关键词: 张家口-渤海地震构造带, 地壳形变, NE向梯度带

Abstract: The Zhangjiakou-Bohai fault zone in the northern part of North China area is a seismotectonic zone controlling the present-day strong earthquake activity. Under the effect of NEE-directing regional principal compressive stress,a series of NE trending active tectonic zones were engendered in this area. Together with the Zhangjiakou-Bohai fault zone,they make up a group of conjugate shear fracturing systems,controlling the occurrence of the present-day strong earthquakes. This paper focuses mainly on the feature of crustal deformation around this fault zone. The secular GPS crustal deformation image has reflected the relatively complete left-lateral strike-slip movement of this active fault zone. However,the crustal deformation images of different stages indicate that a series of NE-trending large-scale anomalous gradient zones appeared along the Zhangjiakou-Bohai zone before the occurrence of moderate strong earthquakes. These anomalous zones have indicated separately the activities of the Tangshan-Hejian,Sanhe-Laishui and Yan-huai-Shanxi seismotectonic zones,and represented the medium term precursors of the moderate strong earthquake along the Zhangjiakou-Bohai fault zone. Furthermore,the results of this study suggest that the crustal deformation that appears before the occurrence of strong earthquake reflects the strain activity in the deep seismogenic zone,while the disorderly and unsystematic crustal deformation images that occur after the occurrence of strong earthquake reflect the adjusting movement of the covering strata.

Key words: Zhangjiakou-Bohai seismotectonic zone, crustal deformation, NE-trending gradient zone