完全库仑破裂应力变化与云南龙陵震群序列的应力触发

计算和研究了1976年云南龙陵Ms73、Ms74双主震产生的完全库仑破裂应力变化及其对后续震群序列的动态和静态应力触发作用.结果显示,龙陵第2主震受到了第1主震的动态和静态库仑破裂应力的触发作用.龙陵双主震的13个后续强余震中,发生在龙陵三角形块体内部的强余震,90%都受到了第1或第2主震的动态库仑破裂应力触发或静态库仑破裂应力触发作用,发生在三角形块体外部的强余震,2/3受到了第2主震的动态库仑破裂应力触发作用.从触发作用的强度和范围上看,第2主震的动态和静态库仑破裂应力触发作用都大于第1主震.就近场而言,受到龙陵双主震的动态和静态库仑破裂应力触发作用的余震数目相当,但动态库仑破裂应力的触发作用范围大于静态库仑破裂应力.     

Study of the Correlations Between Long-Periodic Terrestrial Tides and Occurrence of Earthquakes in the Vrancea Site

Following studies already performed by various authors concerning specific tectonic faults we analyse statistically the possibility that earthquakes in the very active seismological zone of Vrancea (Roumania) may be triggered by the vertical luni-solar tidal oscillations when retaining only the components other than diurnal and semi-diurnal. Our conclusion is that for three of the four leading tides, with periods 18.6 y, 182.62 d, and 13.66 d, the earthquakes occur preferentially during the ascending part of each of the sine oscillation. This property is especially visible for the leading 13.66 d fortnightly wave, and for the 18.6 y wave, for which results are very similar to that of Kilston and Knopoff (1983), who analyzed data related to the San Andreas fault in Southern California. As a complement, we carried out an analysis of earthquake periodicities w.r.t. the variation rate of each of the tides above, given by the half-period sine oscillation, which also leads to interesting possible correlations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Impacts of multiple stressors on mountain communities: Insights from an agent-based model of a Nepalese village

Mountain communities in developing and transitioning countries are experiencing a period of rapid social, economic, and environmental change. While change has long been a feature of mountain life, the rate, magnitude, nature, and number of the transformations now taking place is unprecedented, with profound implications for the sustainability and welfare of mountain communities in the coming years. It is therefore vital that their potential impacts be understood. Considering stressors in isolation can give a false picture as each stressor alters the context within which the other stressors are operating. Holistic approaches are needed. In this paper, a variety of stressors are concurrently simulated within an empirically informed agent-based model of a rural Nepalese mountain community so that their combined impact can be studied. The potential effect of changing fertility rates, increasing crop yield variability, and earthquakes on household finances is considered for the period 2015–2030. Results show that higher fertility rates, increased crop yield variability, and earthquakes all have negative long-term effects on household finances, and that each of these stressors compounds the effect of the other stressors in an additive fashion. Results further highlight heterogeneity in the capacity of households to cope with stressors and demonstrate the important role that happenstance can play in exacerbating the effect of stressors. Our findings suggest that development practitioners should explicitly take multiple stressors into account when considering interventions. They should also contemplate improved microtargeting of households to increase aid effectiveness over the longer term, while recognising that household vulnerability is often dynamic.     

Disaster Mitigation vis-á-vis Time of Occurrence and Magnitude of Earthquakes in India

Earthquakes occurring during the night or early morning hours cause a heavy loss of life. Also, an earthquake occurring in the late evening hours poses serious handicap for disaster mitigation efforts due to failure of electricity and blocking of roads due to fall of debris. The larger aftershocks may cause further damage depending upon the magnitude of the main earthquakes of magnitude 5.5 and more in the Indian region. Out of 7 great earthquakes (M >7.8) in the Indian region during the last 200 years, five (1819, 1897, 1905, 1941 and 1950) have occurred during the evening or early morning hours. About 67% of all the earthquakes of magnitude >7.0 show similar result. It is found that in general, the percentage of earthquakes occurring during evening/early morning is larger than that during the daytime. However, the difference in time of occurrence is not significant at 95% level of confidence using ² test. Keeping in view that most of the earthquakes in India of magnitude more than 6 have caused significant damage in the last decade (Uttarkashi, 1991; Latur, 1993; Jabalpur, 1997; Chamoli, 1999) and have occurred in the night/ early morning, disaster management plans need to be designed for awareness and education separately for the night and day times.The limitations of the seismic zoning map with reference to the earthquakes in Koyna (1967) and Latur (1993) have suggested to safeguard the life and property of the Indian population from the effects of future damaging earthquakes which should be failsafe instead of following the code and then introduce further changes in the code. It is suggested that residential houses (including tall structures) should be built for a design earthquake of magnitude 6.0 in all the three seismic zones namely I, II and III demarcated by the Bureau of Indian Standards giving proper weight age to site response. However, for Zones IV and V usual code recommendations may be followed through micro zoning of important, densely populated or most vulnerable areas.     

Probabilistic methodology for the analysis of paleoliquefaction features

Paleoliquefaction features can be used to estimate lower bounds on the magnitude and ground motion associated with the earthquake that caused the liquefaction feature. The engineering back-analysis of paleoliquefaction features is usually conducted using state of the practice liquefaction-triggering analysis methodologies. Recent studies have shown that these methodologies are associated with variable probabilities of liquefaction depending on the soil parameters. This would imply that estimates of magnitude and ground motion intensity obtained from these methodologies would not be consistent for all soil sites. Moreover, these estimates could be unconservative. In this paper, the use of a probabilistic methodology for the back-analysis of paleoliquefaction features is proposed. The proposed methodology permits the incorporation of model and parameter uncertainty into the analysis and results in more robust estimates of past magnitude and a measure of the uncertainty associated with these predictions. Previously published paleoliquefaction data are used to demonstrate the applicability of the proposed method. Magnitude estimates obtained with the proposed method do not differ significantly from those obtained using deterministic methodologies, but the proposed methodology permits a quantification of the uncertainty associated with magnitude estimates.     

Effects of the tip depth of a pre-existing fracture on surface fault ruptures in cemented clay

Ground deformation due to faulting can cause serious damage to buildings and structures. Much attention has been devoted to understanding fault rupture propagation in uncemented soil. However, the effects of a pre-existing fracture in cemented soil on surface fault ruptures are not fully understood. This paper describes a numerical parametric study to investigate the mechanism of normal fault rupture propagation through cemented clay. Special attention was paid to the effect of the location (or tip depth) of a pre-existing fracture on the mechanism. The numerical model and model parameters adopted were verified through two centrifuge model tests. The results show that a zone of influence consisting of a tensile failure zone and a differential settlement zone was induced by normal faulting in cemented clay both with and without a pre-existing fracture. The width of this zone of influence increased with the tip depth of the pre-existing fracture. The effects of the tip depth of a pre-existing fracture on the width of the zone of influence were more significant when the tip was located at a shallow depth.     

Lessons learned and need for instrumented liquefaction sites

Instrumented sites provide essential information for understanding and modeling of ground response and ground deformation. For example, significant new lessons were learned from responses at the Wildlife Liquefaction Array (WLA) including: (1) soil softening led to lengthening of period of transmitted ground motions; (2) soil softening also led to attenuation of short-period spectral accelerations (<0.7 s); (3) amplification of long period motions (>0.7 s) occurred due to liquefaction-induced ground oscillation; and (4) ground oscillation led to a continued rise of pore water pressures after strong ground shaking ceased. A new and expanded instrumented site is being developed 70 m downstream from the old WLA site as part of the NSF Network for Earthquake Engineering Simulation (NEES). The new site has more accelerometers, piezometers and ground deformation measurement devices and the data will be streamed to the NEES-grid in near real time.     

饱和砂土地震液化机理分析及地基处理的应用

饱和砂土在地震荷裁作用下极易发生液化,对工程建筑造成危害。本文探讨了地震力作用下饱和砂土的液化机理。通过实例说明cFG桩和碎石桩共用具有明显的抗液化效果,并提出CFG桩和碎石桩抗液化,需进一步研究的问题。     

华北地区地下水开采对地壳应力的影响

近50年来华北地区遭受持续大面积过量开采地下水,已形成区域地下水漏斗、地面沉降、地陷地裂等地质灾害.然而,地下水的抽取减小了地壳的载荷,造成地壳应力场变化,这一点至今尚未被充分认识.为探索华北地区地下水超采对地壳应力场的影响,本文建立了二维有限元模型,定量计算地下水超采引起地壳变形和应力场变化.结果表明:华北地区地下水开采会引起地表抬升达+12.4cm;漏斗区上、中地壳的水平拉应力增量分别达到70kPa和35kPa;而在地下水开采区外围,水平压应力增量达20kPa;而华北地区构造主压应力积累速率约为0.5kPa·a-1.通过对比华北地区1980年前后5级以上地震的分布状况,本文认为地下水开采对区域构造应力场的扰动不可忽略,其卸载过程可能对华北地区大地震孕震过程存在减缓作用.     

Magnitude scaling factors in liquefaction triggering procedures

A revised magnitude scaling factor (MSF) relationship for CPT-based and SPT-based liquefaction triggering analyses is presented in this paper. The revised MSF relationship incorporates functional dependency on the soil characteristics [represented by clean sand equivalent penetration resistances in the present form] as well as on earthquake magnitude. The revisions in MSF are based on the examination of cyclic testing results for a broad range of soil types and densities, analyses of strong ground motion records to develop relationships for the equivalent number of loading cycles for different soil properties, and the synthesis of those results into an MSF relationship suitable for implementation in practice. A separate study [2] showed that use of the revised MSF relationship in CPT-based and SPT-based liquefaction triggering procedures is well-supported by the case history databases. Other factors known to fundamentally influence the MSF are discussed.