Crustal anisotropy of Huailai--Yanqing region in North China
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摘要: 提出了利用人工爆破P波走时反演地壳介质方位各向异性参数的方法. 在假定介质是弱各向异性介质的情况下， 使用扰动理论得到了线性化的反演公式， 其中待反演的弱各向异性参数是P波走时的线性函数. 如果在反演公式中参考走时取相同震中距接收点的P波平均走时， 那么所获得的弱各向异性参数与参考介质速度的选取无关. 反演得到的弱各向异性参数可以看作是不同震中距和不同深度范围内介质的等效弱各向异性参数. 等效弱各向异性参数在一定程度上反映了不同深度范围内水平方向相速度随方位的变化. 这种变化可能是不同时期构造应力作用的结果. 2007年中国地震局在首都圈怀来地区实施了一次大吨位人工爆破实验, 以爆破点为中心， 布设了高密度的地震观测台网和台阵. 台站相对于爆破点具有360°的全方位覆盖， 所得到的地震记录数据为研究怀来、 延庆地区地壳介质P波方位各向异性提供了必要条件. 我们通过走时反演获得了与水平方位相关的弱各向异性参数， 并对弱各向异性参数进行坐标变换， 得到了能够直观描述岩石弱各向异性的具有水平对称轴的横向各向同性介质， 给出了对应的3个独立弱各向异性参数及其对称轴方位， 讨论了介质各向异性与构造应力场的关系. 结果表明该地区地壳介质存在明显的方位各向异性， 其最大值约为4.6%.Abstract: A method for inversion of seismic anisotropy in crustal medium is presented by using the P-wave travel times from an explosive source. Perturbation theory is used to derive the linear inversion formulae when the medium is taken as a weakly anisotropic (WA) medium. The WA parameters in the formulae are a linear function of the P-wave travel time. The WA parameters are independent of the reference velocity when the reference travel time takes an average of P-wave travel times. The WA parameters from inversion can be considered as effective anisotropic parameters concerning the medium within different source-distances and different depths， which reflect the variation of phase velocity with azimuth. The azimuthal variation of phase velocity was probably caused by the tectonic force in different history periods. A large exploration was carried out in Huailai--Yanqing region near Beijing by China Earthquake Administration in 2007. A high density seismic network was deployed around the explosive point with azimuthal coverage of 360°， which provides an abundant data set for studying P wave seismic anisotropy of crustal medium. Travel time tomography in weakly anisotropic medium was used to calculate WA parameters. In order to get a comprehensive understanding of the studied medium， the inverted WA parameters were transformed to a new coordinate system related to a HTI (transverse isotropy with a horizontal axis of symmetry) medium. Three WA parameters and the orientation of symmetry axis of the medium were obtained. The relation of the WA parameters to tectonic stress field was discussed. The result shows that there exists an obviously azimuthal anisotropy (maximum 4.6%) in the crust of studied region.
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Keywords:
- crust medium /
- anisotropy /
- seismic ray /
- travel time /
- inversion
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图 1 怀来、 延庆地区地震台站及P波走时插值点分布图
Figure 1. Distribution of seismic stations and interpolation points in Huailai- -Yanqing region
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图 3 W1区(-10°，170°)内与震中距40 km (a)、 50 km (b)、 60 km (c)、 80 km (d)、 100 km (e)、 120 km (f)、 140 km (g)和150 km (h)相对应深度内的等效介质相速度曲线
实线表示通过反演得到的一般弱各向异性介质的相速度； “+”表示由“观测”走时计算得到的相速度；虚线表示由坐标变换和最小二乘算出的相应的相速度； 红色直线代表P波相速度的参考各向同性速度
Figure 3. Effective phase velocity curves corresponding to different epicenter distances of 40 km (a)，50 km (b)，60 km (c)，80 km (d)，100 km (e)，120 km (f)，140 km (g) and 150 km (h) in the W1 area
Solid line denotes effective phase velocity from inversion of general weakly anisotropic media,cross denotes the result from the “observed” values,dashed line represents the phase velocity calculated by coordinate transformation and least squares fitting,and red straight line represents the reference velocity of P wave
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图 4 W2区(170°，350°)内与震中距为40 km (a)、 50 km (b)、 60 km (c)、 80 km (d)、 100 km (e)、120 km (f)、 140 km (g)和150 km (h)相对应深度内的等效介质相速度曲线
实线表示通过反演得到的一般弱各向异性介质的相速度； “+”表示由“观测”走时计算得到的相速度；虚线表示由坐标变换和最小二乘算出的相应的相速度； 红色直线代表P波相速度的参考各向同性速度
Figure 4. Effective phase velocity curves corresponding to different epicenter distances of 40 km (a)，50 km (b)，60 km (c)，80 km (d)，100 km (e)，120 km (f)，140 km (g) and 150 km (h) in the W2 area
Solid line denotes effective phase velocity from inversion of general weakly anisotropic media,cross denotes the result from the “observed” values,and dashed line represents the phase velocity calculated by coordinate transformation and least squares fitting, and red straight line represents the reference velocity of P wave
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图 5 W1区(-10°，170°)走时拟合图. 实线表示反演值，“+”表示“观测值”
Figure 5. Travel time fitting in the W1 area.Solid line denotes inversion values，and cross denotes “observed” values
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图 6 W2区(170°，350°)走时拟合图. 实线表示反演值，“+”表示“观测值”
Figure 6. Travel time fitting in W2 area. Solid line denotes inversion values，and cross denotes “observed” values
表 1 W1区域(-10°,170°)内弱各向异性参数反演值
Table 1 Weak anisotropy parameters in the W1 area (-10°,170°)
下载: 导出CSV
表 2 W2区域(170°,350°)内弱各向异性参数反演值
Table 2 Weak anisotropy parameters in the W2 area (170°,350°)
下载: 导出CSV
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