Based on the gravity and aeromagnetic data of the Muli-Yanyuan arc structure belt,combined with the methods of 3D apparent density inversion,aeromagnetic vertical magnetization and upward continuation,the deep seismogenic environment of the Muli-Yanyuan arc structure belt and its adjacent area in Sichuan Province are studied.Through the extraction of potential field separation and anomaly characteristics of the gravity and aeromagnetic data,the variation information of apparent density,the distribution scope of magnetic media,the shape,amplitude and gradient of gravity and magnetic anomaly at different depth of the earth's crust are obtained.The inversion of 3D apparent density indicates that the distribution characteristics of apparent density anomaly in the Muli-Yanyuan arc structure belt are different.A wide range of low density anomaly occurs in middle and lower crust in the Jinpingshan,west section of the arc structure belt,indicating that the middle and lower crustal medium of western Sichuan Plateau is relatively weak,a condition favorable for the plastic flow of crustal medium.The existence of deep low density anomaly results in the stress accumulation in the shallow brittle crust,which is the driving force for the intense structural deformation and frequent earthquake activity.Aeromagnetic △T vertical magnetization result reveals that abnormal distribution occurs on both sides of the arc structure belt,tectonic boundary faults have obvious mark of deep geophysical field,and most of the earthquakes occurred in the displaced crust and in the deep faults within upper mantle.Upward continuation contour map of 10 km and 20 km of aeromagnetic △T indicates that the epicenter of Yanyuan-Ninglang Ms 5.7 (2012) is located in the aeromagnetic anomaly mutation area.It is considered that there are apparent differences of magnetic medium within the crust or between the basements,the discrepancy of magnetic medium leads to the stress accumulation,bring about fractures,provide seismogenic environment for the deep Ms 5.7 earthquake.%探讨川西高原木里-盐源弧形构造带重磁异常分布特征与地震孕育和发生的深部孕震环境及地震构造背景.基于木里-盐源弧形构造带的重力和航磁资料,采用重力三维视密度反演、航磁化极和延拓等位场数据处理方法,获得壳内不同深度的视密度变化信息和磁性介质的分布范围及重磁异常的形态、幅值大小及梯度变化等特征.综合三维视密度反演结果表明:沿木里-盐源弧形构造带两侧的视密度异常展布特征各异,锦屏山弧形构造带西侧区域大范围分布的低密度异常反映了川西高原的中下地壳深度范围物质较为软弱.深部低密度异常的存在容易造成浅部脆性地壳内应力积累,是驱动木里-盐源地区构造变形作用和地震频繁活动的深部动力环境.航磁△T化极结果显示研究区两侧不同区域呈现不同的异常展布特征,构造边界断裂具有明显的深部地球物理场标志,中强震多数发生在断错地壳和上地幔的深大断裂及其附近.根据航磁△T向上延拓10 km和20 km图像可知,2012年发生在盐源与宁蒗交界处Ms 5.7地震,其震中位置处于航磁异常的突变带上,该区域壳内块体或基底性质之间存在明显的介质磁性差异,易于造成应力积累并在构造应力作用下发生破裂,这是此次Ms 5.7地震的深部孕震条件.
展开▼
