从南大西洋裂解过程解密大陆漂移的驱动力

南大西洋裂解造成的非洲和南美洲的大陆分离到了广泛认可,该区域也与大陆漂移学说的诞生密切相关。但大陆漂移的驱动力从其提出至今一直存在争议,定量化分析大西洋裂解过程中板块运动的驱动力显得尤为重要。我们研究了南大西洋两侧被动大陆边缘盆地区域的两条深反射地震勘探剖面,在构造地质解译基础上,详细估算了非洲大陆的莫霍面倾角,得到了沿莫霍面地壳重力滑移剪切力的大小,用于解释大西洋裂解过程中非洲大陆运动的动力机制。结果说明,非洲大陆板块在地幔上涌形成的倾斜界面上能够产生强大的重力滑移力,且南部驱动力大于中部。大陆板块依靠连续的地幔热上涌和重力滑移力会持续漂移。该模型能够合理解释大西洋上诸多线状分布的大陆残片的成因机制,也能合理解释南大西洋南部宽度大于中部的内在原因,最后对南大西洋的打开过程进行了精细的构造演化史恢复。该研究为板块运动提供了一个新的动力模式,为认识板块运动驱动力提供了更为精确的约束信息。     

Comparative Hydrodynamics during Events along a Barrier Island: Explanation for Overwash

A fairly undeveloped barrier island along the mid-Atlantic coast, Onslow Beach, was exposed to two differing, yet sequential meteorological events in the fall of 2008. The response of the barrier island differed significantly enough to warrant investigation into the causes of aberrant overwash locations. Tropical Storm Hanna generated high significant wave heights for a short period of time and caused overwash events along the southern portion of Onslow Beach. The Nor’easter and subsequent wind shift after TS Hanna generated significant wave heights that were lower than during TS Hanna, yet more locations of overwash were recorded along the beach. Data from NOAA wave buoys and a nearshore deployed AWAC were analyzed to understand the underlying physics behind the recorded differences in barrier island response. These data were also used to validate a coupled hydrodynamic (ADCIRC) and waves (SWAN) model to investigate the alongshore variability. Low frequency variability, on the order of days, and tidal timing of shoreward high significant wave heights contributed to the recorded variability.     

The use of minimum curvature surface technique in geoid computation processing of Egypt

According to the wide spread use of satellite-based positioning techniques, especially Global Navigation Satellite Systems (GNSS), a greater attention has been paid to the precise determination of geoid models. As it is known, leveling measurements require high cost and long time in observation process that make it not convenient for the practical geodetic purposes. Thus obtaining the orthometric heights by GNSS is the most conventional way of determining these heights. Verifying this goal was the main objective behind the current research. The current research introduces a numerical solution of geoid modeling by applying a surface fitting for a few sparse data points of geoid undulation using minimum curvature surface (MCS). The MCS is presented for deriving a system of linear equations from boundary integral equations. To emphasize the precise applicability of the MCS as a tool for modeling the geoid in an area using GPS/leveling data, a comparison study between EGM2008 and MCS geoid models, is performed. The obtained results showed that MCS technique is a precise tool for determining the geoid in Egypt either on regional and/or local scale with law distortion at check points.     

Historical sand injections on the Mediterranean shore of Israel: evidence for liquefaction hazard

The abundant silt and sand along the coastal plain of Israel have long been considered susceptible to liquefaction, but previous searches have failed to find field evidence for it. We report the first finding of typical liquefaction features and silty sand injections in trenches that were excavated behind a fourth century Byzantine dam on the Taninim Creek, some 850 m inland of the Mediterranean shore. The trenches revealed a series of flame-shape injections of silty sand that penetrate the overlying clay-rich soil. The injections are largest and most frequent within several meters of the point where the dam is badly damaged on the seaward side, which we interpret as a possible result of a large wave. Three features make the sand injections special: (1) their lower extent is commonly asymmetric with dominant southeastward vergence, away from the breach in the dam, (2) zigzag shapes characterize the upper parts of many injections, and (3) the size and frequency of the injections diminish gradually with distance from the dam until they completely disappear some 100 m away from it. We suggest that the sand injections can be explained by overpressure that was induced either directly by earthquake shaking or by a tsunami wave that breached the dam, filled the reservoir behind the dam and increased the pressure on the water-saturated silt and sand layers and triggered liquefied sand injections. The movement of water sloshing back and forth in the lake accounts for the zigzag shape of the injections. The similarity to structures that were observed in Thailand after the great 2004 tsunami and other palaeotsunami observations lead us to prefer the tsunami origin of the liquefaction features. Based on the stratigraphic position, the archeological context, and the historical accounts, we suggest that an earthquake of November 25, 1759 is the most plausible trigger of the sand injections, either directly or via earthquake-induced tsunami. The observations demonstrate the vulnerability of the densely populated coastal plain to liquefaction.     

Monthly-mean wind stress along the coast of the north Indian Ocean

Monthly-mean wind stress and its longshore and offshore components have been computed using the bulk aerodynamic method for each of a string of 36 two-degree-latitude by two-degree-longitude squares along the coast of the north Indian Ocean. The data source for the computation is the sixty-year mean resultant winds of Hastenrath and Lamb. The main features exhibited by the components, taking the longshore components as positive (negative) when the Ekman transport is away from (towards) the coast, are: (1) Along the coasts of Somalia and Arabia, the magnitude of the wind stress is among the highest in the north Indian Ocean, and its direction is generally parallel to the coastline. This results in a longshore component which is large (as high as 2·5 dyne/cm²) and positive during the southwest monsoon, and weaker (less than 0·6 dyne/cm²) and negative during the northeast monsoon. (2) Though weak (less than 0·2 dyne/cm²) during the northeast monsoon, the monthly-mean longshore component along the west coast of India remains positive throughout the year. The magnitude of the offshore component during the southwest monsoon is much larger than that of the longshore component. (3) The behaviour of the wind stress components along the east coast of India is similar to that along the Somalia-Arabia coast, but the magnitudes are much smaller.     

Radial Deformations Induced by Groundwater Flow on Deep Circular Tunnels

Summary. The magnitude and distribution of ground deformations around a tunnel are often monitored during construction and provide key information about ground-support interaction and ground behavior. Thus it is important to determine the effects of different parameters on ground deformations to accurately and effectively evaluate what contributes to ground and support behavior observed during excavation. This paper investigates one such relation: the effects of seepage on radial deformations. A number of numerical analyses have been conducted with the following assumptions: deep circular unsupported tunnel, elastic ground, isotropic far field stresses, dry ground or saturated ground with steady-state water seepage. The analyses cover a wide range of tunnel sizes, effective stresses, and pore pressures. Results from the numerical simulations confirm previous analytical solutions for normalized radial deformations behind the face (i.e. on the tunnel side of the face) of a tunnel excavated in dry ground, and have been used to propose a new analytical formulation for normalized radial displacements ahead and behind the tunnel face for both dry and saturated ground with water flow. Water seepage substantially increases the magnitude and distribution of the normalized radial deformations ahead of the face and at the tunnel face, but does not change much the displacement distribution behind the tunnel face.     

Fold crest degradation and slump deposition in the deepwater Niger Delta,Nigeria

This study uses 3D seismic reflection dataset to investigate the time-integrated growth and degradation of fault-propagation folds and associated slump deposits in the deepwater Niger Delta. The results of the analysis showed that slump deposits are sourced almost entirely from the degradation of growing fold crests—resulting in the development of seaward-facing scarps on the forelimbs of these folds. The scarps develop parallel to fold axes and have dips that range between 4° SW and 20° SW. The constant presence of fan-shaped slump deposits downdip of these scarps suggests a direct relationship to fold crest degradation. Moreover, the spatio-temporal distribution of the slumps also shows a direct relationship with the heights of seabed scarps along the strike of growing folds. Slumps have short run-out distances (generally less than 5 km); however, they also travel long distances downslope (up to 10 km) across growing structures by exploiting relay areas between growing scarps.     

The Keiva ice marginal zone on the Kola Peninsula, northwest Russia: a key component for reconstructing the palaeoglaciology of the northeastern Fennoscandian Ice Sheet

One of the key elements in reconstructing the palaeoglaciology of the northeastern sector of the Fennoscandian Ice Sheet is the Keiva ice marginal zone (KIZ) along the southern and eastern coast of Kola Peninsula, including the Keiva I and II moraines. From detailed geomorphological mapping of the KIZ, primarily using aerial photographs and satellite images, combined with fieldwork, we observed the following. (1) The moraines display ice contact features on both the Kola side and the White Sea side along its entire length. (2) The Keiva II moraine is sloping along its length from c. 100 m a.s.l. in the west (Varzuga River) to c. 250 m a.s.l. in the east (Ponoy River). (3) The KIZ was partly overrun and fragmented by erosive White Sea-based ice after formation. From these observations we conclude that the KIZ is not a synchronous feature formed along the lateral side of a White Sea-based ice lobe. If it was, the moraines should have a reversed slope. Rather, we interpret it to be time transgressive, formed at a northeastward-migrating junction between a warm-based Fennoscandian Ice Sheet expanding from the west and southwest into the White Sea depression, and a sluggish cold-based ice mass centred over eastern Kola Peninsula. In contrast to earlier reconstructions, we find it unlikely that an ice expansion of this magnitude was a mere re-advance during the deglaciation. Instead, we propose that the KIZ was formed during a major expansion of a Fennoscandian Ice Sheet at a time pre-dating the Last Glacial Maximum.     

Evidence of paleoearthquakes from trench investigations across Pinjore Garden fault in Pinjore Dun, NW Himalaya

The Pinjore Garden Fault (PGF) striking NNW-SSE is now considered one of the active faults displacing the younger Quaternary surfaces in the piggyback basin of Pinjore Dun. This has displaced the older Kalka and Pinjore surfaces, along with the other younger surfaces giving rise to WSW and SW-facing fault scarps with heights ranging from 2 to 16 m. The PGF represents a younger branch of the Main Boundary Thrust (MBT) system. An ~ 4m wide trench excavated across the PGF has revealed displacement of younger Quaternary deposits along a low angle thrust fault. Either side of the trench-walls reveals contrasting slip-related deformation of lithounits. The northern wall shows displacement of lithounits along a low-angle thrust fault, while the southern wall shows well-developed fault-related folding of thick sand unit. The sudden change in the deformational features on the southern wall is an evidence of the changing fault geometry within a short distance. Out of five prominent lithounits identified in the trench, the lower four units show displacement along a single fault. The basal unit ‘A’ shows maximum displacement of aboutT _o = 2.85 m, unit B = 1.8 m and unit C = 1.45 m. The displacement measured between the sedimentary units and retro-deformation of trench log suggests that at least two earthquake events have occurred along the PGF. The units A and D mark the event horizons. Considering the average amount of displacement during one single event (2 m) and the minimum length of the fault trace (~ 45 km), the behaviour of PGF seems similar to that of the Himalayan Frontal Fault (HFF) and appears capable of producing large magnitude earthquakes.     

灾害救助超宽带电磁生命探测方法的模拟研究

将超宽带电磁探测技术引入灾害救助方法技术之中,以充分发挥其穿透障碍物能力强、探测距离大、分辨率高和携带目标信息量丰富等优势。为了研究超宽带电磁波在有障碍物下的探测目标人体的可行性,论文以穿墙探测障碍物后的生命体模型为例,使用时域有限差分方法模拟超宽带电磁波的三维传播过程;给出一个呼吸周期内的人体胸部几个变化状态下的人体回波信号的模拟结果,给出多个状态下的微动信号的加强积分曲线。模拟结果显示了超宽带电磁波能高分辨率地反映墙体与障碍物后面的目标人体,能检测到人体呼吸所致的胸部运动信号,因而超宽带电磁探测技术用于灾害现场生命检测是理论可行的,在穿透墙体及障碍物方面具有独特优势。研究成果为城市灾害以及紧急事件现场生命救助提供了电磁探测方法与数理模拟技术,并为建立一种新型、非接触、有效的灾害救助实用技术打下了基础。     

Assessing the threat to Western Australia from tsunami generated by earthquakes along the Sunda Arc

A suite of tsunami spaced evenly along the subduction zone to the south of Indonesia (the Sunda Arc) were numerically modelled in order to make a preliminary estimate of the level of threat faced by Western Australia from tsunami generated along the Arc. Offshore wave heights from these tsunami were predicted to be significantly higher along the northern part of the west Australian coast than for the rest of the coast south of the town of Exmouth. In particular, the area around Exmouth may face a higher tsunami hazard than other areas of the West Australian coast nearby. Large earthquakes offshore of Java and Sumbawa are likely to be a greater hazard to WA than those offshore of Sumatra. Our numerical models indicate that a magnitude 9 or above earthquake along the eastern part of the Sunda Arc has the potential to significantly impact a large part of the West Australian coastline. The Australian government reserves the right to retain a non-exclusive, royalty free license in and to any copyright.     

晚更新世以来南天山阿克苏地区地壳缩短率

作者研究南天山中段阿克苏—库车山前带活动断层,发现断层切过托木尔峰山麓第四纪冰碛物和阶地,形成2条断层崖。通过测量阶地和冰碛物的变形量,推断阶地和冰碛物的沉积年代,估算南天山中段阿克苏地区晚更新世以来的地壳缩短率可能为1.85mm/a。这个缩短率与库尔勒地区(2mm/a)和柯坪地区(1.8mm/a)的地壳缩短率一致,但是小于南天山西段喀什地区(10±2mm/a)和天山东段玛纳斯地区(6±3mm/a)的地壳缩短率,表明天山不同地段的地壳缩短率存在明显差异。     

Understanding processes of sediment bleaching in glacial settings using a portable OSL reader

Analysis of a high‐resolution suite of modern glacial sediments from Jostedalen, southern Norway, using a portable optically stimulated luminescence (OSL) reader, provides insights into the processes of sediment bleaching in glacial environments at the catchment scale. High‐magnitude, low‐frequency processes result in the least effective sediment bleaching, whereas low‐magnitude, high‐frequency events provide greater bleaching opportunities. Changes in sediment bleaching can also be identified at the scale of individual bar features: tails of braid‐bars and side‐attached bar deposits have the lowest portable reader signal intensities, as well as the smallest conventional OSL residual doses. In addition to improving our understanding of the processes of sediment bleaching, portable reader investigations can also facilitate more rapid and comprehensive modern analogue investigations, which are commonly used to confirm that the OSL signals of modern glacial sediments are well bleached.     

Distribution and abundance of tidal marshes along the coast of maine

Planimetry studies of coastal geology maps prepared by the Maine Geological Survey show that there is more than an order of magnitude more tidal marsh area in the state of Maine than documented in previously published estimates. The highly convoluted coast of Maine, which is approximately 5,970 km long, contains almost 79 km² of salt marsh, far more than any other New England state, New York, or the Bay of Fundy region. Reasonable estimates for the per-unit primary productivity of salt marshes lead to projections of total marsh productivity on the order of 10¹⁰ g dry weight yr^?1 for the Maine coast and 10¹¹ g dry weight yr^?1 for the Gulf of Maine as a whole. Distribution of tidal marsh area is strongly controlled by coastal geomorphology, which varies considerably along the coast of Maine. The salt marsh area is concentrated in the southwestern coastal region of arcuate bays, where marshes have developed behind sandy beaches. A series of long islands and bedrock peninsulas in the south-central portion of the coast also provides sheltered areas where large marshes occur. Northeast of Penobscot Bay salt marshes become more numerous and smaller in average areal extent. A lack of protection from waves, along with limited sources of glacio-fluvial and glacio-marine sediments, restricts the occurrence of salt marshes in that region to the frignes of coves and tidal rivers.     

Statistics for Modeling Heavy Tailed Distributions in Geology: Part I. Methodology

Many natural phenomena exhibit size distributions that are power laws or power law type distributions. Power laws are specific in the sense that they can exhibit extremely long or heavy tails. The largest event in a sample from such distribution usually dominates the underlying physical or generating process (floods, earthquakes, diamond sizes and values, incomes, insurance). Often, the practitioner is faced with the difficult problem of predicting values far beyond the highest sample value and designing his system either to profit from them, or to protect against extreme quantiles. In this paper, we present a novel approach to estimating such heavy tails. The estimation of tail characteristics such as the extreme value index, extreme quantiles, and percentiles (rare events) is shown to depend primarily on the number of extreme data that are used to model the tail. Because only the most extreme data are useful for studying tails, thresholds must be selected above which the data are modeled as power laws. The mean square error (MSE) is used to select such thresholds. A semiparametric bootstrap method is developed to study estimation bias and variance and to derive confidence limits. A simulation study is performed to assess the accuracy of these confidence limits. The overall methodology is applied to the Harvard Central Moment Tensor catalog of global earthquakes.     

Interpretation of moment measures derived from water-lain sediments

ABSTRACT Although mean size provides a useful value for characterizing sediments, it is an inadequate expression of the interaction between sediments and hydraulic regimes and many authors have explored the significance of other statistical moment measures in different parts of the particle size spectrum. Few publications, however, have described such relationships over a very wide range of particle sizes. Using a combined data base (of marine and lacustrine sediments) we have been able to demonstrate the following: The distributions of standard deviation (and entropy) values are repeated in similar form on either side of the sand size class, describing mixtures between sands and gravel and sands and clay. Skewness is modified by the hydraulic response of the sediment and a skewness divide is present at about 2.7 φ, where it coincides with the minimum shear velocity required to initiate particle motion. This divide may be used to distinguish between high and low energy regimes. Kurtosis, like standard deviation, is a measure of the mixing of end member populations but it is more sensitive to changes in the tails of distribution curves. The maximum positive peak of kurtosis, at about 2.5 φ, lies close to the skewness divide (in our data). By using skewness/kurtosis plots it is possible to define separate field distribution for sediments which, to a greater or lesser degree, appear to conform to hydraulic equilibrium. By using sediments which are close to equilibrium (and lie at or close to our skewness/kurtosis boundary curves, it should be possible to define hydraulic shear velocities, at the time of formation. Values may be established by empirical relationships or by defining the break-point (traction and intermittent suspension) between linear components of individual sample φ size/probability distribution plots. Such relationships should be enhanced by using hydraulic equivalent diameters instead of sieve diameters. In fine sediments, the use of fractional (silt: clay) ratios offers sensitivity at a level comparable to that of skewness/kurtosis plots and may be more easily computed.     

Preservation of planar laminae due to migration of low-relief bed waves over aggrading upper-stage plane beds: comparison of experimental data with theory

Experimental studies of the formation of planar laminae by migration of low-relief bed waves over aggrading upper-stage plane beds show that the average thickness of laminae at a point increases with both aggradation rate and the variance of the heights of bed waves passing that point. In general, the preserved laminae represent only a small proportion (generally less than 50%) of the height of the largest bed waves in the population. The theory developed by Paola & Borgman (1991), relating the probability density function of stratal thickness to that of bed wave height for the case of no net aggradation, was adapted for aggrading conditions and shown to agree well with data. These results suggest that the theory can be used to estimate, from the distribution of thickness of planar laminae, either the distribution of bed wave heights, mean aggradation rate, mean bed wave length or mean bed wave celerity, provided the other parameters can be estimated.     

不规则波在岸礁地形增水变化规律试验

波浪增水抬升了岸礁礁坪的平均水位,对岸礁后方陆域安全有重要影响。通过水槽试验,研究了不规则波况下岸礁礁坪的增水。试验组次为3种礁坪水深、4种有效波高和4种谱峰周期的组合。试验结果表明:①岸礁地形上的波浪要素需至少统计200个波才能达到稳定;②不规则波列在岸礁上破碎过程比规则波列复杂,同一波列中波高较大的波以卷破形式在礁前斜坡上破碎,波高较小的波以崩破形式在礁坪上破碎或者不破碎;③礁坪上最大增水值随入射波周期的增大而增大、随礁坪水深的增大而减小,并与入射波波高呈正比。结合试验数据,发现基于规则波试验得出的Gourlay礁坪增水公式,在使用有效波高和谱峰周期作为代表波要素时,公式能良好地预测不规则波在岸礁礁坪上的最大增水值。     

Bedform climbing in theory and nature

Where bedforms migrate during deposition, they move upward (climb) with respect to the generalized sediment surface. Sediment deposited on each lee slope and not eroded during the passage of a following trough is left behind as a cross-stratified bed. Because sediment is thus transferred from bedforms to underlying strata, bedforms must decrease in cross-sectional area or in number, or both, unless sediment lost from bedforms during deposition is replaced with sediment transported from outside the depositional area. Where sediment is transported solely by downcurrent migration of two-dimensional bedforms, the mean thickness of cross-stratified beds is equal to the decrease in bedform cross-sectional area divided by the migration distance over which that size decrease occurs; where bedforms migrate more than one spacing while depositing cross-strata, bed thickness is only a fraction of bedform height. Equations that describe this depositional process explain the downcurrent decrease in size of tidal sand waves in St Andrew Bay, Florida, and the downwind decrease in size of transverse aeolian dunes on the Oregon coast. Using the same concepts, dunes that deposited the Navajo, De Chelly, and Entrada Sandstones are calculated to have had mean heights between several tens and several hundreds of metres.     

龙门山断层破裂的频率非平稳特性

为研究地震过程中的频率非平稳特性,对近年来龙门山断层发生的两次大地震:汶川大地震和芦山大地震的近断层地震记录进行频谱分析。结果表明:相对于芦山地震有较大走向滑动分量的汶川地震,大多数位于汶川地震断层滑动前方的台站接收到更高的频率成分,位于断层滑动后方的台站接收到的地震波频率较低;尽管芦山地震断层相对汶川地震有较小的走向滑动分量,但仍然可以得出与汶川地震相同的结论,不同的是虽然芦山地震沿断层面向上方向分量大,但是其同一台站东西、南北、竖直三方向分量记录幅值相当。把芦山地震三分量记录变换到走向和沿断层面向上方向,证实了沿断层面向上方向高频成分更丰富。在断层滑动前方接收到的地震波频率较高,在断层滑动后方接收到的频率较低,这正是多普勒效应影响的结果。由于多普勒效应的客观存在,其对频率非平稳特性的影响与震源、传播路径和场地效应一样具有普遍性;所以,工程场地接收到的地震波的频率不仅取决于震源、传播路径、场地效应,还取决于断层滑动速度(多普勒效应)。