurrenceinterval of surface-faulting earthquakes on the middle ISTL is 680 to 825years (Gofukuji fault) and 1258 to 1510 years in the northern ISTL. These data suggest the most recent event on both northern and middleISTL occurred in 841 A.D. The results highlight the high seismic potentialof the ISTL. Additional studies of the entire ISTL are needed to define theextent of the next rupture.     

Soil Condition and Seismic Characteristics in Shiga Prefecture,Japan

The seismic observation network in Shiga prefecture iscomposed of four systems.All the observation data obtained were used to estimatethe dynamic characteristicsof the surface soils around the lake and the seismichazard for Hikone City. Since1995, 21 records have been obtained at the USP site ofwhich those for the 1999Shigaken-Hokubu Earthquake and 1999 Wakayarnaken-HokubuEarthquake wereused in this study. Transfer functions of the surfacesoil in each ward of ShigaPrefecture based on the Mt. Kojin site (KJY), whichhas relatively hard soil, werecalculated from the records of the two earthquakes.Assuming a scenario earthquakeat the Hyakusaiji fault close to Hikone City, maximumground motions were obtainedfor 15 city blocks in order to establish a damageestimation for that city. The numberof collapsed wooden houses and the damage ratiowere calculated based on the distributionof construction of wooden houses by year. Blockswith a larger ratio of old, wooden housesin Hikone showed a close correlation with blockswith a large ratio of aged persons inthem, indicative that weak persons, such as theelderly, may find themselves in a severesituation during a natural disaster.     

Critical damping of structures with elastically supported visco‐elastic dampers

This paper presents a new formulation for critical damping of structures with elastically supported visco‐elastic dampers.Owing to the great dependence of damper performance on the support stiffness, this model is inevitable for reliable modelling of structures with visco‐elastic dampers. It is shown that the governing equation of free vibration of this model is reduced to a third‐order differential equation and the conventional method for defining the critical damping for second‐order differential equations cannot be applied to the present model. It is demonstrated that the region of overdamped vibration is finite in contrast to that (semi‐infinite) for second‐order differential equations and multiple critical damping coefficients exist. However, it turns out that the smaller one is practically meaningful. Copyright © 2001 John Wiley & Sons, Ltd.