1954年山丹7 1/4 级地震的孕震构造和发震机制探讨

Abstract

Located in the front margin of the northeastern Tibetan Plateau, the Longshoushan fault zone is one of the boundary faults between the Hexi corridor and the Alxa block. Although the resolution of observation is limited, the 1954 M7(1/4) earthquake on the Longshoushan fault zone is a rare big event in this zone with modern seismological observation and documentation. During this earthquake, one continuous dextral-normal rupture occurred with a length of only 7 km along a transfer fault connecting two secondary faults of the Longshoushan fault zone. However, the dextral-normal sense of this surface rupture is contrary to the tectonic kinematics in this area. Based on recent field investigation and mapping, two phenomena were discovered. Firstly, except the one appeared on the transform fault, no surface rupture zone is found on the branches of the Longshoushn fault zone. Secondly, the main damage caused by earthquake is distributed on the region between the northern and the southern branches of the Longshoushan fault zone. These two findings indicate that the 1954 earthquake was caused by the main faults rather than the transfer fault of the Longshoushan fault zone, and the co-seismic surface rupture on the transfer fault is only a form of stress release at local portion. According to calculation of static co-seismic Coulomb stress change, the surface rupture could be triggered by an earthquake on the southern branch of the Longshoushan fault zone. Furthermore, in consideration of the possibility of dynamic triggering along the northern branch of the Longshoushan fault zone, the surface rupture should be the co-seismic response to the earthquake occurred on the main branches of the Longshoushan fault zone. Relocation of the small earthquakes also shows that a narrow inverted north-extending triangle about 10 km wide was formed between the northern and the southern faults of the Longshoushan fault zone.