Cross-shore variation of long-term average longshore current velocity in the nearshore zone

The cross-shore variation of long-term average longshore current velocity was investigated on the basis of a 15-year data set of longshore current, wave and wind. The longshore current velocities were measured once a day along a 427-m-long pier. The results show that the direction of the long-term average longshore current velocity away (>200 m) from the shore was the opposite to that near the shore. The southward current was dominant offshore, whereas the northward current was dominant near the shore. The cross-shore variation of the long-term average longshore current velocity was formed owing to a difference between the wave and wind conditions when the northward and southward currents developed. When the northward current developed, the offshore wave height was relatively small and the frequency of the southward wind velocity was almost equal to that of the northward one. As a result, the northward current developed only near the shore and decayed outside the narrow surf zone. On the other hand, when the southward current developed, the offshore wave height was relatively large and the southward wind velocity was predominant, which resulted in the southward current developing not only in the wide surf zone but also outside the surf zone. The superposition of the two cross-shore variations produced a cross-shore variation of the long-term average longshore current velocity with a northward velocity near the shore and a southward velocity away from the shore.     

渤海海峡及周边区域地壳结构的层析成像特征

渤海海峡是连接中国东部山东半岛和辽东半岛的重要途径,其跨海通道的地壳稳定性研究受到高度关注.本文利用地震层析成像方法重建三维P波速度模型,揭示了渤海海峡及周边区域地壳和上地幔的构造特征.结果表明,渤海海峡的速度结构存在明显的非均匀性,海峡北部地壳速度较高,结构较为完整,断层活动不明显,与现今较弱的地震活动相吻合,但是地壳底部存在低速薄层,它有可能成为地壳和上地幔之间的滑脱带,需要开展进一步的研究加以确认.相比之下,海峡南部地壳速度偏低,附近区域地震活动频繁,与张家口—蓬莱断裂带通过于此有着密切的联系,该断裂持续不断的地震活动对海峡南部的地壳结构产生了较大的影响.在渤海南部,郯庐断裂带东、西两侧的地壳结构明显不同,西侧速度偏高,东侧至渤海海峡速度偏低,这一特征可能与此地区广泛发育的断层和地震活动有关.另外,受华北克拉通破坏及地幔上涌的影响,渤海地区地壳深部和上地幔速度偏低,郯庐断裂带及渤海海峡附近显示出深部热流的活动迹象,反映了岩石圈减薄和软流圈的局部抬升.     

黄海的地壳速度结构与中朝—扬子块体拼合边界

利用中国、韩国和ISC台站的地震走时数据反演了黄海地区的地壳P波速度结构,对比重力异常和断裂体系、Pn波速度及其各向异性,分析了不同地球物理异常的相互关系以及黄海东部和西部的结构差异,为厘定黄海东部断裂暨中朝—扬子块体的拼合边界提供了新的信息.反演结果表明,北黄海和南黄海西部具有沉积盆地的地壳结构特征,P波速度明显偏低且深度较大,说明盆地内部沉积层较厚、沉降幅度较大,以北黄海、南黄海海州湾和苏北—南黄盆地最为突出.南黄海中部、胶东半岛、辽东半岛和朝鲜半岛显示出构造隆起区的地壳速度特征,其中南黄海中部的高速异常具有北东方向的伸展痕迹,与胶东地区的区域构造走向趋于一致,但是与朝鲜半岛的高速异常并不相连,其间存在明显的分界.据此推测南黄海与朝鲜半岛之间可能存在一个近南北方向的深断裂——黄海东部断裂,至于该断裂是否可以作为中朝—扬子块体在海区的拼合边界,尚需获取黄海东部及朝鲜半岛更详细的相关资料提供依据.     

Poleward expansion of the westward electrojet depending on the solar wind and IMF parameters

The effect of the interplanetary parameters on the latitudinal position of the substorm westward electrojet is studied in the work. The data from the IMAGE chain of magnetic stations and POLAR and WIND satellites for the period close to the solar activity minimum (1995–1996) and for the period of the solar activity maximum (2000) have been used for this purpose. It has been indicated that the electrojet poleward edge reaches, on average, higher latitudes at a higher solar wind velocity and at a larger (B _s ) IMF southward component. It has been indicated that the average latitude of the westward electrojet center increases with increasing solar wind velocity and decreases with increasing IMF southward component, as a result of which the electrojet center is, specifically, not observed at high geomagnetic latitudes at large values of the IMF southward component.     

小江断裂带周边地区三维P波速度结构及其构造意义

作为青藏高原的东南边界,小江断裂带在高原物质的侧向逃逸中发挥着重要的作用.本文利用流动地震台阵及固定台站的走时观测资料,对小江断裂带及周边区域的壳幔三维P波速度结构进行了研究.结果表明,在中上地壳,小江断裂带内部主要为低速异常,其东侧主要为高速异常.在中下地壳,小江断裂带中部为低速异常,北部和南部主要为高速异常,其中北部的高速异常可延伸到地表附近,南部的高速异常可一直延伸到上地幔.我们推测,小江断裂带中部的低速异常与深部热作用有关;北部的高速异常可能是晚古生代地幔柱活动导致大量基性和超基性幔源物质侵入地壳引起的,它的存在对青藏高原物质向南逃逸起到了一定的阻挡作用,可能是导致川滇活动块体北部次级块体快速抬升的重要因素;南部顶界面向北倾斜的高速异常体对川滇活动块体向南滑移起到了进一步的阻挡作用,导致其上覆的中上地壳低速异常区发生较强的变形和强烈的地震活动,同时在上地幔深度范围起到了稳定的作用,使其南部区域的介质受青藏高原物质向南挤出的影响明显减小.     

Experiments using a long-time-scale shelf circulation model of relevance to the Labrador Current

Experiments are described using a three-dimensional, shelf circulation model. The model geometry consists of a rectangle in latitude-longitude space with a shelf-slope region bordering the northern and western boundaries and a deep ocean region in the southeast. Relatively light water is flushed in through the northern boundary and allowed to exit through the southern boundary, a situation of relevance to the southward flowing Labrador Current. In an earlier paper, we showed the downstream development of a shelf break current. In that paper, bottom friction was parallel to bottom geostrophic velocity. In this paper, bottom friction is parallel to bottom velocity. This leads to a more diffuse downstream jet. We show that changing the density contrast across the front does not change its width. On the other hand, a sharper front is obtained when a small trough is introduced into the bottom topography. We also describe an experiment in which the density of the inflowing water is varied seasonally. This leads to a seasonal redistribution of the southward transport across the shelf, similar to a suggestion made by Myers et al. [(1989) Seasonal and interannual variability of the Labrador Current and West Greenland Current. Department of Fisheries and Oceans, Canada] for the Newfoundland Shelf. This redistribution results from the seasonal pulsing of fresh water down the shelf, which, in turn, influences transport through the Joint Effect of Baroclinicity And Relief (JEBAR), and is similar to the mechanism proposed by Lazier and Wright [(1993) Journal of Physical Oceanography, 23, 659–678].Other results concern the splitting of the shelf break jet. We show that in the previous paper, the splitting of the jet was influenced by the numerical formulation of the outflow condition at the southern boundary. We also show that the splitting can be suppressed by specifying the density of water flowing into the model domain through the southern boundary, rather than allowing this to be determined by the previous history of mixing and outflow on the boundary.     

Crustal Structure of the Southern Korean Peninsula from Seismic Waves Generated by Large Explosions in 2002 and 2004

In order to investigate the velocity structure of the southern part of the Korean peninsula, seismic refraction profiles were obtained along a 294-km WNW-ESE line and a 335-km NNW-SSE line in 2002 and 2004, respectively. Seismic waves were generated by detonating 500–1000 kg explosives in drill holes at depths of 80–150 m. The seismic signals were recorded by portable seismometers at nominal intervals of 1.5–1.7 km. Separate velocity tomograms were derived from first arrival times using a series expansion method of travel-time inversion. The raypaths indicate several mid-crust interfaces including those at approximate depths of 2–3, 15–17, and 22 km. The Moho discontinuity with refraction velocity of 7.8 to 8.4 km/s has a maximum depth of 37–39 km under the southern central portion of the peninsula. The Moho becomes shallower as the Yellow Sea and the East Sea are approached on the west and east coasts of the peninsula, respectively. The depth of the 7.6 km/s velocity contour varies from 29.4 km to 36.5 km. The discrepancy in depth between the seismological Moho and the interpreted critically refracting interface may result from the presence of a gradual transition between the crust and mantle. The velocity tomograms show particular crustal structures including (1) the existence of an over 70-km wide low-velocity zone centered at 6–7 km depth under the Okchon fold belt and Ryeongnam massif, (2) existence of high-velocity materials under the Gyeongsang basin, and (3) the downward extension of the Yeongdong fault to depths greater than 10 km.     

Further studies of the Perkins stability during Space Weather Month

A simple integral equation describing the height of the mid-latitude ionosphere is presented and compared with observations and a more sophisticated model. At any instant, the ionosphere moves toward an equilibrium altitude, if it exists, which is determined by a combination of southward wind and eastward electric field. If no equilibrium exists, the layer simply falls at its terminal velocity or at a wind or electric field-aided rate. Including a simple recombination model provides an even more complete characterization of the nighttime ionosphere. By comparing the measured and predicted content, an estimate of the flux from the plasmasphere is possible.     

南北地震带北段的地壳速度结构及其构造启示

收集和拾取了“中国地震科学台阵”探测项目在南北地震带北段布设的680个流动地震台站和中国地震台网217个固定台站所记录的地震事件的P波和S波初至到时,通过层析成像研究获得了南北地震带北段水平网格间距为0.33°×0.33°的地壳P波和S波速度分布。结果显示:在30 km深度上青藏高原东北部表现为显著的整体性低速异常,低速异常区向南延伸至龙门山断裂,以106°E为界线将秦岭造山带分为西侧的低速异常和东侧的高速异常,并沿银川—河套地堑向东北展布,向北穿过河西走廊,在阿拉善地块表现为低速异常,这可能暗示了青藏高原向东的扩展被较为坚固的四川盆地和秦岭造山带阻挡,而向北的扩展可能影响到了河西走廊至阿拉善地块,并沿银川—河套地堑影响到鄂尔多斯西北缘;在50 km深度上,阿拉善地块、祁连造山带东段显示高速异常,有可能是阿拉善地块向祁连东段下方俯冲所致。研究区内大部分地震分布在P波和S波高低速异常相间及速度剧烈变化的地区,M≥6.0强震几乎全部投影在30 km深度的低速异常区域内,说明强震发生的背景可能与地震源区下方的低速区有关。      

Barotropic wind-driven circulation patterns in a closed rectangular basin of variable depth influenced by a peninsula or an island

We study how a coastal obstruction (peninsula or coastal island) affects the three-dimensional barotropic currents in an oblong rectangular basin with variable bathymetry across the basin width. The transverse depth profile is asymmetric and the peninsula or island lies in the middle of the long side of the rectangle. A semi-spectral model for the Boussinesq-approximated shallow water equations, developed in Haidvogel et al. and altered for semi-implicit numerical integration in time in Wang and Hutter, is used to find the steady barotropic state circulation pattern to external winds. The structural (qualitative) rearrangements and quanti2tative features of the current pattern are studied under four principal wind directions and different lengths of the peninsula and its inclination relative to the shore. The essentially non-linear relationships of the water flux between the two sub-basins (formed by the obstructing peninsula) and the corresponding cross-sectional area left open are found and analysed. It is further analysed whether the depth-integrated model, usually adopted by others, is meaningful when applied to the water exchange problems. The flow through the channel narrowing is quantitatively estimated and compared with the three-dimensional results. The dynamics of the vortex structure and the identification of the up-welling/down-welling zones around the obstruction are discussed in detail. The influence of the transformation of the peninsula into a coastal island on the global basin circulation is considered as are the currents in the channel. The geometric and physical reasons for the anisotropy of the current structure which prevail through all obtained solutions are also discussed.Dedicated to Professor L.A. Mysak on the occasion of his sixtieth birthday.     

堪察加地区现今地壳运动与变形特征研究

利用俄罗斯堪察加地区1995～2005年的GPS观测数据,研究了该区现今地壳水平运动速度场特征.在球坐标系中解算了各应变率分量,分析了应变率场的空间分布特征,并与地震学和地质学研究结果进行了综合对比分析.结果表明,堪察加半岛北部的微板块边界并不明显,堪察加南部测站运动速度大于中部和北部地区,愈靠近东部板块汇聚区,测站速度越大.从东海岸到西海岸,测站水平速度存在明显的梯度衰减特征,水平运动方向与太平洋板块向西北的俯冲方向基本一致.各应变率分量具有东部海岸大于中部和西海岸、从东至西呈梯度衰减的特点.堪察加大部分地区处于EW和NS向压缩状态,局部存在拉张.面应变率结果显示绝大部分为压缩区;刚性转动结果表明大部分地区表现为顺时针转动,北部地区和南端顺时针旋转性明显.东部有效应变率明显大于西部地区,东西向梯度衰减关系明显.主压应变率明显大于主张应变率,特别是在东海岸地区.主压应变率方向与中等以上地震的主压应力轴在水平方向的投影方向基本一致.地壳变形场在空间分布上的不一致性主要与太平洋板块在堪察加半岛东南侧的俯冲深度、俯冲方位角、俯冲倾角和俯冲带的耦合强度有关.     

利用地震背景噪声成像技术反演三峡库区及邻近地区地壳剪切波速度结构

本文利用三峡库区重庆遥测台网及邻近地区台网2012年1月至12月期间的连续波形记录,通过噪声互相关,提取了这些台站间的经验格林函数.并通过获取的经验格林函数瑞利面波频散信息,反演出该区域5~40 s瑞利面波相速度分布.周期5 s及10 s的相速度分布结果显示：四川盆地具有较厚的沉积盖层,其中盆地中部沉积层相对偏浅.而周期20 s及35 s的相速度分布结果显示：中下地壳的扬子克拉通整体呈现较高的相速度分布,具有相对坚硬的中下地壳结构.这其中吉首—常德的武陵山区附近出现明显高速异常,可能与江南古陆古火山岛链（雪峰—九岭弧形古火山群）活动有关,火山活动导致大量的基性-超基性幔源物质侵入地壳,造成了武陵山重力异常,并一定程度上阻挡了云贵高原和湘鄂西部的物质向南逃逸;在地形上,此处恰好对应我国地形第二阶梯向第三阶梯的过渡带,并将华南地块分隔为东西两大部分.最后我们在瑞利面波相速度成像的基础上,反演出该区域三维剪切波速度分布,结果显示四川盆地周边的龙门山、米仓山、大巴山、七曜山和大娄山等地质构造与地壳剪切波速度结构具有明显对应关系.研究结果有助于深入研究该区的深部地质构造特征与地震发生机理.     

利用Rayleigh波相速度和群速度联合反演青藏高原东北缘S波速度结构

利用青藏高原东北缘地区固定和流动地震台网2007年8月到2012年1月期间记录的远震波形,运用小波变换频时分析方法分别测定了1216和653条周期从15到140 s的台站间基阶Rayleigh相速度和群速度频散曲线.通过对上述频散进行反演,重构了青藏高原东北缘分辨率高达0.5°×0.5°的2-D相速度和群速度分布图.然后通过对所提取到的每个格网点Rayleigh波相速度和群速度频散进行联合反演,得到了研究区下方一维S波速度结构.最后通过线性插值,得到了青藏高原东北缘下方地壳上地幔三维S波结构.结果表明,印度板块向北俯冲已经达到班公-怒江缝合带附近;在柴达木盆地北部祁连山下面我们发现了亚洲板块,且其没有表现出明显的向南俯冲的迹象;在两大板块中间,我们观测到延伸到250 km深度的低速异常,该低速异常可能是地幔物质底辟上涌现象造成的.     

Development of a modelling methodology for simulation of long-term morphological evolution of the southern Baltic coast

The Darss–Zingst peninsula at the southern Baltic Sea is a typical wave-dominated barrier island system which includes an outer barrier island and an inner lagoon. The formation of the Darss–Zingst peninsula dates back to the Littorina Transgression onset about 8,000 cal BP. It originated from several discrete islands, has been reshaped by littoral currents, wind-induced waves during the last 8,000 years and evolved into a complex barrier island system as today; thus, it may serve as an example to study the coastal evolution under long-term climate change. A methodology for developing a long-term (decadal-to-centennial) process-based morphodynamic model for the southern Baltic coastal environment is presented here. The methodology consists of two main components: (1) a preliminary analysis of the key processes driving the morphological evolution of the study area based on statistical analysis of meteorological data and sensitivity studies; (2) a multi-scale high-resolution process-based model. The process-based model is structured into eight main modules. The two-dimensional vertically integrated circulation module, the wave module, the bottom boundary layer module, the sediment transport module, the cliff erosion module and the nearshore storm module are real-time calculation modules which aim at solving the short-term processes. A bathymetry update module and a long-term control function set, in which the ‘reduction’ concepts and technique for morphological update acceleration are implemented, are integrated to up-scale the effects of short-term processes to a decadal-to-centennial scale. A series of multi-scale modelling strategies are implemented in the application of the model to the research area. Successful hindcast of the coastline change of the Darss–Zingst peninsula for the last 300 years validates the modelling methodology. Model results indicate that the coastline change of the Darss–Zingst peninsula is dominated by mechanisms acting on different time scales. The coastlines of Darss and the island of Hiddensee are mainly reshaped by long-term effects of waves and longshore currents, while the coastline change of the Zingst peninsula is due to a combination of long-term effects of waves and short-term effects caused by wind storms.     

Fault tectonic analysis of Kii peninsula,Southwest Japan: Preliminary approach to Neogene paleostress sequence near the Nankai subduction zone

The southern part of the Outer Zone of Southwest Japan including the Kii peninsula belongs to the tectonic ‘shadow zone’, where fewer conspicuous active faults and less Quaternary sediments develop than in the Nankai subduction zone and Inner Zone of Southwest Japan. In order to study the paleostress sequence of the Kii peninsula, we analyzed fault‐slip data and tension gashes at pilot sites of Early–Middle Miocene forearc sediments and Late Cretaceous accretionary complex. According to the results, six faulting events are reconstructed in sequence: (i) east–west extension (normal faulting); (ii) east–west compression and north–south extension (strike‐slip faulting); (iii) NNW–SSE compression and ENE–WSW extension (strike‐slip faulting); (iv) northeast–southwest compression and northwest–southeast extension (strike‐slip faulting); (v) WNW–ESE compression (strike‐slip or reverse faulting); and (vi) NNE–SSW extension. The north–south to NNW–SSE trending dyke swarm of Middle Miocene age in the Kii peninsula is thought to be related to Event 3, implying that Event 3 was active at least during the Middle Miocene. Because Event 6 is recognized solely at a site, the overall latest faulting event seems to be Event 5. Assuming that the compression results from the motion of the crust or plate, the compression direction of Event 5 is in good accordance with the present‐day WNW crustal velocity vectors of the Kii peninsula. The stress trajectory map of Southeast Korea and Southwest Japan reveals that the current compression directions of the Kii peninsula correspond to the combinatory stress fields of the Himalayan and Philippine Sea tectonic domains.     

华南地区上地幔P波三维速度结构和动力学意义:来自有限频层析成像的证据

利用155个宽频带流动地震仪记录的连续地震波形数据,通过有限频层析成像技术,反演获得了华南地区上地幔的高分辨率P波三维速度模型.结果显示,大致以江南造山带为界,研究区域南部的华夏块体的大部分区域上地幔存在一个清晰的低速异常构造,而研究区北部的扬子克拉通的大部分区域上地幔则存在高速异常结构,并且这些速度异常体都向下延伸到地幔转换带.一个重要的结果是在(27°N,118°E)处观测到通过410-km界面的上涌流,并且在上升的过程中逐渐向西和向北扩展,显示为华夏块体深部200～400 km深度的大范围低速异常,可能为华夏块体广泛分布的新生代岩浆活动提供深部来源.更重要的是华夏块体通过410-km界面上涌流在上涌的过程中向北延伸,越过江南造山带"侵入"到扬子克拉通的南部地区,造成了扬子克拉通较厚的岩石圈对应的高速异常体向南倾斜的假象.最后,位于117°E(郯庐断裂的南端)以东的扬子克拉通岩石圈已经被"活化",即被来自南部热的软流圈物质替而代之.同时,推断在华夏块体下方地幔转换带内低速异常体可能是与海南地幔柱有关.海南地幔柱和(27°N,118°E)410-km界面上涌流的关系还有待于今后更大范...     

Response of the reconnection electric field and polar cap potential to the IMF and velocity of solar wind

It is well known that the cross polar cap potential is saturated under a strong interplanetary electric field and is often said to be related to the ionospheric currents. To investigate the other factors influencing this phenomenon, a global magnetohydrodynamics simulation not including the feedback from the ionosphere to the magnetosphere was conducted. The simulation results showed that an increase in the southward IMF causes a smaller increase in the cross polar cap potential than that caused by an increase in the solar wind velocity. This difference was caused by the transportation of reconnected magnetic field lines towards the tail.     

Shallow/upper crustal shear wave structure of the Tehran region (Central Alborz,Iran) from the inversion of Rayleigh wave dispersion measurements

Surface wave dispersion curves from microearthquakes are used to obtain group velocity dispersion maps. The calculation of the local dispersion curves for each grid point from these maps then produces the input data to retrieve the 3D shear wave velocity model of the Tehran region. The group velocity maps indicate that the tomographic results agree well with the three main tectonic features and the geological units in the study area. The tomographic maps generally possess high-velocity structures across most of the mountain belts (Central Alborz and east-southeast mountains), whereas the Tehran Basin correlates to a low-velocity structure. Increasing the period in the study area highlights four independent low-velocity zones that reflect faults and fault junction systems. The shear wave velocity profiles indicate that the depth to bedrock exhibits southward variation ranging from ~?300 m to ~?1500 m. We also focus our analysis on the existence of faults within the shear wave profiles and discuss the low shear wave velocity anomalies deeper than 2 km result from the main fault structures (e.g., North Tehran, North-South Rey and Parchin). Furthermore, we argue that the dip angle of the North Tehran fault varies along fault strike, whereas the North-South Rey fault possesses a constant dip angle. Moreover, initial model uncertainties and checkerboard resolution tests are used to identify reliable and robust anomaly features in the 3D shear wave velocity model and 2D tomographic maps, respectively. Microearthquake analysis provides an effective approach for studying the upper crustal structure heterogeneity, especially the fault structure, of the Tehran region.     

El Nino演变不同阶段东亚大气环流年际异常型的数值模拟

给定1948～1999年逐月变化的全球观测的海表温度分布，使用全球大气环流模式（CCM3/NCAR）模拟了大气对海表温度变化的响应，利用SVD和合成检验方法，分析了El Nino发展阶段夏季、成熟阶段冬季以及衰亡阶段夏季东亚大气环流的年际异常型.结果表明：El Nino发展阶段夏季，中国东北、朝鲜半岛以及日本海附近为高度负异常中心，西太平洋副高偏弱、偏东，东亚夏季风增强；El Nino成熟阶段冬季，东亚大槽加强，东亚北部冬季风加强；El Nino衰亡阶段夏季，西太平洋副高偏强、偏南、西伸，东亚夏季风减弱；El Nino事件在其衰亡阶段夏季与东亚大气环流异常的关系最紧密，其次是成熟阶段冬季，最后是发展阶段夏季.模拟的El Nino演变不同阶段东亚大气环流年际异常型易于解释以往研究中观测分析揭示的由El Nino造成的我国东部气温和降水异常型.     

Central polar cap convection response to short duration southward Interplanetary Magnetic Field

Central polar cap convection changes associated with southward turnings of the Interplanetary Magnetic Field (IMF) are studied using a chain of Canadian Advanced Digital Ionosondes (CADI) in the northern polar cap. A study of 32 short duration (1 h) southward IMF transition events found a three stage response: (1) initial response to a southward transition is near simultaneous for the entire polar cap; (2) the peak of the convection speed (attributed to the maximum merging electric field) propagates poleward from the ionospheric footprint of the merging region; and (3) if the change in IMF is rapid enough, then a step in convection appears to start at the cusp and then propagates antisunward over the polar cap with the velocity of the maximum convection. On the nightside, a substorm onset is observed at about the time when the step increase in convection (associated with the rapid transition of IMF) arrives at the polar cap boundary.