华北北部地区地壳三维P波速度结构的双差地震层析成像

利用区域固定台站和华北科学探测台阵记录的10 461个近震事件的183 909个Pg波绝对走时和495 753个相对走时数据,采用双差地震层析成像获得华北北部(37.5°～41.5°N,111.5°～119.5°E)范围内的地壳三维P波速度结构模型。结果表明:研究区内各主要构造单元具有明显不同的速度结构特征,速度异常的走向与区域构造的走向一致,浅层速度图像很好地反映了地表地质和岩性的变化;重定位后的大部分地震集中在0～20km的深度上,主要位于低速区的内部或高速和低速区的交界部位;三河—平谷和唐山地震震源区中、下地壳的低速异常可能是流体的显示。结合前人成果和本文模型所揭示的深、浅结构,我们认为太平洋板块在中国东部之下的俯冲和滞留引起板块脱水、软流圈物质上涌等一系列过程,软流圈热物质到达上地幔顶部并沿超壳断裂上侵进入地壳,致使上地幔顶部和下地壳中的含水矿物发生脱水作用产生流体,流体继续上移造成中、上地壳发震层的弱化,从而导致大地震的发生。因此华北北部地区的强震活动,以及地壳结构的非均匀性应是与太平洋板块俯冲、滞留引起的深部过程密切相关的。

Double-difference Seismic Tomography of 3D P-wave Velocity Crustal Structure under the Northern Part of North China

We determined a 3D P-wave velocity model of the crust under the northern part of North China (37.5°~41.5°N,111.5°~119.5°E) by using double-difference seismic tomography and jointly inverting 183909 absolute arrival times and 495753 differential arrival times from 10461 local events recorded by permanent seismic network and North China Seismic Array. Our results show considerable differences in the velocity structure of the upper crust between the main tectonic units in the study area. The trends of velocity anomalies are consistent with the trends of geological structures. The velocity images of the upper crust reflect well the surface geological and lithology features. The majority of hypocenters is located between 0~20 km depth and located in the low velocity anomalies or in the conjunctional areas of low and high velocity anomalies. Significant low velocity anomalies are found in the source areas of the 1976 Tangshan earthquake (M7.8) and the 1679 Sanhe-Pinggu earthquake (M8.0), which are considered to be associated with fluids. Combining previous studies with our present model, we think that the deep subduction and stagnancy of the Pacific slab have resulted in deep slab dehydration and upwelling of hot asthenospheric materials, and the asthenospheric materials have reached the top of uppermantle and broken into crust along the deep faults which may cut through the crust, then the hydrous minerals in the top of uppermantle and lower crust are dehydrated to produce fluids, and these fluids further cause the weakening of the seismogenic layer in the middle and upper crust. Hence, we believe that the crustal heterogeneity and the seismotectonics under the northern part of North China are closely related to a series of deep dynamic processes associated with the deep subduction and stagnancy of the Pacific slab under East China.