青藏高原东缘虎牙断裂带南北段运动方式差异性机理:来自数值模拟的约束  

Mechanisms of the different senses of fault slip in the north and south segments of the Huya Fault zone,eastern Tibetan Plateau:Constraints from numerical modeling

作　　者：刘皓晴 李玉江 陈连旺 LIU HaoQing;LI YuJiang;CHEN LianWang(National Institute of Natural Hazards,Ministry of Emergency Management,Beijing 100085,China)

机构地区：[1]应急管理部国家自然灾害防治研究院,北京100085

出　　处：《地球物理学报》2023年第7期2757-2771,共15页Chinese Journal of Geophysics

基　　金：国家自然科学基金项目(42174120,41874116);中央公益性科研院所基本科研业务专项(ZDJ2019-16)资助。

摘　　要：虎牙断裂带作为青藏高原物质东向扩展的前缘断裂之一,其运动方式和强震活动类型表现出显著的南北差异.研究虎牙断裂带运动方式的差异性机理,对于认识地震发生机制与高原东向扩展模式有着重要意义.本文构建包含虎牙断裂带的三维黏弹性有限元模型,研究介质流变性差异与断层几何形态对区域地壳变形及断层三维滑动速率的影响.数值实验结果表明,在青藏高原物质东向挤出的动力学背景下,在虎牙断裂带南段,中下地壳介质流变性横向差异控制着断层以逆冲性质为主的运动,且随着中下地壳断层两侧流变性差异的增大,断层西侧物质的水平运动更易发生向垂向运动的转换.在断层两侧流变强度差1~2个数量级时,断层倾向滑动速率与走向滑动速率的比值达3.3~4.0,表现出显著的逆冲运动,与松潘—平武强震所反映的断层运动特征基本一致.相反,在断裂带北段,考虑断层两侧中下地壳较小的流变性差异时,断层即表现出明显的逆冲运动,这与九寨沟MS7.0等强震反映的断层以走滑性质为主的运动明显不符,表明虎牙断裂带北段可能不存在中下地壳介质流变性的横向差异,断层以走滑为主的运动方式主要受断裂带几何展布控制.研究结果为认识青藏高原东缘同一构造区内断层运动方式差异与发震机制以及高原东向扩展模式提供了理论依据.As one of the frontal faults of the eastward expansion of the Tibetan Plateau,the Huya Fault zone shows significant north-south differences in the senses of fault slip and types of strong earthquakes.The mechanism of different senses of fault slip is crucial for revealing the eastward expansion pattern of the plateau.In this study,we constructed a three-dimensional viscoelastic finite element model that embedded the Huya Fault zone,to investigate the effects of rheological heterogeneity and fault geometry on crustal deformation and three-dimensional fault slip rates.The results show that,with the continuous eastward extrusion of the Tibetan Plateau,in the south segment of the Huya Fault zone,the lateral rheological heterogeneity controls the thrust-dominated fault movement,and with the increase of the difference in rheological viscosity on both sides of the fault in the mid-lower crust,the horizontal movement on the west side of the fault is more likely to be transferred to the vertical movement.When a viscosity contrast between the two sides of the fault is about two orders of magnitude,the ratio between the dip-slip rate and the strike-slip rate reaches 3.3~4.0,showing a significant thrust motion,which is consistent with the sense of fault slip reflected by the strong Songpan—Pingwu earthquakes.On the contrary,in the north segment of the fault,even with the small lateral rheological heterogeneity in the mid-lower crust,the fault can show obvious thrust motion,which is in contrast to the strike-slip motion demonstrated by the MS7.0 Jiuzhaigou earthquake,suggesting that there may be no lateral rheological heterogeneity in the mid-lower crust in the north segment of the Huya Fault zone,and fault geometry is the main factor controlling the dominated strike-slip fault movement.This study provides a theoretical basis for understanding the spatial different senses of fault slip and seismogenic mechanism in the eastern margin of the Tibetan Plateau,and also the eastward expansion pattern of the plateau.

关 键 词：青藏高原东缘 虎牙断裂带 介质流变差异 地壳变形 数值模拟 

分 类 号：P315[天文地球—地震学] P313[天文地球—固体地球物理学]