SEISMIC GEOLOGICAL CHARACTERISTICS OF TONGHAI EARTHQUAKE IN 1970
Zhang Sichang and Liu Bochi (Yunnan Seismological Bureau)
On January 5, 1970, a strong earthquake of magnitude 7.7 occurred in Tonghai region, Yunnan province. As a result, a 48 km-long seismic fault emerged along the north-west trending Qujiang fault. Taking all the other materials with consideration, the fracture connected with the Tonghai earthquake is shown to be mainly a clockwise dislocation between rock bodies in both sides along the fracture, thus producing in the shallow part an upthrust in the NE wall, and in the deeper part a normal fault in nature for the vertical component of dislocation due to the change in the dip by the fracture plane.The geological conditions of Tonghai earthquake and strong earthquake's cluster in Tonghai region are: 1. The Qujiang fault that has been highly active ever since the Yanshan period to Holocene has formed a weak plane in the crust; 2. Under the influence of the recent stress field, stress concentration in Tonghai region is due to regional large scale tectonic activities; 3. The Qujiang fault is horizontally dislocated by transverse fault, resulting in discontinuity along strike of the fault and in the bending and confluence of the major structures in Tonghai region, which leads to the lock of seismogenetic structure.