Study On The Mechanics Of Cross-Fault Leveling Change By The Finite Element Method

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.