三峡水库区上地壳三维精细速度结构成像

摘要/Abstract

摘要： 使用2009年3月至2010年6月三峡台网和三峡水库库区加密观测台站48个台站记录到的4967个地震事件的到时数据, 采用双差层析成像方法, 反演得到了三峡水库区地震震源参数及库区分辨率达0.05°的上地壳三维速度结构。反演结果显示: 研究区速度结构显示的横向不均性反应了库区介质结构较复杂。 S波速度以位于秭归盆地与黄陵背斜界面部位的香溪河一带为界, 在香溪河以东至三峡大坝在内的区域均呈现完整的高V_S分布; 以西区域包括秭归盆地、高桥断裂以及长江流域显示明显的大范围低V_S分布。秭归盆地的V_P、 V_S低速分布说明盆地的沉积层厚度在6 km至8 km之间, 在盆地边缘及香溪一带V_S低速异常较V_P显著, 可能在盆地边缘发生了流体扩散、渗透。重定位结果显示, 水库区地震事件沿巴东地区、泄滩以西的长江沿岸以及仙女山地区三个区域, 与研究区A字型架构地震构造带分布一致。巴东地区的3条近东西向条带状的地震发生在低或弱低V_S区内, 且可见这些低速区与浅层低速区联通, 推测这些地震的发生与巴东地区岩性条件易于流体渗透有关。其中2013年12月的M_S5.1地震, 正好位于地震自浅至深形成的条带底部, 同时是P、 S波的局部高速区。 2017年6月两次M4级地震位于P波高速区、 S波弱高速区内, 2014年3月27日的M_S4.3和2014年3月30日M_S4.7地震均位于P、 S波高速区边缘。

关键词: 三峡水库区, 双差层析成像, 三维速度结构, 精定位

Abstract: In this paper, tomoDD method is applied to determine detailed 3D velocity structure of the upper crust and the hypocenter parameters simultaneously in the Three Gorges reservoir area using the phase arrival times from 4967 seismic events recorded by 48 instruments. The resolution of the inversion result is 0.05°. The following conclusions are drawn:The heterogeneity of medium is obvious, reflecting the complex media structure in the reservoir area. The V_S distribution is bounded by the Xiangxi River between the Zigui Basin and the Huangling anticline. It shows a high V_S distribution to the east of the Xiangxi River including the reservoir dam. To its west, there are obvious large-scale low V_S distributions, including the Zigui basin, the Gaoqiao fault, and the Yangtze River. The distributions of low V_P and V_S in the Zigui Basin show that the sedimentary thickness of the basin is between 6 km and 8 km respectively. The low V_S anomaly in the margins of the basin and Xiangxi River is significantly higher than V_P, and fluid diffusion and infiltration may occur at the edge of the basin. The seismic events in the reservoir area, which clusted in the Badong area along the Yangtze River coast west of Xietan and the Xiannvshan area, forms a A-frame pattern, which is consistent with pervious observation. The lower boundary of focal depth is deeper from west to east with a dominant distribution between 0 and 10 km. The earthquakes along Gaoqiao fault in the Badong area occurred in the low or weak V_S areas, implying the relationship between the occurrences of these earthquakes with the lithological conditions in this area. The December 2013 M_S5.1 earthquake is located within a high V_P and V_S area and also at the bottom of a strip formed by shallow to deep earthquakes . The two M4 earthquakes in December 2016 located in the high V_P and V_S zone, the M_S4.1 on March 27, 2014 and the M_S4.7 earthquake on March 30, 2014 are located in the margin of high V_P and V_S areas as well.

Key words: Three Gorges reservoir area, tomoDD, 3D velocity structure, Seismic relocation

中图分类号:

P315.7

张娜, 赵翠萍, 周连庆. 三峡水库区上地壳三维精细速度结构成像[J]. 地震, 2018, 38(4): 37-48.

ZHANG Na, ZHAO Cui-ping, Zhou Lian-Qing. Upper Crust Velocity Structure Tomography of the Three Gorges Reservoir Area based on Dense Observation[J]. EARTHQUAKE, 2018, 38(4): 37-48.

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