Constraining Moho characteristics with frequency-dependence of receiver function and its application
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摘要:
基于不同莫霍面模型的全波形理论地震图，计算了不同频率的接收函数，分析对比了不同形态莫霍面在不同频率上的接收函数变化特征。数值试验结果显示，当莫霍面的形态复杂时，高频接收函数上P−S转换波和多次波会出现多峰值特征。之后对不同形态莫霍面的模型在不同频率的接收函数进行了分类总结，据此判别实际观测资料所表征的莫霍面性质。以位于青藏高原东北缘的高台（GTA）地震台为例，分析了该台站不同频率的接收函数。结果表明，该台站下方莫霍面总体为遵循同一变化规律的速度过渡带，但在沿龙首山断裂方向附近速度变化不同于主要变化方式。基于此，通过对观测结构进行拟合构建了该台站下方地壳及莫霍面模型，并结合地质学和岩石学等方面的结果对这种莫霍面形成的原因进行了探讨，进而推断此种莫霍面是由于多种构造因素以及上地幔热物质上涌引起地区壳幔物质的分异与交换所导致。
Abstract:Based on the full-waveform theoretical seismograms of different Moho velocity models, the receiver functions of different frequencies are calculated, and the performances of different Moho morphologies on receiver functions at different frequencies are analyzed and compared. The numerical results show that when the Moho is complex, the P-S converted wave and the multiple wave will have multi-peak characteristics on the high-frequency receiver functions. Receiver functions of different frequencies based on different Moho velocity models are classified and summarized, and the Moho properties revealed by the actual observation data can be discriminated. Taking the seismic station Gaotai （GTA） in the northeastern margin of Tibe-tan Plateau as an example, this paper analyzed the effects of the receiver functions of different frequencies on the Moho morphology beneath the station. The results show that Moho beneath the station is a velocity transitional zone in which velocity varies in a dominant regular, except for the velocity varies at the azimuth along the Longshoushan fault. Furthermore, we built the Moho velocity model beneath the station by fitting the observation structure. Combining the results of geology and petrology, the causes of this Moho morphology were discussed. It is inferred that various tectonic factors and the upwelling and bottom invasion of the upper mantle thermal materials give rise to the differentiation and exchange of crust-mantle materials in this area, and then the Moho morphology mentioned above is resulted in.
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Keywords:
- receiver function /
- frequency /
- Moho morphology /
- velocity model
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图 1 尖锐莫霍面模型的理论地震图和理论接收函数
（a） 尖锐的莫霍面速度模型；（b） 径向、垂直向理论地震图；（c） 归一化的径向理论接收函数
Figure 1. The synthetic seismogram and receiver function of velocity model with sharp Moho
（a） Velocity model with sharp Moho；（b） The synthetic radial and vertical seismogram；（c） Normalized synthetic radial receiver function
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图 2 H（ f ）＝0.1时α因子与频率f的对应关系
Figure 2. Relationship between α and frequency f with H（ f ）＝0.1
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图 3 尖锐的莫霍面速度模型（a）及其对应的不同频率理论接收函数 （b）
Figure 3. Velocity model with sharp Moho （a） and corresponding synthetic receiver functions with different frequencies （b）
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图 4 速度线性增加的莫霍面模型 （a）及其对应的不同频率理论接收函数 （b）
Figure 4. Velocity model with a gradient transition zone of the Moho （a） and corresponding synthetic receiver functions with different frequencies （b）
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图 5 速度非线性增加的莫霍面模型（左）及其对应的不同频率理论接收函数（右）
图（a）中46—50 km深度之间的速度为梯度变化与急剧变化的组合模型
Figure 5. Velocity models with different non-gradient transition zone of the Moho （left panels）and the corresponding synthetic receiver functions （right panels）
In Fig. （a） Moho is a combination of a gradient transition zone and a sharpness in the depth from 46 km to 50 km
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图 6 GTA台站位置及Pms震相在莫霍面的透射转换点分布
Figure 6. Location of the station GTA and topography of its adjacent areas as well as the transmission conversion point distribution of the Pms phase at Moho interface
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图 7 地震事件相对台站的震中距方位角分布图
Figure 7. Distribution of epicenter azimuths relative to the station
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图 8 实际观测到的径向、垂向地震波形（a）以及归一化的径向接收函数（b）
Figure 8. Radial and vertical components of an observation seismogram （a） and normalized radial receiver function （b）
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图 9 α为1 （a），4 （b）和9 （c）时GTA台站按方位角叠加的接收函数
Figure 9. Receiver functions stacking by back azimuth at the station GTA with α being 1 （a），4 （b） and 9 （c）
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图 10 GTA台站主要高频分裂特征接收函数示例
Figure 10. Examples of the observed receiver functions showing dominant high-frequency splitting features for the station GTA
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图 12 GTA台站下方两种莫霍面速度模型及对应的不同频率理论接收函数
图（a）和图（b）为主要莫霍面速度模型及其对应的不同频率理论接收函数；图（c）和图（d）为110°—120°后方位角范围内可能存在的另一种莫霍面速度模型及其对应不同滤波因子的理论接收函数
Figure 12. Two Moho models beneath the station GTA and their corresponding theoretical receiver functions with different frequencies
Figs. （a） and （b） are the dominant Moho velocity model beneath GTA and its corresponding theoretical receiver functions with different α；Figs. （c） and （d） are another Moho velocity model with back azimuth of 110°−120° and its corresponding theoretical receiver functions with different frequencies
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图 13 GTA下方主要莫霍面模型及110°—120°后方位角莫霍面模型对应的理论接收函数与实际接收函数的对比
图（a）和图（b）分别为α＝1和α＝10时图12a模型理论接收函数与三条实际观测接收函数的对比；图（c）和图（d）分别为α＝1和α＝10时12c模型理论接收函数与三条实际观测接收函数的对比
Figure 13. Comparison of the theoretical receiver functions and the observations with different α corres-ponding to the dominant Moho velocity model and the model at the 110°−120° azimuth beneath GTA
Figs. （a） and （b） show the comparison of the theoretical receiver function of the model of Fig.12a with three observations at α＝1 and α＝10，respectively；Figs. （c） and （d） give the comparison of the theoretical receiver function of the model of Fig.12c with three observations at α＝1 and α＝10，respectively
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