1998年张北地震序列构造应力张量变化特征的研究

根据张北地震序列的地震活动特征,利用张北震区1998年1月～1999年3月间145个3 级左右地震的震源机制解资料,应用Gephart(1990)的应力张量反演方法,研究张北地震序列构造应力张量的总体变化特征和时序变化特征;研究发现强余震发生前,应力张量的波动幅度不是很大,震源及附近地区的构造应力作用较强,强余震发生后,应力因发生破裂而重新分布。而且震源区的构造应力场方向与华北地区的构造应力场方向基本一致,说明张北主震和强余震都是在华北统一构造应力场的作用下发生的。     

岫岩5.4级地震序列主应力变化特征研究

根据1999年岫岩地震序列的活动特点, 将其划分为前震、 余震密集、 余震平静、 余震衰减4个时段, 利用收集整理的1999年6月至2000年9月间该序列中161次2.5级以上地震的震源机制解和Gephart的应力反演方法, 研究了震源区各时段内构造应力场的总体特征和时序变化特征。 结果表明, 序列的主压应力方向与华北地区的构造应力场基本一致, 而且最大主压应力的空间取向经历了主震前相对稳定—主震后分散—最大余震前相对稳定—最大余震后分散的全过程。 该结果对主震和最大余震的预测具有一定的参考价值。     

2000年1月15日姚安6.5级地震的震源断层与震源应力场

用姚安地震序列前震、主震和余震的 78个震源机制解 ,分析了序列震源断层和震源应力场特征。结果表明 ,姚安地震序列的主破裂面是走向N5 0°W、倾角陡立的构造断裂 ,主震、前震和绝大多数余震都发生在主破裂面上。此外 ,NNE NE向构造断裂在序列发展过程中也参与了活动。序列震源应力场以SSE方位、接近水平的主压应力为主 ,与区域构造应力场一致。在序列发展中 ,震源区应力场出现多方位和多作用方式共存的复杂状态 ,震源破裂出现多方向和多表现形式的复杂性特征     

华北地区中小地震应力场的优势方向

根据华北地区61次中、小地震（3.0≤M≤5.5）和10次M≥6.0大地震的震源机制结果的统计分析,得到地震释能应力场的优势方向,主压应力轴为70°-80°,主张应力轴为340°-350°,它们的仰角基本上小于45°。这表明,华北地区处于以北东东向水平压应力和北北西向水平张应力为主的现代构造应力场中。指出了这一地区6级以上地震和震源深度大于17公里的中、小地震应力场方向一致性较好,可能更接近构造应力场方向。华北地区一致性应力场的南缘,可能在秦岭、大别山及长江下游一带。     

大同—阳高2次地震前后震源应力场的变化过程

利用1989、1991年大同－阳高两次地震前后几年的地震记录资料，详细分析了其震源应力场的变化，结果表明，6.1级地震前震源应力场中两个主应力轴的方向曾发生逆转，主压应力轴逆转时间约10个月，主张应力轴逆转时间为2个月。6.1级主震前4个月，两个主应力轴的方向、仰角都恢复到正常状态，且与华北区域应力场的特征基本一致，而1991年5.8级地震前却没有出现上述现象，说明大同－阳高2次地震是在同一震源应力场作用下孕育、发展的，亦说明5.8级地震是6.1级地城的晚期强余震，指出山西地震带6级以上地震的孕育发生似是在华北区域应力场的作用之下完成的。     

2009年云南姚安6.0级地震震源机制与发震构造的分析研究

利用P波、SV波、SH波初动及其振幅比联合反演震源机制解的方法,计算了2009年7月9日发生在云南姚安6.0级地震余震序列的震源机制解,同时结合地震序列的空间分布,对姚安6.0级地震的发震断层性质和震区应力场特征进行综合分析。结果分析表明：（1）姚安6.0级地震发震断层为NWW—SEE向的直立右旋走滑断层,与美国哈佛大学的主震CMT解节面基本一致,也与余震优势方向分布一致,证明结果可靠;（2）震区主压应力场优势方向为NNW—SSE向,与其现今区域构造应力场主压应力NNW—SSE向一致,表明主震应力场主要受到现今区域构造应力场的控制,同时还有一些小的余震与主震应力场不同,表明震区应力场的多样性和复杂性;（3）结合本次地震序列的空间分布、震源机制解特征、震区断裂构造特征综合分析,综合判定姚安6.0级地震的发震构造属于马尾箐断裂。     

2009年云南姚安6.0级地震震源机制与发震构造的分析研究

利用P波、SV波、SH波初动及其振幅比联合反演震源机制解的方法,计算了2009年7月9日发生在云南姚安6.0级地震余震序列的震源机制解,同时结合地震序列的空间分布,对姚安6.0级地震的发震断层性质和震区应力场特征进行综合分析。结果分析表明：（1）姚安6.0级地震发震断层为NWW—SEE向的直立右旋走滑断层,与美国哈佛大学的主震CMT解节面基本一致,也与余震优势方向分布一致,证明结果可靠;（2）震区主压应力场优势方向为NNW—SSE向,与其现今区域构造应力场主压应力NNW—SSE向一致,表明主震应力场主要受到现今区域构造应力场的控制,同时还有一些小的余震与主震应力场不同,表明震区应力场的多样性和复杂性;（3）结合本次地震序列的空间分布、震源机制解特征、震区断裂构造特征综合分析,综合判定姚安6.0级地震的发震构造属于马尾箐断裂。     

大同─阳高2次地震前后震源应力场的变化过程

利用１９８９、１９９１年大同－阳高两次地震前后几年的地震记录资料，详细分析了其震源应力场的变化，结果表明，６．１级地震前震源应力场中两个主应力轴的方向曾发生逆转，主压应力袖逆转时间约１０个月，主张应力轴逆转时间为２个月。６．１级主震前４个月，两个主应力轴的方向、仰角都恢复到正常状态，且与华北区域应力场的特征某本一致，而１９９１年５．８级地震前却没有出现上述现象。说明大同－阳高２次地震是在同一震源应力场作用之下孕育、发展的，亦说明５．８级地震是６．１级地震的晚期强余震。指出山西地震带６级以上地震的孕育发生似是在华北区域应力场的作用之下完成的。     

大同盆地区域应力场变化特征及其与中强地震的关系

通过大量震源机制解结果推断出大同盆地现代地壳应力场的主压应力方向为NEE,仰角变化不定,主张应力方向为NNW—SSE,仰角很小的结论,这与区域构造应力场NE—NEE向挤压作用的结果相符;分析研究认为,1998年1月10日张北6．2级地震、1999年11月1日大同-阳高5．6级地震前于1997年2月出现的P轴趋于一致,同1989年大同-阳高6．1级主震前应力场变化特征很相似;在矛盾符号比低值的过程中,如果应力主轴取向也出现趋于一致的变化,这类异常往往与盆地中等地震有较好的对应关系。山自皂、阳原单台初动向上、向下次数累计百分比在1991年3月26日大同--阳高5．8级地震前具有准同步变化的特点,反映了大震前应力场的一致增强。     

汶川M_W7.9和集集M_W7.6地震前应力场转换现象及其可能的前兆意义

2008年四川汶川MW7.9地震和1999年台湾集集MW7.6地震均为挤压推覆构造环境条件下发生的板内逆断层型地震。本文对比分析了两次地震前的CMT解、震区附近的中小地震震源机制解及其反演的应力场可知,集集地震主震震源机制解与用台湾内陆中西部的CMT解反演得到逆断层类型构造应力场吻合,而震区附近中小地震具有随机发生的性质,反演得到了震前与构造应力场不一致的走向滑动类型的局部应力场,但当局部应力场变化到与构造应力场一致时,数月后发生主震;同样,用青藏高原东部的CMT解震源机制反演得到走向滑动类型的构造应力场,逆冲类型的汶川主震与构造应力场的压应力轴吻合,震区附近中小地震反演得到了与构造应力场一致的区域应力场,但震前局部应力场变化为逆冲类型应力场一致时,随即发生主震。说明逆断层型主震区附近随着震源区应力积累,在震前会出现相似的应力场转换现象,当最终转换到与发生主震的应力状态一致时,表明震源区附近应力已达到相当高的应力水平,是发生大地震的征兆,应引起进一步的关注。     

SCIENCE CHINA Earth Sciences SUBJECT INDEX TO VOLUME 57(2014)

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A Monte Carlo study of rainfall forecasting with a stochastic model

A procedure for short-term rainfall forecasting in real-time is developed and a study of the role of sampling on forecast ability is conducted. Ground level rainfall fields are forecasted using a stochastic space-time rainfall model in state-space form. Updating of the rainfall field in real-time is accomplished using a distributed parameter Kalman filter to optimally combine measurement information and forecast model estimates. The influence of sampling density on forecast accuracy is evaluated using a series of a simulated rainfall events generated with the same stochastic rainfall model. Sampling was conducted at five different network spatial densities. The results quantify the influence of sampling network density on real-time rainfall field forecasting. Statistical analyses of the rainfall field residuals illustrate improvement in one hour lead time forecasts at higher measurement densities.     

Distribution of throughfall and stemflow in multi‐strata agroforestry,perennial monoculture,fallow and primary forest in central Amazonia,Brazil

The partitioning of rain water into throughfall, stemflow and interception loss when passing through plant canopies depends on properties of the respective plant species, such as leaf area and branch angles. In heterogeneous vegetation, such as tropical forest or polycultural systems, the presence of different plant species may consequently result in a mosaic of situations with respect to quantity and quality of water inputs into the soil. As these processes influence not only the water availability for the plants, but also water infiltration and nutrient leaching, the understanding of plant effects on the repartitioning of rain water may help in the optimization of land use systems and management practices. We measured throughfall and stemflow in a perennial polyculture (multi‐strata agroforestry), monocultures of peach palm (Bactris gasipaes) for fruit and for palmito, a monoculture of cupuaçu (Theobroma grandiflorum), spontaneous fallow and primary forest during one year in central Amazonia, Brazil. The effect on rain water partitioning was measured separately for four useful tree species in the polyculture and for two tree species in the primary forest. Throughfall at two stem distances, and stemflow, differed significantly between tree species, resulting in pronounced spatial patterns of water input into the soil in the polyculture system. For two tree species, peach palm for fruit (Bactris gasipaes) and Brazil nut trees (Bertholletia excelsa), the water input into the soil near the stem was significantly higher than the open‐area rainfall. This could lead to increased nutrient leaching when fertilizer is applied close to the stem of these trees. In the primary forest, such spatial patterns could also be detected, with significantly higher water input near a palm (Oenocarpus bacaba) than near a dicotyledonous tree species (Eschweilera sp.). Interception losses were 6·4% in the polyculture, 13·9 and 12·3% in the peach palm monocultures for fruit and for palmito, respectively, 0·5% in the cupuaçu monoculture and 3·1% in the fallow. With more than 20% of the open‐area rainfall, the highest stemflow contributions to the water input into the soil were measured in the palm monocultures and in the fallow. Copyright © 1999 John Wiley & Sons, Ltd.     

Anthropogenic Eutrophication and Red Tide Outbreak in Lacustrine Systems of the Kashmir Himalaya

Red tide, a recurrent phenomenon has become conspicuous in several Kashmir lake ecosystems since 1991. The responsible organism (Euglena pedunculata), a rare flagellate rediscovered in the Kashmir Himalaya (Khan 1993) caused first and unprecedented red tide outbreak, constituting a maximum of 96% of resident numerical phytoplankton density in Dal Lake. At present, conflicting hypotheses exist on the generation of causal assemblage(s) imparting redness to waters: Jeeji Bai (1991) linked its origin to acid precipitation – a fallout of burning oil‐fields during the Gulf War – whilst Khan (1993) holds local factor(s) responsible. Field/experimental studies support the latter contention that the influx of untreated sewage, in unison with warm temperatures, high levels of PhAR, iron and interruption to hydrological flow‐pattern together with absence/or reduction in grazing activity created conducive environmental milieu for red tide outbreak. Dal Lake “red tide” drifted the bloom‐inoculum to other waters, including Lake Wular, where additional ecological niches were carved out, threatening the aesthetic value and biological diversity of Kashmir lakes. Ecological monitoring indicates frequent seasonal red tide occurrence in Dal Lake (including summer‐autumn event of 1998) which testifies its unabated eutrophication status. Further studies are needed on ecological adaptability and biogeographic distribution of this rare and unique red tide‐causing flagellate.