Lg波Q值随震源深度的变化作为地壳本征衰减深度的判据

Lg波的Q_Lg值是描述区域地壳结构及介质衰减特性的重要参数之一，Q_Lg层析成像被广泛应用于地壳衰减结构横向不均匀性的研究中.但是，对Q_Lg在垂直方向上的不均匀性的研究较少.当发现一个地区发生Q_Lg值低时，我们希望进一步了解这种介质衰减深度.本文通过数值模拟合成地震图的方法，通过分析不同震源深度和介质模型计算Q_Lg，判断Lg波发生主要衰减的深度.研究结果显示：（1）当震源深度浅时，上地壳介质衰减对于Lg波Q值的影响明显比下地壳的影响更大.而震源深度深时，两者贡献基本相当；（2）如果上地壳介质衰减强，则随着震源深度的增加，η值逐渐减小；如果下地壳介质衰减强，η值先增大后减小；但如果下地壳存在低速层，则η值持续增大；（3）如果上地壳介质衰减强，则随着震源深度的增加，Q₀逐渐增大；反之如果下地壳介质衰减强，则Q₀逐渐减小.因此，我们可以通过调查某一区域不同深度地震的Lg波衰减规律，来判断这一地区的地壳介质衰减情况.

Dependance of Lg Q on depth as a criterion to determine the depth of intrinsic attenuation in the crust

Q_Lg attenuation is an important parameter used in studies of region crustal structure and media physical nature. Lg Q tomography is widely used in study of crustal horizontal heterogeneity. However, studies of vertical heterogeneity are very few in the literature. When there is a low Q_Lg, it is essential to know the depth that generate it. In this paper, we use synthetic seismograms for models with different focal depth to estimate the depth of the intrinsic Q. Our results show that:(1) when source depth is shallow, upper crust attenuation has obvious impact on Lg wave Q than that in lower crust. When source depth is deep, upper crust and lower crust has similar impact. (2) If upper crust has a strong media attenuation, η decreases as source depth increases. If lower crust has a strong media attenuation, η increases first and then decreases. However, when there is a low-velocity layer in lower crust, η increases as source depth increases. (3) If upper crust has a strong media attenuation, Q₀ increases as source depth increases. Conversely, if lower crust has a strong media decline, Q₀ decreases as source depth increases. Using this conclusion, according to the research of the Lg decline rule of different depths in a region, we can estimate the depth of high attenuation of the studied region.