ON THE SEISMO GEOLOGICAL TYPES OF STRONG EARTHQUAKES IN SICHUAN AND YUNNAN PROVINCES
Zhang Sichang (Hebei Seismological Bureau)
Starting from the analysis of geological background, tectonic features and fracture properties for strong earthquakes, as well as taking the earthquakes in sichuan and Yunnan as examples, we studied the relation between the temporal and spatial distribution patterns and the tectonics for strong shocks. As a result, the stromg esrthquakes were divided into two, A and B, seismogeological types. For type A, the earthquake is due to a continuing fracture. The earthquake-generating structure is a deep and large fault with intensive activity. A variety of earthquake phenomena strictly correspond to the fault. A considerable fracture occurs at a single plane in some direction and appears mainly to be strike-slip. The fracture process behaves as stick-slip. The source fracture can be seen on the ground. The fracture propagates along the fault so as to form a striking linear seismic belt. The seismic energy is released concentrately and the earthquake sequences mostly appear to be mainshock-aftershock type. For type B, the earthquake is due to a newborn fracture. The earthquakes mostly occur in the region where several sets of structure in a fault-block intersacted each other. The fractures often appear in two directions for an earthquake sequence. Some of them appear mainly to be strike-slip, and others dip-slip. But in both of these cases, the fracture process mainly consists of brittle fractures in the rocks. The fracture plane and dislocation are small, and can not be seen on the ground. The energy is released in a decentralized form so that the earthquake sequence appears to have several strong events with comparable magnitude.