Morphometrical analysis of two tropical mountain river basins of contrasting environmental settings, the southern Western Ghats, India

The morphometric analysis of river basins represents a simple procedure to describe hydrologic and geomorphic processes operating on a basin scale. A morphometric analysis was carried out to evaluate the drainage characteristics of two adjoining, mountain river basins of the southern Western Ghats, India, Muthirapuzha River Basin (MRB) in the western slopes and Pambar River Basin (PRB) in the eastern slopes. The basins, forming a part of the Proterozoic, high-grade, Southern Granulite Terrain of the Peninsular India, are carved out of a terrain dominantly made of granite- and hornblende-biotite gneisses. The Western Ghats, forming the basin divide, significantly influences the regional climate (i.e., humid climate in MRB, while semi-arid in PRB). The Survey of India topographic maps (1:50,000) and Shuttle Radar Topographic Mission digital elevation data were used as the base for delineation and analysis. Both river basins are of 6th order and comparable in basin geometry. The drainage patterns and linear alignment of the drainage networks suggest the influence of structural elements. The Rb of either basins failed to highlight the structural controls on drainage organization, which might be a result of the elongated basin shape. The irregular trends in Rb between various stream orders suggest the influence of geology and relief on drainage branching. The Dd values designate the basins as moderate- to well-drained with lower infiltration rates. The overall increasing trend of Rl between successive stream orders suggests a geomorphic maturity of either basins and confirmed by the characteristic I _hyp values. The Re values imply an elongate shape for both MRB and PRB and subsequently lower vulnerability to flash floods and hence, easier flood management. The relatively higher Rr of PRB is an indicative of comparatively steeply sloping terrain and consequently higher intensity of erosion processes. Further, the derivatives of digital elevation data (slope, aspect, topographic wetness index, and stream power index), showing significant differences between MRB and PRB, are useful in soil conservation plans. The study highlighted the variation in morphometric parameters with respect to the dissimilarities in topography and climate.     

Excitation of Resonant Modes along the Japanese Coast by the 1993 and 1983 Tsunamis in the Japan Sea

We observed seiches at 55 ports in Japan facing the Japan Sea and obtained dominant periods from their maximum spectral amplitudes. These periods were mostly determined ranging from 10 to 40 minutes. They were compared with dominant periods of the 1993 Hokkaido Nansei-oki tsunami and the 1983 Nihonkai Chubu-oki tsunami at the same ports. As a result, relations of dominant periods between seiches and tsunamis are classified into three types. The first one is fundamental mode excitation, the second is higher mode excitation and the third is no excitation. Plotting the maximum spectral amplitude normalized at an epicentral distance of 50 km versus the ratio of the tsunami dominant period to the seiche dominant period, we obtained resonance curves having maxima at one. This fact shows a contribution of resonance to the amplification. Thus it is recognized that the dominant period of seiching is an important factor in interpreting amplification and resonance of tsunami.     

In situ Measurements of Tide Gauge Response and Corrections of Tsunami Waveforms from the Niigataken Chuetsu-oki Earthquake in 2007

Linear and nonlinear responses of ten well-type tide gauge stations on the Japan Sea coast of central Japan were estimated by in situ measurements. We poured water into the well or drained water from the well by using a pump to make an artificial water level difference between the outer sea and the well, then measured the recovery of water level in the well. At three tide gauge stations, Awashima, Iwafune, and Himekawa, the sea-level change of the outer sea is transmitted to the tide well instantaneously. However, at seven tide gauge stations, Nezugaseki, Ryotsu, Ogi, Teradomari, Banjin, Kujiranami, and Naoetsu, the sea-level change of the outer sea is not always transmitted to the tide well instantaneously. At these stations, the recorded tsunami waveforms are not assured to follow the actual tsunami waveforms. Tsunami waveforms from the Niigataken Chuetsu-oki Earthquake in 2007 recorded at these stations were corrected by using the measured tide gauge responses. The corrected amplitudes of the first and second waves were larger than the uncorrected ones, and the corrected peaks are a few minutes earlier than the uncorrected ones at Banjin, Kujiranami, and Ogi. At Banjin, the correction was significant; the corrected amplitudes of the first and second upward motion are +103 cm and +114 cm, respectively, while the uncorrected amplitudes were +96 cm and +88 cm. At other tide gauge stations, the differences between the uncorrected and corrected tsunami waveforms were insignificant.     

Effects of Bathymetric Grid-Cell Sizes on Habitat Suitability Analysis of Cold-water Gorgonian Corals on Seamounts

We investigated the effects of the spatial resolution of bathymetry data on the predicted local distribution of large gorgonian corals on the southern Emperor Seamounts in the high seas of North Pacific. Terrain attributes were calculated from bathymetric survey data at six different grid-cell sizes (25–800 m) to investigate how the values of bottom terrain attributes and the prediction of MaxEnt habitat suitability models would vary with grid-cell size. The values of each terrain attribute showed different patterns among different grid-cell sizes, and MaxEnt demonstrated better performance at grid-cell size of 100 m or less than at larger sizes.     

Presence of high nitrate to phosphate ratio subsurface water in the Tsushima Strait during summer

To identify water with an excess nitrate concentration to phosphate ratio and its potential source, the nutrient concentrations in the Tsushima Strait (TSS) were investigated over ten cruises in August and September 2007–2014, excluding 2010. On the basis of the Redfield ratio, water with an excess nitrate concentration of >1 μM (positive ExNOx water) was identified below the surface mixed layer during four cruises in 2011–2013. Positive ExNOx water was present mainly in less-saline (<34) waters with a density of 22–25 σ _θ , and 25–75 m depth. However, in August 2012, positive ExNOx was detected in dense (25–25.5 σ _θ ) and deep (50–110 m depth) waters near the salinity maximum, although the salinity during this period was significantly lower than that in other years. The horizontal length of positive ExNOx water was >100 km across the TSS during two cruises in August 2012 and September 2013, respectively. According to multi-regression analysis conducted on the silicate concentration, temperature, and salinity, the silicate concentration was increased in the less-saline subsurface water. The required amount of original freshwater was 10^8–9 m³ day^?1 based on the excess nitrate concentration. This evidence indicates that the positive ExNOx water originated from large river waters such as the Changjiang. Thus, discharged water from the rivers of the East Asia is contributing to the increased N:P ratio in the Tsushima Warm Current, southern Japan Sea.     

Millennium‐scale recurrent uplift inferred from beach deposits bordering the eastern Nankai Trough,Omaezaki area,central Japan

A flight of Holocene marine terraces on the southwestern coast of Cape Omaezaki of central Japan provides evidence of recurrent millennium‐scale uplift events. We reconstructed the uplift history of these terraces by using facies analysis of drill core and geoslicer samples, environmental analysis of trace fossils, and ¹⁴C age determinations. Coastal uplift can be identified by the displacement of beach deposits such as foreshore deposits, which represent the intertidal swash zone of a wave‐dominated sandy coast. Three levels of former beach deposits facing the Nankai Trough were identified near the coast in the Omaezaki area. The highest of these, dated at about 3020–2880 BC, records a maximum of 2.2–2.7 m of emergence. The middle beach surface, of minimum age 370–190 BC, shows 1.6–2.8 m of emergence. The lowest beach surface, which is older than 1300–1370 AD, records 0.4–1.6 m of emergence. Our analysis of vertical crustal deformation data during the Holocene in this region suggests that rapid and strong uplift was restricted to the southwestern coast of the Omaezaki area and was probably caused by high‐angle thrusting on subsidiary faults branching from the underlying plate boundary megathrust.     

Crustal structure beneath Aso Caldera,Southwest Japan,as derived from receiver function analysis

Aso Volcano experienced a huge pyroclastic eruption 90 thousand years ago, and formed a large caldera (18 km × 25 km). In order to test the hypothesis of a magma body in the mid and lower crust that has been suggested geophysically and geochemically, we investigated seismic velocity discontinuities and velocity structure beneath Aso Caldera using receiver functions and a genetic algorithm inversion. We confirm the existence of the Moho at depths between 30 km and 35 km and a large velocity anomaly should exist in the deep portion of the crust beneath Aso Caldera, from imaging of receiver functions observed only at stations outside the caldera. As a result of a more detailed examination with GA inversion, a low velocity layer is detected at depths between 10 km and 24 km beneath the western part of the caldera. S-wave velocity of the layer is estimated to be 2.0–2.4 km/s. We estimate that the low velocity layer contains at most 15% melt or 30% aqueous fluid. The layer exists near the Conrad and at the same depths as the swarm of the low frequency earthquakes and a compressional and dilatational deformation source which are expected to be caused by fluid movement beneath the middle-eastern part of the caldera. Fluid contained in the layer might be related with huge pyroclastic eruptions of Aso Volcano.     

The effect of a tidal cycle on the dynamics of nutrients in a tidal estuary in the Seto Inland Sea, Japan

A 24 hour time series survey was carried out during a spring tide (tidal range ca.2 m) of May 1995 on a tidal estuary in the Seto Inland Sea, Japan, in the context of an integrated program planned to quantify the dynamics of biophilic elements (carbon, nitrogen and phosphorus) and the roles played by the macrobenthos on the processes. Three stations were set along a transect line of about 1.4 km, which linked the river to the rear to the innermost part of the subtidal zone. Every hour, at each station, measurements were made of surface water temperature, salinity and dissolved oxygen concentration, and surface water was collected for the determination of nutrients [NH₄ ⁺−N, (NO₃ ⁻+NO₂ ⁻)−N, PO₄ ³⁻−P and Si (OH)₄−Si]. During the ebb flow, riverine input of silicate and nitrate+nitrite significantly increased the concentrations of both the intertidal and the subtidal stations. Conversely, during the high tide, river nutrient concentrations were lowered by the mixing of fresh water with sea water. As a result, best (inverse) correlations were found at the river station for salinity against silicate (y=-2.9 Sal.+110.7,r ²=0.879) and nitrate+nitrite (y=-1.3 Sal.+48.4,r ²=0.796). In contrast, ammonium nitrogen concentrations were higher at intermediate salinities. Indeed, no significant correlation was found between salinity and ammonium. The effect of the macrobenthos, which is abundant on the intertidal flat, is discussed as a biological component that influences the processes of nutrient regeneration within the estuary. The effect of the tidal amplitude is an important one in determining the extent of the variations in nutrient concentrations at all three stations, which were stronger between the lower low tide and the higher high tide.     

紫色土集水区氮素收支状况与平衡分析

通过 1999- 2 0 0 0年间定位观测和模拟实验 ,研究了紫色土丘陵集水区生态系统氮素养分的收入与支出参数及系统氮素平衡。结果表明 ,本区农田氮素略有盈余 ,平均为 4 2kg hm2 ,其中氮肥施用量较大 ,平均达到 35 8kg hm2 ;而林地在系统氮素循环和平衡中发挥了重要作用 ,是系统氮素从亏缺转变为盈余驱动因素之一 ;系统中有机厩肥和化肥是主要的氮素来源 ,但损失严重 ,利用率不高 ,应注意由此引起的对大气和水体的农业非点源污染问题。     

Podiform chromitite in the arc mantle: chromitite xenoliths from the Takashima alkali basalt,Southwest Japan arc

Chromitite xenoliths from the Takashima alkali basalt in the Southwest Japan arc are classified into two types: Type 1 chromitite in thin layers in dunite or wehrlite xenoliths; and Type 2 chromitite in discrete xenoliths which has an orbicular texture, previously documented only from podiform chromitites in ophiolites. Type 1 may be equivalent to layered chromitites in ophiolitic cumulates and Type 2 to podiform chromitites in the transition zone of ophiolites. This example of podiform chromitite from the Southwest Japan arc suggest that these podiform chromitites may exist in the upper mantle beneath an arc, where their formation is favored.