2021年青海玛多M_(S)7.4地震的重力挠曲均衡背景与震前重力变化

Gravity flexural isostasy background of the 2021 Madoi(Qinghai)M_(S)7.4 earthquake and gravity change before the earthquake

本文综合利用EIGEN6C4布格重力异常、SIO V15.1地形和流动重力观测数据,研究2021年玛多M S7.4地震的重力挠曲均衡背景和震前重力变化特征.首先,基于岩石圈挠曲均衡模型,结合布格重力异常和地形数据,采用有限差分方法计算了震中及周边地区(青藏高原东北部)岩石圈有效弹性厚度(Te)和挠曲均衡重力异常.结果表明,青藏高原东北部T_(e)为0~100 km,横向差异明显,且与块体构造关系密切.巴颜喀拉块体以北的柴达木块体T_(e)值高达50~80 km,以南的羌塘块体大部分区域的T_(e)大于20 km,五道梁以南出现局部大于30 km的高值区,玉树—德格地区出现局部大于40 km的高值区.巴颜喀拉块体T_(e)为0~20 km,较其南北块体明显偏小,更易于发生形变,从而在南北“夹持”下发生物质东向运动,是青藏高原中部物质东流的主要区域.地震易发生在岩石圈强弱变化的过渡地带(T_(e)变化梯度带),以及T_(e)较小区域的断裂带上.本次地震即发生在巴颜喀拉块体内部T_(e)低值区,震中附近有效弹性厚度约为15 km.震前流动重力变化分析表明,2015年以来3~5年的累积重力变化自西向东呈负-正-负的区域性变化特征,大致以震中为界形成了垂直于断裂带的重力变化高梯度带,主要反映了震前青藏高原物质东流过程中出现的深部构造运动态势.2018年以来的重力变化主要呈围绕震中形成西正-东负的弱区域性变化特征,显示震中地区已处于高应力应变的“固化”状态,地震即发生在重力变化零值线拐弯部位.

In this paper,the flexural isostasy and gravity variations before the 2021 Madoi M S7.4 earthquake were studied using the EIGEN6C4 Bouguer gravity anomalies,SIO V15.1 topography model and the repeated gravimetry data.Firstly,based on the lithospheric flexural isostatic model,combining with Bouguer gravity anomalies and topography data,the effective elastic thickness(T_(e))and the flexural isostatic gravity anomalies of the lithosphere in the epicenter and its surrounding area(northeastern Qinghai-Tibet Plateau)were calculated using the finite difference method.The results show that T_(e) ranges from 0 to 100 km in the northeastern Qinghai-Tibet Plateau,with obvious lateral changes and close relationship with block tectonic structures.To the north of the Bayan Har block,T_(e) of the Qaidam block is as high as 50~80 km,to the south,T_(e) of the Qiangtang block is larger than 20 km,with local high values larger than 30 km in the south of Wudaoliang and larger than 40 km around Yushu-Dege area.T_(e) of the Bayan Har block is 0~20 km,which is smaller than that of the north and south blocks,and it is more prone to deformation.Therefore,the mass in the Bayan Har block moves eastward under the north and south holding,and it is the primary area of eastward material flow in the central part of the Qinghai-Tibet Plateau.Earthquakes are prone to occur in the transition zones of lithospheric strength change(T_(e) gradient zones),as well as on the faults where T_(e) is low.The Madoi M S7.4 earthquake occurred in the area with low T_(e) inside the Bayan Har block,with an effective elastic thickness of about 15 km near the epicenter.The analysis of repeated gravimetry data before the earthquake shows that the 3~5 years′accumulated gravity variations since 2015 present a regional change characteristic of negative-positive-negative from west to east,and a high gradient zone of gravity variation perpendicular to the fault zone is formed with the epicenter as the boundary,which mainly reflects the deep tectonic movement situation in the process of material flow from the Qinghai-Tibet Plateau to east before the earthquake.The gravity variations since 2018 is mainly characterized by a weak regional change from positive in the west to negative in the east around the epicenter,which shows that the epicenter area has been in the“solidification”state of high stress and strain,and the earthquake occurred at the turning point of the zero value line of gravity variations.