摘要
2016年1月21日01时13分在青海省海北州门源县发生了M_S6.4地震,震中位置位于青藏高原东北缘地区祁连造山带内的祁连—海原断裂带冷龙岭断裂部分附近,震源深度约11.4km,震源机制解显示该次地震为一次纯逆冲型地震.我们于2015年7—8月期间完成了跨过祁连造山带紧邻穿过2016年1月21日青海门源M_S6.4地震震中区的大地电磁探测剖面(DKLB-M)和古浪地震大地电磁加密测量剖面(HYFP).本文对所采集到的数据进行了先进的数据处理和反演工作,获得了二维电性结构图.结合青藏高原东北缘地区最新获得的相对于欧亚板块2009—2015年GPS速度场分布特征,1月21日门源M_S6.4地震主震与余震分布特征以及其他地质与地球物理资料等,探讨了门源M_S6.4地震的发震断裂,断裂带空间展布、延伸位置,分析了门源M_S6.4地震孕震环境与地震动力学背景等以及祁连山地区深部构造特征等相关问题.所获结论如下:2016年门源M_S6.4地震震源区下存在较宽的SW向低阻体,推测冷龙岭断裂下方可能形成了明显的力学强度软弱区,这种力学强度软弱区的存在反映了介质的力学性质并促进了地震蠕动、滑移和发生;冷龙岭北侧断裂可能对门源M_S6.4地震主震和余震的发生起控制作用,而该断裂为冷龙岭断裂在青藏高原北东向拓展过程中产生的伴生断裂,表现出逆冲特征;现今水准场、重力场、GPS速度场分布特征以及大地电磁探测结果均表明祁连—海原断裂带冷龙岭断裂部分为青藏高原东北缘地区最为明显的一条边界断裂,受控于青藏高原北东向拓展和阿拉善地块的阻挡作用,冷龙岭断裂附近目前正处于青藏高原北东向拓展作用最强烈、构造转化最剧烈的地区,这种动力学环境可能是门源M_S6.4地震发生的最主要原因,与1927年古浪M_S8.0地震和1954年民勤M_S7.0地震相似,2016年门源M_S6.4地震的发生同样是青藏高原北东向拓展过程中的一次地震事件.
An M_S6.4 earthquake attacked Menyuan County,Qinghai Province on 21 January2016.Its epicenter is located near the Lenglongling section of the Qilian-Haiyuan fault zone in the Qilian orogenic belt,northeastern margin of the Tibetan Plateau.The depth of the source is about 11.4 km,The focal mechanism solutions show that this event is a pure thrusting.Before this quake,we carried out an MT surveys along two profiles across the Qilian orogenic belt during July to August,of which one t the epicenter of the 2016 Menyuan event(DKLB-M)and the other crosses the Gulang area(HYFP).The remote reference,"robust",and phase tensor decomposition techniques were used to process the acquired MT data,and the NLCG twodimensional inversion was made to image the deep electrical structure.In combination with the GPS velocity field from 2009 to 2015 of the northeastern margin of the Tibetan Plateau with respect to stable Eurasia,distributions of main shock and aftershocks of the Menyuan earthquake and relevant geological and geophysical data available,we explored the seismogenic tectonics of the 2016 Menyuan earthquake,including its location and extension of the causative fault,regional tectonic environment and deep structure of the Qilian orogenic belt.The results show that there is a wide SW-dipping low-resistivity body under the epicenter of the Menyuan earthquake,representing a mechanically weak zone below the Lenglongling fault.Its existence might be associated with preparation,slip and occurrence of the Menyuan event.A fault north of Lenglongling might have controlled its main shock and aftershocks,which is an associated fracture of the Lenglongling fault generated during the northward expansion of the Tibetan Plateau,showing an obvious thrusting characteristic.The present day crustal vertical movement,gravity field variations,GPS velocity field and MT surveys all suggest that Lenglongling section of the Qilian-Haiyuan fault is one of the most significant boundary features in the northeastern margin of the Tibetan Plateau.Affected by the Tibetan Plateau′s northeastward expansion and compression from the Alax block,the area around the Lenglongling fault currently is situated in the area with most intense northeastern expansion and tectonic transformation of the plateau margin.Such a dynamic environment may be the main reason for the occurrence of the 2016 Menyuan M_S6.4 earthquake,like the 1927 Gulang M_S8.0 and 1954 Minqin M_S7.0 earthquakes.