【作者】 和平；

【导师】 李志雄；

【作者基本信息】 中国地震局地震预测研究所 ， 固体地球物理， 2010， 硕士

【摘要】 长期以来,人们早就注意到中强地震的发生常常伴随着显著的地壳变形。中、美、俄等国都十分重视利用大地测量观测技术监测地壳运动状态。断层属于地壳形变的薄弱地带,对构造应力场变化最敏感,在捕捉强地震孕育信息中扮演着十分重要的角色。跨断层水准测量作为获取断层形变信息的最主要手段之一,其异常现象常常被看作地震前兆信息的一个非常重要指标。1975年海城7.3级地震预报成功很大程度上依赖于包括距震中200km的金县台观测到的跨断层异常变化等在内的前兆异常。因此,定性定量地解读跨断层水准变化机理不仅有助于人们加深对地震孕育过程中地表形变、断层活动规律的认识,而且为进行野外异常落实、未来震情判断提供重要的参考。这也是实现由地震经验预报到物理性地震预报转型的一次重要尝试。基于上述思路,本文利用有限单元法重点开展了以下研究工作:一、首都圈地区跨断层水准测量监测能力评估。以首都圈地区布设在不同断层上的24处跨断层水准观测为研究对象,通过建立不同地壳分层结构、含不同倾角断层的三维非线性有限元模型,研究断层倾角和走向对断层上下盘垂向差异性运动的变化特征,从理论上探讨跨断层水准观测在何种情况下与区域应力场应力增强相关的信息最显著,对导致测点监测能力差异的可能物理机理加以探讨和分析;同时结合震例研究及R值震兆信度评分结果,对首都圈地区跨断层水准测量的整体监测能力进行初步评价。二、环境变化对跨断层水准观测结果的数值模拟。以2007年八宝山跨断层测点水准测量资料异常变化为例,根据首都圈地质构造环境、震源机制解、大地测量以及观测场地调研等相关资料,建立三维有限元模型,探讨和解释了八宝山测点跨断层水准异常变化的机理。通过数值模拟和综合分析,论文得到以下几点认识:(1)当区域应力场应力增强时,对于倾角介于30°~70°的断层,跨断层水准变化幅度相对较大,异常效果比较明显,当倾角接近90°时,跨断层水准变化幅度趋近零,信息展露最小。(2)对于相同倾角的断层,当其走向与断层两侧水平位移矢量差方向接近垂直时,跨断层水准变化较明显,测点越容易监测到断层的活动,从而更加有效的捕捉断层活动的异常现象。(3)总体而言,首都圈的跨断层水准测量的布设具备较强的地震监测能力。数值模拟结果表明:监测效能比较好的跨断层水准测点主要包括紫荆关、德胜口、小水峪、燕家台、大灰厂、八宝山、张山营、狼山等。以上数值模拟结果均与震例总结和R值评分结果具有很好的一致性。(4)未来对跨断层水准测点进行选址时,应当考虑断层倾角和走向对跨断层水准监测能力的影响,建议将跨断层水准测点尽量布设于倾角介于20°~80°之间且走向与区域构造运动总体方向近垂直的断层上。(5)场地变化时质量迁移对于跨断层水准的影响很小,同时对断层周围最大剪应力的分布没有太大影响。相比而言,质量迁移导致的观测场地岩体渗水条件改变,通过影响断层两侧破裂带介质性质,可能导致地表垂直形变场和跨断层水准值相对明显的扰动,并有可能导致断层附近的应力场的重新分布。此外,就出露地表的八宝山断层,大范围、长期的降雨对其测点跨断层水准观测是有比较显著的影响的。(6) 2007年5月至7月跨断层测量异常引起的机理主要可能归结为两方面,一是降雨影响,二是土石开方导致的渗水条件改变的影响,而场地附近的质量迁移(载荷卸载的重力塌陷作用)对跨断层水准观测结果几乎没有影响。更多还原

【Abstract】 People acknowledged the accompanied crustal deformation during the pregnancy and occurrence of strong earthquakes for a long time. In China, America, Russia, much emphasis has been paid to the obtainment of crustal deformation using geodetic measurement and observational techniques. Fault, deemed as the weak, flexible belt of crust, is most sensitive to the variation of tectonic stress, and thus plays an extremely crucial role in detecting the earthquake precursory information. As one of the main approaches to attain geomorphologic information, the anomaly of cross-fault leveling recording is always regarded as one important indicator for the possible occurrence of imminent earthquakes. The successful prediction of Ms 7.3 Haicheng Earthquake in China attributes much of its accuracy to the obtainment and acknowledge of cross-fault measurement anomaly. Therefore, qualitative and quantitatively understanding of the mechanics of cross-fault leveling change not only boost people’s comprehension of the complex relationships between earthquake and fault behavior, but also provide useful insight for the discretion of cross-fault leveling anomaly and the assessment of the likelihood of future earthquakes, which could also be considered as one valuable endeavor to convert statistical and empirical earthquake prediction to physically predicting.In this dissertation, in order to address the appeal above, we firstly established three dimensional finite element models that involved different fault angles and vertical inhomogeneity to study the impact patterns of fault dip and strike on cross-fault leveling monitoring, using the twenty-four cross-fault leveling stations scattered over different faults in the capital area as research objects. Then combined cross-leveling data analysis, earthquake case study, and R-value assessment, we obtained the preliminary evaluation of cross-fault level monitoring effectiveness in capital area.Moreover, we quantitatively studied the effects of monitoring site condition alteration on cross-fault leveling. Using Babaoshan cross-fault leveling variation as an illustration, we quantitatively estimate the impact of monitoring environment change on cross-fault leveling records using three-dimensional finite element model that takes into account the Capital’s geological structure, focal mechanism solutions, geodetic measurement, and field investigation, and then explained the possible cause for the anomaly in cross-fault leveling change.Our results illustrate that:(1) cross-fault leveling sites located across fault with dip between 30°~70°have relatively more conspicuous cross-fault leveling change in case of tectonic stress accumulation, stronger monitoring effectiveness. While dips of monitored fault have an upright angle, the cross-fault leveling change approximates to zero;(2) to those sites located across fault with similar dip, the cross-fault leveling would demonstrate a more evident change when the fault strike were perpendicular to crustal general motion and thus possess relatively stronger monitoring effectiveness in terms of earthquake prediction;(3) corroborated with the results from earthquake case study, and R-value assessment, cross-fault leveling in the capital area is believed to possess adequate monitoring capacity in terms of earthquake prediction in general. Some sites with satisfactory monitoring capacity include Zijinguan, Deshengkou, Xiaoshuiyu, Yanjiatai, Dahuichang, Babaoshan, Zhangshanying, Langshan etc;(4) the prospective cross-fault leveling stations are recommended to be established to faults with dip between 20°~80°and strike perpendicular to crustal general motion for stronger monitoring effectiveness in terms of earthquake prediction;(5) the gravity collapse, caused by mass loss, bears little effects on cross-fault leveling as well as the layout of shallow stress in surface. While the fluid penetration condition alteration resulted from the same course is proved to have more influences on leveling change and surface stress change based on numerical calculation. The large-scale, lasting precipitation is simulated to cause apparent adjustment in cross-faults leveling for surface-emerged faults like Babaoshan fault;(7) the anomaly in cross-leveling change in Babaoshan site from May to July in 2007 are attributable for two possible factors: lubricating effects from precipitation, and pore penetration condition change due to mass loss, while the mere gravity collapse (mass unloading) are not one factor for its leveling anomaly.更多还原