FUZZY CLUSTER ANALYSIS OF COMPOSITE FAULT PLANE SOLUTIONS OF SMALL EARTHQUAKES THAT OCCURRED IN NORTH CHINA

Composite fault plane solutions of earthquakes between January 1981 and September 1983 in the North China area (35-41N, 112-122 E) are determined by the data of first motion direction of P wave of macroearthqukes. According to the inhomogeneity of the distribution of stress axis directions of various solutions, the fuzzy cluster analysis is made for the directions of maximum, minimum and medium principal stress axes and a map of dynamic cluster spectral system of various solutions is obtained. Cut samples with subordinate degree of greater than 0.93 and divide them into 4 types. The characteristics of various types of directions are analyzed and the relationship between geologic structure and stress field and seismicity is discussed.