Effective stress analyses of liquefaction-induced deformation in river dikes

A series of effective stress analyses is carried out on the seismic performance of river dikes based on the case histories during the 1993 Hokkaido-Nansei-oki and 1995 Hyogoken-Nambu earthquakes in Japan. Seven case histories selected for the analyses involve a crest settlement ranging from none to 2.7 m in the dikes 3–6 m high with evidence of liquefaction at foundation soil. The effective stress model used is based on a multiple shear mechanism and was developed by one of the authors. The soil parameters are evaluated based on the site investigation and laboratory test results. The results of the analyses are basically consistent with the observed performance of the river dikes. In particular, the effective stress model shows a reasonable capability to reproduce the varying degree of settlements depending on the geotechnical conditions of foundation soils beneath the dikes. The analyses also indicate that the effect of a cohesive soil layer mixed with the liquefiable sand layers beneath the dikes can be a primary factor for reducing the liquefaction-induced deformation of dikes.     

Organic facies and geochemical aspects in Neogene neritic sediments of the Takafu syncline area of central Japan: Paleoenvironmental and sedimentological reconstructions

Abstract Organic petrological observations of kerogen macerals and organic geochemical analyses of carbon isotopes of kerogen macerals and biomarkers were conducted on Neogene neritic sediments of the Takafu syncline area of central Japan. The Senmi, Sakainomiya and Lower Shigarami Formations in that area were deposited at the neritic provinces on the southern edge of the paleo‐Japan Sea during the Late Miocene to Early Pliocene. Sedimentary organic matter in these formations was almost terrigenous in origin. Changes in kerogen maceral compositions reflect sedimentological and tectonic histories evaluated in previous studies from sedimentary facies and paleontology. It was found that carbon isotope ratios (δ¹³C) of kerogen macerals increased from ?28‰ to ?25‰ from the Sakainomiya to the lower part of the Lower Shigarami Formations. The cause of that increase was presumably the expansion of C₄ plants into southwest Japan. The timing was concordant with that of the expansion of C₄ plant grasslands in East Asia. The oxicity (oxic to anoxic) conditions of sea bottoms evaluated from pristane/phytane ratios varied. Particularly, in the lower part of the Senmi Formation, layers in which no steroid biomarkers could be detected were found, and had presumably formed under oxic conditions when strong biodegradation had occurred. Concentrations of regular (C₂₇–C₂₉) steranes and dinosteranes were higher in the Sakainomiya and Lower Shigarami Formations. This indicates that dinoflagellates‐dominant primary productions were higher at those stages. In addition, concentrations of diatomaceous biomarkers such as C₂₆ norsterane increased from the Lower Shigarami Formation, thus adding diatoms to the major producers. Furthermore, similar associations between the increases of δ¹³C values of kerogen macerals and concentrations of diatomaceous biomarkers were observed in the Takafu syncline area. Thus, the expansion of C₄ plants was possibly associated with the high production of diatom in the shallow‐marine areas of the paleo‐Japan Sea during the Neogene Period.     

Production and transport of long-chain alkenones and alkyl alkenoates in a sea water column in the northwestern Pacific off central Japan

Sinking particles collected from year-long time-series sediment traps at 1674, 4180, 5687 and 8688 m depths, the underlying bottom sediment at 9200 m depth, and suspended particles from surface and subsurface waters in the northwestern North Pacific off Japan were analyzed for long-chain alkenones and alkyl alkenoates (A&A) which are derived mainly from Gephyrocapsacean algae, especially Emiliania huxleyi and Gephyrocapsa oceanica. Alkenone temperature records in sediment trap samples at 1674 m were almost similar to observed sea surface temperatures (SST) with a time delay of one half to one full month. However, alkenone temperatures in trap samples were about slightly lower than measured SST in late spring to early fall. The lowering might be caused by formation of the seasonal thermocline. Nevertheless, these temperature drops observed in trap samples were smaller than those actually observed in a subsurface layer off central Japan. Vertical profiles of A&A concentrations and alkenone temperatures in suspended particles collected from the subsurface waters in early fall indicated that these compounds were produced mostly in a surface mixed layer above the depth of the chlorophyll maximum even in warm seasons. These results suggested that alkenone temperatures strongly reflected SST rather than the temperatures of thermocline waters in these study areas even in such a warm season. Pronounced maxima in A&A fluxes found in sediment trap samples at 1674 m in late spring to summer showed that A&A productions were highest during the periods of spring bloom, according to a time delay between alkenone temperatures and observed SST. Seasonal patterns of alkenone records in trap samples at 4180 and 5687 m could also preserve SST signals well, suggesting that A&A in deep sea waters were mainly derived from primary products in the surface layer. A&A fluxes tended to decrease with water depth, and the ratios of A&A to particulate organic carbon (POC) rapidly decreased in underlying bottom sediment. This clearly indicates that A&A were decomposed and diluted by other refractory organic materials in either the water column or the sediment–water interface. However, A&A compositions were consistently uniform between the trap samples and the underlying bottom sediments, so that A&A could not qualitatively alter during early diagenetic processes.     

Non‐iterative time integration scheme for non‐linear dynamic FEM analysis

This paper proposes a non‐iterative time integration (NITI) scheme for non‐linear dynamic FEM analysis. The NITI scheme is constructed by combining explicit and implicit schemes, taking advantage of their merits, and enables stable computation without an iteration process for convergence even when used for non‐linear dynamic problems. Formulation of the NITI scheme is presented and its stability is studied. Although the NITI scheme is not unconditionally stable when applied to non‐linear problems, it is stable in most cases unless stiffness hardening occurs or the problem has a large velocity‐dependent term. The NITI scheme is applied to dynamic analysis of the non‐linear soil–structure system and computation results are compared with those by the central difference method (CDM). Comparison shows that the stability of the NITI scheme is superior to that of the CDM. Accuracy of the NITI scheme is verified because its results are identical with those by the CDM in which the time step is set as 1/10 of that for the NITI scheme. The application of the NITI scheme to the mesh‐partitioned FEM is also proposed. It is applied to dynamic analysis of the linear soil–structure system. It yields the same results as a conventional single‐domain FEM analysis using the Newmark β method. This result verifies the usability of mesh‐partitioned FEM analysis using the NITI scheme. Copyright © 2003 John Wiley& Sons, Ltd.     

Petrology of the greenstones of the Lower Sorachi Group in the Sorachi–Yezo Belt, central Hokkaido, Japan, with special reference to discrimination between oceanic plateau basalts and mid-oceanic ridge basalts

Abstract   The Lower Sorachi Group of the Sorachi–Yezo Belt in central Hokkaido, Japan is a peculiar accretionary complex characterized by numerous occurrences of greenstones (metabasalts and diabases), which are mostly composed of aphyric basalts. Clinopyroxene-rich phenocryst assemblage in phyric basalts is different from olivine–plagioclase assemblage in mid-oceanic ridge basalts (MORB). The greenstones are geochemically uniform, and show a lower-Ti trend than MORB in an FeO*/MgO-TiO₂ diagram, mostly plotting on the island arc tholeiite (IAT) field in a TiO₂−10MnO−10P₂O₅ diagram. In a MORB-normalized spider diagram, the greenstones show a flat pattern from P to Y, which are lower than those of normal mid-oceanic ridge basalt (N-MORB). These indicate that the greenstones were derived by a higher degree of partial melting from a depleted mantle similar to a N-MORB source, and experienced olivine–clinopyroxene fractional crystallization. However, a positive spike of Nb in the spider diagram cannot be explained, and may be attributed to mantle heterogeneity. These characteristics are analogous to those of oceanic plateau basalts (OPB) such as in Ontong Java Plateau, Manihiki Plateau and Nauru Basin, suggesting that the greenstones in the Lower Sorachi Group are of oceanic plateau origin. The present study proposes new field divisions to distinguish OPB from MORB in the conventional FeO*/MgO–TiO₂ and TiO₂−10MnO−10P₂O₅ diagrams.     

Effects of coastal erosion on landslide activity revealed by multi-sensor observations

Coastal erosion is becoming an increasingly serious consequence of climate change. This study demonstrates the effects of coastal erosion on landslide activity while considering the amount of erosion and changes in pore water pressure. To determine the factors related to landslide slip generation, we specifically measured the displacement, deformation, pore water pressure, and amount of erosion with high temporal resolution (1 s–1 h) for a coastal landslide in Hokkaido, north-eastern Japan, for 7 months. It has been determined that landslides occur simultaneously with high pore water pressure. Toe erosion events also occurred several times, while the landslide exhibited major displacement. Because toe erosion and the increase in pore water pressure occurred simultaneously, we tried to determine which of the two contributed majorly to the landslide displacement by conducting a stability analysis that incorporates the effects of the two factors. From the actual observed data, toe erosion and the increase in pore water pressure had comparable effects on the destabilization of the studied landslide. Specifically, the time series for the safety factor shows that the landslide in the case with toe erosion was destabilized more than that in the case with no erosion, with a difference of more than 5% in the safety factor. The model with toe erosion provided a better explanation for the landslide displacement. Furthermore, the inclination data suggested that erosion took place at least 1 month before the landslide displacement. This implied that coastal erosion played a role in the preparation and ongoing displacement of the coastal landslide. Inland landslides with toes that are subject to undercutting due to river incision or artificial construction have geomorphological settings that are similar to those of the studied landslide. The knowledge obtained here can contribute to the understanding of destabilization mechanisms and terrain changes related to such landslides. © 2020 John Wiley & Sons, Ltd.     

Assessment of the vertical distribution on seismic ground motion

It is very important for the facilities such as nuclear power plants to infer seismic force loading on the earthquake stability assessment of the building foundation and the surrounding slope. The purpose of this paper was to propose a method to evaluate underground seismic coefficients, taking into account dynamic response along the depth in horizontally multi-layered ground. The dynamic property of the seismic coefficient was analyzed on the basis of earthquake records observed at hard and soft rock sites mostly found in Tertiary deposits and sedimentary ground sites of the Pleistocene and Holocene epoch. The evaluation methods of a vertical distribution on underground seismic coefficients were proposed for a few calculation methods on the classified layered ground. Extended evaluation for underground seismic coefficients was confirmed with respect to some multi-layered ground during strong motion.     

Seasonal variation of bivalve larvae on an exposed sandy beach on Kashima-nada: Tips for the sandy beach recruitment process

Bivalves are often the dominant macrobenthos species in exposed sandy beach environments. However, our understanding of their recruitment processes before post-settlement stages on sandy beaches with highly energetic environments is incomplete. To clarify the characteristics of the free-swimming planktonic stage that affects recruitment efficiency in sandy shore ecosystems, we investigated the temporal (weekly–biweekly) variation of bivalve planktonic larval concentration coupled with oceanographic conditions on an exposed sandy shore on the sea of Kashima-nada, Japan, from summer 2003 to autumn 2005. Larvae were observed throughout the year, but the surge of larval concentration composed of sandy beach and sessile bivalves occurred most prominently in summer, from August to September. The peak concentration of larvae during this season was more than 1000 times higher than in other seasons. The larval concentration was positively correlated with water temperature and northward wind velocity and negatively correlated with each of the nutrient concentrations. On the other hand, chlorophyll a concentration and salinity seemed to have little effect on the larval concentration. Based on this fundamental knowledge, further investigations about planktonic larvae in sandy beaches are needed.     

响应谱与土木构筑物的抗震设计

前言地震学家与土木工程学和建筑学家之间的隔膜之一就是所用的频谱不同。在地震学上为表示地震动的频率特性通常用傅里叶谱,而在工程学方面则用响应谱。尽管各自用各自的频谱给自己带来了方便,但其结果却意想不到地产生了很大的阻碍。本文首先简单介绍响应谱,然后再介...