Interpretation of the GRACE-derived mass trend in Enderby Land, Antarctica

Monthly gravity solutions of the Gravity Recovery and Climate Experiment (GRACE) reveal three areas in Antarctica with striking interannual mass trends. The positive mass trend in Enderby Land, East Antarctica, is poorly understood because of uncertainties in the surface ice-sheet mass balance, post-glacial rebound (PGR), and processing of GRACE data. In this study, we compare the GRACE mass trends with values estimated from in situ snow-stake measurements, and Ice Cloud and land Elevation Satellite (ICESat) data. The mass trends estimated from ICESat data show a strong correlation with GRACE mass trends. In contrast, the snow-stake data show discrepancies with temporal variations in GRACE mass, especially in 2006. The discrepancies are probably associated with basal ice-sheet outflow, which is difficult to observe using snow stakes. We conclude that the bulk of the GRACE mass trend can be explained by snow accumulation and basal ice-sheet outflow.     

Simultaneous assimilation of surface drifter data, satellite and in situ observations for improved estimates of meso-scale variability in the Kuroshio Extension Region

In order to better estimate meso-scale variabilities in the energetic Kuroshio Extension (KE) region, simultaneous assimilation of drifter-derived velocity data, together with satellite and in situ hydrographic data, is attempted by using a high-resolution 4-dimensional variational data assimilation (4D-VAR) system. Our experimental results, both with and without assimilation of drifter data (Exp. Drf and Exp. Ref, respectively) for the period during Aug–Oct 2005, show that the reproduced fields in Exp. Drf better reflect the observed meso-scale features such as the KE meandering jet and associated eddies. The adjoint sensitivity analysis indicates that our 4D-VAR system has the ability to provide a more realistic timeseries of the meandering jet structures that play a key role in the intergyre exchange between the subtropical and subarctic gyres in the North Pacific. In addition, the observed information from the surface drifters works to improve the subsurface structure. These results illustrate the advantage of our 4D-VAR simultaneous assimilation with the addition of drifter-derived surface velocity information.     

Characteristics of Seismoelectric Wave Fields Associated with Natural Microcracks

Properties of seismoelectric waves in relation to natural earthquakes have been investigated. The electromagnetic disturbances were analyzed to test the hypothesis that pulse-like electric variations are directly related to microcracks as source. Because variation is very difficult to detect, there have been few quantitative field investigations. We used selected events with clear S and P phases from the data catalog obtained before the Tohoku earthquake in 2011. The electric strength of the fast P wave (P_f), S wave (S), and electromagnetic wave (EM) associated with formation of cracks of tensile mode were estimated. The co-seismic electric signal accompanied by the S wave has the largest strength, well above the noise level, and the EM wave has the lowest strength. Analytical estimation of the ratio of the strengths of the P_f and EM phases to that of the S phase by use of Pride’s equations gave results partially in agreement with observation (the order was Ap_f > A_s > A_em). The strength of the observed electromagnetic mode is approximately two orders of magnitude larger than that estimated from the theory. We suggest this greater strength can be attributed to the converted modes at layer contracts or to the effect of the boundary between free atmosphere and crust. Overall agreement between observations and theoretical estimates suggests that electromagnetic anomalies, crustal deformation, and groundwater changes can be investigated on the basis of the unified equations for the coupled electromagnetics, acoustics, and hydrodynamics of porous media.     

Assessing flood disaster impacts in agriculture under climate change in the river basins of Southeast Asia

Natural Hazards - This study focused on flood damage assessment for future floods under the impact of climate change. Four river basins of Southeast Asia were selected for the study. They included...     

Influence of the Oyashio Current and Tsugaru Warm Current on the circulation and water properties of Otsuchi Bay,Japan

Mooring and hydrographic observations were conducted from September 2012 to May 2014 at the mouth of Otsuchi Bay, a ria along the Pacific coast of Japan. Our observations quantitatively demonstrated that the circulation and the water properties of Otsuchi Bay are strongly influenced by the Tsugaru Warm Current (TWC) and Oyashio Current (OY) at seasonal and subseasonal time scales. Two bottom-mounted velocity profilers and temperature and salinity measurements beneath the near-surface halocline showed a counterclockwise lateral circulation pattern related to the TWC, which was enhanced from summer to autumn. From winter to early spring, the lateral circulation patterns related to the TWC weakened and the influence of the OY occasionally increased. When the OY was weak, surface flows became an overturning structure, with outflows in the upper layer and inflows in the lower layer. When the OY was strong and passed close to the Sanriku coast, the circulation became highly variable and intermittent. Intrusions of the markedly low-salinity OY water were observed on two occasions and persisted for periods of several weeks to several months. Salinity was sometimes less than 33.7, the lower limit of the typical TWC from late summer to autumn even when the TWC dominates. We suggested that this is the seasonal fluctuations of the TWC itself, as the upstream current of the Tsushima Warm Current is freshened in summer as a result of the influence of the Changjiang River. The surface water was generally fresher in the south of the bay than in the north, suggesting the Coriolis deflection of the river plume.     

Geochemical modeling of sedimentary rocks in the central Hokkaido,Japan: Episodic deformation and subsequent confined basin-formation along the eastern Eurasian margin since the Cretaceous

Late Cretaceous to Paleogene tectonic episodes around the eastern Eurasian margin are described utilizing one-dimensional (1D) basin modeling technique on the basis of organic maturation; vitrinite reflectance and T_max parameter values of the Rock-Eval pyrolysis. Fine-grained marine sediment of the Yezo Supergroup in central Hokkaido is one of the most extensive Cretaceous constituents along the plate margin. To evaluate organic maturation within the Yezo Supergroup, a surface section (Hakkin-zawa) and a deep borehole (MITI Yubari) were selected, from which quite different maturation trends were obtained. Notably low and constant maturation was suggested throughout the thick Hakkin-zawa section, and it was confirmed on the basis of various biomarker analyses. It requires anomalously rapid burial, probably related to thrust-stacking or large-scale slumping, followed by prompt tilting/exhumation event. In sharp contrast, Late Cretaceous strata in the MITI Yubari do not indicate coeval tectonic disturbances in a short period. A wider view of the ancient convergent margin suggests that deformation of the forearc propagated westward during the Cretaceous. High maturation levels in the uppermost thrust sheet around the MITI Yubari are optimized in 1D geochemical modeling on the assumption that a thick missing Paleogene unit has been eroded out as a result of thrust-forming contraction scheme emerged during the Neogene. A similar Cenozoic burial pattern is adopted for the once tilted and exhumed Hakkin-zawa section in order to match the present maturation levels. Although such an active subsidence of a ‘forearc’ is generally interpreted as an effect of the subduction erosion, heavy mineral composition of the shallow marine to fluvial Paleogene implies uplift and exhumation of ultramafic rocks to the east (trench-side). Thus the confined basin-formation in central Hokkaido is to be understood in a different tectonic framework that may reflect transcurrent motions on the plate margin.     

An Interpretation of the Seafloor Spreading History of the West Enderby Basin between Initial Breakup of Gondwana and Anomaly C34

The seafloor spreading evolution in the Southern Indian Ocean is key to understanding the initial breakup of Gondwana. We summarize the structural lineaments deduced from the GEOSAT 10 Hz sampled raw altimetry data as well as satellite derived gravity anomaly map and the magnetic anomaly lineation trends from vector magnetic anomalies in the West Enderby Basin, the Southern Indian Ocean. The gravity anomaly maps by both Sandwell and Smith 1997, J. Geophys. Res. 102, 10039–10054 and 10 Hz raw altimeter data show almost the same general trends. However, curved structural trends, which turn from NNW–SSE in the south to NNE–SSW in the north, are detected only from gravity anomaly maps by 10 Hz raw altimeter data just to the east of Gunnerus Ridge. NNE–SSW structural trends and magnetic anomaly lineation trends that are perpendicular to them are observed between the Gunnerus Ridge and the Conrad Rise. To the west of Gunnerus Ridge, structural elements trend NNE–SSW and magnetic polarity changes are normal to them. In contrast, almost NNW–SSE structural trends and ENE–WSW magnetic polarity reversal strikes are dominant to the east of Gunnerus Ridge. Curved structural trends, which turn from WNW–ESE direction in the south to NNE–SSW direction in the west, and magnetic polarity reversal strikes that are almost perpendicular to them are observed just south of Conrad Rise. The magnetic polarity reversals may be parts of the Mesozoic magnetic anomaly sequence that formed along side of the structural lineaments before the long Cretaceous normal polarity superchron. Curved structural trends, detected only from gravity anomaly maps by 10 Hz raw altimeter data, most likely indicate slight changes in spreading direction from an initial NNW–SSE direction to NNE–SSW. Our results also suggest that these curved structural trends are fracture zones that formed during initial breakup of Gondwana.     

Wells turbine with end plates for wave energy conversion

In order to improve the performance of the Wells turbine for wave energy conversion, the effect of end plate on the turbine characteristics has been investigated experimentally by model testing. As a result, it is found that the characteristics of the Wells turbine with end plates are superior to those of the original Wells turbine, i.e., the turbine without end plate and the characteristics are dependent on the size and position of end plate. Furthermore, by using a computational fluid dynamics (CFD), reason of the performance improvement of the turbine has been clarified and the effectiveness of the end plate has been demonstrated.