Issues in characterizing heterogeneity and connectivity in non-multiGaussian media

The performances of kriging, stochastic simulations and sequential self-calibration inversion are assessed when characterizing a non-multiGaussian synthetic 2D braided channel aquifer. The comparison is based on a series of criteria such as the reproduction of the original reference transmissivity or head fields, but also in terms of accuracy of flow and transport (capture zone) forecasts when the flow conditions are modified. We observe that the errors remain large even for a dense data network. In addition some unexpected behaviours are observed when large transmissivity datasets are used. In particular, we observe an increase of the bias with the number of transmissivity data and an increasing uncertainty with the number of head data. This is interpreted as a consequence of the use of an inadequate multiGaussian stochastic model that is not able to reproduce the connectivity of the original field.     

A Survey of Techniques for Predicting Earthquake Ground Motions for Engineering Purposes

Over the past four or five decades many advances have been made in earthquake ground-motion prediction and a variety of procedures have been proposed. Some of these procedures are based on explicit physical models of the earthquake source, travel-path and recording site while others lack a strong physical basis and seek only to replicate observations. In addition, there are a number of hybrid methods that seek to combine benefits of different approaches. The various techniques proposed have their adherents and some of them are extensively used to estimate ground motions for engineering design purposes and in seismic hazard research. These methods all have their own advantages and limitations that are not often discussed by their proponents. The purposes of this article are to: summarise existing methods and the most important references, provide a family tree showing the connections between different methods and, most importantly, to discuss the advantages and disadvantages of each method.

Probabilistic aftershock hazard assessment II: application of strong ground motion simulations

Probabilistic aftershock hazard assessment (PAHA) has been introduced by Wiemer (Geophys Res Lett 27:3405–3408, 2000). The method, in its original form, utilizes attenuation relations in evaluating peak ground velocity (PGV) exceedence probability. We substitute the attenuation relations together with their uncertainties by strong ground motion simulations for a set of scenarios. The main advantage of such an approach is that the simulations account for specific details of the aftershock source effects (faulting style, slip distribution, position of the nucleation point, etc.). Mean PGVs and their standard deviations are retrieved from the simulation results obtained by the new hybrid k-squared source model, and they are used for the PAHA analysis at a station under study. The model chosen for the testing purposes is inspired by the Izmit A25 aftershock (M _w  = 5.8) that occurred 26 days after the mainshock. The PAHA maps are compared with (1) those obtained by the use of attenuation relations and (2) the peak values of ten selected strong-motion recordings written by the aftershock at epicentral distances <50 km. We conclude that, although the overall hazard decay with increasing fault distance is similar, the PAHA maps obtained by the use of simulations exhibit remanent radiation pattern effect and prolongation in the strike direction due to the directivity effect pronounced for some of the scenarios. As regard the comparison with real data, we conclude that the PAHA maps agree with observed peak values due to appropriate attenuation model adopted in the analysis.     

Lava flow hazard at Nyiragongo volcano,D.R.C.

The 2002 eruption of Nyiragongo volcano constitutes the most outstanding case ever of lava flow in a big town. It also represents one of the very rare cases of direct casualties from lava flows, which had high velocities of up to tens of kilometer per hour. As in the 1977 eruption, which is the only other eccentric eruption of the volcano in more than 100 years, lava flows were emitted from several vents along a N–S system of fractures extending for more than 10 km, from which they propagated mostly towards Lake Kivu and Goma, a town of about 500,000 inhabitants. We assessed the lava flow hazard on the entire volcano and in the towns of Goma (D.R.C.) and Gisenyi (Rwanda) through numerical simulations of probable lava flow paths. Lava flow paths are computed based on the steepest descent principle, modified by stochastically perturbing the topography to take into account the capability of lava flows to override topographic obstacles, fill topographic depressions, and spread over the topography. Code calibration and the definition of the expected lava flow length and vent opening probability distributions were done based on the 1977 and 2002 eruptions. The final lava flow hazard map shows that the eastern sector of Goma devastated in 2002 represents the area of highest hazard on the flanks of the volcano. The second highest hazard sector in Goma is the area of propagation of the western lava flow in 2002. The town of Gisenyi is subject to moderate to high hazard due to its proximity to the alignment of fractures active in 1977 and 2002. In a companion paper (Chirico et al., Bull Volcanol, in this issue, 2008) we use numerical simulations to investigate the possibility of reducing lava flow hazard through the construction of protective barriers, and formulate a proposal for the future development of the town of Goma.     

气候变化对滇金丝猴分布的潜在影响

分析气候变化对动物分布的影响,对气候变化影响下保护生物多样性具有重要的意义。利用CART(classification and regression tree,分类和回归树)生态位模型,采用A1、A2、B1和B2气候变化情景,模拟分析了气候变化对我国滇金丝猴分布范围及空间格局的影响趋势。结果显示：气候变化后,滇金丝猴目前适宜分布范围将减小,新适宜及总适宜范围将扩大,在1991-2020年时段较大,从1991-2020年时段到2081-2100年时段随气候变化时间段延长而逐渐缩小,其中A1情景下变化最大,B1情景下变化最小。气候变化后,滇金丝猴目前适宜分布区东北部及南部适宜范围将缩小,西部和西北及东南部适宜范围将扩大。气候变化后,滇金丝猴目前适宜、新适宜和总适宜分布区范围与我国年均气温和年降水量变化呈负相关。多元回归分析表明,滇金丝猴目前适宜、新适宜和总适宜分布范围均随我国年均气温升高和年降水量增加而减少,其中气温变化影响比降水量变化影响大。因此,气候变化后,近期将使滇金丝猴目前分布适宜分布范围减少,新适宜分布范围将扩大,随气候变化程度增强,新适宜及总适宜分布范围都将减小。     

中国土地覆盖时空变化未来情景分析

在对HLZ (Holdridge life zone) 分类系统与土地覆盖类型分类系统之间的差异性进行对比分析的基础上,根据土地覆盖类型与HLZ生态系统类型的最大对应概率,构建了基于栅格的土地覆盖边际转换模型。采用基于HadCM3 A1FI、A2a、B2a等三种未来气候变化情景数据模拟获得的中国HLZ生态系统时空变化的系列栅格数据,运行模型后获得相应时段中国未来土地覆盖时空变化情景系列数据。引入土地覆盖类型景观指数及平均中心系列模型和构造平均中心偏移距离及偏移方向的计算模型,对中国未来土地覆盖的景观指数变化及其平均中心的偏移距离、偏移方向及偏移趋势进行综合分析。基于HadCM3 A1FI、A2a、B2a三种情景模拟结果分析表明：在2000~2099年间耕地、草地、湿地、水域、冰川雪被等土地覆盖类型面积逐渐减少,林地、建设用地、荒漠等土地覆盖类型面积逐渐增加,沙漠面积有所减少。其中,林地增加速度最快 (平均每10年增加2.34%),裸露岩石减少速度最快 (平均每10年减少2.38%)。     

不同水资源情景下干旱区未来土地利用/覆盖变化模拟——以黑河中上游张掖市为例

应用回归分析方法确定对研究区土地利用/覆盖变化有重要贡献的10种驱动因子,用线性规划方法确定模型输入文件中模拟期间每年的各种土地利用类型面积.用ArcView空间分析的方法建立驱动力文件,利用SPSS13.0软件分析每种驱动力的权重(β值),建立Logistic方程,并将其作为模型输入文件.建立可利用水资源总量分别为18.0×10⁸m³,26.5×10⁸m³,35.0×10⁸m³的3种情景假设,应用CLUE-S模型模拟张掖市2001-2020年土地利用/覆盖变化变化.模拟结果显示:1)三种情景下,耕地的面积都在减少,耕地面积的减少量与可利用水资源总量呈负相关;2)林草地面积在3种情景下均增加,林草地面积的变化与可利用水资源总量呈正相关;3)在3种情景假设下,水域面积的变化都不明显;4)城镇用地面积变化与水量也呈正相关;5)未利用地面积持续减少.     

利用随机天气模式及多种插值方法生成逐日气候变化情景的研究

针对目前大气环流模式在用于气候变化影响评估研究中时间分辨率较低的局恨性, 以及气候情景的要求和气候变化影响研究的需要, 结合GCM的模拟试验结果, 利用随机天气模式WGEN生成了中国东北地区未来气候变化的逐日情景, 其中包含了可能的气候变率信息, 可与作物动力模式等气候影响模式嵌套, 研究作物生长发育及其产量的可能变化, 及气候变率变化的可能影响等.     

未来气候变化对淮河流域径流深的影响

本文运用多元回归方法,建立有关气候-径流深的数学模型,并用该模型预测在未来气候变化的15种可能情景下淮河三个代表子流域径流深的变化。结果表明:年径流深随年降水量的增加而增加,随年均温度的升高而减少;不同流域对各种气候变化的响应存在着明显的差异,反映出整个淮河流域不同自然地理条件的影响;不同季节的径流深对各种气候变化的响应也存在明显的差异,体现了季风气候对径流的影响。文章还特别关注了暖干天气组合下径流深的变化,提出这种极端气候情景对工农业生产和国民经济建设有着严重的负面影响     

Oscillations In Galaxies

Oscillations in galaxies have been investigated by numerical simulations. The various models used have density distributions corresponding to that of polytrope of index n in the range 0 ≤ n ≤ 4 and their evolution has been followed for more than 70 crossing times. The kinetic energy shows regular and smooth oscillations for models with n = 0, 1 and 2 whereas in other models it shows noisy oscillation. The oscillation in kinetic energy is observed to have a period of 3 crossing time irrespective of the density and size of the galaxy. The amplitude of oscillation is seen to decrease as the central density of the galaxy increases. This revised version was published online in July 2006 with corrections to the Cover Date.