CRUSTAL STRUCTURE OF TANGSHAN EPICENTRAL REGION AND ITS RELATION TO THE SEISMOGENIC PROCESS OF A CONTINENTAL EARTHQUAKE

In the present paper, the authors discuss the different phases in DSS profile records obtained from Tangshan, and the methods of their interpretations. The DSS data of some profiles running through the Tangshan epicentral region show that the crustal structure in this region is characterized by some special features, which may bear some significant relations to the cause of the occurence of a great earthquake in continintal region.In this area, the earths crust are of three different types, each of which exhibits some peculiar features in velocity structure. An abrupt change of depth of Moho and hence a certain type of faulting usually occurs at or near the boundary between two areas of different types of crust. The difference between different crustal structures may be largely due to the difference in degree of vertical migration of-materials from the uppermost mantle into the crust. The crustal strusture of the epicentral region exhibits the highest degree of material migration from the uppermost mantle in to the lower crust. However, the present crustal structure is a collective fossil inherited from a long span of geological time. Therefore, it is still not very clear whether the migration processes which seem to have been very prominent in the past, are still working effectively at the present. A comparison has been made of the boundaries between different types of crustal structure with the geological map in which the Tertiary surface layer has been stripped off. If is striking that the boundary between different crustal structures is in coincidence with a certain ancient geological map. It is possible that in such an area as Beijing-Tianjing-Tangshan, which is not very far from the coast, different kinds of small ancient geological bodies have been brought together and cemented by ancient global tectonic processes. The main shock of Tangshan earthquake of 1976 occured in the crustal structure of type Ⅱ and near the boundary of the crustal structure of type Ⅱ and type Ⅲ. Therefore, either the occurence of Tang-shan earthquake is due to vertical migration of material from the uppermost mantle into the crust, or it is caused by the reactivation of the weak zone while the whole region is at present under the action of a certain tectonic force. However, it is quite certain that in the area with crustal structure of type Ⅱ, the middle crustal layer has negligible thickness or is totally absent. Since the upper part of the crust may be brittle, it will easily collapse under the action of present tectonic stress.