关键词: 有序迁移, 中强震, 强震

Abstract: The dynamic pattern of moderate-strong earthquakes in Yunnan region has been traced in this paper. It is discovered that a sequential migration of moderate-strong earthquakes occurs 1～4 years before the active period of strong earthquakes in Yunnan region; the sequentially migrated moderate-strong earthquakes indicate the general layout of M ≥6.7 strong earthquakes during the active period, while about 80% of the strong earthquakes occur within a range of 150km from the epicenters of these moderate-strong earthquakes. The first strong earthquake of the active period occurs within 2 days to 11 months after the termination of sequential migration of moderate-strong earthquakes, and about 75% of the first strong earthquakes occur within 3 months. During the active periods Ⅰ and Ⅲ in Yunnan region, 8 of 10 strong earthquakes occur within the range of 190km from the apexes of tetragon formed by linking the sequentially migrated epicenters of moderate-strong earthquakes(such as the epicenters of Huaning, Ludian, Jianchuan, Longling, Qiaojia, and Jinning earthquakes, as well as those of the Jiangcheng, Ruili, Zhongdian, Yongshan, and Jinghong earthquakes), while about 60% of the strong earthquakes occur within a range of 50～90km from the epicenter locations of moderate-strong earthquakes. The M 7.7 Tonghai earthquake of January 5, 1970 occurred on the eastern side of the tetragon mentioned above, showing an equidistant layout of 230km from Yongshan in the north to Dongchuan, Tonghai and Jiangcheng in the south. The relationship between sequential migration of moderate-strong earthquake and the occurrence of strong earthquake for the active periods Ⅱ and Ⅳ has a little difference from that for the periods Ⅰ and Ⅲ. Sequential migrations of moderate-strong earthquakes from Lancang to Lufeng, Shiping and Tengchong, as well as from Shidian to Ninglang, Huidong and Lancang make up tetragons, while the strong earthquakes occur at the starting or ending positions of the migration, i.e. within a range of 140km from both ends of the diagonal lines of the tetragons, such as Lancang and Lufeng, as well as Lancang and Ninglang. About 75% of the strong earthquakes occur within a range of 100km from the epicenters of moderate-strong earthquakes. The strong earthquakes densely occur in the southwest Yunnan region, where is the starting or ending places of sequential migrations of moderate-strong earthquakes. The Tonghai and Longling earthquakes of magnitude 7 did not occur until the active period Ⅲ, located at the other ends of the diagonal line, 50km from Shiping and Yuxi earthquake epicenters, and 150km from Tengchong earthquake epicenter that occurred before the active period Ⅱ. The starting moderate-strong earthquakes of the migration sequence during the active periods Ⅰ and Ⅲ were M 6.0 double shocks, which are rarely seen in Yunnan. The ending moderate-strong earthquake occurred within a range of 50～70km from the epicenter of a certain moderate-strong earthquake from the migration sequence. The starting moderate-strong earthquake before the active period Ⅱ was M 6.0, and both the starting and ending moderate-strong earthquakes before the active period Ⅳ were all M 5.0 double shocks, possessing a certain peculiarity. Some earthquake examples, such as equidistant migration of strong earthquakes, sequential migration of moderate-strong earthquakes during the later stage of the active period, sequential migration before and after a great earthquake and oriented migration of moderate-strong earthquakes are also introduced in this paper. However, further study is needed for understanding the mechanism of sequential migration of seismicity.

Key words: sequential migration, moderate-strong earthquake, strong earthquake