The Longling-Lancang fault zone is a newly emerging fault zone, which is composed of multisecondary faults, distributed in the obligate or clustered types, and characterized by the zoning of active faults, earthquake faults and earthquakes. Its kinematic feature is dextral extensive and the formation epoch is early-middle Pleistocene. It is still active in the later period. The rupture in the future tends to cut across the tectonically-blocked segment, discontinuous segment of the fault and then through the whole fault zone. The formation of the nealy emerging fault zone has close relation with the uplift of the Qingzang Plateau. The composite action by the material fluid from the north to the south and the intrusion of Asamu block towards northeast makes the Dianmian block to rotate in a counter-clock wise direction and forms a new dextral shear, NNW-trending fault zone in the central area of the block.

The Longling earthquake of M7.3 and M7.4 in 1976 and the Lancang Gengma earthquake of M7.2 and M7.6 in 1988 are two typical double main shocks occured in Longlin Lancang fault zone,Yunnan. Based on the seismicity analysis of the events,it is found that they have different characters. The Longling earthquake occurred in a block which composed mainly of the granite in which the structure is comparatively homogeneous. The focus machanism analyses of the Longling earthquake give the direction of compressive stress in NNW,the trace lines of the principle stress axis extends mainly in SN to NE. The direction of the seismogenic structures are in NNW to SN and NE to NEE. There are two intensity areas of 9 degree of which the long axes are nearly right crossed each other. The Lancang Gengma earthquake occured near an ancient plate boundary with complex seismotectonic background. The directions of principal compressive stress of the two main shocks given by focal mechanism analyses are different. The Gengma M7.2 earthquak occurred in an area in which the tectonic stress is mainly tensional in NW direction. While the principal compressive stress field of the Longling M7.6 earthquake is characterized by the fan shape in NS direction. The seismogenic structures of the Langcang Gengma earthquakes are of en echelon in NW direction. The axes of meizoseismic areas of two main shocks are parallel each other. Based on the comparison and analysis,the X-type model and triggering model of seismic rupturing for the double main shocks are proposed,and their features and formative condition are also disscussed.

Based on the observations of four major shocks, nonlinear correlations between the anomalous amplitude of precursory information and epicentral distance are discussed in this paper. It is found that the maximal anomalies are not often at epicentre but on the edge of the source area. It is suggested that the gaps enclosed by the anomalous peaks can be used to predict occurrence of an earthquake.