唐山M_(S)7.8震区地壳三维密度结构重力反演

Study on gravity inversion of three-dimensional density structure of the crust in Tangshan M_(S)7.8 earthquake area

1976年唐山M_(S)7.8地震位于燕山褶皱与华北平原的过渡部位,震区已有地壳结构研究仍不够精细,对其孕震构造条件的认识也存在一些争议.综合唐山震区较高分辨率的布格重力异常数据和最新全球重力场模型WGM2012,基于小波多尺度信号分离技术和空间域迭代反演算法获得震区的莫霍面起伏和地壳内部密度扰动分布图像.结果表明:唐山震区位于太原—延庆布格重力异常梯度带(亦即著名中国东部重力梯级带的中段)和莫霍面陡变带的东侧,其等值线从北北东向往北东向弯曲,莫霍面深度处于32~37 km之间,唐山震中位于莫霍面陡变带和隆起区过渡部位;震区地壳内部密度分布呈现南高北低、浅部分散高密度体随深度增加逐步聚合特征,前者反映华北平原与燕山褶皱之间构造运动的差异性,后者反映浅部高密度体主要源于深部上地幔岩浆作用;从垂直与平行唐山断裂的密度结构剖面可以发现,唐山断裂贯穿莫霍面,在水平拉张和莫霍面上隆挤压作用下其深部(约>7 km)先存破裂面受到上地幔岩浆的侵入作用而逐步张开、越往上岩浆扩张越大,其浅部(约7 km深以内)相对均匀,可能是因地表不断沉积较新地层并成岩而阻止了岩浆上侵,从而在高密度体顶部形成应力应变易积累的闭锁区,这可能是唐山地震发生在高密度体顶部并呈现浅层新生花状破裂特征的重要原因.结合已有成果,从物质迁移运动角度提出了唐山地震的岩浆上侵与断层联合作用发震模式,同时为唐山地震孕震构造条件的地震测深推测提供了新的佐证.

The Tangshan M_(S)7.8 earthquake in 1976 was located at the transitional part between the Yanshan fold and the North China Plain.The research on the existing crustal structure in the earthquake area is still not fine enough,and there are some controversies in the understanding of its seismogenic tectonic conditions.Combining the high-resolution Bouguer gravity anomaly data in Tangshan earthquake area with the latest global gravity field model WGM2012,based on wavelet multi-scale signal separation technology and spatial domain iterative inversion algorithm,the Moho surface fluctuation and the inner crust density disturbance distribution images in the earthquake area are obtained.The results show that the Tangshan earthquake area is located on the east side of Taiyuan-Yanqing Buge gravity anomaly gradient belt(that is,the middle part of the famous gravity gradient belt in eastern China)and the Moho steep belt,its isoline curves from NNE to NE,and the Moho depth is between 32~37 km.The epicenter of Tangshan is located in the transition between Moho steep belt and uplift area.The internal density distribution of the crust in the earthquake area presents the characteristics of high in the south and low in the north,and the scattered high-density bodies in the shallow part gradually aggregate with increasing depth.The former reflects the difference of tectonic movement between the North China Plain and Yanshan fold,while the latter reflects that the shallow high-density bodies mainly originate from deep upper mantle magmatism.From the density structure profile of the vertical and parallel Tangshan fault,it can be found that the Tangshan fault runs through the Moho surface,and its deep(about>7 km)pre-existing fracture surface is gradually opened by the intrusion of upper mantle magma under the action of horizontal tension and uplift compression of Moho surface,and the magma expands more and more as it goes up.The relatively uniform shallow part(within a depth of about 7 km)may be due to the continuous deposition of new strata and diagenesis on the surface,which prevents magma from invading,thus forming a locking area where stress and strain are easy to accumulate at the top of high-density body.This may be an important reason why the Tangshan earthquake occurred on the top of the high-density body and showed the characteristics of flower-like rupture in the shallow.Combining with the existing achievements,the seismogenic model of the Tangshan earthquake by the combined action of magmatic upwelling and faults is put forward from the point of view of material migration movement,and at the same time,it provides new evidence for the seismic sounding speculation of the seismogenic structural conditions of the Tangshan earthquake.