FEATURES OF THE DEEP COMPRESSION ZONE IN THE BEIJING TIANJIN TANGSHAN REGION AND ITS RELATION TO EARTHQUAKES

The axis of the deep compression zone in the Beijing Tianjin Tangshan region is oriented in a SE NW direction along the line of Ningbo Changping. The NE trending M plane and C plane across the zone display an arcuate shape conex toward the NW. The lower crust and top of the upper mantle thicken in the lenticular form. The NE trending faults at shallow depths also bend toward the NW. Stress measurements indicate that the present stress field of the zone is still subjected to NW compression. The deep compression is produced by sea floor spreading of the Bohai Sea. This compression zone has controlled the occurrence of earthquakes. In particular its NE side is a zone of strong earthquakes.