冷龙岭地区2016年青海门源6.4级地震发震构造特征

摘要

青海门源北部的冷龙岭断裂带于2016年发生MS6.4地震,该地震与1986年发生的另一次MS6.4地震的震源机制解截然不同,并且二者与冷龙岭主断层几乎表现为纯左旋走滑的性质也差异很大.为明确2次地震的发震构造特征,分析二者与冷龙岭主断层的关系,利用Sentinel-1A数据干涉处理得到了2016年地震的同震形变场,并结合高分辨率影像、余震精定位及GPS资料对断层构造样式及地震构造特征进行了研究.研究结果表明,2016年地震发生在冷龙岭断裂带西段北侧的分支断层上,地震造成分支断层两侧发生背斜式褶曲隆升.2次地震分别发生在分支断层的东西两端,其两端分别向冷龙岭主断层弯曲收敛并交会,在平面上形成左行右阶与左行左阶的几何结构,在区域整体剪切环境下,分别发生剪切挤压与剪切拉张,因而造成2次地震的震源机制解分别表现为压性与张性为主又兼具一定的走滑性质,综合认为分支断层与主断层之间的构造组合样式是导致2个不同震源机制解的主要因素.2次地震活动皆反映了冷龙岭断裂带整体表现为左旋走滑运动特征,该构造变形机制也体现了祁连-海原构造带对青藏高原东北缘的构造调节作用,即来自于青藏板块的构造运动在受到北部戈壁-阿拉善块体及东部华北克拉通块体的阻挡后,在青藏高原东北缘通过一系列左旋走滑断裂的调节,构造运动方向逐渐发生顺时针旋转.门源地震使得对冷龙岭断裂带地震地质问题的深入研究更加紧迫.%On January 21 2016,an earthquake of MS6.4 hit the Lenglongling fault zone(LLLFZ)in the NE Tibetan plateau,which has a contrary focal mechanism solution to the Ms 6.4 earthquake occurring in 1986.Fault behaviors of both earthquakes in 1986 and 2016 are also quite different from the left-lateral strike-slip pattern of the Lenglongling fault zone.In order to find out the seismogenic structure of both earthquakes and figure out relationships among the two earthquakes and the LLLFZ,InSAR co-seismic deformation map is constructed by Sentinel-1A data.Moreover,the geological map,remote sensing images,relocation of aftershocks and GPS data are also combined in the research.The InSAR results indicate that the co-seismic deformation fields are distributed on both sides of the branch fault(F2)on the northwest of the Lenglongling main fault(F1),where the Earth`s surface uplifts like a tent during the 2016 earthquake.The 2016 and 1986 earthquakes occurred on the eastern and western bending segments of the F2 respectively,where the two parts of the F2 bend gradually and finally join with the F1.The intersections between the F1 and F2 compose the right-order and left-order alignments in the planar geometry,which lead to the restraining bend and releasing bend because of the left-lateral strike-slip movement,respectively.Therefore,the thrust and normal faults are formed in the two bending positions.In consequence,the focal mechanism solutions of the 2016 and 1986 earthquakes mainly present the compression and tensional behaviors,respectively,both of which also behave as slight strike-slip motion.All results indicate that seismic activity and tectonic deformation of the LLLFZ play important parts in the Qilian-Haiyuan tectonic zone,as well as in the NE Tibetan plateau.The complicated tectonic deformation of NE Tibetan plateau results from the collisions from three different directions between the north Eurasian plate,the east Pacific plate and the southwest Indian plate.The intensive tectonic movement leads to a series of left-lateral strike-slip faults in this region and the tectonic deformation direction rotates clockwise gradually to the east along the Qilian-Haiyuan tectonic zone.The Menyuan earthquake makes it very important to reevaluate the earthquake risk of this region.