Effects of a covering layer in a circular-arc canyon on incident plane SV waves

Introduction The effect of local topographies on seismic wave propagation is one of the most attractive topics in seismology. It may be resolved by either an analytical method or a numerical method. Here, the analytical method is the wave function expansion method; numerical methods include the finite difference, finite element, boundary integration equation, and discrete wave number method, etc. The advantage of these numerical methods is that they can be applied to local inhomogeneity and ir…     

Determining anti-plane responses induced by oblique-truncated semicircular canyon using systematic hybrid method with mapping function

This paper proposes a novel strategy for the investigation of displacement amplitude ($\left|u_y\right|$) near and along an oblique-truncated semicircular canyon subjected to shear horizontal (SH) waves. Transfinite interpolation (TFI) was used to obtain the coordinates of nodes and determine the sequence of node numbering in the inner finite region including the canyon. The hybrid method, comprising finite element method and a Lamb series, was applied in conjunction with TFI to study the effects of canyon geometry, incident angle of SH waves ($\theta$), and dimensionless frequency ($\eta$) on $\left|u_y\right|$. We detailed the amplification of $\left|u_y\right|$ in the illuminated zone and variations in $\left|u_y\right|$ due to canyon-decay-effect along the canyon surface as well as the decay of $\left|u_y\right|$ resulting from the shield effect in the shadow zone. Interestingly, oblique-effects play an important role in the magnification of $\left|u_y\right|$ along the inclined bottom of canyons, and variations in $\theta$ and $\eta$ dominate the patterns of $\left|u_y\right|$.     

汶川地震诱发山地斜坡震动的地形放大效应

汶川地震诱发大量山地斜坡次生灾害具有明显的地形分布特征,对其地形放大效应研究具有重要的理论及实际意义.斜坡震害调查表明,宏观地貌单元对次生地质灾害的发育类型具有控制作用,中高山区以大中型滑坡、崩塌为主,低山及丘陵区以山体震裂、小型崩塌为主;峡谷形态对次生地质灾害的发育密度具有明显影响,其中“V”型峡谷发育密度高于“U”型峡谷;单薄山脊、条形山体、坡型转折部位、凸出地形等微地貌地形放大效应明显.监测显示,不同方向水平分量地震波峰值(PHA)相差可达10余倍,研究表明:地形尺寸与地震波波长的耦合作用使得地震动水平分量放大显著.     

起伏地形条件下电阻率法三维反演

地形起伏下的三维反演问题是当前电阻率法研究的一个难题.为了更好地解决上述问题,采用加权正则化共轭梯度法实现起伏地形三维电阻率反演算法.该方法引入了加权正则化思想,显著降低了迭代时目标函数发散的问题,提高了反演稳定性.对比了两种消除反演中地形影响的方法,结果表明,直接带地形的电阻率法三维反演具有更好的分辨率,能有效地消除地形所造成的误差,但在起伏角度偏大,如河流、堤坝等接近垂直角度时,使用此方法会使得反演发散得不到满意的结果;此时采用基于地形校正的方法有一定的效果.     

地形对涡量传播和台风切向风速变化的参数敏感性

设计了一个高分辨率的浅水模式，以研究地形参数和纬向基流在涡旋Rossby波传播和台风切向风速变化中的作用。结果表明，无纬向基流条件下，岛屿地形的水平尺度对涡旋Rossby波传播的影响表现在：随地形水平方向尺度的增大，扰动涡度场环绕地形顺时针方向旋转的趋势增强，局域风速增幅减小，增强了平均风速的减弱。纬向东风基流条件下，热带气旋的局域风速及其平均切向风速的演变受台风与地形间距的改变以及地形作用时间长短的影响：当台风逐渐靠近地形时，局域风速增幅增强，平均风速逐渐减小；当台风逐渐远离地形时，出现相反的变化。     

A buried submarine canyon in the northwestern South China Sea: architecture,development processes and implications for hydrocarbon exploration

High-resolution multichannel seismic data enables the discovery of a previous, undocumented submarine canyon(Huaguang Canyon) in the Qiongdongnan Basin, northwestern South China Sea. The Huaguang Canyon with a NW orientation is 140 km in length, and 2.5 km to 5 km in width in its upper reach and 4.6 km to 9.5 km in width in its lower reach. The head of the Huaguang Canyon is close to the Xisha carbonate platform and its tail is adjacent to the Central Canyon. This buried submarine canyon is formed by gravity flows from the Xisha carbonate platform when the sea level dropped in the early stage of the late Miocene(around 10.5 Ma). The internal architecture of the Huaguang Canyon is mainly characterized by high amplitude reflections, indicating that this ancient submarine canyon was filled with coarse-grained sediments. The sediment was principally scourced from the Xisha carbonate platform. In contrast to other buried large-scale submarine canyons(Central Canyon and Zhongjian Canyon) in the Qiongdongnan Basin, the Huaguang Canyon displays later formation time,smaller width and length, and single sediment supply. The coarse-grained deposits within the Huaguang Canyon provide a good environment for reserving oil and gas, and the muddy fillings in the Huaguang Canyon have been identified as regional caps. Therefore, the Huaguang Canyon is a potential area for future hydrocarbon exploration in the northwestern South China Sea. The result of this paper may contribute to a better understanding of the evolution of submarine canyons formed in carbonate environment.     

Large-scale Circulation Control of the Occurrence of Low-level Turbulence at Hong Kong International Airport

This study identifies the atmospheric circulation features that are favorable for the occurrence of low-level turbulence at Hong Kong International Airport [below 1600 feet(around 500 m)]. By using LIDAR data at the airport, turbulence and nonturbulence cases are selected. It is found that the occurrence of turbulence is significantly related to the strength of the southerly wind at 850 h Pa over the South China coast. On the other hand, the east–west wind at this height demonstrates a weak relation to the occurrence. This suggests that turbulence is generated by flow passing Lantau Island from the south. The southerly wind also transports moisture from the South China Sea to Hong Kong, reducing local stability. This is favorable for the development of strong turbulence. It is also noted that the strong southerly wind during the occurrence of low-level turbulence is contributed by an anomalous zonal gradient of geopotential in the lower troposphere over the South China Sea. This gradient is caused by the combination of variations at different timescales. These are the passage of synoptic extratropical cyclones and anticyclones and the intraseasonal variation in the western North Pacific subtropical high. The seasonal variation in geopotential east of the Tibetan Plateau leads to a seasonal change in meridional wind, by which the frequency of low-level turbulence is maximized in spring and minimized in autumn.     

Topography-Controlled Soil Water Content and the Coexistence of Forest and Steppe in Northern China

The semi-arid forest-steppe ecotone in China is characterized by a patchy pattern of forest and steppe, with forest patches restricted to shady slopes. To address the effect of topography on forest distribution through regulation of available water, we calculated evaporation as a function of slope aspect and inclination. Field vegetation records from randomly selected sites with minimum slope inclination were used to test the simulated forest distribution. Seasonal and diurnal changes of surface soil temperature and moisture of shady and sunny slopes were recorded. Soil water content was measured during two growing seasons on both sunny and shady slopes with the same forest type at three sites located along the mean annual precipitation (MAP) gradient. Evaporation decreases with slope inclination on shady slopes, but increases with inclination on sunny slopes. The shady slope received 35% of the annual direct solar radiation received by the sunny slope when the slope inclination was 25°, and the contrast in annual direct solar radiation between the shady and sunny slopes further widens as slope inclination increases. Steeper shady slopes can support forests in dryer climates, with log-linear regression revealing a minimum slope inclination for forest distribution along the MAP gradient. The simulated minimum slope inclination for forest growth was larger than the observed minimum inclination, and the difference was greater in wetter conditions. A larger forest area fraction was considered to lead to a reduction in soil temperature and evaporation, as verified by soil temperature and moisture records and soil water content measurements. The slope-specific forest distribution in the semi-arid region of China can be explained by a topography-controlled soil water supply. Lower evaporation, resulting from lower direct solar radiation on shady slopes, allows shady slopes to retain a water supply sufficient for sustaining forests, and the existence of forests on shady slopes further reduces evaporation. Different tree species coexist at the xeric timberline due to regulation by slope inclination and aspect.     

喜马拉雅山脉地震带主要地质灾害与地形地貌关系——以西藏日喀则地区为例

喜马拉雅山脉是欧亚板块和印度洋板块挤压碰撞形成的巨大山脉,历史上曾多次发生7级以上地震,引发大量次生地质灾害。收集以往资料,结合野外调查和遥感解译,利用喜马拉雅山脉地震带（日喀则段）地貌图和GIS的数据管理与空间分析功能,对日喀则地区主要地质灾害进行分析,绘制地质灾害分布图和密度等值线图。结果表明：①研究区主要地质灾害集中分布,共5个集中区;②主要地质灾害发育于侵蚀剥蚀大起伏高山和冰缘作用的大起伏极高山地貌,河谷平原及冰碛丘陵次之,其他地貌特征发育灾害较少;③滑坡、崩塌灾害易发于海拔3500 m以下阳坡（90°~270°）,在南东向敏感性最强,滑坡、崩塌分别易发于斜坡坡度15°~45°和35°~90°;④泥石流灾害多发生于流域面积小于5 km2,相对高差大于100 m的沟谷中,纵坡大于212.56‰的"V"形谷中尤为发育。     

黄海夏季潮汐锋区环流的数值研究

基于Blumberg＆Mellor三维非线性环流模型(ECOM)并结合Mellor＆Yamada的湍封闭模型，对黄海M2分潮、密度环流、锋区环流等进行了的数值模拟：潮汐模拟结果较好的体现了黄、渤海M2潮波传播系统，验证了模型的可行性；模拟密度环流的量阶和方向均与夏季实测环流保持一致，体现了密度环流是夏季总环流的重要组成部分；两个断面锋区环流的模拟结果显示锋区位置不同环流特征也不相同，锋区对应的上升流特征与锋区表面通常观测到冷水现象吻合，锋区上层水体沿潮汐锋方向流速较强。另外，数值试验结果显示了地形和潮混合对潮汐锋的形成及锋区环流有重要影响。