A STUDY OF THE FOCAL PROCESS OF THE ZHAO TONG EARTHQUAKE (M=7.l), YUNNAN PROVINCE OF MAY 11, 1974 FROM THE DIRECTIVITY FUNCTION OF RAYLEIGH WAVES

Based on the data of the first P-motion recorded at more than 100 seismographic stations located in China and other parts of the world, the fault plane solution of the Zhao-Tong earthquake of May 11, 1974, Yunnan Province was determined. Using the directivity function and its generalized form of surface waves, the nodal plane striking N45°E has been preferred as the fault plane of this earthquake. The direction of propagation of the fracture is northeast with a velocity of 1.3 km/sec and the total length of the fracture 53 km. The estimation of the seismic moment of this earthquake by spectra density analysis gives a value of 6.5×10²⁵ dyne-cm. From the result thus obtained and in the light of the seismicity and tectonic features of the Zhao-Tong region, it is believed that this earthquake was caused by the action of eastwest tectonic stress field, under which the northeast striking Qiaojia-Lianfeng major fault continued to be fractured toward the northeast, so as to link-up with the Huaying-Yibin major fault by breaking through the gap between them.From the above viewpoint for the model of focal process, we suppose that under the action of the same regional tectonic stress field, the conjugating geological figures, namely, the northwest striking Yiliang-Shuicheng fault and the Emei-Yanjin tectonic structural belt might be seismically dangerous.