地球物理学报  2015, Vol. 58 Issue (10): 3571-3582   PDF    
SsPmp震相地壳探测方法
刘震1,2, 田小波1, 朱高华1,2, 梁晓峰1, 段耀晖1,2, 张洪双3, 滕吉文1    
1. 中国科学院地质与地球物理研究所, 北京 100029;
2. 中国科学院大学, 北京 100049;
3. 中国地质科学院地质研究所, 北京 100037
摘要:SsPmp波是远震S波经地表反射转换的P波在莫霍面发生反射后被地表台站接收得到的震相.震中距在30°~50°之间的远震S波震相经地表反射转换的P波射线参数较大,在莫霍面发生全反射,使得台站接收的SsPmp波具有较强的能量,能够从地震记录中清楚地识别出来,为探测台站附近的莫霍面形态提供新的途径.本文通过合成理论地震图分析了SsPmp震相与地壳厚度、射线参数和Pn波速度之间的关系.结果表明:对于水平界面,地壳厚度只影响SsPmp与Ss波之间的相对到时差;Pn波速度只影响SsPmp的相位;射线参数既对SsPmp波的相对到时有影响,也会引起SsPmp波的相位变化.对于复杂的界面,SsPmp反映的深度与速度梯度最大的深度接近,而反映的Pn波速度与实际的Pn波速度一致.
关键词虚拟震源地震测深     全反射     地壳厚度     莫霍面     Pn波速度    
Probing the Moho interface using SsPmp waves
LIU Zhen1,2, TIAN Xiao-Bo1, ZHU Gao-Hua1,2, LIANG Xiao-Feng1, DUAN Yao-Hui1,2, ZHANG Hong-Shuang3, TENG Ji-Wen1    
1. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. University of Chinese Academy of Sciences, Beijing 100049, China;
3. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
Abstract: The Moho is one of the most important discontinuities in the earth. Its shape is associated with tectonic deformation and evolution of the crust. In orogenic belts, such as the Tibetan plateau, the crustal thickness is about 60~80 km, however, in extensional regions, it is only 30~40 km even less than the average global value.Detecting the depth of the Moho is helpful to understand tectonic environments. The virtual deep seismic sounding (VDSS), a new method to measure the crustal thickness, can detect the Moho robustly. A systematic study of VDSS, however is absent now. In this paper, we use synthetic theoretical seismograms to analyze the seismic phase SsPmp in VDSS and its application in estimation of crustal thickness.
Teleseismic S waves convert into P waves at the ground, and these down-going P waves will be reflected by the Moho, so SsPmp waves can be received after Ss phases. The most suitable epicentral distance for this observation is between 30°~50°, in which the waveforms of VDSS is protected from interfering with other seismic phases and the SsPmp becomes a prominent phase. As the ray parameters increase, the down-going P waves can be fully reflected and the energy of SsPmp will become very strong. We analyze the relationship between the delay time and the phase shift of SsPmp and the ray parameter, uppermost mantle velocity and the thickness of the crust by synthetic seismograms. Our study suggests that the crustal thickness can be measured robustly by waveform fitting with a single model even if the Moho is a complex transition layer.
The relation between SsPmp phases and the crustal thickness, ray parameter and uppermost mantle velocity (Pn velocity) was analyzed by synthetic waveforms. Differences of crustal thickness only cause the variation in the delay time of the SsPmp phases relative to Ss phases. Different Pn velocities only result in the phase shift of SsPmp variation. As the Pn velocity become faster, the phase shift becomes larger. With the ray parameter increasing, the delay time between SsPmp and Ss decreases, and the phase shift becomes larger.
In general, SsPmp can be used to detect the thickness of the crust. This phase is powerful than the Ps used in receiver function. SsPmp, as a full-reflection phase, is strong enough to neglect these reflections and multiples from shallow crustal structure.
Key words: Virtual deep seismic sounding     Full reflection     Crustal thickness     Moho     Pn velocity    
1 引言

莫霍面是地球内部重要的间断面之一,其形态可直接反映地壳的构造变形和构造环境.稳定的克拉通地区的莫霍面较为平滑,地壳厚度接近全球大陆平均地壳厚度(30~40 km)(Cawood et al.,2013Zhang et al.,2011bTeng et al.,20132014Zeng et al.,1995),例如我国的鄂尔多斯高原(李英康等,2014徐树斌等,2013)和四川盆地(李志伟等,2011楼海等,2008);挤压造山带地区的莫霍面起伏较剧烈,地壳厚度通常较大,例如青藏高原及其周边地区受印度-欧亚板块汇聚的影响,莫霍面深 度从周边盆地的40~50 km(Sinha,1987Tapponnier et al.,2001管烨等,2001李志伟等,2011楼海等,2008)急剧加深到高原地区的60~80 km(Zhang et al.,2011aGao et al.,2013Shi et al.,2009Xu et al.,2010丁志峰等,1999孙长青等,2013赵金仁等,2005李永华等,2006卢占武等,2006);相反,拉张环境地区的莫霍面相对较浅,例如我国东部地区,地壳厚度只有30~40 km(Li et al.,2013Zhang et al.,2014a嘉世旭和张先康,2005罗艳等,2008郭震等,2012葛粲等,2011叶卓等,2013危自根和陈凌,2012).因此,通过研究莫霍面形态和地壳厚度有助于理解研究区域的构造环境,并为深部精细探测提供约束.

深地震测深一般使用炸药作为爆炸源激发地震波(Zhang et al.,2013高锐等,2002徐涛等,2014白志明和王椿镛,2006),可以事先知道爆破的准确时间和位置,避免了震源误差造成的影响,并且可以根据需要选择观测地点,所以可以得到精度很高的走时曲线和地壳结构信息,但是深埋在地下的爆炸不但成本高,而且会对地表造成破坏.Tseng等(2009)Yu等(20122013),Chen等(2013)利用震中距在30°~50°之间的天然地震S波震相在地表激发的下行P波在莫霍面反射的震相(记为SsPmp),得出了青藏高原、华北以及鄂尔多斯地区的地壳厚度.这些成功的应用实例表明SsPmp震相能量大,且无需炸药作为爆炸源,避免了对地表生态环境的破坏,因此,可以用于探测地壳厚度.

该方法利用S波在地表激发的P波探测地壳厚度,激发点类似于人工源炮点,故称为虚拟震源地 震测深(Virtual Deep Seismic Sounding,简称VDSS).

2 基本原理

利用SsPmp震相探测地壳厚度的原理如图 1所示.

图 1 虚拟震源地震测深方法原理示意图(a)为Ss、Sp、SsPmp震相路径示意图; (b)表示在给定模型下(地壳厚度60 km,地壳内P波速度为6.2 km·s-1,Pn波速度为8.1 km·s-1,射线参数0.13 s·km-1)Ss、Sp、SsPmp震相到时之间的关系.Fig. 1 A diagram shows the principle of virtual deep seismic sounding Fig. (a) shows the paths of Ss, Sp and SsPmp; (b) A seismogram synthesized through a certain model shows the relationship of the arrival time among the different phases (Model parameter: the crustal thickness of 60 km, the velocity of P wave in the crust is 6.2 km·s-1, the uppermost mantle velocity is 8.1 km·s-1, the ray parameter is 0.13 s·km-1).

地震波在传播过程中遇到速度界面会发生反射、折射并且会发生P波和S波之间的转换.当震中距满足一定条件时(约为30°~50°之间),远震S波震相射线参数大约在0.12~0.14 s·km-1之间,S波入射到地表会有一部分能量转换成下行P波,在Pn速度(大约为8.1 km·s-1)足够大的情况下,下行P波会在莫霍面发生全反射.转换波的能量几乎全部被反射回地表,在地表可以清楚观测到该震相.所以利用该震相可以研究莫霍面的深度.SsPmp震相与S震相到时差可以表示为TSsPmp-Ss=2H(1/V2P-p2β)1/2,其中TSsPmp-Ss表示转换反射波与直达S波之间的到时差,H表示地壳厚度,VP表示地壳中P波平均速度,pβ表示射线参数.可以看出,SsPmp与Ss的到时差主要依赖于地壳厚度、地壳内平均P波速度以及射线参数,并且随着地壳厚度的增厚,地壳内平均P波速度的减小以及射线参数减小而增大.

为了研究上述参数对SsPmp震相到时和振幅的影响,通过合成不同地壳结构参数下的理论地震 图(Herrmann and Wang,1985)研究不同参数变化对SsPmp震相的影响.

3 SsPmp随地壳速度模型的变化3.1 SsPmp震相与射线参数的关系

图 2所示,图 2a给出了合成理论地震图用到的速度模型,地壳厚度为60 km,地壳内P波速度为6.2 km·s-1,Pn波速度为8.1 km·s-1.图 2b是根据图 2a中的速度模型在不同射线参数情况下S波从下向上穿过莫霍面正演得到的地震波垂向分量 与Ss子波反褶积后(Ammon et al.,1990Ammon,1991)的波形.图 2c给出了SsPmp在不同射线参数下波峰波谷与S波到时差的关系.图 2d是SsPmp震相波峰、波谷振幅以及峰谷差与射线参数的关系.射线参数在0.124 s·km-1时下行P波在莫霍面发生全反射,可以看出,随着射线参数的增大,SsPmp震相与直达Ss波震相之间的到时差不断减小;射线参数较大时波峰的到时更接近理论到时,而射线参数较小时,波谷的到时和理论到时比较接近;在达到全反射之前,波谷振幅随着射线参数增大而增大,没有出现波峰,当射线参数增大到全反射后,波形会出现波峰,并且波峰逐渐增大,波谷有所减小.可见,超临界角反射产生的相位差是导致波形出现变化的原因(Zhang et al.,2012).

图 2 SsPmp随射线参数变化特点(a)表示合成理论地震图用到的速度模型:地壳厚度60 km,地壳内P波速度为6.2 km·s-1,Pn波速度为8.1 km·s-1; (b) 根据图(a)中的速度模型在不同射线参数情况下S波从下向上穿过莫霍面正演得到的地震波垂向分量与子波反褶积后的波形; (c) 给出了SsPmp在不同射线参数下波峰波谷与S波到时差的关系.实线为理论模型计算的结果,点线为SsPmp波谷到时,虚线为波峰到时; (d) SsPmp震相波峰、波谷振幅以及峰谷差与射线参数的关系,虚线表示波峰,点线表示波谷,实线为峰谷差.Fig. 2 The relationship between the ray parameter and SsPmp(a) shows the velocity model used to synthesize seismogram. The crustal thickness is 60 km, the velocity of P wave in the crust is 6.2 km·s-1, the uppermost mantle velocity is 8.1 km·s-1; (b) The waveform shows in is synthesized from the model showed in figure (a), and the ray parameter is increased from 0.11 s·km-1 to 0.15 s·km-1; (c) shows the variation of the arrival time of the peak and trough with the ray parameter, the dashed line denote the arrival time of peak, the dotted line denote the arrival time of trough, the solid line denote the arrival of the SsPmp phase; (d) shows the variation of the amplitude of the peak, trough and peak-trough difference with the ray parameter, the dashed line denote the amplitude of peak, the dotted line denote the amplitude of trough, the solid line denote the peak-trough difference.

相位变化引起波形变化的原理如图 3所示:图 3a为一系列单一频率的余弦函数,在零相位时合成图 3b所示的波形信号.图 3c是波形发生不同的相移之后叠加产生的波形,可以看出,在信号发生超前和滞后半个周期的变化会导致波形翻转;在测试的射线参数范围内,地震波信号大约有半个周期的相移.

图 3 地震波形出现相位差之后波形的改变(a) 表示一系列单一频率的余弦信号; (b) 所示波形为(a)中的单一频率的波形零相移叠加而成;(c) 表示当(a)中的单一频率的余弦信号发生了一定周期的相位变化叠加后波形发生变化的结果.Fig. 3 The phase shift will cause the waveform transform(a) shows a list of single frequency waveform, which can form the waveform showed; (b) The phase shift of each frequency increase from -180° to 180° ; (c) The transformation of the waveform was show.
3.2 SsPmp震相与地壳厚度的关系

图 4给出了SsPmp震相与地壳厚度的关系:分别对三个不同的射线参数(0.11 s·km-1,蓝线;0.13 s·km-1,黑线;0.15 s·km-1,红线)进行理论合成地震图.地壳厚度(图 4a)从60 km增厚到85 km,地壳P波速度为6.2 km·s-1,Pn速度为8.1 km·s-1.随着地壳厚度的不断增厚,SsPmp与Ss的到时差不断增大,但是波形不会发生明显的变化.图 4b给出了射线参数为0.13 s·km-1情况下,不同地壳厚度引起的波形变化.图 4c给出了三个射线参数下波峰波谷到时与理论到时随地壳厚度变化的关系.图 4d给出了三个射线参数下波峰波谷的振幅以及峰谷差随地壳厚度变化的关系.由此可见,SsPmp与Ss震相之间的到时差随着地壳增厚而增大,但地壳厚度的变化基本不会引起SsPmp的波形变化.

图 4 SsPmp震相与地壳厚度的关系(a)表示合成理论地震图用到的速度模型:地壳厚度从60 km增厚到85 km,地壳P波速度为6.2 km·s-1,Pn速度为8.1 km·s-1; (b) 根据图(a)中的速度模型在射线参数为0.13 s·km-1的情况下,不同地壳厚度情况下S波从下向上穿过莫霍面正演得到的地震波垂向分量与子波反褶积后的波形; (c) SsPmp在不同地壳厚度下波峰波谷与S波到时差的关系.实线为理论模型计算的结果,点线为SsPmp波谷到时,虚线为波峰到时; (d) SsPmp震相波峰、波谷振幅以及峰谷差与地壳厚度的关系,虚线表示波峰,点线表示波谷,实线为峰谷差.蓝线,0.11 s·km-1;黑线,0.13 s·km-1;红线,0.15 s·km-1.Fig. 4 The relationship between the crustal thickness and SsPmp(a) shows the velocity model used to synthesize seismogram. The crustal thickness is from 60 km to 85 km, the velocity of P wave in the crust is 6.2 km·s-1, the uppermost mantle velocity is 8.1 km·s-1; (b) The waveform shows is synthesized from the model showed in figure (a), and the crustal thickness is increased from 60 km to 85 km, the ray parameter is 0.13 s·km-1; (c) shows the change of the arrival time of the peak and trough with the crustal thickness, the dashed line denote the arrival time of peak, the dotted line denote the arrival time of trough, the solid line denote the arrival of the SsPmp phase; (d) shows the variation of the amplitude of the peak, trough and peak-trough difference with the crustal thickness, the dashed line denote the amplitude of peak, the dotted line denote the amplitude of trough, the solid line denote the peak-trough difference. The red line denote the ray parameter is fixed at 0.15 s·km-1, the black line denote the ray parameter is fixed at 0.13 s·km-1, the blue line denote the ray parameter is fixed at 0.11 s·km-1.
3.3 SsPmp震相与Pn波速度的关系

图 5所示为Pn波速度对SsPmp的影响.本测试中使用的速度模型如图 5a所示,地壳厚度为60 km,地壳内P波速度为6.2 km·s-1,Pn波速度从7.5 km·s-1增加到9.0 km·s-1.同样进行了同 图 4中测试相同的三种射线参数情况的理论合成.图 5b给出了射线参数为0.13 s·km-1情况下,使用不同Pn波速度时的波形变化.可以看出,Pn波速度的变化不会影响SsPmp与Ss之间的到时差(图 5c),而对波形影响较大(图 5b图 5d),推测可能是反射导致的相位差对Pn波速度改变比较敏感.当射线参数为0.11 s·km-1时,下行P波在莫霍面的 入射角小于临界角,没有发生全反射,所以,从波形上看,SsPmp几乎没有出现波峰,而波谷振幅随着射线参数增大而不断增大.射线参数为0.13 s·km-1时,Pn波速度从小到大的变化会使下行P波临界角减小,从而发生全反射,出现波峰,并且不断增大;而波谷在经过临界角以后逐渐减小.当射线参数为0.15 s·km-1时,入射角远大于临界角,波形不再有明显的变化.

图 5 SsPmp震相与Pn波速度的关系(a)表示合成理论地震图用到的速度模型:地壳厚度为60 km,Pn波速度从7.5 km·s-1增大到9.0 km·s-1,地壳P波速度为6.2 km·s-1; (b) 根据图(a)中的速度模型在射线参数为0.13 s·km-1,不同Pn波速度情况下S波从下向上穿过莫霍面正演得到的地震波垂向分量与子波反褶积后的波形; (c) SsPmp在不同Pn波速度下波峰波谷与S波到时差的关系.实线为理论模型计算的结果,点线为SsPmp波谷到时,虚线为波峰到时; (d) SsPmp震相波峰、 波谷振幅以及峰谷差与Pn波速度的关系, 虚线表示波峰,点线表示波谷,实线为峰谷差.蓝线,0.11 s·km-1;黑线,0.13 s·km-1;红线,0.15 s·km-1.Fig. 5 The relationship between the P wave velocity at the uppermost mantle and SsPmp(a) shows the velocity model used to synthesize seismogram. The crustal thickness is from 60 km, the velocity of P wave in the crust is 6.2 km·s-1, the uppermost mantle velocity is increased from 7.5 km·s-1 to 9.0 km·s-1; (b) The waveform shows is synthesized from the model showed in figure (a), and the uppermost mantle velocity is increased from 7.5 km·s-1 to 9.0 km·s-1, the ray parameter is 0.13 s·km-1; (c) shows the change of the arrival time of the peak and trough with the uppermost mantle velocity, the dashed line denote the arrival time of peak, the dotted line denote the arrival time of trough, the solid line denote the arrival of the SsPmp phase; (d) shows the variation of the amplitude of the peak, trough and peak-trough difference with the upper mantle velocity, the dashed line denote the amplitude of peak, the dotted line denote the amplitude of trough, the solid line denote the peak-trough difference. The red line denote the ray parameter is fixed at 0.15 s·km-1, the black line denote the ray parameter is fixed at 0.13 s·km-1, the blue line denote the ray parameter is fixed at 0.11 s·km-1.
4 VDSS方法的适用条件

本文根据IASP91模型计算了S波的走时曲线(震源深度10 km)(Crotwell et al.,1999Winchester and Crotwell,1999),如图 6所示:S波在15°~30°之间因为受地幔过渡带的影响出现了S波三岔震相.这一震中距范围内的S波震相识别容易发生相互混淆,所以应用VDSS方法只能选取震中距大于30°的地震事件.而震中距越大,射线入射角越小,所以太大的震中距会导致SsPmp无法在莫霍面达到全反射,这一最大震中距大约在50°,如图 7所示.

图 6 IASP91模型下的S波走时曲线Fig. 6 S wave travel time curve calculate by IASP91 model

图 7 震源深度10 km根据IASP91模型合成理论地震图(a) 截取垂向分量与S子波信号反褶积的结果;(b) 径向分量;(c) 垂向分量.Fig. 7 A seismogram synthesized by IASP91 model, and the source depth is 10 km(a) The waveform shown is the result which is deconvolution of the vertical component and S wavelet; (b) The waveform shown is radial component; (c) The waveform shown is vertical component.

我们利用IASP91模型(Kennett and Engdahl,1991)合成理论地震图(参考了Wang(1999)提供的 程序),以震源深度10 km为例,如图 7所示.图 7a7b7c依次为截取垂向分量与S子波信号反褶积的结果(Yu et al.,2013)、径向分量和垂向分量.因为P波的振动方向与传播方向一致,所以垂向分量在理论到时附近发现一个明显的波峰,而径向分量没有.反褶积之后的结果同样显示在理论到时之后出现一个波峰.震中距在50°以外,到时差延迟随着震中距的变化明显变快,经过分析,由于这时的射线参数不足以使转换P波在莫霍面发生全反射,这是震中距较小的位置发生全反射后产生Pn波的震相,即SsPnp(图 7中黑线为理论计算震相到时差,55°以外根据SsPnp路径计算的到时差).

综上所述,可以通过对震中距在30°~50°之间的远震S波波形中Ss和SsPmp震相的识别探测地壳厚度.

5 复杂地壳结构对虚拟震源地震测深(VDSS)的影响

接收函数利用界面产生的Ps转换波与直达P波进行反褶积计算可以得到界面信息(刘启元和邵学钟,1985吴庆举等,2007徐强和赵俊猛,2008).接收函数对速度界面反应敏感,是探测深部速度界面的有效方法.但是通常莫霍面不是一个简单的速度跃变(Meissner,1973),而是缓慢变化的一个具有一定厚度的层,因此,接收函数中多出现复杂的莫霍面转换震相(Tian et al.,2011Xu et al.,2014Zheng et al.,2006吴庆举和曾融生,1998司少坤等,2012).针对复杂的莫霍面结构,本文做了如下方面的研究,并与接收函数的结果作了比较.

图 8展示了同时存在速度界面和速度渐变层 模型SsPmp震相与简单速度界面合成地震图的拟合情况.地壳内速度为 6.2 km·s-1,Pn波速度为8.2 km·s-1,射线参数为0.125 s·km-1.存在速度变化的层厚度分别为5 km,10 km,15 km.模型1速度从6.2 km·s-1连续变化到8.2 km·s-1,模型2在速度渐变层的上面存在一个1 km·s-1的速 度突变,模型3在速度渐变层下存在一个1 km·s-1 的速度突变.与单一速度跃变模型合成波形的拟合结果表明,所获得的深度更接近模型速度出现跃变的深度,约等于速度变化量对深度的加权平均,而波形对应的Pn波速度更接近上地幔顶部的Pn波速度.

图 8 复杂的速度渐变界面对SsPmp的影响Fig. 8 The influence of complex velocity gradient discontinuityon SsPmp

图 9所示:地壳速度为6.2 km·s-1,Pn速度为 8.2 km·s-1,合成入射S波射线参数为0.125 s·km-1和入射P波射线参数为0.06 s·km-1.模型1为50 km深以下有10 km连续变化的速度 渐变层,速度从6.2 km·s-1逐渐增大到8.2 km·s-1,模型2在50~60 km深度之间给定均匀速度7.2 km·s-1.

图 9 SsPmp对不同界面的反应与P波接收函数的比较Fig. 9 Different responses to various discontinuities between SsPmp and P wave receiver function

图 9中每个速度模型的右侧给出了理论合成的接收函数(红线)和虚拟震源地震测深(蓝线)的波形,黑色虚线是利用单层地壳模型进行波形拟合的结果.可以看出,对于接收函数来说,速度界面缓慢变化会使波形变宽,振幅减小,分辨率被降低,如图 9a所示.在实践中,当接收函数中出现两个到时比较接近的波峰时,很难判断哪一个指示的是莫霍面,如图 9b会出现两个Ps转换波震相.接收函数得到的界面深度分别为:模型1:56 km;模型2:51 km和60 km.而虚拟震源地震测深在两种形式的模型下都只出现一个震相,且振幅较大,容易识别,波形 拟合的结果显示,与模型1拟合较好的简单模型为地壳厚度55 km,波形对应的Pn速度均为8.2 km·s-1. 与模型2拟合较好的简单模型为地壳厚度59 km,波形对应的Pn波速度为8.2 km·s-1.由此可见,接收函数对莫霍面以外的界面同样有响应,并且受到莫霍面形态的影响;而虚拟震源地震测深方法基本不受壳内结构和莫霍面形态的影响,能简单清楚地识别莫霍面震相.

6 实际数据对比

我们对比了接收函数和虚拟震源地震测深两种方法在实际应用中探测的地壳厚度结果.假设地壳 厚度大约为40 km左右,SsPmp波(射线参数0.125~0.14 s·km-1)在莫霍面的反射点分布在距离台站50~80 km范围内,而接收函数P-S波(射线参数 0.04~0.08 s·km-1)转换点位于台站正下方15 km 以内.延安台(YAAN)周围地势平坦,降低了莫霍面横向变化对两种方法探测结果的影响.所以我们利用延安台2007—2009两年的宽频带天然地震数据,分别用两种方法探测该地区地壳厚度,如图 10所示.图 10a展示了筛选得到的82条质量较好的接收函数,通过深度-速度比(H-K)扫描方法(地壳P波平均速度6.3 km·s-1)得到该台站附近地壳厚度为44±1.17 km(速度比为1.74±0.031),图 10a红线给出Pms、PpPms以及PsPms+PpSms理论到时.图 10c为筛选得到13条SsPmp震相清晰的地震事件波形,黑线为实际数据波形,射线参数(RP)范围从0.1292~0.14 s·km-1,反方位角(BAZ)分布在150°~281°,莫霍面反射点的Pn波速度VPn约为8.1 km·s-1(Pei et al.,2007);灰线为理论合成波形(地壳P波平均速度6.3 km·s-1),地壳厚度(H)在43~45 km.两种方法结果比较接近,证明了虚拟震源地震测深方法的可行性.

图 10 (a) YAAN台筛选得到的质量较好的接收函数,Pms、PpPms以及PsPms+PpSmS理论到时如红线所示;(b)H-K扫描所得到的结果,地壳厚度为44±1.17 km (速度比为1.74±0.031);(c)筛选得到的SsPmp震相清晰的事件,射线后分别给出了射线参数(RP)、地壳厚度(H)、Pn波速度(VPn)和反方位角的结果(BAZ)Fig. 10 Receiver functions of YAAN station are shown in figure (a).The red lines denote the arrival time of Pms,PpPms and PsPms+PpSmS.(b) The H-K scan result shows that the crustal thickness is about 44±1.17 km (VP/VS is 1.74±0.031).The ray parameter,crustal thickness,velocity of upper most mantle and back azimuth are followed the waveform of SsPmp we select in figure (c).
7 结论

综上所述,可以利用震中距在30°~50°之间的远震S波转换的SsPmp震相探测地壳厚度.该震相的到时差及波形主要受到射线参数、地壳厚度、地壳平均P波速度以及Pn波速度等因素的影响.经过分析合成理论地震图可以得出以下几点结论:

(1)地壳厚度的改变只影响SsPmp与Ss震相之间的到时差,与SsPmp的相位差无关,地壳厚度增厚,SsPmp延迟Ss到达的时间越长,SsPmp震相波形不会发生改变;

(2)Pn波速度的变化只影响SsPmp的相位,与SsPmp和Ss震相之间的到时差无关,Pn波速度增大,SsPmp在莫霍面的临界角减小,导致SsPmp震相在莫霍面发生全反射,波形发生改变,波形从只有一个波谷逐渐出现波峰;

(3)射线参数既影响SsPmp与Ss之间的到时差还会影响SsPmp的相位,射线参数增大,SsPmp延迟Ss到达的时间越短,SsPmp相位在超过临界角后发生相移,SsPmp震相的波形随之改变,波形从只有一个波谷逐渐出现波峰;

(4)对于复杂的地壳模型,虚拟震源地震测深方法拟合得到的Pn波速度接近真实的Pn波速度,深度与速度梯度最大的深度相近,并且基本不受壳内界面的影响.

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