1970年1月5日在云南通海发生的7.7级地震,仅次于1973年炉霍7.9级地震和1976年唐山7.8级地震,是近二十多年来发生在我国的第三个大震。

The seismic structure that caused the 1970 Tonghai earthquake of M = 7.7 is the NW-trend Qujiang fault along which the Qujiang river runs. This fault experienced a complex oblique motion. To be exact, it is a pivotal fault with both dextral shearing and scissoring movement under the NNW-trend principle compressions! stress since the Quaternary.Its dextral motion is shown by the corresponding offset of tributaries of the Qu-jiang river which run across the fault, the offset of alluvial fans, and of a formerly con-tinuous early Pleistocene ancient valley. These phenomena mainly occur in the central section of the Qujiang fault, which has had a dextral displacement of 1,000 m since the Mid-Pleistocene.The scissoring or pivotal motion of the Qujiang fault is shown by the following facts: along its western section the fault plane dips SW with the southern hanging wall thrusting against the northern footwall. In this area the Neogene planation surface and the early Pleistocene river terrace on the southern block are higher than those on the northern block. Along the eastern section of the fault, the situation is reversed, the fault plane dips NE with the northern block thrusting against the southern one. Here the Neogene planation surface on the northern side and the early Pleistocene river ter-race along the main fault stand higher than those on the southern side.In addition, three (western, central and eastern) Quaternary transverse upwarps are formed along the Qujiang fault at its intersections with faults of other directions. These upwarps appear to be represented by dome-like deformation of the Neogene pla-nation surface and the early Pleistocene terraces of the Qujiang river. The western and central upwarps caused interruptions, inversions and deviations of the Qujiang river. The central upwarp is situated immediately adjacent to the pivotal region of the Qu-jiang fault, where the dip of the fault plane changes from southwest to northeast, and both vertical crustal movement and lateral displacement along the fault are most con-spicuous. These intense crustal deformation reflects that there is a concentration of tectonic stress in this region. It was where the 1970 Tonghai earthquake of M = 7.7 and its "early foreshock" of M=5.2 in 1965 took place.Another important phenomenon is that in addition to the two earthquakes men-tioned above, there are aftershocks and the historic earthquakes occurred in central and eastern sections of the Qujiang fault. It is worthy to note that the Tonghai earthquake region is situated at the apex of a southeastward-moving wedge-like block bounded on the east by the NS-trend sinistral Xiaojiang fault and on the west by the NW-trend dextral Honghe fault. Under the action of NNW-trend principle compressional stress, the central and eastern sections of the Qujiang fault are isubjected to much stronger dextral shearing compression than the western section, hence the higher seismic activities occured.