泉州盆地地震效应特征的一维等效线性和二维非线性分析的比较

采用等效线性动粘弹性模型描述土的动力非线性特性,基于一维等效线性波传法,对泉州盆地地震效应进行了分析;同时,采用修正Martin-Seed-Davidenkov动粘弹塑性模型描述土的动力非线性特性,对泉州盆地非线性地震效应进行了大尺度二维精细化有限元分析,研究了地形地貌和土层横向不均匀性对地震效应的影响。将两种分析结果进行对比,结果表明：①随着基岩输入地震动强度增大,地表峰值加速度PGA放大效应总体呈现减小趋势,中震与小震、大震与小震的地表PGA放大系数之比依次为0.83~0.99、0.72~0.97;②该盆地Ⅲ类场地处,基岩、地表起伏不大,且土层横向分布较均匀,两种方法计算得到的地震效应特征类似;基岩或地表起伏剧烈、土层横向分布明显不均匀的Ⅱ类场地上,二维非线性分析给出的地表PGA放大系数明显大于一维等效线性结果,两种方法得到的地表加速度反应谱及PGA随土层深度的变化特征存在显著差异,二维非线性分析给出的地表加速度反应谱大多呈现双峰甚至多峰现象,且PGA在土层特定深度处存在聚集效应,使PGA随土层深度的变化呈现非单调性。     

Dynamic mechanisms controlling the topography of Longmenshan area

The Longmenshan fault, which defines the eastern edge of the Tibetan Plateau, is one of the steepest margins of the plateau with a sharp elevation drop of about 4 km over a distance less than 100 km across the Longmenshan fault. The mechanism which is responsible for controlling and maintaining the elevation difference is highly debated. Using multiple observations including seismic velocity model, Moho depth, effective elastic thickness of the lithosphere, we conducted a quantitative study for elucidating the contributions from crust and lithospheric mantle by an integrated analysis of lithospheric isostasy and flexure. It is shown that the topography of the Longmenshan fault is supported by both lithospheric isostasy and flexure statically, and lower crustal channel flow and mantle convection dynamically. Different mechanisms have different weights for contribution to the topography of the Songpan-Ganzi block and the Sichuan Basin. The static and dynamic support contribute roughly the same to the topographic difference of ~4 km between the two sides of the Longmenshan fault. The static topographic difference of ~2 km is mainly resulted from the lithospheric isostasy, while the dynamic one of ~2 km is contributed by the uprising of the accumulated material in the lower crust beneath the Songpan-Ganzi block and the downward drag force caused by the upper mantle convection under the Sichuan Basin. It is thus suggested that the lower crustal flow and upper mantle convection are dynamic forces which should be taken into account in the studies on the dynamics in the Longmenshan and surrounding regions.     

The effects of rock uplift and rock resistance on river morphology in a subduction zone forearc,Oregon, USA

Relationships between riverbed morphology, concavity, rock type and rock uplift rate are examined to independently unravel the contribution of along-strike variations in lithology and rates of vertical deformation to the topographic relief of the Oregon coastal mountains. Lithologic control on river profile form is reflected by convexities and knickpoints in a number of longitudinal profiles and by general trends of concavity as a function of lithology. Volcanic and sedimentary rocks are the principal rock types underlying the northern Oregon Coast Ranges (between 46°30′ and 45°N) where mixed bedrock–alluvial channels dominate. Average concavity, θ, is 0·57 in this region. In the alluviated central Oregon Coast Ranges (between 45° and 44°N) values of concavity are, on average, the highest (θ = 0·82). South of 44°N, however, bedrock channels are common and θ = 0·73. Mixed bedrock–alluvial channels characterize rivers in the Klamath Mountains (from 43°N south; θ = 0·64). Rock uplift rates of ≥0·5 mm a⁻¹, mixed bedrock–alluvial channels, and concavities of 0·53–0·70 occur within the northernmost Coast Ranges and Klamath Mountains. For rivers flowing over volcanic rocks θ = 0·53, and θ = 0·72 for reaches crossing sedimentary rocks. Whereas channel type and concavity generally co-vary with lithology along much of the range, rivers between 44·5° and 43°N do not follow these trends. Concavities are generally greater than 0·70, alluvial channels are common, and river profiles lack knickpoints between 44·5° and 44°N, despite the fact that lithology is arguably invariant. Moreover, rock uplift rates in this region vary from low, ≤0·5 mm a⁻¹, to subsidence (<0 mm a⁻¹). These observations are consistent with models of transient river response to a decrease in uplift rate. Conversely, the rivers between 44° and 43°N have similar concavities and flow on the same mapped bedrock unit as the central region, but have bedrock channels and irregular longitudinal profiles, suggesting the river profiles reflect a transient response to an increase in uplift rate. If changes in rock uplift rate explain the differences in river profile form and morphology, it is unlikely that rock uplift and erosion are in steady state in the Oregon coastal mountains. Copyright © 2006 John Wiley & Sons, Ltd.     

Restrictions on Deep Flow Across the Shelf-break and the Role of Submarine Canyons in Facilitating such Flow

The shelf-break acts as a separator between the coastal ocean and the open ocean. Circulation (particularly deep near-bottom flow) is restricted from crossing the bathymetry. Eddies become elongated in the region of the shelf-break restricting exchange. An estimate of the horizontal eddy diffusivity over the shelf-break of less than 10m²s^-1 is found from a numerical model. Various mechanisms are responsible for the weak cross-isobath flow that does occur. One is the increase of the Rossby number over small-scale topography such as submarine canyons. Along-shore flow (in the direction opposite to Kelvin wave propagation) generates upwelling through submarine canyons. A review of upwelling through submarine canyons is given. The deep cross-shelf flow generated by the canyons is shown to be as significant as the wind-driven upwelling in some regions. Examples for the reduction of flow across the shelf-break and for upwelling through canyons are taken from the West Coast of Vancouver Island.     

Geophysical implications of Izu–Bonin mantle wedge hydration from chemical geodynamic modeling

Using two-dimensional dynamic models of the Northern Izu–Bonin (NIB) subduction zone, we show that a particular localized low-viscosity (η_LV =  3.3 × 10¹⁹ − 4.0 × 10²⁰ Pa s), low-density (Δρ ∼ −10 kg/m³ relative to ambient mantle) geometry within the wedge is required to match surface observations of topography, gravity, and geoid anomalies. The hydration structure resulting in this low-viscosity, low-density geometry develops due to fluid release into the wedge within a depth interval from 150 to 350 km and is consistent with results from coupled geochemical and geodynamic modeling of the NIB subduction system and from previous uncoupled models of the wedge beneath the Japan arcs. The source of the fluids can be either subducting lithospheric serpentinite or stable hydrous phases in the wedge such as serpentine or chlorite. On the basis of this modeling, predictions can be made as to the specific low-viscosity geometries associated with geophysical surface observables for other subduction zones based on regional subduction parameters such as subducting slab age.     

数字图像技术在重力异常地形校正中的应用

通过对地形体及其密度信息数字图像化,利用计算机仿真能较精确地计算出地形体在任意一点的重力场强度。在重力异常地形校正时,用各测点的实测数据减去相应测点的地形体重力场强度,即得到地形改正和中间层改正后的重力场强度。这样,重力异常校正中的地形改正和中间层改正可一并完成,既简化了校正的工作步骤,又提高了准确性。通过地形体实例,分别用数字图像仿真计算和积分精确计算其重力场强度,结果表明,二者计算结果十分相近。用数字图像仿真计算地形体重力场强度,误差较小,准确性较高,该方法完全适合重力异常地形校正。     

侧扫声纳在琼州海峡跨海通道地壳稳定性调查中的应用

为开展琼州海峡跨海通道地壳稳定性评价,利用Sonar Beam S-150D侧扫声纳系统在琼州海峡海底开展了地形地貌调查,获得了琼州海峡跨海通道工程区的侧扫声纳影像。通过数据分析、图像判读,分析了地质灾害类型及其可能的危害。在琼州海峡海底发现软土、活动沙波、岸坡及活动构造等微地貌单元,跨海通道工程施工、运行阶段都将遭受相关地质灾害的危害。这些资料和分析为跨海通道工程地壳稳定性评价提供了基础资料,为工程规划线路优选、建设实施提供了技术支撑。研究表明侧扫声纳技术可应用于海底地壳稳定性调查与评价工作中。      

The causes and processes responsible for rocky desertification in karst areas of southern China

The karst region of southern China is a fragile ecological zone with widespread rocky desertification. This paper describes the rocky desertification occurring in this region in terms of both natural and anthropogenic factors. During geological time periods, the region’s changing environment governed the natural rocky desertification processes, whereas during historical and modern times, anthropogenic processes have been superimposed on these natural processes. Human activities have accelerated and exacerbated rocky desertification. The period from the beginning to the middle of the Qing dynasty was an important transitional period in which human activities began to exert a particularly strong influence on rocky desertification. Since then, the effect of anthropogenic factors has increasingly exceeded the effect of natural factors. The rocky desertification process in southern China’s karst region combines surface ecological processes (including vegetation degradation and loss, soil erosion, surface water loss, and bedrock solution) with a reduction of the land’s biological productivity, leading to degradation that produces rocky desert. Controlling rocky desertification requires comprehensive development of sustainable agriculture and economic development that provides employment alternatives to agriculture and thereby promotes the rehabilitation of rocky desertified land.     

东南极内陆格罗夫山地区冰厚及冰下地形特征

中国第30次南极科学考察队格罗夫山分队(CHINARE30, 2013-2014年)利用雪地车载深层探冰雷达在东南极格罗夫山地区开展了测线总长度超过200 km大范围、高分辨率的冰厚及冰下地形调查, 获得了哈丁山北部和萨哈罗夫岭与阵风悬崖之间详细的冰厚及冰下地形特征. 通过对雷达数据分析表明, 哈丁山北部区域平均冰厚为580 m, 最大冰厚超过1 000 m, 出现在该区域的东北方向, 而东南方向冰厚相对较小; 萨哈罗夫岭与阵风悬崖之间区域的平均冰厚为610 m, 最大冰厚超过1 100 m, 该区域槽谷发育十分成熟, 槽谷形态近似呈U型. 通过对雷达剖面影像的筛选和分析, 推测在格罗夫山地区可能存在2个液态冰下湖泊.     

中国近海海面地形的计算与研究

介绍了海面地形的研究现状,并根据几何水准求得了中国近海的海平面倾斜及其精度.给出应用水文资料计算中国近海海面地形的公式及其计算结果.探讨了由球函数模型计算中国近海海面地形的理论和方法,并对不同方法进行了比较,得出了一些有益的结论.