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

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

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,Q_0逐渐增大;反之如果下地壳介质衰减强,则Q_0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.

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,ηdecreases as source depth increases.If lower crust has a strong media attenuation,ηincreases first and then decreases.However,when there is a low-velocity layer in lower crust,ηincreases as source depth increases.(3)If upper crust has a strong media attenuation,Q 0 increases as source depth increases.Conversely,if lower crust has a strong media decline,Q 0 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.