崔輝輝， 周元澤*， 石耀霖， 王曉冉， 李國(guó)輝

1 中國(guó)科學(xué)院計(jì)算地球動(dòng)力學(xué)重點(diǎn)實(shí)驗(yàn)室， 北京　100049 2 中國(guó)科學(xué)院大學(xué)地球科學(xué)學(xué)院， 北京　100049

華北克拉通東部滯留板塊下方低速異常的地震三重震相探測(cè)

崔輝輝1,2， 周元澤1,2*， 石耀霖1,2， 王曉冉1,2， 李國(guó)輝1,2

1 中國(guó)科學(xué)院計(jì)算地球動(dòng)力學(xué)重點(diǎn)實(shí)驗(yàn)室， 北京100049 2 中國(guó)科學(xué)院大學(xué)地球科學(xué)學(xué)院， 北京100049

摘要本文基于中國(guó)數(shù)字地震臺(tái)網(wǎng)記錄的中國(guó)東北與俄羅斯交界地區(qū)發(fā)生于2011年5月10日的一個(gè)深源地震的P波寬頻帶波形資料，研究了華北克拉通東部660 km間斷面附近的P波速度結(jié)構(gòu).通過(guò)一維射線(xiàn)追蹤擬合P波三重震相的相對(duì)到時(shí)，并進(jìn)行觀(guān)測(cè)波形與理論地震圖的對(duì)比，發(fā)現(xiàn)華北克拉通東部660 km間斷面下沉約15～20 km，其上方存在厚度約115～120 km的高速異常，P波速度升高1.5%～2.0%，應(yīng)為滯留的太平洋俯沖板塊；660 km間斷面下方存在局部的低速異常，P波速度降低0.6%～0.9%，該異常可能與滯留板塊從其底部向下地幔頂部脫水或坍塌進(jìn)入下地幔深處滯留體的脫水有關(guān)，也可能與板塊深俯沖及板塊碎片崩塌所引起的地幔熱物質(zhì)上涌有關(guān).關(guān)鍵詞P波三重震相； 660 km間斷面； 低速異常； 下地幔頂部； 板塊脫水

1引言

俯沖板塊將地球表面的水帶入地球深部，深刻地影響著地球表層和地球內(nèi)部動(dòng)力學(xué)過(guò)程(汪品先, 2009; Faccenda, 2014).俯沖板塊直接進(jìn)入下地?；蛘咂脚P的滯留體坍塌進(jìn)入下地幔，均可能在下地幔的溫壓環(huán)境中脫水(Ohtani, 2005).在下地幔頂部，水可由超水相B(Mg10Si3O14(OH)4)的分解而產(chǎn)生(Ohtani et al., 2001)；在下地幔深部，水則可由存儲(chǔ)于俯沖板塊或滯留體中的水相D(MgSi2O4(OH)2)分解而產(chǎn)生(Ohtani et al., 1997; Yuen et al., 2007; Faccenda, 2014).前人對(duì)俯沖和滯留板塊脫水研究主要側(cè)重于板塊上方的脫水作用，一般認(rèn)為與大地幔楔(big mantle wedge)的成因有關(guān)(如，Stern, 2012; Zhao et al., 2012; Lei et al., 2013)，而對(duì)于板塊下方的脫水及其影響的研究則不多見(jiàn).

在西北太平洋地區(qū)，俯沖的海洋巖石圈在地幔轉(zhuǎn)換帶底部近水平停滯于660 km間斷面(以下簡(jiǎn)稱(chēng)660)附近，形成了滯留板塊(Fukao et al., 1992, 2009).Lawrence和Wysession(2006)基于地震波衰減成像發(fā)現(xiàn)東亞地區(qū)660～1400 km深度存在低S波品質(zhì)因子(Qμ)異常，認(rèn)為與俯沖板塊通過(guò)水相D分解而產(chǎn)生的水有關(guān).區(qū)域地震層析成像揭示了滯留板塊在中國(guó)東部及周邊地區(qū)地幔轉(zhuǎn)換帶內(nèi)是普遍存在的(Li and van der Hilst, 2010)，部分板塊因重力不穩(wěn)定性而崩塌進(jìn)入下地幔(Zhao et al., 2011)；在部分區(qū)域滯留板塊下方存在一定的低速異常(Huang and Zhao, 2006; Li et al., 2008).Tang等(2014)發(fā)現(xiàn)長(zhǎng)白山火山可能源自俯沖滯留板塊下方的拆沉物質(zhì).對(duì)俯沖和滯留板塊脫水所對(duì)應(yīng)的低速異常區(qū)域的地震學(xué)精細(xì)探測(cè)有助于深入理解相關(guān)的地球動(dòng)力學(xué)過(guò)程.

基于高密度地震臺(tái)網(wǎng)資料的三重震相分析可有效應(yīng)用于地球內(nèi)部地震波速精細(xì)結(jié)構(gòu)探測(cè)(Tajima and Grand, 1995; Song et al., 2004; Chen and Tseng, 2007; Chu et al., 2012).前人利用P波或S波三重震相獲得了較多東北亞地區(qū)地幔轉(zhuǎn)換帶的速度結(jié)構(gòu).Tajima和Grand(1995,1998)通過(guò)P波三重震相研究得出了M3.11模型，顯示西北太平洋地區(qū)660下沉了30 km，其上方存在與俯沖板塊相關(guān)的高速層.Wang等(2006)通過(guò)SH波研究表明東北亞地區(qū)660下沉至730 km，下沉幅度達(dá)到了70 km.Wang和Chen(2009)通過(guò)P波和SH波分析顯示日本俯沖帶在660之上存在與俯沖板塊相關(guān)的高速異常，南千島下方660至760 km存在可能與下地幔上升熱柱有關(guān)的低速異常.Zhang等(2012)通過(guò)SH波分析得出朝鮮半島下方660下沉約15～20 km，且在地幔轉(zhuǎn)換帶下部存在S波高速異常.Wang和Niu(2010)通過(guò)P波分析，提出中國(guó)東北地區(qū)660為30～70 km厚的梯度層，可能與橄欖石和石榴子石組分分解相關(guān)的多重相變有關(guān).P波和SH波聯(lián)合約束顯示中國(guó)東北地區(qū)660為35 km厚的梯度層(NEChina-P、NEChina-S模型)，且地幔轉(zhuǎn)換帶內(nèi)含有約0.2～0.3 wt%的水(Ye et al., 2011; 葉玲玲和李娟, 2012; Li et al., 2013).前人對(duì)西太平洋地區(qū)地幔轉(zhuǎn)換帶研究所得出的速度模型存在一定差異，這與因所用地震事件或臺(tái)站的不同，相應(yīng)的射線(xiàn)路徑存在差異有關(guān).

合適震級(jí)的地震可以提供相對(duì)簡(jiǎn)單的震源時(shí)間函數(shù)和足夠高的信噪比；而密集且相對(duì)均勻分布的地震臺(tái)可以提供高質(zhì)量的地震波形資料，為研究地球深部精細(xì)結(jié)構(gòu)橫向變化提供數(shù)據(jù)支持.本文基于中國(guó)數(shù)字地震臺(tái)網(wǎng)(CDSN，China Digital Seismograph Network) (鄭秀芬等, 2009)記錄的中國(guó)東北與俄羅斯交界地區(qū)發(fā)生于2011年5月10日的Mb5.4深源地震的寬頻帶波形資料(圖1)，利用P波三重震相方法研究了華北克拉通東部660附近的P波速度結(jié)構(gòu)，并以此探討了可能與之相關(guān)的地球動(dòng)力學(xué)過(guò)程.

圖1　本文所用地震位置和臺(tái)站分布圖震源球標(biāo)出了震中位置，黑色三角形表示地震臺(tái)站，A—C為基于方位角12°間隔所劃分的三個(gè)子區(qū)域.白色五角星為P波拐點(diǎn)在地表的投影. A子區(qū)域拐點(diǎn)的深度范圍為553～821 km， B子區(qū)域?yàn)?52～833 km， C子區(qū)域?yàn)?52～845 km. 白色虛線(xiàn)為和達(dá)-貝尼奧夫帶等深線(xiàn)(Gudmundsson and Sambridge,1998).黑色虛線(xiàn)標(biāo)出了Huang和Zhao(2006)一文中3條層析成像剖面(43°N、41°N及39°N)的位置.Fig.1　Map showing locations of earthquake (beach ball) and stations (black triangles) used in the studyA—C denote three sub-regions divided by the azimuth interval of 12°. White stars represent surface projections of P-wave turning points. Depths of turning points range 553～821 km in sub-region A, 552～833 km in sub-region B and 552～845 km in sub-region C. Contours shown in white dashed lines indicate the Wadati-Benioff zone (Gudmundsson and Sambridge,1998). Black dashed lines indicate the location of three tomography profiles (43°N, 41°N and 39°N) in Huang and Zhao (2006).

2方法和數(shù)據(jù)

2.1三重震相方法

圖2　660附近P波三重震相的射線(xiàn)路徑圖(a)和理論走時(shí)、波形圖(b)理論模型為IASP91 (Kennett and Engdahl, 1991)，震源深度為552.0 km.660上方的回折波(AB)， 660之上的反射波(BC)及660下方的回折波(CD)分別用點(diǎn)線(xiàn)、實(shí)線(xiàn)和虛線(xiàn)表示.Fig.2　Ray paths (a) and theoretical travel times, synthetic waveforms (b) of P-wave triplication near the 660 km discontinuity A deep source with focal depth of 552.0 km and the IASP91 model (Kennett and Engdahl, 1991) are used in the calculation. The turning wave above the 660 km discontinuity (AB) (hereafter as 660), the reflection on the 660 (BC) and the turning wave below the 660 (CD) are shown in dotted, solid and dashed lines, respectively.

地震波在經(jīng)過(guò)地球內(nèi)部速度間斷面時(shí)，由于上下速度的跳變，地震射線(xiàn)會(huì)出現(xiàn)與間斷面相關(guān)的三重震相(圖2a).660附近的三重震相包括間斷面上方的回折P波(AB)、間斷面上的反射P波(BC)及間斷面下方的回折P波(CD).三重震相通過(guò)射線(xiàn)拐點(diǎn)1)地震射線(xiàn)路徑的最低點(diǎn)定義為拐點(diǎn)，在拐點(diǎn)處射線(xiàn)方向是水平的且射線(xiàn)的入射角為90°(Shearer,2009).對(duì)徑向速度結(jié)構(gòu)進(jìn)行采樣，其采樣區(qū)域和拐點(diǎn)深度依賴(lài)于震源深度及震中距的變化(Tajima et al., 2009; 眭怡和周元澤, 2015).對(duì)單個(gè)臺(tái)站記錄的三重震相而言，射線(xiàn)路徑在地表淺層和巖石圈大致相同，主要路徑差別集中在間斷面附近，多重P波受臺(tái)站下方橫向不均勻性的干擾較小(Wang and Niu, 2010).在一定程度上，臺(tái)陣或者密集臺(tái)網(wǎng)提供的波形資料可降低淺層橫向不均勻性的影響(Tseng and Chen, 2007; Zhang et al., 2012).如圖2b所示，在震中距10°～30°范圍內(nèi)三重震相的理論走時(shí)曲線(xiàn)AB、BC和CD呈回折形態(tài)，其相對(duì)到時(shí)和波形振幅變化能有效約束間斷面附近的速度結(jié)構(gòu)(葉玲玲和李娟, 2012; 李國(guó)輝等, 2014).

本文使用動(dòng)態(tài)射線(xiàn)追蹤法(Tian et al., 2007)計(jì)算不同速度模型的理論走時(shí)曲線(xiàn)，并基于反射率法(Wang,1999)合成理論地震圖，通過(guò)試錯(cuò)法(trial and error method)得到各子區(qū)域擬合效果最優(yōu)的速度模型.

2.2數(shù)據(jù)收集及處理

為避免410 km間斷面三重震相的干擾，本文所選地震的震源深度大于410 km，震級(jí)為Mb5.4，震源參數(shù)(表1)引自國(guó)際地震中心目錄(International Seismological Centre, 2012)，震源機(jī)制解引自Global CMT(Centroid-Moment-Tensor)質(zhì)心矩張量解1)1) http:∥www.globalcmt.org/(Dziewonski et al., 1981; Ekstr?m et al., 2012).

表1　地震震源參數(shù)表

注： a) 經(jīng)重定位的震源深度.

震源深度的誤差會(huì)影響三重震相走時(shí)的形態(tài)(Wang and Niu, 2010)，因此對(duì)震源深度需要進(jìn)行重新定位或檢驗(yàn).國(guó)際地震中心(ISC)目錄提供了基于pP震相定位的震源深度，范圍為545～560 km，最新定位的深度為554.9 km.我們從美國(guó)地震學(xué)研究聯(lián)合會(huì)2)2) http:∥www.iris.edu/獲取了30°～95°震中距內(nèi)的全球地震臺(tái)網(wǎng)(GSN，Global Seismographic Network)波形資料(圖3a)，手動(dòng)讀取震相而獲得了pP-P走時(shí)差；基于IASP91(Kennett and Engdahl, 1991)及CRUST 2.0模型(Bassin et al., 2000)，擾動(dòng)震源深度使得pP-P的理論走時(shí)差與觀(guān)測(cè)走時(shí)差的均方根最小(圖3b)，重定位后震源深度為552.0 km.該地震的P波信噪比較高，P波三重震相較清晰，而S波信噪比相對(duì)較低，因此本文僅采用P波三重震相進(jìn)行研究.

圖3　地震震源深度重定位(a) 中震源球標(biāo)出了震中位置，黑色三角形是重定位所用臺(tái)站，黑色實(shí)線(xiàn)為P波射線(xiàn)路徑在地表的投影. (b) 中X軸為震源擾動(dòng)深度， Y軸為相對(duì)于IASP91模型的pP-P相對(duì)時(shí)差的殘差均方根.Fig.3　Focal depth relocation of the earthquake(a) Beach ball represents the epicenter location, black triangles represent the stations used in the relocation, and the black lines are the surface projection of P-wave ray paths. (b) X axis is the perturbation of the focal depth; Y axis is the RMS of the time residuals of pP-P relative intervals relative to the IASP91 model.

我們對(duì)中國(guó)數(shù)字地震臺(tái)網(wǎng)記錄的原始地震資料進(jìn)行了去均值、去線(xiàn)性趨勢(shì)處理，去除儀器響應(yīng)后進(jìn)行0.05～1.0 Hz的帶通濾波，最后將速度記錄積分成為位移記錄.該地震與660相關(guān)的三重震相出現(xiàn)在10°～30°震中距范圍內(nèi)，P波射線(xiàn)拐點(diǎn)的投影集中在華北克拉通東部地區(qū)(圖1)，因此P波三重震相可約束其下方660附近的速度結(jié)構(gòu).為分析結(jié)構(gòu)的橫向變化，本文以12°的方位角間隔將臺(tái)站劃分為A、B和C三個(gè)子區(qū)域(圖1)，各子區(qū)域的觀(guān)測(cè)波形見(jiàn)圖4.

2.3正演測(cè)試

基于對(duì)觀(guān)測(cè)波形特征的分析(圖4)，在IASP91模型基礎(chǔ)上構(gòu)建了三個(gè)改進(jìn)模型進(jìn)行正演測(cè)試(圖5)：660上方存在一個(gè)高速層(模型I)、660下沉至675 km(模型II)及660下方存在低速異常(模型III).通過(guò)改進(jìn)模型與IASP91模型的走時(shí)及波形的對(duì)比，得出不同模型下P波三重震相的變化特征，并驗(yàn)證這些特征在觀(guān)測(cè)波形中是否具備.

圖5給出了IASP91及3個(gè)改進(jìn)模型的P波走時(shí)曲線(xiàn)及合成波形圖.對(duì)于IASP91模型而言，AB震相終止震中距(尖端B)為20.5°，CD震相起始震中距(尖端C)為11.4°，AB和CD震相在震中距15.7°相交(O點(diǎn)).

當(dāng)660上方存在一個(gè)厚度為100 km的高速層，P波速度最大異常值為1.0%(模型I，圖5a)時(shí)，尖端B′(27.6°)消失明顯晚于尖端B，AB′震相在15.0°～27.6°震中距內(nèi)斜率大于AB震相；尖端C′(11.6°)稍晚于尖端C，B′C′震相的長(zhǎng)度明顯大于BC震相；C′D′震相較CD震相提前約0.2 s，AB′與C′D′震相的相對(duì)時(shí)差小于AB與CD震相的相對(duì)時(shí)差.

圖4　A、B和C三個(gè)子區(qū)域觀(guān)測(cè)波形圖(a)—(c)中，橫軸為折合走時(shí)，縱軸為震中距；黑色實(shí)線(xiàn)為由IASP91模型計(jì)算的P波三重震相的走時(shí)，字母A、B、C、D和O是對(duì)各震相的標(biāo)記.Fig.4　Observed waveforms for sub-regions A, B and C X axis is the reduced travel time, and Y axis is the epicentral distance; The travel times of P-wave triplication calculated =with the IASP91 are shown in black lines, and the phases are marked with the symbols A, B, C, D and O.

當(dāng)660下沉至675 km(模型II，圖5b)時(shí)，交點(diǎn)O′點(diǎn)(16.5°)明顯晚于交點(diǎn)O (15.7°)；尖端C′(12.1°)晚于尖端C，尖端B′(21.5°)稍晚于尖端B，B′C′震相的長(zhǎng)度稍大于BC震相；C′D′震相較CD震相延后約0.6 s；AB′與C′D′震相的相對(duì)時(shí)差小于AB與CD震相的相對(duì)時(shí)差.

當(dāng)660下方存在低速異常，P波速度降低1.0%(模型III，圖5c)時(shí)，C′D′震相的斜率較CD震相明顯變小，且兩震相的相對(duì)時(shí)差隨震中距變大而變大；AB′與AB震相基本重合；尖端C′(12.0°)稍晚于尖端C；交點(diǎn)O′(16.1°)稍晚于交點(diǎn)O；AB′與C′D′震相的相對(duì)時(shí)差小于AB與CD震相的相對(duì)時(shí)差.

3結(jié)果

從A子區(qū)域?qū)嶋H觀(guān)測(cè)資料可看出觀(guān)測(cè)三重震相與IASP91模型的理論到時(shí)之間沒(méi)有明顯的系統(tǒng)性時(shí)間偏差(圖6a)，AB′震相的斜率在13°～20°震中距內(nèi)明顯大于IASP91模型的理論AB震相，且尖端B′震中距(25.5°)明顯大于IASP91模型的預(yù)測(cè)值(20.5°)，可確定660之上存在一個(gè)高速層；交點(diǎn)O′(18.0°)明顯大于IASP91模型的預(yù)測(cè)值(15.7°)，且尖端C′震中距(13.0°)稍大于IASP91模型的預(yù)測(cè)值(11.4°)，且AOC′區(qū)較AOC區(qū)呈加寬形態(tài)，這表明660可能出現(xiàn)了下沉；存在速度梯度的間斷面可使尖端C′的震中距明顯變大，觀(guān)測(cè)波形中尖端C′和尖端C的震中距差較小(1.6°)，預(yù)示660傾向于一速度跳躍界面.C′D′震相與IASP91模型的預(yù)測(cè)震相CD的相對(duì)時(shí)差隨震中距變大而變大，且C′D′的斜率小于預(yù)測(cè)震相CD的斜率，這表明在660之下存在低速異常.通過(guò)正演試錯(cuò)測(cè)試，我們得到了能很好擬合觀(guān)測(cè)波形的一維P波速度模型ENCC-A(圖6b和6c).該模型表明在660上方存在120 km厚的高速層，P波速度異常最大值為1.5%；660下沉至680 km；660下方存在低速異常，P波速度降低0.9%；660附近的速度跳變?yōu)?.73%，小于IASP91模型的5.8%.

圖5　不同速度模型下的走時(shí)與理論波形圖模型I：660上方存在一個(gè)高速層；模型II：660下沉至675 km；模型III：660下方存在低速異常.正演模擬所用的震源深度為552.0 km.Fig.5　Travel times and synthetic seismograms for different velocity modelsModel I: A high-velocity layer above the 660; Model II: A 15 km depression of the 660; Model III: A low-velocity anomaly below the 660. A source with the focal depth of 552.0 km is used in the forward modeling.

圖6　A子區(qū)域觀(guān)測(cè)波形(a)與擬合波形(b)及速度模型圖(c)(a)和(b)中的黑色和灰色實(shí)線(xiàn)分別是由IASP91和ENCC-A模型計(jì)算的理論走時(shí); (c)中黑色實(shí)線(xiàn)為IASP91速度模型，灰色實(shí)線(xiàn)為ENCC-A模型.Fig.6　Observed waveforms (a), synthetic waveforms (b) and velocity models (c) for sub-region ABlack and gray lines in (a) and (b) represent the travel times calculated with the IASP91 and ENCC-A models, respectively. Black and gray lines in (c) indicate the IASP91 and ENCC-A models, respectively.

圖7　B子區(qū)域觀(guān)測(cè)波形(a)與擬合波形(b)及速度模型圖(c)(a)和(b)中的黑色和灰色實(shí)線(xiàn)分別是由IASP91和ENCC-B模型計(jì)算的理論走時(shí).(c)中黑色實(shí)線(xiàn)為IASP91速度模型，灰色實(shí)線(xiàn)為ENCC-B模型.Fig.7　Observed waveforms (a), synthetic waveforms (b) and velocity models (c) for the sub-region BBlack and gray lines in (a) and (b) represent the travel times calculated with the IASP91 and ENCC-B models, respectively. Black and gray lines in (c) indicate the IASP91 and ENCC-B models, respectively.

圖8　C子區(qū)域觀(guān)測(cè)波形(a)與擬合波形(b)及速度模型圖(c)(a)和(b)中的黑色和灰色實(shí)線(xiàn)分別是由IASP91和ENCC-C模型計(jì)算的理論走時(shí); (c)中黑色實(shí)線(xiàn)為IASP91速度模型，灰色實(shí)線(xiàn)為ENCC-C模型.Fig.8　Observed waveforms (a), synthetic waveforms (b) and velocity models (c) for the sub-region CBlack and gray lines in (a) and (b) represent the travel times calculated with the IASP91 and ENCC-C models, respectively. Black and gray lines in (c) indicate the IASP91 and ENCC-C models, respectively.

B子區(qū)域三重震相的形態(tài)與A子區(qū)域類(lèi)似(圖7a)，C′D′震相與預(yù)測(cè)震相CD的相對(duì)時(shí)差較A區(qū)稍小.通過(guò)類(lèi)似方法，得到了B子區(qū)域最適速度模型ENCC-B(圖7b和7c).該模型表明，660上方存在115 km厚的高速層，P波速度異常最大值為1.5%；660下沉至675 km；660下方存在低速異常，P波速度降低0.6%；660附近的速度跳變?yōu)?.93%，小于IASP91模型的5.8%.

C子區(qū)域IASP91模型的預(yù)測(cè)震相OD與P波初至大致重合(圖8a).通過(guò)類(lèi)似方法，得到了C子區(qū)域最適速度模型ENCC-C(圖8b和8c).該模型表明660上方存在120 km厚的高速層，P波速度異常最大值為2.0%；660下沉至680 km；660下方不存在低速異常；660附近的速度跳變量為5.20%，小于IASP91模型的5.8%.

4討論與結(jié)論

我們選取了A子區(qū)域最適模型ENCC-A與前人的部分研究結(jié)果作對(duì)比：M3.11模型(Tajima and Grand, 1995, 1998)，NEChina-P模型(Ye et al., 2011; 葉玲玲和李娟, 2012; Li et al., 2013)及255-270模型(Wang和Niu (2010)一文中255°～270°方位子區(qū)域的模型).由圖9a可見(jiàn)，ENCC-A模型與其他模型在660之上均存在高速異常，但異常的厚度和大小存在一定差異；上述模型的不同之處是ENCC-A在660之下存在低速異常.

由圖9b—9d可見(jiàn)，M3.11模型的AB震相明顯早于觀(guān)測(cè)波形，尖端B震中距達(dá)29.4°(圖9b)，這可能由660上方的高速異常過(guò)大引起；OC震相稍滯后于觀(guān)測(cè)P波，反映出間斷面下沉幅度稍大.Li等(2013)在對(duì)P波到時(shí)分析中發(fā)現(xiàn)三重震相與參考模型的走時(shí)之間存在系統(tǒng)性偏差，而在NEChina-P模型的淺部加入了Fukao(1977)的速度結(jié)構(gòu)以消除偏差，在本文的研究中未觀(guān)測(cè)到系統(tǒng)性偏差(圖4)，因此采用了與Tajima和Grand(1998)類(lèi)似的方法將NEChina-P模型的走時(shí)曲線(xiàn)做了平移消除(圖9c).平移后的AB震相與觀(guān)測(cè)波形一致性較好；該模型中660為一梯度層，尖端C(14.7°)震中距明顯變大.255-270模型的AB震相滯后于觀(guān)測(cè)波形(圖9d)，說(shuō)明該模型中660之上的高速異常偏??；相比于觀(guān)測(cè)波形，CD震相起始震中距(15.9°)變大，這也與該模型中660為一梯度層有關(guān).M3.11、NEChina-P及255-270模型所對(duì)應(yīng)的OD震相與觀(guān)測(cè)P波初至的相對(duì)時(shí)差隨震中距變大而變大，不能很好地?cái)M合觀(guān)測(cè)波形.ENCC-A模型中660之下存在0.9%的低速異常，其對(duì)應(yīng)的OD震相可大致擬合觀(guān)測(cè)P波.

圖9　ENCC-A模型與前人部分速度模型的走時(shí)對(duì)比圖(a)中灰色實(shí)線(xiàn)為IASP91模型，黑色虛線(xiàn)為ENCC-A模型，紫色實(shí)線(xiàn)為M3.11模型，紅色實(shí)線(xiàn)為NEChina-P模型，藍(lán)色實(shí)線(xiàn)為255-270模型.(b)、(c)和(d)中黑色實(shí)線(xiàn)是B子區(qū)域觀(guān)測(cè)波形， 黑色虛線(xiàn)為ENCC-A的走時(shí)曲線(xiàn)，紫色、紅色及藍(lán)色實(shí)線(xiàn)分別對(duì)應(yīng)M3.11、NEChina-P、255-270模型的理論走時(shí)，紅色虛線(xiàn)是將NEChina-P的走時(shí)向左平移1.4 s后的走時(shí)曲線(xiàn).Fig.9　Travel-time comparison of ENCC-A model and velocity models in previous work(a) IASP91, ENCC-A, M3.11, NEChina-P and 255-270 models are shown in gray, black dashed, purple, red and blue lines, respectively; (b), (c) and (d) Black lines represent the observed waveforms of sub-region B, the travel times calculated with ENCC-A, M3.11, NEChina-P and 255-270 models are shown in black dashed, purple, red and blue lines, and the red dashed line represent the travel time of NEChina-P after the left shift of 1.4 s.

本文研究結(jié)果顯示華北克拉通東部660之上存在厚度約115～120 km的高速層，P波速度升高1.5%～2.0%.根據(jù)前人三重震相的相關(guān)研究結(jié)果(140±20 km)(Tajima and Grand, 1995; Wang and Niu, 2010; Zhang et al., 2012; Li et al., 2013)，該高速異常應(yīng)為滯留的太平洋俯沖板塊.所探測(cè)到的滯留板塊厚度明顯大于俯沖大洋板塊的厚度(76～77 km) (周春銀等, 2010)，滯留板塊在地幔轉(zhuǎn)換帶內(nèi)發(fā)生折曲(Zou et al., 2008; Tang et al., 2014)和內(nèi)部變形(Schmid et al., 2002; Fukao et al., 2009)是板塊增厚可能的因素.利用660 km深度附近的地幔熱膨脹系數(shù)?lnVP/?T=-6.5×10-5K-1(Cammarano et al., 2003)，轉(zhuǎn)換帶內(nèi)滯留板塊的高速異常所引起的理論低溫異常為230～300 K；利用負(fù)的克拉伯龍斜率-2.8 MPa/K(Hirose, 2002)和地幔壓力梯度參數(shù)35 MPa/km(Fossen,2010)，理論低溫異常所對(duì)應(yīng)660的下沉幅度約18～24 km.本文所探測(cè)到華北克拉通東部地區(qū)660下沉約15～20 km，與理論值大致相當(dāng).冷的滯留板塊導(dǎo)致地幔轉(zhuǎn)換帶出現(xiàn)低溫環(huán)境，從而出現(xiàn)了660的下沉(Lebedev et al., 2002; Tang et al., 2014).

此外，模型ENCC-A和ENCC-B顯示該區(qū)域660下方存在P波低速異常，P波速度降低0.6%～0.9%；ENCC-C顯示660之下不存在低速異常，這表明低速異?？赡苁蔷植看嬖诘?我們選取了Huang和Zhao(2006)一文中圖8a和圖10a，10b作為對(duì)照，分別為43°N、41°N及39°N的層析成像剖面圖.43°N和41°N剖面主要穿過(guò)了A、B子區(qū)域P波射線(xiàn)的拐點(diǎn)(圖1)，且均顯示在滯留板塊下方存在低速異常，橫向經(jīng)度范圍約120°E—130°E，深度范圍約660～1000 km，P波低速異常約0.5%～1.0%；39°N剖面主要穿過(guò)了C子區(qū)域P波射線(xiàn)的拐點(diǎn)(圖1)，顯示滯留板塊下方不存在低速異常.由此可見(jiàn)，A、B子區(qū)域低速異常與層析成像結(jié)果類(lèi)似，但其分布位置(115°E—128°E)偏西約2°～5°.利用A、B子區(qū)域尖端B′的震中距(25.5°，25.0°)可確定三重震相能相對(duì)準(zhǔn)確約束的最大深度分別為777 km和772 km(圖6c和7c)，徑向深度探測(cè)范圍相對(duì)有限.在震中距大于尖端B′之后，僅C′D′震相存在，三重震相不能較好地約束更深范圍(至821～833 km)的低速異常.結(jié)合25°～35°震中距觀(guān)測(cè)P波初至的變化趨勢(shì)和層析成像結(jié)果(Huang and Zhao, 2006)，滯留板塊下方的低速異?？赡艽嬖谙鄬?duì)較深的展布.

相比于高水容量的地幔轉(zhuǎn)換帶(Bercovici and Karato, 2003)，下地幔礦物的儲(chǔ)水能力相對(duì)較小(Murakami et al., 2002; Ohtani, 2005).俯沖板塊在地幔轉(zhuǎn)換帶底部經(jīng)過(guò)相變后會(huì)變得更為致密(van Mierlo et al., 2013)，超水相B分解所釋放的水被壓入下地幔(Ohtani et al., 2001; Yuen et al., 2007; Fukao et al., 2009)；由于滯留板塊在本文研究區(qū)域地幔轉(zhuǎn)換帶內(nèi)是普遍存在的，在滯留板塊下方可形成局部相對(duì)封閉的環(huán)境，而板塊底部脫水所釋放的水可賦存于下地幔頂部.與此同時(shí)，因坍塌進(jìn)入下地幔的滯留體中水相D的分解也可能對(duì)這部分低速有貢獻(xiàn).本文所探測(cè)到的低速異常可能與滯留板塊從其底部向下地幔頂部脫水或坍塌進(jìn)入下地幔深處滯留體的脫水有關(guān).

此前，Wang和Chen(2009)認(rèn)為亞洲東北部的南千島地區(qū)660下方的低速異常可能與下地幔上升熱柱有關(guān)；本文所涉及華北克拉通東部地區(qū)的層析成像結(jié)果(Huang and Zhao, 2006)顯示該區(qū)域滯留板塊下方的低速異常并未源于下地幔深部.因此，該低速異?？赡懿皇窍碌蒯Ｉ畈康臒嵩此?Zhao等(2012)提出西太平洋板塊下方的低速異?？赡芘c板塊深俯沖及部分板塊碎片崩塌所引起的地幔熱物質(zhì)的上涌有關(guān).如果該低速異常與熱物質(zhì)上涌有關(guān)，那么利用700 km深度附近的地幔熱膨脹系數(shù)?lnVP/?T=-2.8×10-5K-1(Cammarano et al., 2003)換算得出高溫異常約為251～323 K.關(guān)于下地幔頂部低速異常的來(lái)源或者成因，值得更進(jìn)一步的研究和探討.

需要指出，我們研究剖面上的少量臺(tái)站記錄與三重震相理論走時(shí)和波形擬合上存在偏差，如A子區(qū)域DCD、WUH臺(tái)，B子區(qū)域HSH、YULG臺(tái)等，這些可能與深部小尺度散射體的存在有關(guān)，未來(lái)在臺(tái)站分布和地震事件可能的情況，可以進(jìn)一步開(kāi)展二維或三維波形模擬的研究.

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(本文編輯何燕)

Seismic detection of a low-velocity anomaly under the stagnant slab beneath the eastern North China Craton with P-wave triplication

CUI Hui-Hui1,2, ZHOU Yuan-Ze1,2*, SHI Yao-Lin1,2, WANG Xiao-Ran1,2, LI Guo-Hui1,2

1KeyLaboratoryofComputationalGeodynamics,ChineseAcademyofSciences,Beijing100049,China2CollegeofEarthScience,UniversityofChineseAcademyofSciences,Beijing100049,China

AbstractThe broadband P waveforms from a deep earthquake of 10 May 2011 at the border of Northeast China and Russia, recorded by the China Digital Seismograph Network, are used to study the P-wave velocity structure around the 660 km discontinuity beneath the eastern North China Craton. Through relative travel-time fitting of P-wave triplication using the 1D raytracing method and comparison between the observed and synthetic seismograms, we find a ～15～20 km depression of the 660 km discontinuity. A high-velocity layer is revealed above the 660 km discontinuity with the thickness of ～115～120 km and velocity increase of 1.5%～2.0%, and the layer should be the stagnant slab of the Pacific Plate. A local low-velocity anomaly is also revealed below the 660 km discontinuity with the velocity decrease of 0.6%～0.9%. The low-velocity anomaly is possibly related with the dehydration of the stagnant slab from its bottom to the top of lower mantle or the dehydration of the slab fragments that have collapsed into the deep lower mantle, or probably associated with the hot material upwelling induced by the slab deep subduction and the collapse of slab fragments.

KeywordsP-wave triplication; 660 km discontinuity; Low-velocity anomaly; The top of the lower mantle; Slab dehydration

基金項(xiàng)目國(guó)家自然科學(xué)基金(41274092，41474070)資助.

作者簡(jiǎn)介崔輝輝，博士研究生，主要從事地球深部結(jié)構(gòu)及地震學(xué)方面的研究.E-mail:cuihuihui12@mails.ucas.ac.cn E-mail:yzzhou@ucas.ac.cn;yzzhou@gmail.com

*通訊作者周元澤，教授，主要從事地球內(nèi)部結(jié)構(gòu)與地震波傳播以及地震信號(hào)分析等方面的教學(xué)科研工作.

doi:10.6038/cjg20160413 中圖分類(lèi)號(hào)P315 致謝中國(guó)地震局地球物理研究所國(guó)家數(shù)字測(cè)震臺(tái)網(wǎng)數(shù)據(jù)備份中心(10.7914/SN/CB)和美國(guó)地震學(xué)研究聯(lián)合會(huì)數(shù)據(jù)管理中心為本研究提供了地震波形數(shù)據(jù)，李娟研究員和王寶善研究員提供了中國(guó)東北地區(qū)地幔轉(zhuǎn)換帶的研究結(jié)果，審稿人對(duì)文章提出了寶貴的修改意見(jiàn)和建議，在此一并表示感謝.

收稿日期2015-06-02， 2015-12-22收修定稿

崔輝輝， 周元澤， 石耀霖等. 2016. 華北克拉通東部滯留板塊下方低速異常的地震三重震相探測(cè).地球物理學(xué)報(bào)，59(4):1309-1320,doi:10.6038/cjg20160413.

Cui H H, Zhou Y Z, Shi Y L, et al. 2016. Seismic detection of a low-velocity anomaly under the stagnant slab beneath the eastern North China Craton with P-wave triplication.ChineseJ.Geophys. (in Chinese),59(4):1309-1320,doi:10.6038/cjg20160413.