Influence of flooding on groundwater flow in central Cambodia

Cambodia is affected by flooding from the Mekong, Tonle Sap and Bassac rivers every year, which harms human populations and damages property, as well as alters the water quality in aquifer systems. The objective of this paper is to highlight the effects of river flooding on groundwater flow using numerical simulation. A two-dimensional groundwater flow model coupled with a groundwater recharge model was applied to the research area in central Cambodia. River level variation was included in model processes, and flood areas and periods were assigned. The results showed that during flooding periods, floodwater from the three rivers played an important role in recharging groundwater. During the dry season, Tonle Sap River received groundwater supply from the northwest, and levels in the Bassac and Mekong River dropped to lower than the groundwater level. This study improves understanding of the surface water and groundwater flow system in the study area.     

Variable Tsunami Sources and Seismic Gaps in the Southernmost Kuril Trench: A Review

In the southernmost Kuril Trench, the tsunami source regions vary their along-trench extent even among earthquakes occurring within the same segment. Recent studies suggest that the tsunami source of the 1952 Tokachi-oki earthquake (M 8.1) differs from but partially overlaps with that of the 2003 Tokach-oki earthquake (M 8.0). Furthermore, the along-trench extent among the earthquakes seems to differ between deep and shallow portions of the subduction interface. A seismic gap has been recognized along the deep subduction interface between the sources of the 1952 and 1973 earthquakes. We propose that the gap is now larger, including both shallow to deep portions of the interface between the 1973 and 2003 earthquakes. Variability in spatial extent of large subduction earthquakes in both along-trench direction and trench-normal direction makes it difficult to forecast future earthquakes in the southernmost Kuril Trench.     

Prediction of ground motion in the Osaka sedimentary basin associated with the hypothetical Nankai earthquake

We studied the long-period ground motions in the Osaka sedimentary basin, Japan, which contains a 1- to 3-km thickness of sediments and is the site of many buildings or construction structures with long-natural period. We simulated the broadband ground motions likely to be produced by the hypothetical Nankai earthquake: the earthquake expected to give rise to the most severe long-period ground motion within the basin. For the simulation, we constructed multiscale heterogeneous source models based on the Central Disaster Management Council of Japan (CDMC) source model and adopted a hybrid computation method in which long-period motion and short-period motion are computed using a 3-D finite difference method and the stochastic Green’s function method, respectively. In computing long-period motions, we used a 3-D structure model of the crust and the Osaka sedimentary basin. The ground motions are estimated to have peak velocities of 50–90 cm/s, prolonged durations exceeding 300 s, and long predominant periods of 5–10 s in the area with great thickness of sediments. The predominant periods are in agreement with an approximate evaluation by 4 H/V _s where H and V _s are the thickness of the sediment and the average S wave velocity, respectively.     

Experiments on the ocean sequestration of fossil fuel CO2: pH measurements and hydrate formation

We have carried out a series of in situ experiments to investigate the formation of a CO₂ hydrate (CO₂:5.75 H₂O) for the purpose of evaluating scenarios for ocean fossil fuel CO₂ disposal with a solid hydrate as the sequestered form. The experiments were carried out with a remotely operated vehicle in Monterey Bay at a depth of 619 m. pH measurements made in close proximity to the hydrate–seawater interface showed a wide range of values, depending upon the method of injection and the surface area of the hydrate formed. Rapid injection of liquid CO₂ into an inverted beaker to form a flocculant mass of hydrate resulted in pH initially as low as 4.5 within a few centimeters of the interface, decaying slowly over 1–2 h towards normal seawater values as dense CO₂ rich brine drained from the hydrate mass. In a second experiment, slower injection of the liquid CO₂ to produce a simple two-layer system with a near planar interface of liquid CO₂ with a thin hydrate film yielded pH values indistinguishable from the in situ ocean background level of 7.6. Both field and laboratory results now show that the dissolution rate of a mass of CO₂ hydrate in seawater is slow but finite.     

日本大沼湖沉积物碳氮比值、有机碳同位素特征及其近400年的古气候环境意义

通过对日本大沼湖沉积物样品总有机碳含量(TOC)及其稳定同位素、总氮含量(TN)和C/N 值的测定, 结合沉积岩芯火山灰和 14C 年代, 分析了大沼湖沉积物中有机质的来源, 探讨了沉积物有机碳同位素的主要影响因素。结果表明: 大沼湖沉积物中有机质主要来源于自生藻类, 受陆源输入的影响较小; 沉积物δ13C 值指示了夏季温度的变化。过去400年来, 大沼湖地区存在1660~1730年和1780~1860年两个明显的冷期, 与邻近地区冰芯、树轮和湖泊沉积物记录的冷期基本一致, 分别对应于Maunder和Dalton太阳黑子极小期。     

Rapid Identification of Water-Conducting Fractures Using a Trace Methane Gas Measurement

Identification of water-conducting fractures is important for the safety assessment of underground projects in crystalline rocks at geological disposal sites. We applied a portable methane gas analyzer by wavelength-scanned cavity ring-down spectroscopy to detect the water-conducting fractures in the underground tunnel of the Mizunami Underground Research Laboratory, central Japan. The tunnel is excavated in granite with CH₄-rich groundwater. Two approaches were taken to obtain the profile of CH₄ concentration along the gallery walls: (1) “Scan by walking” at the speed of 0.5 m/s and (2) monitoring for 30 s at 0.5 or 1 m intervals. In the Scan by walking approach, the peaks of the CH₄ concentration corresponded well with the occurrence of high water flow rate fractures. Thus, this method is useful for rapid identification of major water-conducting fractures. Monitoring at constant intervals takes more time than the Scan by walking approach; however, this method can largely detect occurrences of fractures with low fluid fluxes.     

Magmatic unmixing in spinel from late Precambrian concentrically-zoned mafic–ultramafic intrusions, Eastern Desert, Egypt

Spinel is widespread in the ultramafic core rocks of zoned late Precambrian mafic–ultramafic complexes from the Eastern Desert of Egypt. These complexes; Gabbro Akarem, Genina Gharbia and Abu Hamamid are Precambrian analogues of Alaskan-type complexes, they are not metamorphosed although weakly altered. Each intrusion is composed of a predotite core enveloped by pyroxenites and gabbros at the margin. Silicate mineralogy and chemistry suggest formation by crystal fractionation from a hydrous magma. Relatively high Cr₂O₃ contents are recorded in pyroxenes (up to 1.1 wt.%) and amphiboles (up to 1.4 wt.%) from the three plutons. The chrome spinel crystallized at different stages of melt evolution; as early cumulus inclusions in olivine, inclusions in pyroxenes and amphiboles and late-magmatic intercumulus phase. The intercumulus chrome spinel is homogenous with narrow-range of chemical composition, mainly Fe³⁺-rich spinel. Spinel inclusions in clinopyroxene and amphibole reveal a wide range of Al (27–44 wt.% Al₂O₃) and Mg (6–13 wt.% MgO) contents and are commonly zoned. The different chemistries of those spinels reflect various stages of melt evolution and re-equilibration with the host minerals. The early cumulus chrome spinel reveals a complex unmixing structures and compositions. Three types of unmixed spinels are recognized; crystallographically oriented, irregular and complete separation. Unmixing products are Al-rich (Type I) and Fe³⁺-rich (Type II) spinels with an extensive solid solution between the two end members. The compositions of the unmixed spinels define a miscibility gap with respect to Cr–Al–Fe³⁺, extending from the Fe³⁺–Al join towards the Cr corner. Spinel unmixing occurs in response to cooling and the increase in oxidation state. The chemistry and grain size of the initial spinel and the cooling rate control the type of unmixing and the chemistry of the final products. Causes of spinel unmixing during late-magmatic stage are analogous to those in metamorphosed complexes. The chemistry of the unmixed spinels is completely different from the initial spinel composition and is not useful in petrogenetic interpretations. Spinels from oxidized magmas are likely to re-equilibrate during cooling and are not good tools for genetic considerations.     

Snow ripples and their contribution to the mass transport in drifting snow

Morphological characteristics of snow ripples formed by drifting snow were investigated as functions of wind velocity in a cold wind tunnel at -15 °C. Wave-length, wave height and migration rate of snow ripples increased from 5 to 20 cm, 3 to 5 mm and 1 to 8 cm/min, respectively, with increasing wind velocity from 5 to 7 m/s. Measured size distributions of snow particles in snow ripples showed sorting of large particles in ridges, suggesting that the snow ripple migration is caused by creeping of large particles. The snow drift rate caused by creep, that is, by the ripple migration, was estimated to amount, at least, to 6% of the total snow drift rate.     

Seismic behaviour of pile groups by hybrid experiments

Non-linear seismic soil-pile interaction was studied with a hybrid procedure that used a pseudo-dynamic testing (PDT) method modified to account for frequency dependence and developed for foundation-soil systems. The numerical scheme used in the conventional PDT was improved by the introduction of a time-dependent pseudo-forcing function derived from the frequency-dependent dynamic characteristics of the system by Hilbert transformation in the frequency domain. Single, 2-, 3- and 9-pile group foundation models were used, their mechanical characteristics later being determined from static and forced vibration dynamic tests. Amplitude scaling was used for three recorded accelerograms. Data recorded during an earthquake at the site of the experiments revealed that the proposed methodology predicts well seismic nonlinear interaction and accounts for frequency dependence and non-linearity in the time domain.     

Use of tsunami waveforms for earthquake source study

Tsunami waveforms recorded on tide gauges, like seismic waves recorded on seismograms, can be used to study earthquake source processes. The tsunami propagation can be accurately evaluated, since bathymetry is much better known than seismic velocity structure in the Earth. Using waveform inversion techniques, we can estimate the spatial distribution of coseismic slip on the fault plane from tsunami waveforms. This method has been applied to several earthquakes around Japan. Two recent earthquakes, the 1968 Tokachi-oki and 1983 Japan Sea earthquakes, are examined for calibration purposes. Both events show nonuniform slip distributions very similar to those obtained from seismic wave analyses. The use of tsunami waveforms is more useful for the study of unusual or old earthquakes. The 1984 Torishima earthquake caused unusually large tsunamis for its earthquake size. Waveform modeling of this event shows that part of the abnormal size of this tsunami is due to the propagation effect along the shallow ridge system. For old earthquakes, many tide gauge records exist with quality comparable to modern records, while there are only a few good quality seismic records. The 1944 Tonankai and 1946 Nankaido earthquakes are examined as examples of old events, and slip distributions are obtained. Such estimates are possible only using tsunami records. Since tide-gauge records are available as far back as the 1850s, use of them will provide unique and important information on long-term global seismicity.