Calibration shifts in Scintrex CG‐3M gravimeters with an application to detection of microgravity changes at Iwo‐tou caldera,Japan

Calibration shift seriously influences gravity values measured using Scintrex CG‐3M gravimeters. We calibrated three Scintrex CG‐3M gravimeters three times (1999, 2003 and 2006) over eight years, using a calibration line with a gravity difference of 1.38 Gal. The scale factor correction coefficients (calibration factors) obtained here range from 0.9998–1.0005. The calibration factors vary with time by 89 ppm, ?102 ppm and ?126 ppm between the 1999–2003 surveys. The calibration shifts of two of the three gravimeters decreased to about 20 ppm or less in the second interval, the other remained about the same (142 ppm). The results indicate that they shifted at rates on the order of 10 ppm/year even several years after manufacturing. The large shift in calibration factors indicates that they must be corrected using calibrations done before and after the measurements to perform microgravity measurements when gravity differences between a reference gravity site and survey sites are on the order of a hundred milligals (mGal) or more. The results also indicate that the calibration factors change gradually with time, so their interpolation provides a good practical approximation for a specific survey time. We applied the time‐dependent calibration factors to microgravity monitoring at the Iwo‐tou caldera, Japan, where the gravity difference between the base site on the island and the reference site on Honshu (the mainland of Japan) is about 870 mGal mainly due to the 11° latitude difference. Gravity surveys were conducted every two years from 1998–2006. The correction of scale factors estimated from the repeated calibration surveys leads to satisfactory measurements, in which the average of the absolute differences between two Scintrex CG‐3M instruments in five surveys is reduced from 207μGal to 19μGal; for three of those surveys, it is less than 10 μGal. This result demonstrates the importance of repeated calibration surveys.     

Effect of dust event timing on glacier runoff: sensitivity analysis for a Tibetan glacier

The impact of the timing of dust deposition on glacier runoff was evaluated using a glacier mass‐balance model with a newly improved scheme to track a dusted layer in a snow layer of a glacier. The lowering of surface albedo due to the dusted layer appearing leads to a drastic increase of glacier runoff even under the same meteorological conditions. Calculations of seasonal sensitivity, the relationship between dusted date and resulting runoff, have shown that dust deposition during a melting season might cause a drastic mass outflow from a glacier through changing the surface albedo during the melting season. Copyright © 2006 John Wiley & Sons, Ltd.     

Effects of transport of pore water and material heterogeneity on strain localization of fluid‐saturated gradient‐dependent viscoplastic geomaterial

The purpose of the present paper is to clarify the effects of permeability and initial heterogeneity on the strain localization of fluid‐saturated cohesive soil modelled by a strain gradient‐dependent poro‐viscoplastic constitutive model. The effects of permeability and gradient parameters on the growth rate of the fluctuation were obtained by a linear instability analysis. Deformation behaviour of clay specimens modelled as a viscoplastic model with a second order strain gradient during shear was numerically analysed by a soil–water coupled FEM under both globally undrained and partially drained conditions. It was found that the deformation pattern and the stress–strain curve greatly depend on the permeability, the drainage conditions and the initial non‐homogeneous properties. Copyright © 2005 John Wiley & Sons, Ltd.     

Unstructured grid generation using LiDAR data for urban flood inundation modelling

Inundation disasters, caused by sudden water level rise or rapid flow, occur frequently in various parts of the world. Such catastrophes strike not only in thinly populated flood plains or farmland but also in highly populated villages or urban areas. Inundation of the populated areas causes severe damage to the economy, injury, and loss of life; therefore, a proper management scheme for the disaster has to be developed. To predict and manage such adversity, an understanding of the dynamic processes of inundation flow is necessary because risk estimation is performed based on inundation flow information. In this study, we developed a comprehensive method to conduct detailed inundation flow simulations for a populated area with quite complex topographical features using LiDAR (Light Detection and Ranging) data. Detailed geospatial information including the location and shape of each building was extracted from the LiDAR data and used for the grid generation. The developed approach can distinguish buildings from vegetation and treat them differently in the flow model. With this method, a fine unstructured grid can be generated representing the complicated urban land features precisely without exhausting labour for data preparation. The accuracy of the generated grid with different grid spacing and grid type is discussed and the optimal range of grid spacing for direct representation of urban topography is investigated. The developed method is applied to the estimation of inundation flows, which occurred in the basin of the Shin‐minato River. A detailed inundation flow structure is represented by the flow model, and the flow characteristics with respect to topographic features are discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Improvement of GPS/acoustic seafloor positioning precision through controlling the ship’s track line

The precision of GPS/acoustic seafloor positioning was improved by introducing a hull-mounted onboard system in March 2008, which allows us to conduct acoustic ranging measurements with the vessel sailing along the pre-determined track lines, while the early system before 2008 could only adopt the uncontrollable drifting observation. The continuity of the positioning results due to the transition was first confirmed through the comparison between results from sailing and drifting observations conducted in parallel. Using the data acquired for about 3 years since 2008, the repeatability of the determined position for the sailing observation was evaluated to be about 2 cm in root mean squares in the horizontal component, significantly better than that for the early drifting observation. The improvement of positioning precision probably resulted from the improvement of geometric distribution of acoustic ranging data by controlling the track lines. It was also shown that the sailing observation allows to obtain reliable results with a smaller amount of data. Comparison between the results in different sea regions suggests that positioning precision is better in the region along the Nankai Trough than in the region along the Japan Trench, probably because of the complicated acoustic velocity structure of seawater often observed in the latter. Furthermore, the precision of height determination was also improved, which leads us to expect that vertical crustal movement will be detectable in the future through accumulation of data as well as further technology development.     

Paleoenvironmental interpretations of Quaternary reef deposits based on comparisons of 10 selected modern and fossil larger foraminifera from the Ryukyu Islands, Japan

Abstract A new method for reconstructing depositional environments of larger foraminifera‐bearing limestones is proposed. First, depth and spatial distributions of empty tests of 10 foraminiferal taxa in a 1–2 mm size fraction were examined using 32 surface sediment samples collected from depths shallower than 200 m, located to the west of Miyako Island, Ryukyu Islands, northwest Pacific. Distributional ranges of empty tests in the 1–2 mm size fraction appear to be more limited than those including other size fractions in previous reports, partly because larger empty tests of each taxon are less easily transported than smaller ones. Multivariate analyses (Q‐mode cluster analysis and non‐metric multidimensional scaling ordination) based on binary (presence/absence) data of the 10 taxa delineate four sample groups, each of which corresponds to different depositional environments: nearshore zone/bay inlet; back‐reef to fore‐reef; outer shelf to shelf slope; and shelf slope distant from coral reefs. Next, these modern data were applied to reconstruct the depositional environment of a rock section distributed in the Shiratorizaki area (Irabu Island, Ryukyu Islands), which consists of larger foraminiferal limestone of the Pleistocene Ryukyu Group. Multivariate analyses were performed on the fossil plus the modern foraminiferal data to explore the possible relationship of the fossil associations with variations in modern associations, demonstrating that the fossil foraminiferal associations resemble the modern outer shelf associations. The modern analog technique was also applied to estimate paleobathymetry using fossil foraminiferal data. The results indicate that the section studied was deposited in outer shelf environments at depths between 53.5 and 98.6 m. These paleoenvironmental interpretations are consistent with previous studies based on sedimentary facies and the computer‐based expert system. The modern dataset and methods used in our work would be particularly useful for paleoenvironmental reconstructions of Quaternary reef and shelf carbonates along active margins of the northwest Pacific.     

Impact of mesoscale eddies on particulate organic carbon flux in the western subarctic North Pacific

The mechanism that controls particulate organic carbon (POC) flux in the deep sea differs depending on the season and sea. The POC produced in the western subarctic North Pacific are known to be transported to the deep sea efficiently, but the direct relationship between the POC flux and physical processes is still unclear. In this study, we evaluated the effect of mesoscale eddies on POC flux in the western subarctic North Pacific. The seasonal and interannual variabilities of POC flux were investigated using data from a time-series sediment trap deployed at 4810 m at station K2 (47°N, 160°E) from 2005 to 2018. POC flux was high during May–November, appearing to reflect spring and fall blooms at the ocean surface. POC flux also showed interannual variability, with twelve peaks that were mostly affected by enhanced bloom just before the peak. Nine peaks of the twelve peaks were affected by mesoscale eddies, which enhanced bloom around K2 by extending the area with a high chlorophyll-a concentration along the coastal region into the offshore region, suggesting that mesoscale eddies strongly impact the interannual variability of POC flux at K2.

     

Horizontal distribution and population dynamics of the dominant mysid Hyperacanthomysis longirostris along a temperate macrotidal estuary (Chikugo River estuary, Japan)

The estuarine turbidity maximum (ETM) that develops in the lower salinity areas of macrotidal estuaries has been considered as an important nursery for many fish species. Mysids are one of the dominant organisms in the ETM, serving as a key food source for juvenile fish. To investigate the horizontal distribution and population dynamics of dominant mysids in relation to the fluctuation of physical conditions (temperature, salinity, turbidity, and freshwater discharge), we conducted monthly sampling (hauls of a ring net in the surface water) along the macrotidal Chikugo River estuary in Japan from May 2005 to December 2006. Hyperacanthomysis longirostris was the dominant mysid in the estuary, usually showing peaks of density and biomass in or close to the ETM (salinity 1–10). In addition, intra-specific differences (life-cycle stage, sex, and size) in horizontal distribution were found along the estuary. Larger males and females, particularly gravid females, were distributed upstream from the center of distribution where juveniles were overwhelmingly dominant. Juveniles increased in size toward the sea in marked contrast with males and females. The findings suggest a possible system of population maintenance within the estuary; gravid females release juveniles in the upper estuary, juveniles grow during downstream transport, young males and females mature during the upstream migration. Density and biomass were primarily controlled by seasonal changes of temperature, being high at intermediate temperatures (ca. 15–25 °C in late spring and fall) and being low at the extreme temperatures (ca. 10 °C in midwinter and 30 °C in midsummer). High density (up to 666 ind. m⁻³) and biomass (up to 168 mg dry weight m⁻³) of H. longirostris were considered to be comparable with those of copepods in the estuary.     

Imagining the borderlands: Contending stories of a resource frontier in Muang Sing

Muang Sing in Luang Namtha province, an administrative district of northern Laos bordering Myanmar and China, has been portrayed by the Lao government and international development agencies as a ‘poor’ rural region in need of development. To developers, Muang Sing's remoteness from major towns and the livelihoods of ethnic people such as the Akha in the uplands based on swidden agriculture and opium production characterized ‘poverty’. To address this rural poverty, state and development agencies devised land use zoning that would demarcate and regulate various land uses in line with a rural development plan for the district. This vision for regulated development began to go awry, however, as farmers and traders in Muang Sing launched their own rapid social and economic changes. In contrast to the official image of a backward rural district in need of outside assistance, this paper portrays farmers and local entrepreneurs of the Muang Sing borderlands as actively transforming their lives and agricultural landscapes. This paper challenges the official version of a remote, poor district untouched by regional trade through a focus on narratives of local people. Ethnographic research reveals the dynamic micro‐processes of agrarian transformation during recent decades to highlight the centrality of borderland people in reworking their lives and agricultural landscapes through cross‐border relationships in China.