劉震， 田小波， 朱高華，2, 梁曉峰， 段耀暉， 張洪雙， 滕吉文

1 中國(guó)科學(xué)院地質(zhì)與地球物理研究所， 北京 100029 2 中國(guó)科學(xué)院大學(xué)， 北京 100049 3 中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所， 北京 100037

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SsPmp震相地殼探測(cè)方法

劉震1,2， 田小波1， 朱高華1，2, 梁曉峰1， 段耀暉1,2， 張洪雙3， 滕吉文1

1 中國(guó)科學(xué)院地質(zhì)與地球物理研究所， 北京 100029 2 中國(guó)科學(xué)院大學(xué)， 北京 100049 3 中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所， 北京 100037

SsPmp波是遠(yuǎn)震S波經(jīng)地表反射轉(zhuǎn)換的P波在莫霍面發(fā)生反射后被地表臺(tái)站接收得到的震相.震中距在30°～50°之間的遠(yuǎn)震S波震相經(jīng)地表反射轉(zhuǎn)換的P波射線參數(shù)較大，在莫霍面發(fā)生全反射，使得臺(tái)站接收的SsPmp波具有較強(qiáng)的能量，能夠從地震記錄中清楚地識(shí)別出來(lái)，為探測(cè)臺(tái)站附近的莫霍面形態(tài)提供新的途徑.本文通過(guò)合成理論地震圖分析了SsPmp震相與地殼厚度、射線參數(shù)和Pn波速度之間的關(guān)系.結(jié)果表明：對(duì)于水平界面，地殼厚度只影響SsPmp與Ss波之間的相對(duì)到時(shí)差；Pn波速度只影響SsPmp的相位；射線參數(shù)既對(duì)SsPmp波的相對(duì)到時(shí)有影響，也會(huì)引起SsPmp波的相位變化.對(duì)于復(fù)雜的界面，SsPmp反映的深度與速度梯度最大的深度接近，而反映的Pn波速度與實(shí)際的Pn波速度一致.

虛擬震源地震測(cè)深； 全反射； 地殼厚度； 莫霍面； Pn波速度

1 引言

莫霍面是地球內(nèi)部重要的間斷面之一，其形態(tài)可直接反映地殼的構(gòu)造變形和構(gòu)造環(huán)境.穩(wěn)定的克拉通地區(qū)的莫霍面較為平滑，地殼厚度接近全球大陸平均地殼厚度(30～40 km)(Cawood et al., 2013；Zhang et al., 2011b；Teng et al., 2013, 2014；Zeng et al., 1995)，例如我國(guó)的鄂爾多斯高原(李英康等，2014；徐樹斌等，2013)和四川盆地(李志偉等，2011；樓海等，2008)；擠壓造山帶地區(qū)的莫霍面起伏較劇烈，地殼厚度通常較大，例如青藏高原及其周邊地區(qū)受印度-歐亞板塊匯聚的影響，莫霍面深度從周邊盆地的40～50 km(Sinha，1987；Tapponnier et al.，2001；管燁等，2001；李志偉等，2011；樓海等，2008)急劇加深到高原地區(qū)的60～80 km(Zhang et al., 2011a；Gao et al., 2013；Shi et al., 2009；Xu et al., 2010；丁志峰等，1999；孫長(zhǎng)青等，2013；趙金仁等，2005；李永華等，2006；盧占武等，2006)；相反，拉張環(huán)境地區(qū)的莫霍面相對(duì)較淺，例如我國(guó)東部地區(qū)，地殼厚度只有30～40 km(Li et al., 2013；Zhang et al., 2014a；嘉世旭和張先康，2005；羅艷等，2008；郭震等，2012；葛粲等，2011；葉卓等，2013；危自根和陳凌，2012).因此，通過(guò)研究莫霍面形態(tài)和地殼厚度有助于理解研究區(qū)域的構(gòu)造環(huán)境，并為深部精細(xì)探測(cè)提供約束.

深地震測(cè)深一般使用炸藥作為爆炸源激發(fā)地震波(Zhang et al.，2013；高銳等，2002；徐濤等，2014；白志明和王椿鏞，2006)，可以事先知道爆破的準(zhǔn)確時(shí)間和位置，避免了震源誤差造成的影響，并且可以根據(jù)需要選擇觀測(cè)地點(diǎn)，所以可以得到精度很高的走時(shí)曲線和地殼結(jié)構(gòu)信息，但是深埋在地下的爆炸不但成本高，而且會(huì)對(duì)地表造成破壞.Tseng等(2009)，Yu等(2012，2013)，Chen等(2013)利用震中距在30°～50°之間的天然地震S波震相在地表激發(fā)的下行P波在莫霍面反射的震相(記為SsPmp)，得出了青藏高原、華北以及鄂爾多斯地區(qū)的地殼厚度.這些成功的應(yīng)用實(shí)例表明SsPmp震相能量大，且無(wú)需炸藥作為爆炸源，避免了對(duì)地表生態(tài)環(huán)境的破壞，因此，可以用于探測(cè)地殼厚度.

該方法利用S波在地表激發(fā)的P波探測(cè)地殼厚度，激發(fā)點(diǎn)類似于人工源炮點(diǎn)，故稱為虛擬震源地震測(cè)深(Virtual Deep Seismic Sounding，簡(jiǎn)稱VDSS).

2 基本原理

利用SsPmp震相探測(cè)地殼厚度的原理如圖1所示.

圖1 虛擬震源地震測(cè)深方法原理示意圖(a)為Ss、Sp、SsPmp震相路徑示意圖； (b)表示在給定模型下(地殼厚度60 km，地殼內(nèi)P波速度為6.2 km·s-1，Pn波速度為8.1 km·s-1，射線參數(shù)0.13 s·km-1)Ss、Sp、SsPmp震相到時(shí)之間的關(guān)系.Fig.1 A diagram shows the principle of virtual deep seismic soundingFig. (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).

為了研究上述參數(shù)對(duì)SsPmp震相到時(shí)和振幅的影響，通過(guò)合成不同地殼結(jié)構(gòu)參數(shù)下的理論地震圖(HerrmannandWang, 1985)研究不同參數(shù)變化對(duì)SsPmp震相的影響.

3 SsPmp隨地殼速度模型的變化

3.1 SsPmp震相與射線參數(shù)的關(guān)系

如圖2所示，圖2a給出了合成理論地震圖用到的速度模型，地殼厚度為60 km，地殼內(nèi)P波速度為6.2 km·s-1，Pn波速度為8.1 km·s-1.圖2b是根據(jù)圖2a中的速度模型在不同射線參數(shù)情況下S波從下向上穿過(guò)莫霍面正演得到的地震波垂向分量與Ss子波反褶積后(Ammon et al., 1990；Ammon, 1991)的波形.圖2c給出了SsPmp在不同射線參數(shù)下波峰波谷與S波到時(shí)差的關(guān)系.圖2d是SsPmp震相波峰、波谷振幅以及峰谷差與射線參數(shù)的關(guān)系.射線參數(shù)在0.124 s·km-1時(shí)下行P波在莫霍面發(fā)生全反射，可以看出，隨著射線參數(shù)的增大，SsPmp震相與直達(dá)Ss波震相之間的到時(shí)差不斷減??；射線參數(shù)較大時(shí)波峰的到時(shí)更接近理論到時(shí)，而射線參數(shù)較小時(shí)，波谷的到時(shí)和理論到時(shí)比較接近；在達(dá)到全反射之前，波谷振幅隨著射線參數(shù)增大而增大，沒(méi)有出現(xiàn)波峰，當(dāng)射線參數(shù)增大到全反射后，波形會(huì)出現(xiàn)波峰，并且波峰逐漸增大，波谷有所減小.可見，超臨界角反射產(chǎn)生的相位差是導(dǎo)致波形出現(xiàn)變化的原因(Zhang et al.,2012).

相位變化引起波形變化的原理如圖3所示：圖3a為一系列單一頻率的余弦函數(shù)，在零相位時(shí)合成圖3b所示的波形信號(hào).圖3c是波形發(fā)生不同的相移之后疊加產(chǎn)生的波形，可以看出，在信號(hào)發(fā)生超前和滯后半個(gè)周期的變化會(huì)導(dǎo)致波形翻轉(zhuǎn)；在測(cè)試的射線參數(shù)范圍內(nèi)，地震波信號(hào)大約有半個(gè)周期的相移.

3.2 SsPmp震相與地殼厚度的關(guān)系

圖4給出了SsPmp震相與地殼厚度的關(guān)系：分別對(duì)三個(gè)不同的射線參數(shù)(0.11 s·km-1，藍(lán)線；0.13 s·km-1，黑線；0.15 s·km-1，紅線)進(jìn)行理論合成地震圖.地殼厚度(圖4a)從60 km增厚到85 km，地殼P波速度為6.2 km·s-1，Pn速度為8.1 km·s-1.隨著地殼厚度的不斷增厚，SsPmp與Ss的到時(shí)差不斷增大，但是波形不會(huì)發(fā)生明顯的變化.圖4b給出了射線參數(shù)為0.13 s·km-1情況下，不同地殼厚度引起的波形變化.圖4c給出了三個(gè)射線參數(shù)下波峰波谷到時(shí)與理論到時(shí)隨地殼厚度變化的關(guān)系.圖4d給出了三個(gè)射線參數(shù)下波峰波谷的振幅以及峰谷差隨地殼厚度變化的關(guān)系.由此可見，SsPmp與Ss震相之間的到時(shí)差隨著地殼增厚而增大，但地殼厚度的變化基本不會(huì)引起SsPmp的波形變化.

3.3 SsPmp震相與Pn波速度的關(guān)系

圖5所示為Pn波速度對(duì)SsPmp的影響.本測(cè)試中使用的速度模型如圖5a所示，地殼厚度為60 km，地殼內(nèi)P波速度為6.2 km·s-1，Pn波速度從7.5 km·s-1增加到9.0 km·s-1.同樣進(jìn)行了同圖4中測(cè)試相同的三種射線參數(shù)情況的理論合成.

圖2 SsPmp隨射線參數(shù)變化特點(diǎn)(a)表示合成理論地震圖用到的速度模型：地殼厚度60 km，地殼內(nèi)P波速度為6.2 km·s-1，Pn波速度為8.1 km·s-1; (b) 根據(jù)圖(a)中的速度模型在不同射線參數(shù)情況下S波從下向上穿過(guò)莫霍面正演得到的地震波垂向分量與子波反褶積后的波形; (c) 給出了SsPmp在不同射線參數(shù)下波峰波谷與S波到時(shí)差的關(guān)系.實(shí)線為理論模型計(jì)算的結(jié)果，點(diǎn)線為SsPmp波谷到時(shí)，虛線為波峰到時(shí); (d) SsPmp震相波峰、波谷振幅以及峰谷差與射線參數(shù)的關(guān)系，虛線表示波峰，點(diǎn)線表示波谷，實(shí)線為峰谷差.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.

圖3 地震波形出現(xiàn)相位差之后波形的改變(a) 表示一系列單一頻率的余弦信號(hào)； (b) 所示波形為(a)中的單一頻率的波形零相移疊加而成；(c) 表示當(dāng)(a)中的單一頻率的余弦信號(hào)發(fā)生了一定周期的相位變化疊加后波形發(fā)生變化的結(jié)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.

圖4 SsPmp震相與地殼厚度的關(guān)系(a)表示合成理論地震圖用到的速度模型：地殼厚度從60 km增厚到85 km，地殼P波速度為6.2 km·s-1，Pn速度為8.1 km·s-1; (b) 根據(jù)圖(a)中的速度模型在射線參數(shù)為0.13 s·km-1的情況下，不同地殼厚度情況下S波從下向上穿過(guò)莫霍面正演得到的地震波垂向分量與子波反褶積后的波形; (c) SsPmp在不同地殼厚度下波峰波谷與S波到時(shí)差的關(guān)系.實(shí)線為理論模型計(jì)算的結(jié)果，點(diǎn)線為SsPmp波谷到時(shí)，虛線為波峰到時(shí); (d) SsPmp震相波峰、波谷振幅以及峰谷差與地殼厚度的關(guān)系，虛線表示波峰，點(diǎn)線表示波谷，實(shí)線為峰谷差.藍(lán)線，0.11 s·km-1；黑線，0.13 s·km-1；紅線，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.

圖5 SsPmp震相與Pn波速度的關(guān)系(a)表示合成理論地震圖用到的速度模型：Pn波速度從7.5 km·s-1增大到9.0 km·s-1，地殼P波速度為6.2 km·s-1; (b) 根據(jù)圖(a)中的速度模型在射線參數(shù)為0.13 s·km-1，地殼厚度為60 km時(shí)，不同Pn波速度情況下S波從下向上穿過(guò)莫霍面正演得到的地震波垂向分量與子波反褶積后的波形; (c) SsPmp在不同Pn波速度下波峰波谷與S波到時(shí)差的關(guān)系.實(shí)線為理論模型計(jì)算的結(jié)果，點(diǎn)線為SsPmp波谷到時(shí)，虛線為波峰到時(shí); (d) SsPmp震相波峰、波谷振幅以及峰谷差與Pn波速度的關(guān)系，虛線表示波峰，點(diǎn)線表示波谷，實(shí)線為峰谷差.藍(lán)線，0.11 s·km-1；黑線，0.13 s·km-1；紅線，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.

圖6 IASP91模型下的S波走時(shí)曲線Fig.6 S wave travel time curve calculate by IASP91 model

圖7 震源深度10 km根據(jù)IASP91模型合成理論地震圖(a) 截取垂向分量與S子波信號(hào)反褶積的結(jié)果； (b) 徑向分量； (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.

圖5b給出了射線參數(shù)為0.13 s·km-1情況下，使用不同Pn波速度時(shí)的波形變化.可以看出，Pn波速度的變化不會(huì)影響SsPmp與Ss之間的到時(shí)差(圖5c)，而對(duì)波形影響較大(圖5b、圖5d)，推測(cè)可能是反射導(dǎo)致的相位差對(duì)Pn波速度改變比較敏感.當(dāng)射線參數(shù)為0.11 s·km-1時(shí)，下行P波在莫霍面的入射角小于臨界角，沒(méi)有發(fā)生全反射，所以，從波形上看，SsPmp幾乎沒(méi)有出現(xiàn)波峰，而波谷振幅隨著射線參數(shù)增大而不斷增大.射線參數(shù)為0.13 s·km-1時(shí)，Pn波速度從小到大的變化會(huì)使下行P波臨界角減小，從而發(fā)生全反射，出現(xiàn)波峰，并且不斷增大；而波谷在經(jīng)過(guò)臨界角以后逐漸減小.當(dāng)射線參數(shù)為0.15 s·km-1時(shí)，入射角遠(yuǎn)大于臨界角，波形不再有明顯的變化.

4 VDSS方法的適用條件

本文根據(jù)IASP91模型計(jì)算了S波的走時(shí)曲線(震源深度10 km)(Crotwell et al., 1999；Winchester and Crotwell, 1999)，如圖6所示：S波在15°～30°之間因?yàn)槭艿蒯＿^(guò)渡帶的影響出現(xiàn)了S波三岔震相.這一震中距范圍內(nèi)的S波震相識(shí)別容易發(fā)生相互混淆，所以應(yīng)用VDSS方法只能選取震中距大于30°的地震事件.而震中距越大，射線入射角越小，所以太大的震中距會(huì)導(dǎo)致SsPmp無(wú)法在莫霍面達(dá)到全反射，這一最大震中距大約在50°，如圖7所示.

我們利用IASP91模型(Kennett and Engdahl, 1991)合成理論地震圖(參考了Wang(1999)提供的程序)，以震源深度10 km為例，如圖7所示.圖7a、7b、7c依次為截取垂向分量與S子波信號(hào)反褶積的結(jié)果(Yu et al., 2013)、徑向分量和垂向分量.因?yàn)镻波的振動(dòng)方向與傳播方向一致，所以垂向分量在理論到時(shí)附近發(fā)現(xiàn)一個(gè)明顯的波峰，而徑向分量沒(méi)有.反褶積之后的結(jié)果同樣顯示在理論到時(shí)之后出現(xiàn)一個(gè)波峰.震中距在50°以外，到時(shí)差延遲隨著震中距的變化明顯變快，經(jīng)過(guò)分析，由于這時(shí)的射線參數(shù)不足以使轉(zhuǎn)換P波在莫霍面發(fā)生全反射，這是震中距較小的位置發(fā)生全反射后產(chǎn)生Pn波的震相，即SsPnp(圖7中黑線為理論計(jì)算震相到時(shí)差，55°以外根據(jù)SsPnp路徑計(jì)算的到時(shí)差).

綜上所述，可以通過(guò)對(duì)震中距在30°～50°之間的遠(yuǎn)震S波波形中Ss和SsPmp震相的識(shí)別探測(cè)地殼厚度.

5 復(fù)雜地殼結(jié)構(gòu)對(duì)虛擬震源地震測(cè)深(VDSS)的影響

接收函數(shù)利用界面產(chǎn)生的Ps轉(zhuǎn)換波與直達(dá)P波進(jìn)行反褶積計(jì)算可以得到界面信息(劉啟元和邵學(xué)鐘，1985；吳慶舉等，2007；徐強(qiáng)和趙俊猛，2008).接收函數(shù)對(duì)速度界面反應(yīng)敏感，是探測(cè)深部速度界面的有效方法.但是通常莫霍面不是一個(gè)簡(jiǎn)單的速度躍變(Meissner, 1973)，而是緩慢變化的一個(gè)具有一定厚度的層，因此，接收函數(shù)中多出現(xiàn)復(fù)雜的莫霍面轉(zhuǎn)換震相(Tian et al., 2011；Xu et al., 2014；Zheng et al., 2006；吳慶舉和曾融生，1998；司少坤等，2012).針對(duì)復(fù)雜的莫霍面結(jié)構(gòu)，本文做了如下方面的研究，并與接收函數(shù)的結(jié)果作了比較.

圖8展示了同時(shí)存在速度界面和速度漸變層模型SsPmp震相與簡(jiǎn)單速度界面合成地震圖的擬合情況.地殼內(nèi)速度為6.2 km·s-1，Pn波速度為8.2 km·s-1，射線參數(shù)為0.125 s·km-1.存在速度變化的層厚度分別為5 km，10 km，15 km.模型1速度從6.2 km·s-1連續(xù)變化到8.2 km·s-1，模型2在速度漸變層的上面存在一個(gè)1 km·s-1的速度突變，模型3在速度漸變層下存在一個(gè)1 km·s-1的速度突變.與單一速度躍變模型合成波形的擬合結(jié)果表明，所獲得的深度更接近模型速度出現(xiàn)躍變的深度，約等于速度變化量對(duì)深度的加權(quán)平均，而波形對(duì)應(yīng)的Pn波速度更接近上地幔頂部的Pn波速度.

如圖9所示：地殼速度為6.2 km·s-1，Pn速度為8.2 km·s-1，合成入射S波射線參數(shù)為0.125 s·km-1和入射P波射線參數(shù)為0.06 s·km-1.模型1為50 km深以下有10 km連續(xù)變化的速度漸變層，速度從6.2 km·s-1逐漸增大到8.2 km·s-1，模型2在50～60 km深度之間給定均勻速度7.2 km·s-1.

圖9中每個(gè)速度模型的右側(cè)給出了理論合成的接收函數(shù)(紅線)和虛擬震源地震測(cè)深(藍(lán)線)的波形，黑色虛線是利用單層地殼模型進(jìn)行波形擬合的結(jié)果.可以看出，對(duì)于接收函數(shù)來(lái)說(shuō)，速度界面緩慢變化會(huì)使波形變寬，振幅減小，分辨率被降低，如圖9a所示.在實(shí)踐中，當(dāng)接收函數(shù)中出現(xiàn)兩個(gè)到時(shí)比較接近的波峰時(shí)，很難判斷哪一個(gè)指示的是莫霍面，如圖9b會(huì)出現(xiàn)兩個(gè)Ps轉(zhuǎn)換波震相.接收函數(shù)得到的界面深度分別為：模型1：56 km；模型2：51 km和60 km.而虛擬震源地震測(cè)深在兩種形式的模型下都只出現(xiàn)一個(gè)震相，且振幅較大，容易識(shí)別，波形擬合的結(jié)果顯示，與模型1擬合較好的簡(jiǎn)單模型為地殼厚度55 km，波形對(duì)應(yīng)的Pn速度均為8.2 km·s-1.與模型2擬合較好的簡(jiǎn)單模型為地殼厚度59 km，波形對(duì)應(yīng)的Pn波速度為8.2 km·s-1.由此可見，接收函數(shù)對(duì)莫霍面以外的界面同樣有響應(yīng)，并且受到莫霍面形態(tài)的影響；而虛擬震源地震測(cè)深方法基本不受殼內(nèi)結(jié)構(gòu)和莫霍面形態(tài)的影響，能簡(jiǎn)單清楚地識(shí)別莫霍面震相.

6 實(shí)際數(shù)據(jù)對(duì)比

我們對(duì)比了接收函數(shù)和虛擬震源地震測(cè)深兩種方法在實(shí)際應(yīng)用中探測(cè)的地殼厚度結(jié)果.假設(shè)地殼厚度大約為40 km左右，SsPmp波(射線參數(shù)0.125～0.14 s·km-1)在莫霍面的反射點(diǎn)分布在距離臺(tái)站50～80 km范圍內(nèi)，而接收函數(shù)P-S波(射線參數(shù)0.04～0.08 s·km-1)轉(zhuǎn)換點(diǎn)位于臺(tái)站正下方15 km以內(nèi).延安臺(tái)(YAAN)周圍地勢(shì)平坦，降低了莫霍面橫向變化對(duì)兩種方法探測(cè)結(jié)果的影響.所以我們利用延安臺(tái)2007—2009兩年的寬頻帶天然地震數(shù)據(jù)，分別用兩種方法探測(cè)該地區(qū)地殼厚度，如圖10所示.圖10a展示了篩選得到的82條質(zhì)量較好的接收函數(shù)，通過(guò)深度-速度比(H-K)掃描方法(地殼P波平均速度6.3 km·s-1)得到該臺(tái)站附近地殼厚度為44±1.17 km(速度比為1.74±0.031)，圖10a紅線給出Pms、PpPms以及PsPms+PpSms理論到時(shí).圖10c為篩選得到13條SsPmp震相清晰的地震事件波形，黑線為實(shí)際數(shù)據(jù)波形，射線參數(shù)(RP)范圍從0.1292～0.14 s·km-1，反方位角(BAZ)分布在150°～281°，莫霍面反射點(diǎn)的Pn波速度VPn約為8.1 km·s-1(Pei et al., 2007)；灰線為理論合成波形(地殼P波平均速度6.3 km·s-1)，地殼厚度(H)在43～45 km.兩種方法結(jié)果比較接近，證明了虛擬震源地震測(cè)深方法的可行性.

7 結(jié)論

綜上所述，可以利用震中距在30°～50°之間的遠(yuǎn)震S波轉(zhuǎn)換的SsPmp震相探測(cè)地殼厚度.該震相的到時(shí)差及波形主要受到射線參數(shù)、地殼厚度、地殼平均P波速度以及Pn波速度等因素的影響.經(jīng)過(guò)分析合成理論地震圖可以得出以下幾點(diǎn)結(jié)論：

(1) 地殼厚度的改變只影響SsPmp與Ss震相之間的到時(shí)差，與SsPmp的相位差無(wú)關(guān)，地殼厚度增厚，SsPmp延遲Ss到達(dá)的時(shí)間越長(zhǎng)，SsPmp震相波形不會(huì)發(fā)生改變；

(2) Pn波速度的變化只影響SsPmp的相位，與SsPmp和Ss震相之間的到時(shí)差無(wú)關(guān)，Pn波速度增大，SsPmp在莫霍面的臨界角減小，導(dǎo)致SsPmp震相在莫霍面發(fā)生全反射，波形發(fā)生改變，波形從只有一個(gè)波谷逐漸出現(xiàn)波峰；

(3) 射線參數(shù)既影響SsPmp與Ss之間的到時(shí)差還會(huì)影響SsPmp的相位，射線參數(shù)增大，SsPmp延遲Ss到達(dá)的時(shí)間越短，SsPmp相位在超過(guò)臨界角后發(fā)生相移，SsPmp震相的波形隨之改變，波形從只有一個(gè)波谷逐漸出現(xiàn)波峰；

(4) 對(duì)于復(fù)雜的地殼模型，虛擬震源地震測(cè)深方法擬合得到的Pn波速度接近真實(shí)的Pn波速度，深度與速度梯度最大的深度相近，并且基本不受殼內(nèi)界面的影響.

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(本文編輯 何燕)

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

1InstituteofGeologyandGeophysics,ChineseAcademyofSciences,Beijing100029,China2UniversityofChineseAcademyofSciences,Beijing100049,China3InstituteofGeology,ChineseAcademyofGeologicalSciences,Beijing100037,China

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.

Virtual deep seismic sounding; Full reflection; Crustal thickness; Moho; Pn velocity

10.6038/cjg20151012.

Liu Z, Tian X B, Zhu G H,et al. 2015. Probing the Moho interface using SsPmp waves.ChineseJ.Geophys. (in Chinese),58(10):3571-3582,doi:10.6038/cjg20151012.

國(guó)家自然科學(xué)基金項(xiàng)目(41274066，41340040，41104034)與深部探測(cè)技術(shù)與實(shí)驗(yàn)研究專項(xiàng)SinoProbe-02-02(201311155)共同資助.

劉震，男，1985年生，博士研究生，主要從事地球殼幔結(jié)構(gòu)方面的研究. E-mail: liuzhen@mail.iggcas.ac.cn

10.6038/cjg20151012

P315

2014-12-25，2015-09-16收修定稿

劉震， 田小波， 朱高華等. 2015. SsPmp震相地殼探測(cè)方法.地球物理學(xué)報(bào)，58(10):3571-3582,