Upper and middle crustal deformation of an arc–arc collision across Hokkaido, Japan, inferred from seismic refraction/wide-angle reflection experiments

The Hidaka Collision Zone (HCZ), central Hokkaido, Japan, is a good target for studies of crustal evolution and deformation processes associated with an arc–arc collision. The collision of the Kuril Arc (KA) with the Northeast Japan Arc (NJA), which started in the middle Miocene, is considered to be a controlling factor for the formation of the Hidaka Mountains, the westward obduction of middle/lower crustal rocks of the KA (the Hidaka Metamorphic Belt (HMB)) and the development of the foreland fold-and-thrust belt on the NJA side. The “Hokkaido Transect” project undertaken from 1998 to 2000 was a multidisciplinary effort intended to reveal structural heterogeneity across this collision zone by integrated geophysical/geological research including seismic refraction/reflection surveys and earthquake observations. An E–W trending 227 km-long refraction/wide-angle reflection profile found a complicated structural variation from the KA to the NJA across the HCZ. In the east of the HCZ, the hinterland region is covered with 4–4.5 km thick highly undulated Neogene sedimentary layers, beneath which two eastward dipping reflectors were imaged in a depth range of 10–25 km, probably representing the layer boundaries of the obducting middle/lower crust of the KA. The HMB crops out on the westward extension of these reflectors with relatively high Vp (>6.0 km/s) and Vp/Vs (>1.80) consistent with middle/lower crustal rocks. Beneath these reflectors, more flat and westward dipping reflector sequences are situated at the 25–27 km depth, forming a wedge-like geometry. This distribution pattern indicates that the KA crust has been delaminated into more than two segments under our profile. In the western part of the transect, the structure of the fold-and-thrust belt is characterized by a very thick (5–8 km) sedimentary package with a velocity of 2.5–4.8 km/s. This package exhibits one or two velocity reversals in Paleogene sedimentary layers, probably formed by imbrication associated with the collision process. From the horizontal distribution of these velocity reversals and other geophysical/geological data, the rate of crustal shortening in this area is estimated to be greater than 3–4 mm/year, which corresponds to 40–50% of the total convergence rate between the NJA and the Eurasian Plate. This means that the fold-and-thrust belt west of the HCZ is absorbing a large amount of crustal deformation associated with plate interaction across Hokkaido Island.     

Lake-level changes during the past 100,000 years at Lake Baikal, southern Siberia

Lake-level changes inferred from seismic surveying and core sampling of the floor of Lake Baikal near the Selenga River delta can be used to constrain regional climatic history and appear to be correlated to global climate changes represented by marine oxygen isotope stages (MIS). The reflection pattern and correlation to the isotope stages indicate that the topset and progradational foreset sediments of the deltas formed during periods of stable lake levels and warm climatic conditions. During warm stages, the lake level was high, and during cold stages it was low. The drop in the lake level due to cooling from MIS 5 through MIS 4 is estimated to be 33-38 m; from MIS 3 through MIS 2, it fell an additional 11-15 m. Because the lake level is chiefly controlled by evaporation and river input, we infer that more water was supplied to Lake Baikal during warm stages.     

Generalized optimization of polarimetric contrast enhancement

A generalized optimization of polarimetric contrast enhancement (GOPCE) is proposed in this letter. For this problem, it is not only necessary to find the optimal polarization states such that the received power ratio of a desired target and clutter is maximal, but also necessary to find three optimal coefficients such that the ratio of two factors associated with the desired target and clutter is maximal, where both the factors consist of three parameters, i.e., the Cloude entropy and two special similarity parameters. The optimal coefficients of the GOPCE are obtained by solving an eigenvalue problem. Using an example, we demonstrate that the GOPCE can be employed for detecting roads in a forest area by using polarimetric synthetic aperture radar data.     

Evaluating the susceptibility of landslide landforms in Japan using slope stability analysis: a case study of the 2016 Kumamoto earthquake

This study analyzed 267 landslide landforms (LLs) in the Kumamoto area of Japan from the database of about 0.4 million LLs for the whole of Japan identified from aerial photos by the National Research Institute for Earth Science and Disaster Resilience of Japan (NIED). Each LL in the inventory is composed of a scarp and a moving mass. Since landslides are prone to reactivation, it is important to evaluate the sliding-recurrence susceptibility of LLs. One possible approach to evaluate the susceptibility of LLs is slope stability analysis. A previous study found a good correlation (R ² = 0.99) between the safety factor (F _s ) and slope angle (α) of F _s  = 17.3α ^?0.843. We applied the equation to the analysis of F _s for 267 LLs in the area affected by the 2016 Kumamoto earthquake (M _j  = 7.3). The F _s was calculated for the following three cases of failure: scarps only, moving mass only, and scarps and moving mass together. Verification with the 2016 Kumamoto earthquake event shows that the most appropriate method for the evaluation of LLs is to consider the failure of scarps and moving mass together. In addition, by analyzing the relationship between the factors of slope of entire landslide and slope of scarp for LLs and comparing the results with the Aso-ohashi landslide, the largest landslide caused by the 2016 Kumamoto earthquake, we also found that morphometric analysis of LLs is useful for forecasting the travel distance of future landslides.     

Control strategy for variable damping element considering near‐future excitation influence

A control strategy is proposed for variable damping elements (VDEs) used together with auxiliary stiffness elements (ASEs) that compose a time‐varying non‐linear Maxwell (NMW) element, considering near‐future excitation influence. The strategy first composes a state equation for the structural dynamics and the mechanical balance in the NMW elements. Next, it establishes a cost function for estimating future responses by the weighted quadratic norms of the state vector, the controlled force and the VDEs' damping coefficients. Then, the Euler equations for the optimum values are introduced, and also approximated by the first‐order terms under the autoregressive (AR) model of excitation information. Thus, at each moment t_k, the strategy conducts the following steps: (1) identify the obtained seismic excitation information to an AR model, and convert it to a state equation; and (2) determine VDEs' damping coefficients under the initial conditions at t_k and the final state at t_k+L, using the first‐order approximation of the Euler equations. The control effects are examined by numerical experiments. Copyright © 2000 John Wiley & Sons, Ltd.     

GAME-TIBET青藏高原云和降水综合观测概况及初步结果

对中日合作青藏高原能量水分循环试验项目中云和降水的多普勒雷达观测使用的气象观测仪器和获取的资料等情况进行了总结,并给出了那曲地区水的日变化,不同 雨量计测量降水的对比分析,风速对测量降水影响,多普勒雷达资料的个例分析及云的数值模拟的等初步结果。     

Contribution of different functional groups to the diet of major predatory fishes at a seagrass meadow in northeastern Japan

We examined the variation in habitat use and diet of three dominant fish species (Myoxocephalus brandti, Pholidapus dybowskii, and Pholis crassispina) in a seagrass meadow in the Akkeshi-ko estuary in northeastern Japan, where broad and dense Zostera marina beds exist, using a semi-quantitative census of the fishes and analyses of their stomach contents. Differences among the three fish species in the temporal variation in abundance of each age class (mainly 1- and 2-year age classes) indicated that the temporal pattern of utilization of the seagrass meadow were different among them. In the semi-quantitative dietary analysis, two prey categories, i.e., taxonomic group (order and suborder) and functional group, were used to explain the variation in prey composition with size-dependent changes. The six prey functional groups were classified based on the ecological traits of the prey, i.e., trophic level, size, and life type (habitat and behavior). Ontogenetic shifts in prey of the three fish species could be fully explained by a combination of the two prey categories, and not by the use of only one category (taxonomic or functional group). The pattern of ontogenetic shifts in prey differed among the fish species and size (age) classes. These results indicate that segregation of habitat (seagrass meadow) and prey group (taxonomic and functional group) is performed among the three species, which may contribute to their coexistence in this estuary.     

Effects of seawater acidification on hydrolytic enzyme activities

We have investigated the effects of seawater acidification on the activities of leucine aminopeptidase (LAPase), β-glucosidase (BGase), phosphatase (P-ase), α-glucosidase (AGase), and lipase (L-ase), which are important promoters of degradation of marine organic matter, including proteins, carbohydrates, organic phosphorus compounds, and lipids. Seawater samples were collected from a eutrophic coastal area, from Tokyo Bay, and from oligotrophic pelagic waters outside the Kuroshio Current. Enzyme activities were measured using fluorogenic substrates added to the seawater samples, which were acidified from pH 8.2 to 5.6 by a chemical buffer. Spontaneous hydrolysis of the substrates was shown to be negligible in heat-inactivated control samples, thus validating our results. LAPase was the most sensitive to acidification; enzyme activity rapidly decreased from pH 8.2 to 7.8, corresponding to a realistic scenario of ocean acidification. L-ase activity also decreased with acidification. Activities of P-ase and BGase were relatively constant across the pH levels examined, suggesting that their activity is not appreciably influenced by acidification. The effect of acidification on P-ase activities differed between the coastal and semipelagic samples, and this was likely due to freshwater influence at the nearshore station. Because of the low activity of AGase in the sample, the effect of acidification on this enzyme could not be examined. The effects of acidification on enzyme activity appear to vary depending on enzyme type and location, but we conclude that acidification will cause changes in the cycling of organic matter in marine ecosystems, in particular to proteinous and lipid substances.     

Element transport by dehydration of subducted sediments: Implication for arc and ocean island magmatism

High-pressure experiments on a natural pelite have been conducted at 2–11-GPa pressures in order to evaluate contributions of subducted sediments to arc and ocean island magmatism. Obtained phase relations suggest that, at least in modern subduction zones, subsolidus dehydration of chlorite and phengitic muscovite in the subducted sediments, rather than partial melting, is a predominant process in overprinting sediment components onto the magma source region. Trace element compositions of sediment-derived fluids are estimated based on dehydration experiments at 5.5 GPa and 900/1300°C. Pb is effectively transported by fluids relative to other elements. This results in the Pb enrichment for arc basalts by fluids, generated by the dehydration of subducted sediments, together with altered mid-ocean ridge basalt (MORB), and complementary depletion of Pb in subducted sediments. Inferred arc magma compositions obtained by model calculations based on the present experimental results agree well with a natural primitive arc basalt composition. A large increase in the U/Pb ratio in the subducted sediments at deeper levels than major dehydration depths results in a high Pb isotopic ratio through radioactive decay after long periods of isolation. Combined with other isotopic ratios such as Sr and Nd, it is possible to produce the EM II source, one of the enriched geochemical reservoirs for ocean island basalt magmas, by mixing of a small amount of subducted sediments with depleted or primitive mantle.