青藏高原中部日干配错断裂第四纪活动特征及2020年7月23日西藏尼玛Mw6.4地震发震构造分析

Characteristics of Quaternary Activities along the Riganpei Co Fault and Seismogenic Structure of the July 23,2020 Mw6.4 Nima Earthquake,Central Tibet

日干配错断裂位于青藏高原中部,是“V”型共轭走滑构造中班公湖—怒江缝合带以北的一条NEE–SWW走向左行走滑断裂,在调节青藏高原南北向挤压和东西向伸展过程中起着重要的作用。在2008年1月9日及2020年7月23日,先后在该断裂南西端和北东支分别发生6级以上强震。因此,查明该断裂的晚第四纪活动性及其与区域强震活动的关系,有助于更好地认识该地区的活动构造变形样式及其控震特征。本文通过高精度卫星影像解译,结合野外调查和测量,研究了日干配错断裂的构造地貌特征和晚第四纪滑动速率,并分析了2020年7月23日西藏尼玛Mw6.4地震的发震构造。结果表明,日干配错断裂具有显著的线性展布特征,切割了第四纪地质地貌单元,形成一系列典型的构造地貌。断裂总体走向NEE60°~80°,延伸规模约360 km,累积位错量可能达到14~15 km。该断裂晚更新世以来的平均走滑速率为0.2~0.3 mm/a,远小于前人在班公湖—怒江缝合带南侧左行走滑共轭断裂上获得的结果,指示班怒带南北两侧可能存在不同的构造变形机制和动力学背景。2020年尼玛Mw6.4地震是日干配错断裂北侧依布茶卡地堑活动的结果,发震断层应为该地堑的东缘边界正断层。上述研究结果可为更深入理解第四纪期间的青藏高原中部近东西向伸展变形过程及强震活动机制提供新的参考。

The Riganpei Co fault(RPCF),located to the north of the Bangong Co–Nujiang suture(BNS),is a ENE–WSW striking left-lateral fault that belongs to the V-shaped conjugate strike-slip fault system of central Tibet.It plays an important role in accommodating coeval east-west extension and NS-trending shortening in the interior of the Tibetan Plateau.Two large earthquakes(M>6)occurred on January 9,2008 and July 23,2020 near the southwestern end and northeastern normal splay of the fault,respectively.Therefore,determining the Late Quaternary activity of the fault and its relationship with regional large earthquakes is essential to the better understanding of the active deformation behavior and its influence on regional earthquake nucleation.In this paper,the authors studied the Late Quaternary slip-rate along the RPCF and its tectonic-geomorphologic characteristics based on the interpretation of high-resolution satellite images and field surveys.The seismogenic fault of the Mw6.4 Nima earthquake on July 23,2020 was also discussed.The results indicate that the RPCF has a significant linear distribution characteristics.It cut through the Quaternary geological and geomorphic units,forming a series of typical fault related tectonic geomorphologic phenomena.The RPCF strikes NEE 60°~80°for~360 km,with a long-term cumulative displacement of 14~15 km.The late Pleistocene slip rate of the sinistral RPCF is about 0.2~0.3 mm/a,which is significantly lower than the values along the dextral faults south of the BNS.This implies that the mechanisms and dynamics are most likely different in the north and in the south of the BNS.The 2020 Nima Mw6.4 earthquake resulted from recent activity along the Yibug Caka graben,and the seismogenic fault was most likely the normal fault bounding its eastern side.The results obtained by the authors provide a new reference for the deeper understanding of the EW extensional deformation process and the mechanism of large earthquakes in central Tibet during the Quaternary.