Research On The Segmentation And Seismic Activity Characteristics Of Zhangjiakou-Bohai Structural Zone

The Zhangjiakou-Bohai tectonic zone is an important north-west tectonic zone in the North China and even in eastern China. It is also a famous strong earthquake zone . The seismic activity of the fault is closely related to the seismic risk analysis of Beijing, Tianjin and Bohai rim, and is also related to the seismic zonation and the earthquake prevention and disaster reduction works. Choosing Zhangjiakou-Bohai tectonic zone as the research region and object, deeply discussing the segmental activity and seismic activity features are of great theoretical and realistic significance for scientific evaluation to seismic risk and preventing and relieving the earthquake disaster of the north area of north China.A large number of field observation and research date suggested that Zhangjiakou-bohai tectonic zone which has huge scale, is a complicated tectonic zone with complex architecture, and intersect with NE-trending fault zones in many places. The tectonic zone has segmental active feature and the geological features, seismic activity features of these segments are different. Based on the geological features, aeromagnetic anomaly, crustal deformation, gravity anomaly, moho form and the moderate to small earthquakes activities, we give a more reasonable definition and study the seismic activity features of this tectonic zone. The seismic activity features,migration and evolution of strong earthquakes and seismic risk of each segment are also discussed. Main contents as follow:1,Basis for the geological features, the NW-trending fault and deep depression distribution, aeromagnetic anomaly, crustal deformation, gravity anomaly, deep structure and the moderate to small earthquakes activities, we give a more reasonable definition of this tectonic zone. In the northwest it starts from Zhangbei county, ends in the Bohai sea area. It is 900km long or more on the whole, and has different width, the northwestward which just 60km is narrow and the other end is wide which the widest is 190km. 2,Based on the field geology, topography, crustal deformation, geophysical field, seismic parameter and the relation between NW-trend and NE-trend faults, we divided the Zhangjiakou-Bohai tectonic zone into 4 segments: Zhangjiakou segment, Beijing segment, Tangshan segment and Penglai segment, those segments have different seismic period and episode.3,According to the study, we found Zhangjiakou-Bohai fault since A.D.1484 can be divided into two seismic active periods and these two periods are consistent with the 3rd and 4th seismic active period of the North China respectively. Comparing the seismic intensity of the two active periods, we found each period have different characteristics, for example, in the third active period, earthquakes have clustering characteristics and the clusters periodically expression, in the 4th seismically active period, the quasi periodically recurrence and the seismic interval periodic between 22years and 32years. There are three migration phenomena among the four segments, i e., migration between adjacent segments, migration between the inter-segments and migration in a single segment.4,The seismic active intensity of each segments are different in the same active period or in the different active periods, can be divided into main active segments which can occur earthquakes(Ms≥7.0) and the participate active segments that can occur earthquakes(Ms≤6.0) .In the third active period, Beijing and PengLai segments were the main activities, Zhangjiakou and Tangshan segments were the participate activities. In the 4th seismically active period, Zhangjiakou and Tangshan segments were the main activities, Beijing and Penglai segments were the participate activities.5,Analyzing the characteristics of migration of single earthquakes or Earthquake Swarm, we thought the next earthquake may appear in Zhangjiakou segment or Beijing segment, but there maybe a impossibility of earthquakes with Ms5.5 in the ten years later. Especially for Beijing segment, it is very impossibility to occur earthquake with Ms≥7.0 in the 4th seismically active period,Through analysis of seismic structures of strong earthquakes of four segments, it is suggested that the causative structures of strong seismic areas generally NE-trending fault zones, and the NW-trending fault zones are controlling structure, they controlled the locations of the strong earthquakes. In the end ,from the viewpoint of the main activities will help to ease next segments earthquake stress accumulation and reducing earthquake intensity for parallel strike faults with the same lateral, the main activities and participate activities appear alternately had been explained.