Crustal structure and seismic activity in the Hanzhong basin and its adjacent areas
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摘要: 为了进一步探查汉中盆地的深部动力学机制和孕震构造特征，本文基于背景噪声成像、多频接收函数和面波联合反演以及莫霍面Ps震相时深转换方法反演了汉中盆地及其邻区的地壳S波速度和厚度，并进一步对比分析了研究区深部结构与地震活动性之间的关系。结果表明：汉中盆地不同区域的浅表沉积厚度和速度存在差异；部分区域莫霍面处的速度变化平缓，Ps震相与P震相的振幅比＜0.2；汉中盆地内部鲜有地震发生，其周边10 km范围内地震分布主要受到断层控制；4—16 km震源深度上下界面大致对应于低速体底层和高速体顶层。本文获得的非均匀分布的沉积厚度、渐变的壳幔过渡带结构与汉中盆地长期处于秦岭构造带、大巴山褶皱带以及青藏地块交界区的三联点构造位置密切相关。Abstract: In order to explore the deep dynamic mechanism and seismogenic structural characteristics in the Hanzhong basin, this paper used the ambient noise tomography, joint inversion of the multi-frequency receiver function and surface wave, and the time-depth conversion method of Ps phase from the Moho to image the crustal S-wave velocity and thickness in the Hanzhong basin and its adjacent areas. Then, we further analyzed the relationship between the crustal structure and seismic activity in the studied region. The results show that the sedimentary thickness and velocity of shallow surface are different in different areas of the Hanzhong basin; in some areas of the studied region Moho surface varies gently with a Ps/P ratio less than 0.2. There are few earthquakes in the Hanzhong basin, and the earthquake distribution within 10 km around the basin is mainly controlled by faults. The upper and lower interfaces of the focal depth （4−16 km） correspond to the bottom of the low-velocity region and the top of the high-velocity region, respectively. The uneven sedimentary thickness and gradual crust-mantle transitional zone obtained in this paper are closely related to the triple junction tectonic position of the Hanzhong basin among the Qinling tectonic belt, the Dabashan folded belt and the Tibetan Plateau. The crustal structure and seismicity characteristics obtained in this paper can provide useful information for the study on the deep dynamic mechanism and seismogenic environment in the Hanzhong basin.
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Keywords:
- Hanzhong basin /
- crustal structure /
- seismic activity /
- ambient noise tomography /
- receiver function
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图 1 汉中盆地及其邻区地质构造背景（邓起东等，2002） （a）以及2009年以来的地震和台站分布（b）
图中红色圆圈为4次M5—5½ 历史地震。F1：勉略断裂；F2：汉中北缘断裂；F3：青川断裂；F4：茶坝—林庵寺断裂；F5：梁山南缘断裂；F6：汉中南缘断裂，下同
Figure 1. Regional geological tectonic settings （Deng et al，2003） （a） and distribution of earthquakes since 2009 and stations （b） in the Hanzhong basin and its adjacent regions
The red circles represent the four historical earthquakes with M5−5½ . F1：Mianlüe fault；F2：Hanzhong north edge fault；F3：Qingchuan fault；F4：Chaba-Lin’ansi fault；F5：Liangshan south edge fault；F6：Hanzhong south edge fault；the same below
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图 2 典型台站对垂直分量波形之间的互相关函数（a），邻域算法反演结果（b），群速度频散曲线（c，黄色线条所示）及其拟合结果（d）
Figure 2. Vertical-component cross-correlation functions （a），neighborhood algorithm inversion results （b），group velocity dispersion measurement denoted by the yellow curve （c） and the fitting of group velocity dispersion curves （d） for typical station-pairs
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图 3 汉中盆地及其邻区浅层频散曲线路径覆盖（a）和不同深度h的S波速度分布（b−f）
Figure 3. Path coverage of dispersion curves （a） and distribution of S-wave velocity at different depths h （b−f） in the Hanzhong basin and its surrounding areas
（a） h＝0.3 km；（b） h＝0.7 km；（c） h＝1.1 km；（d） h＝1.7 km；（e） h＝2.5 km；（f） h＝3.9 km
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图 4 多频接收函数及面波频散联合反演示例图
图（a），（b）和（c）分别为典型台站NSHT反演所得S波速度以及接收函数和频散曲线的拟合情况，其中图（b）中F为拟合系数，G为高斯系数，D为平均震中距；图（d）为研究区6个宽频带地震台的联合反演结果
Figure 4. The schematic diagrams for the joint inversion of multi-frequency receiver function and surface wave dispersion
Figs. （a），（b） and （c） show the inverted S-wave velocity，fitting for the receiver functions and dispersion curves for a typical station NSHT. In Fig. （b），F，G and D are the fitting coefficient，Gaussian factor and average epicentral distance，respec-tively. Fig. （d） shows the inversion results for six stations equipped with broadband seismometers in the studied region
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图 5 16个台站远震接收函数叠加后的前8 s波形（a）和两个典型台站S22 （b）和LUYA （c）叠加前的接收函数波形
Figure 5. The stacked teleseismic receiver functions from −1 s to 8 s for 16 stations （a）and original receiver functions for two typical stations S22 （b） 和LUYA （c）
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图 6 汉中盆地及其邻区地形（a）和地壳厚度（b）
图（b）中黄色三角形旁边的数字标明该处的地壳厚度值，单位为km
Figure 6. Topography （a） and crustal thickness （b） in Hanzhong basin and its adjacent areas
In Fig.（b），the numbers next to the yellow triangles （stations） show the crustal thickness values with unit of km beneath the stations
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图 7 2009年1月1日至2018年9月1日汉中盆地及其周边10 km范围内的地震分布（a）及最大震级统计分析（b）
Figure 7. Distribution of the earthquakes in the Hanzhong basin and its surrounding areas （＜10 km） from 1 January 2009 to 1 September 2018 （a） and statistic on corresponding maximum magnitude （b）
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