The "Safety Island" Theory By Multi-Scales Approaching And Optimum Seeking And Its Application

Through looking back the history of the study on regional crust stability, and analysing the present situation and the challenge being faced in this area, the author has proposed a new theory and a new method on evaluating regional crust stability for great engineerin9 site setection in active tectonic areas. And then, it is applied quantitatively to Black Mountain Gorge Project in the Middle Reaches of Yellow River.According to the characteristics of active tectonic areas and experiences on the construction of earth or rock-fill dams in strong earthquake regions, the author has established "Safety Island" theory system of Multil-scale approaching and optimum seeking and its working process. The bases of the theory are discussed in the plate tectonics and physical philosophy. This is a great development to " Safety Island" thought that Li Siguang proposed in 1965 and 1967, and Hu Haitao has been persisting in and studying since 1979. "Safety Island" theory of Multi-scale approaching and optimum seeking consists of several aspects. First, based on the scale, the acting pattern or strength of boundary fault systems and the similarity of the inner properties and the active state in land masses, a series of Laud masses are determined from large to small size. Second, one or several rather stable land masses can be selected out in every scale land masses. Third, a stability criterion of "Safety Island" is expressed quantitatively by a concept called stableLevel (Δf) calculated on the pattern recognition theory. Fourth, the author divides "Safety Island" into three types in the concept of safety period based on the state of geological bodies and large earthquake recurrence intervals, which are permanently, relatively, and temporarily stable "Island".According to the "Safety Island" theory of multi-scale approachin9 and optimum seeking, the author has studied dam site selection in the Black Mountain Gorge of Yellow River. From regionalization of active fault systems in China, the first scale approaching and optimum seeking has been obtained. In the same way, second, third and fourth scales approaching and optimum seeking have been realised. And finally, Black Mountain Gorge land mass is selected out by analysing regional geological and seismological data.Black Mountain Goroe is situated in the border area between Gan Su Province and Ning Xia Hui Nationality Autonomous Region. Geologically, it is located between Zhongwei--Tongxin (once occurring of Ms 7.5 earthquake) and South West Hua Mountain (once occurring of Ms 8.5 earthquake) active fault systems. This is an area affected by strong earthquakes. The author has done field investigations for many times and analysed geological, seismological, geophysical, and active tectonic data. It is discussed that differentiation between west and east parts of Xiang Mountain land mass has been there all neotectonic period. Fractal theory (B. B. Mandelbrot 1977, 1982) is applied to the segregative characteristics of active fault systems and the intensity of neotectonism. Water system inverse method (A. E. Scheidegger, 1977, 1982) is used to calculate principal stress field during neotectonic period. Eventually, the fifth and sixth scales " Safety Island" approaching and optimum seeking are finished, and expressed quantitatively by Cora-3 method modified from pattern recognition theory. According to the stable level (Δf), it is concluded that two dam sites of Da Liushu in the east end and Xiao Guanyin in the west end of Black Mountain Gorge are all on the relatively stable land masses.In brief, this paper has propoesd "Safety Island" theory of Multi-scale approaching and optimum seeking, and has applied it to the selection of dam site in Black Mountain Gorge of Yellow River. It offers a new thought and an practical example for selecting, evaluating, and making a strategic decision to greal engineering slte in active tectonic regions.