2016年青海门源M_S6.4地震前重力变化

Gravity changes before the Menyuan,Qinghai M_S6.4 earthquake of 2016

利用青藏高原东北缘2011—2015年期间的流动重力观测资料,系统分析了区域重力场变化及其与2016年1月21日青海门源MS6.4地震发生的关系,结合GNSS、水准观测成果和区域地质构造动力环境,进一步研究了区域重力场变化的时空分布特征及其机理.结果表明:(1)测区内重力场异常变化与祁连山断裂带在空间上关系密切,反映沿祁连山断裂带(段)在2011—2015年期间发生了引起地表重力变化效应的构造活动或变形.(2)门源MS6.4地震前,测区内先出现了较大空间范围的区域性重力异常,到临近发震前显示出相对闭锁的现象,且围绕震中区周围出现四象限分布特征的局部重力变化,地震发生在重力反向变化过程中,并出现显著的四象限分布特征的重力异常变化,其中,青海门源与甘肃天祝一带重力差异变化达100×10-8 m·s-2以上.(3)区域重力场动态演化大体反映了青藏高原东北缘物质北东流的动态效应,门源震中附近区域地壳受挤压变形显著、面压缩率和重力剧烈变化的特征最为显著.(4)重力场的空间分布及其随时间变化与地壳垂直与水平运动及地质构造活动等观测结果有一定的对应关系,强震易发生在重力变化四象限分布中心地带或正、负异常区过渡的高梯度带上.

Using mobile gravity measurements from 2011 to 2015in northeastern margin of Tibetan plateau,we have systematically analyzed the regional gravity field changes and their relation to the MS6.4 Menyuan earthquake on January 21,2016.Combined with GNSS,leveling observations and regional geology,the time-space distribution of the gravity field and its mechanism were further studied.The results show that in the space domain,the gravity field variation is closely linked with the Qilianshan fault.The relation shows that tectonic activity or deformation caused surface gravity changes along the Qilianshan fault during 2011 and 2015.During the early period before MS6.4 Menyuan earthquake,regional gravity anomalies in a large area appeared in the measurement region.Gravity changes showed relative locking before the impending earthquake,and gravity field variation distributed in a four-quadrant form.The earthquake happened in the process of gravity reverse changes and the anomaly variations near Menyuan and Tianzhu were over 100×10-8 m·s-2.Dynamic evolution characteristics of the gravity field generally reflect the dynamic effects of the moving material of the northeastern margin of Tibetan plateau which spread toward east,and the Menyuan epicenter zone by the earth′s compression deformation,where compression ratio and gravity change are most significant.The spatial distribution of the gravity field and its changes over time and vertical and horizontal crustal movement and tectonic activity observations have certain corresponding relationships.To study the relation between regional gravity changes and seismic activity is a meaningful method that is developed in recent years.Gravity dynamic variation has important implications for location prediction,which can provide experience and reference for medium-term forecast.Great earthquakes tend to occur at the center of four quadrant distribution of gravity changes or the gradient/transit belts from positive to negative anomalies.The MS6.4 Menyuan earthquake occurred nearby the zero contour close to the center of four quadrant distribution of gravity changes.