武振波， 徐濤， 武澄瀧， 張明輝，田小波， 滕吉文

1 中國科學(xué)院地質(zhì)與地球物理研究所，巖石圈演化國家重點(diǎn)實(shí)驗(yàn)室， 北京　100029 2 中國科學(xué)院大學(xué)， 北京　100049 3 中國科學(xué)院青藏高原地球科學(xué)卓越創(chuàng)新中心， 北京　100101

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利用接收函數(shù)反演青藏高原西部地殼S波速度結(jié)構(gòu)

武振波1,2， 徐濤1,3*， 武澄瀧1,2， 張明輝1,2，田小波1,3， 滕吉文1

1 中國科學(xué)院地質(zhì)與地球物理研究所，巖石圈演化國家重點(diǎn)實(shí)驗(yàn)室， 北京100029 2 中國科學(xué)院大學(xué)， 北京100049 3 中國科學(xué)院青藏高原地球科學(xué)卓越創(chuàng)新中心， 北京100101

摘要相對(duì)于寬闊的腹地，青藏高原西部南北向?qū)挾葍H約600 km，卻記錄了印度和歐亞板塊匯聚的深部過程及其響應(yīng).本文用22臺(tái)寬頻帶流動(dòng)地震臺(tái)站在西緣構(gòu)建了一條南北向探測(cè)剖面(～80°E，TW-80試驗(yàn)).利用接收函數(shù)反演剖面下方S波速度結(jié)構(gòu)，綜合西部已有的寬頻帶探測(cè)結(jié)果，分析認(rèn)為：印度板塊向北俯沖可能已到達(dá)班公湖—怒江縫合帶附近，俯沖過程中下地殼發(fā)生榴輝巖化；喀拉昆侖斷裂帶、班公湖—怒江縫合帶、阿爾金斷裂帶均為切穿地殼的深斷裂，莫霍面發(fā)生錯(cuò)斷；喀拉昆侖斷裂帶和龍木錯(cuò)斷裂帶之間的中上地殼沒有發(fā)現(xiàn)連續(xù)的S波低速體，說明可能缺乏解耦層，支持青藏高原西部地殼為整體縮短增厚模式.

關(guān)鍵詞青藏高原； 接收函數(shù)； S波速度結(jié)構(gòu)； 印度板塊； 莫霍面

The receiver function method was used to study the crustal shear-wave velocity structure along the survey line. Firstly, we selected the similar receiver functions for each station by discarding the weird waves, then stacked the rest of results to obtain waves of high signal-to-noise ratio. Secondly, the inversion was operated with the Neighborhood algorithm.

The Neighborhood algorithm made use of the geometrical constructs known as Voronoi cells to derive the search in the parameter space. It is known that this algorithm produces a self-adaptive search behavior. The inversion results of all stations are shown as a 2D image. As we expected, the shear-wave velocity structure along the survey line coincides with the migration of common conversion point (CCP) of the receiver functions. The earthquake ofMS>4.0 mainly happened beneath the Karakorum fault zone and the Longmu Co fault zone, which are featured by low shear-wave velocity in the upper-middle crust at both ends of the profile.

The subduction of Indian plate under the Tibetan plateau probably reaches the Bangong-Nujiang suture based on the comprehensive analysis in this study and the previous work on the western Tibetan plateau. The Karakorum fault zone, Bangong-Nujiang suture and the Altyn-Tagh fault zone cut through the whole crust leading to the Moho offset. The absence of low shear-wave velocity in the upper-middle crust between the Karakorum fault zone and the Longmu Co fault zone probably indicates the lack of the decoupling layer, which supports the assumption that western Tibet has been experienced crustal shortening and thickening.

1引言

印度和歐亞板塊在50 Ma發(fā)生匯聚碰撞(Molnar & Tapponnier, 1975; Rowley, 1996)，造就了地球上最宏偉的高原——“青藏高原”，為我們研究陸陸碰撞過程和高原隆升機(jī)制提供了最佳實(shí)驗(yàn)場(chǎng).目前，青藏高原西部主要的地球物理探測(cè)剖面如圖1a所示，包括中法合作探測(cè)剖面(Wittlinger et al., 2004)、“Hi-CLIMB”計(jì)劃(Nábělek et al., 2009)、“ANTILOPE”計(jì)劃(Zhao et al., 2010，2014)及南側(cè)印度平原兩條剖面(Caldwell et al., 2013; Rai et al., 2006).

印度板塊在青藏高原下的俯沖位置歷來是地球科學(xué)研究的熱點(diǎn)問題.遠(yuǎn)震體波研究顯示在青藏高原下方，印度板塊俯沖距離存在東西向差異(Zhao et al., 2010; Li et al., 2008).橫波分裂研究可能為此提供了一種解釋：印度巖石圈板片向北俯沖過程中，俯沖角度東陡西緩，導(dǎo)致俯沖距離東近西遠(yuǎn)，板片發(fā)生撕裂和斷離(Chen et al., 2015).關(guān)于高原內(nèi)部各塊體邊界處的地表斷裂在地下空間如何展布也尚未達(dá)成共識(shí)，它們與地殼變形密切相關(guān).為此，張忠杰等在青藏高原西部開展了人工源和天然源聯(lián)合地震探測(cè)工作(TW-80試驗(yàn)，Zhang et al., 2014; Wu et al., 2015).綜合已有的觀測(cè)剖面，共組成4條地球物理探測(cè)走廊，如圖1a所示.TW-80試驗(yàn)接收函數(shù)研究揭示該剖面塊體邊界下方莫霍面存在錯(cuò)斷，中法合作剖面接收函數(shù)和層析成像結(jié)果顯示莫霍面在班公湖—怒江縫合帶錯(cuò)斷～10 km，羌塘塊體下方加深至90 km，北至阿爾金斷裂帶突然抬升為50～60 km(Wittlinger et al., 2004).而INDEPTH深部地震探測(cè)結(jié)果顯示雅魯藏布江縫合帶和班公湖—怒江縫合帶下方?jīng)]有莫霍層錯(cuò)動(dòng)(趙文津等，2008；Kind et al., 2002)，表明青藏高原東西部地殼變形方式存在明顯差異.為研究西部地殼變形方式，我們?cè)诮邮蘸瘮?shù)研究的基礎(chǔ)上進(jìn)一步反演S波速度，分析殼內(nèi)速度結(jié)構(gòu)特征.

2研究區(qū)構(gòu)造背景

TW-80試驗(yàn)在青藏高原西緣80°E附近構(gòu)建了一條南北向剖面，實(shí)施人工源寬角地震和天然源寬頻帶地震聯(lián)合探測(cè)(Zhang et al., 2014).寬頻帶臺(tái)站位置如圖1所示，共22臺(tái)地震儀(Reftek-72A 數(shù)據(jù)采集器，周期范圍50Hz-30/60s CMG3-ESP 地震儀)，臺(tái)間距15～20 km，剖面全長(zhǎng)400多公里，記錄時(shí)間從2011年11月至2013年11月，采樣頻率40 Hz.圖1b中顯示剖面橫跨數(shù)個(gè)構(gòu)造單元：甜水海塊體、羌塘塊體、拉薩塊體及喜馬拉雅塊體.塊體之間以縫合帶或斷裂帶為界，構(gòu)造線走向均呈北西向，都可以和高原東部的主要構(gòu)造帶相接.此地印度和歐亞板塊碰撞十分強(qiáng)烈，發(fā)育大型走滑喀拉昆侖斷裂和龍木錯(cuò)斷裂，數(shù)條逆沖斷裂，及新生代拉分盆地.由北向南出露三條蛇綠巖帶：班公湖—怒江蛇綠巖帶、獅泉河蛇綠巖帶和雅魯藏布江蛇綠巖帶，均為特提斯洋演化的產(chǎn)物(Lacassin et al., 2004).

圖1　研究區(qū)域和寬頻帶流動(dòng)臺(tái)站位置(a) 紅色三角形為TW-80試驗(yàn)寬頻帶地震臺(tái)站，不同顏色菱形代表其他研究者布置的寬頻帶臺(tái)站; (b) 紫色圓圈代表1900—2015年發(fā)生的MS>4.0 級(jí)以上地震，右上角紅色圓圈是本文所用地震事件.HB：喜馬拉雅塊體；LB：拉薩塊體；QB：羌塘塊體；TSH：甜水海塊體；SG：松潘—甘孜塊體；Qaidm：柴達(dá)木盆地；Tarim：塔里木盆地；MBT：主邊界逆沖斷裂；MCT：主中央逆沖斷裂；IYS：雅魯藏布江縫合帶；BNS：班公湖—怒江縫合帶；JS：金沙江縫合帶；AKMS：阿尼瑪卿縫合帶；LMF：龍木錯(cuò)斷裂；DWT：多瑪—烏江逆沖斷裂；MT：曼冬錯(cuò)北逆沖斷裂；SF：獅泉河逆沖斷裂；KF：喀喇昆侖斷裂. Zhada：札達(dá)縣；Shiquanhe：獅泉河地區(qū)； Rutog：日土縣；Domar：多瑪鄉(xiāng).Fig.1　Study area and passive-source broad-band seismic stations(a) Red triangles denote broad-band seismic stations used in this study, the other coloured diamonds denote previous broad-band seismic stations; (b) The purple circles represent the seismic events MS>4.0 between 1900 and 2015 year. The red circles in the right-corner denote the seismic events used to compute the receiver functions. HB: Himalaya block; LB: Lhasa block; QB: Qiangtang block; TSH: Tianshuihai block; SG: Songpan-Garzê block; Qaidm: Qaidm Basin; Tarim: Tarim Basin; MBT: Main Boundary Thrust of the Himalayan system; MCT: Main Central Thrust of the Himalayan system; IYS: Yarlung Zangbo River suture; BNS: Bangong-Nujiang suture; JS：Jinsha River suture；AKMS: A′nyêmaqên-Kunlun-Mustagh suture; LMF: Longmu Co fault; DWT: Domar-Wujiang thrust; MT: Mandong-Cuobei thrust; SF: Shiquanhe fault; KF: Karakorum fault. Zhada: Zanda county; Shiquanhe: Shiquanhe city; Rutog: Rutog county; Domar: Domar country.

甜水海塊體和羌塘塊體以龍木錯(cuò)斷裂隔開，該斷裂帶SW走向，分布火山巖(李海兵等，2006).羌塘塊體和拉薩塊體以班公湖—怒江縫合帶為界，研究區(qū)內(nèi)是該縫合帶西段.很多地球物理研究認(rèn)為印度板塊巖石圈地幔已俯沖到該邊界附近(Zhao et al., 2010; Kind and Yuan, 2010;余大新等, 2014).南端雅魯藏布江縫合帶是拉薩塊體和喜馬拉雅塊體的分界線，為印度和歐亞兩大板塊于晚侏羅—白堊紀(jì)匯聚后，殘留的新特提斯洋洋殼和巖石圈(Tapponnier et al., 1981).其北部是右旋走滑喀拉昆侖斷裂帶，圖1b中可看到印度河(印度境內(nèi))—獅泉河(中國境內(nèi))被右行錯(cuò)開約120 km (李海兵等，2008).兩側(cè)發(fā)育新生代盆地：札達(dá)盆地和噶爾盆地，其中札達(dá)盆地新生代地層厚約750 m，磁性地層學(xué)研究表明該盆地受喀拉昆侖斷裂活動(dòng)控制(王世鋒等，2008).

3研究方法

3.1接收函數(shù)計(jì)算

利用接收函數(shù)研究地殼上地幔及地幔過渡帶已成為一種重要的地球物理探測(cè)手段(Kind et al., 2002; Chen and Ai, 2009;Tian et al., 2005a,b; Ai et al., 2007; Shi et al.,2009; 吳慶舉和曾融生，1998；劉啟元等，1996；李永華等，2006).本文數(shù)據(jù)處理包含兩步：計(jì)算單個(gè)臺(tái)站疊加接收函數(shù)，反演臺(tái)站下方S波速度.我們采用時(shí)域迭代反褶積算法計(jì)算接收函數(shù)(Ligorría and Ammon, 1999)，遠(yuǎn)震事件的震中距為30°～90°，回折點(diǎn)位于下地幔，可有效避開上地幔過渡帶三重相或地核影區(qū)的影響(?akr et al., 2000).為了提高反演結(jié)果的穩(wěn)定性，計(jì)算接收函數(shù)時(shí)采用兩種高斯濾波器——濾波系數(shù)分別為1.0和2.5，對(duì)應(yīng)截止頻率0.5 Hz、1.2 Hz.由于事件分布方位不均勻，為了提高信噪比，壓制橫向不均勻性，以臺(tái)站為中心將方位角劃分為不同小區(qū)域(每10°劃為一個(gè)網(wǎng)格)，小區(qū)域內(nèi)不同地震事件的接收函數(shù)進(jìn)行疊加，剔除明顯異常的結(jié)果，把剩下的不同區(qū)域疊加結(jié)果再相加，用最終的高信噪比疊加波形進(jìn)行反演.圖2a是AL04臺(tái)挑選出的接收函數(shù)，對(duì)應(yīng)高斯系數(shù)1.0、2.5，最上面為疊加波形，可以看到信噪比很高，Moho面產(chǎn)生的轉(zhuǎn)換震相PmS非常清晰.每個(gè)臺(tái)站做類似處理，用兩個(gè)疊加接收函數(shù)反演S波速度.圖2b為單個(gè)臺(tái)站對(duì)應(yīng)的兩種疊加波形，其中AL08位于獅泉河斷裂帶上(圖1)，接收函數(shù)信噪比低，我們用鄰近臺(tái)站接收函數(shù)的平均值來代替.

3.2接收函數(shù)反演

接收函數(shù)反演是很強(qiáng)的非線性反演問題，目前主要有兩種處理方法：一種是線性化反演方程，轉(zhuǎn)化為線性反演問題；另一種是直接進(jìn)行非線性反演.對(duì)于接收函數(shù)線性化反演，Ammon等(1990)的研究最具代表性，由于采用Randall算法計(jì)算微分地震圖和跳躍反演技術(shù)，效率極高而被地球物理學(xué)家廣泛采用(徐鳴潔等，2005；彭恒初等，2012；劉啟民等，2014；Rai et al., 2006).Ammon等研究表明，線性反演強(qiáng)烈依賴初始模型.為克服此缺陷，很多全局優(yōu)化算法被引入該領(lǐng)域，如模擬退火(Sen and Stoffa, 1991)、遺傳算法(Sambridge and Drijkoningen, 1992)等.此外還有一些新方法，劉啟元等(1996)根據(jù)Tarantola (1987) 的非線性反演理論，提出接收函數(shù)徑向與垂向分量復(fù)譜比的非線性反演.Julià等(2000)、胡家富等(2005)進(jìn)一步發(fā)展了接收函數(shù)和面波頻散聯(lián)合反演技術(shù).

圖2　單個(gè)臺(tái)站的接收函數(shù)(a) AL04臺(tái)挑選出的兩種接收函數(shù)，最上面為疊加波形，左側(cè)為各接收函數(shù)相應(yīng)的射線參數(shù)；(b) 單個(gè)臺(tái)站兩種疊加接收函數(shù)，分別對(duì)應(yīng)高斯系數(shù)2.5和1.0.Fig.2　The selected receiver functions for each station(a) The receiver functions for the station AL04 filtered by the Gaussian filter 1.0 and 2.5, respectively. The top panels are the stacked results. The left panel gives the corresponding ray parameter for every single receiver function; (b) Two stacked results for each station, corresponding with the Gaussian filter 1.0 and 2.5, respectively.

本文采用相鄰算法(Neighbourhood algorithm，簡(jiǎn)稱NA 算法)進(jìn)行接收函數(shù)反演.NA算法是Sambridge (1999a, 1999b) 基于非線性反演理論提出的一種全局算法，具有較強(qiáng)的自適應(yīng)搜索能力，用一組模型集代替搜索全局最優(yōu)解，可避免反演解陷入目標(biāo)函數(shù)的局部最小域，在地球物理反演中得到廣泛應(yīng)用(Sherrington et al., 2004; Bannister et al., 2004; Hetényi et al., 2006; Snoke and Sambridge, 2002; 賀傳松等，2004).算法引入幾何學(xué)概念——“維諾圖”.在d維模型空間中給定一組隨機(jī)采樣點(diǎn)ns，這些離散點(diǎn)將模型空間分割成ns個(gè)區(qū)域，稱為維諾單元(Voronoi cell)，每個(gè)維諾單元內(nèi)的點(diǎn)到該采樣點(diǎn)的距離都是最近的，用L2準(zhǔn)則測(cè)算.對(duì)每個(gè)維諾單元構(gòu)建失配函數(shù)，從失配值最低的nr個(gè)區(qū)域重采樣，自適應(yīng)實(shí)現(xiàn)高失配、低失配區(qū)域的稀少采樣和密集采樣.

模型空間內(nèi)的地層包含如下參數(shù)：地層厚度、頂面S波速度、底面S波速度及層內(nèi)P-S波速度比，層內(nèi)速度為線性變化.圖3是兩個(gè)模型實(shí)例.圖3a、3b對(duì)應(yīng)模型1，模型參數(shù)及反演搜索空間見表1.圖3b中黑實(shí)線為合成的含噪聲接收函數(shù)(采樣頻率25 Hz，記錄時(shí)長(zhǎng)30 s，共876個(gè)數(shù)據(jù)點(diǎn))，相當(dāng)于觀測(cè)值，紅實(shí)線為最佳反演結(jié)果合成的接收函數(shù).可見，NA算法可以很好重建接收區(qū)的地殼速度結(jié)構(gòu).本研究區(qū)構(gòu)造復(fù)雜，實(shí)際計(jì)算得到的接收函數(shù)除了Moho面的轉(zhuǎn)換震相PmS外，還含有多個(gè)殼內(nèi)間斷面轉(zhuǎn)換震相，致使PmS震相模糊或消失.因此，我們引入含沉積層、結(jié)晶基底的模型2，與圖3c、3d相對(duì)應(yīng)，模型參數(shù)及反演搜索空間見表2.結(jié)果顯示，此方法可以很好擬合殼內(nèi)轉(zhuǎn)換波震相.

圖3　接收函數(shù)反演模型實(shí)例(a)(b)對(duì)應(yīng)模型1，(c)(d)對(duì)應(yīng)模型2.(b)(d)中黑實(shí)線為合成的含噪聲接收函數(shù)，紅實(shí)線為最佳反演結(jié)果合成的理論值；(a)(c)中兩條虛線指示模型搜索空間，黑實(shí)線為設(shè)定的真實(shí)地球模型，灰色區(qū)域?yàn)閿M合度高的1000個(gè)反演模型集， 紅實(shí)線為最佳結(jié)果，白實(shí)線為1000個(gè)模型集的平均值.Fig.3　Two model examples of the NA algorithm to obtain the S-wave velocities(b)(d)The black line denotes the input receiver function, the red line denotes the synthetics based on the best data-fitting inversion result；(a)(c)The black dashed lines denote the parameter space, the gray lines denote the 1000 acceptable inversion results, the red line denotes the best data-fitting model, the white line denotes the average of the ensemble of 1000 models.

表1　模型1(上)及模型空間參數(shù)(下)

表2　模型2(上)及模型空間參數(shù)(下)

4S波速度反演結(jié)果

利用上述NA算法反演單個(gè)臺(tái)站下方S波速度.為增強(qiáng)反演結(jié)果可對(duì)比性，設(shè)定相同的模型搜索空間：沉積層、結(jié)晶基底、上地殼、中地殼和下地殼，模型參數(shù)見表3.針對(duì)反演過程的參數(shù)設(shè)置為ns=100，nr=20，迭代次數(shù)為500，共獲得50100個(gè)反演模型，取失配值最小的1000個(gè)模型計(jì)算平均值作為最終解.圖4為臺(tái)站AL00、AL1011和AL21的反演結(jié)果.高斯系數(shù)2.5計(jì)算的接收函數(shù)含更多高頻成分，除了來自莫霍面的轉(zhuǎn)換震相，還包含多個(gè)殼內(nèi)震相，而1.0計(jì)算的接收函數(shù)PmS信噪比很高，卻損失了殼內(nèi)探測(cè)精度，兩者相互約束可以提供更豐富的速度信息.三個(gè)臺(tái)站橫跨喜馬拉雅塊體、拉薩塊體和羌塘塊體，PmS-P波的到時(shí)差從～7.2 s(AL00)增至～9.5 s(AL21)，說明由南向北莫霍面逐漸加深，反演結(jié)果與之一致(圖7).圖5是所有臺(tái)站接收函數(shù)與波形擬合結(jié)果.

表3　反演設(shè)定的模型空間

圖6為本文反演的單個(gè)臺(tái)站S波速度.圖7為剖面下方二維S波速度結(jié)構(gòu).TW-80試驗(yàn)用接收函數(shù)方法系統(tǒng)研究了該剖面下方的殼內(nèi)結(jié)構(gòu)和莫霍面形態(tài)，為便于結(jié)果對(duì)比，我們把接收函數(shù)疊加成像的結(jié)果投影在該速度圖上.據(jù)觀察，莫霍面與速度值4.0的等值線趨勢(shì)大致吻合，研究區(qū)西側(cè)Rai 等(2006)反演結(jié)果類似.剖面下方莫霍面平均深～70 km，拉薩塊體下方50 km 附近存在S波速度為4.0～4.1 km·s-1的局部高速體.從地震活動(dòng)(圖1b中紫色圓圈)的分布來看，地震主要集中于該剖面兩端.南端喀拉昆侖斷裂帶下方，震源深度達(dá)53 km，北端龍木錯(cuò)斷裂帶下方，震源深度達(dá)60 km，表明這兩條走滑斷裂至少向下延伸到50～60 km.斷裂活動(dòng)在其周圍形成的剪切破碎區(qū)域在S波速度圖上表現(xiàn)為低速特征，對(duì)應(yīng)剖面兩端中上地殼的低速異常,而兩條斷裂帶之間的中上地殼缺乏連續(xù)低速異常分布.

圖8為研究區(qū)莫霍面結(jié)構(gòu)及其橫向變化，測(cè)線位置見圖1a.西側(cè)○a線綜合Rai等(2006)和Wittlinger等(2004)北段剖面的接收函數(shù)結(jié)果，○b線由本文剖面、Caldwell等(2013)觀測(cè)位置及Wittlinger等(2004)觀測(cè)剖面的中段組成，○c、○d線分別為Zhao等(2010)、Nábělek等(2009)接收函數(shù)結(jié)果.可見，莫霍面南北向變化趨勢(shì)相似，從南端印度板塊～40 km向北逐漸加深，羌塘塊體達(dá)～80 km，北至阿爾金斷裂帶下方又抬升至～60 km.整體來看，青藏高原西部主要塊體邊界切穿地殼，莫霍面發(fā)生錯(cuò)斷(如喀拉昆侖斷裂、班公湖—怒江縫合帶和阿爾金斷裂，圖8中黑色虛線).圖7速度結(jié)構(gòu)顯示拉薩塊體下方～50 km處存在不連續(xù)高速間斷面，該剖面接收函數(shù)結(jié)果在此深度范圍有相應(yīng)的轉(zhuǎn)換波震相(Zhang et al., 2014)，綜合南側(cè)Caldwell等(2013)接收函數(shù)結(jié)果，推測(cè)此為MHT滑脫面向北延伸.Hi-CLIMB探測(cè)剖面(～85°E)觀測(cè)到拉薩塊體南部也存在一組殼內(nèi)轉(zhuǎn)換波震相(Nábělek et al., 2009)，往東接收函數(shù)研究顯示青藏高原南部下地殼存在速度間斷面(Schulte-Pelkum et al., 2005)，結(jié)合本研究結(jié)果，推測(cè)該間斷面下方為高速異常.綜合分析認(rèn)為，印度板塊在青藏高原下的俯沖可能已到達(dá)班公湖—怒江縫合帶附近，向北俯沖過程中印度下地殼發(fā)生榴輝巖化，根據(jù)藏南地區(qū)大地電磁結(jié)果推測(cè)由于不同部位含水量差異致使榴輝巖相變程度有所不同(金勝等，2007；魏文博等，2009).

圖4　臺(tái)站AL00、AL1011、AL21的接收函數(shù)反演結(jié)果左圖均為模型搜索空間及擬合度高的1000個(gè)模型集，線條含義參考圖3.右圖黑實(shí)線為實(shí)際觀測(cè)值，紅實(shí)線為擬合最好的反演模型合成的理論接收函數(shù)，上面波形對(duì)應(yīng)高斯系數(shù)2.5，下面波形對(duì)應(yīng)高斯系數(shù)1.0.Fig.4　The inversion results of the AL00，AL1011，AL21 stationsThe left panel denotes the parameter space and the ensemble of good data-fitting 1000 models, the meaning of the colorful lines is similar with the Fig.3. The right panel denotes the observed receiver functions (the black line) and the synthetics (the red line)obtained from the best data-fitting inversion result.

圖5　單個(gè)臺(tái)站接收函數(shù)反演與波形擬合結(jié)果左圖黑線為單臺(tái)采用高斯系數(shù)1.0計(jì)算的疊加接收函數(shù)，紅線為擬合最佳的反演模型合成的理論值；右圖類似，對(duì)應(yīng)高斯系數(shù)2.5.Fig.5　The fitness between the synthetics obtained from the inversion result and the observed receiver functions for all stationsThe left panel denotes the stacked receiver functions with Gaussian filter 1.0 for each station, the black lines represent stacked result and the red lines represent the synthetics obtained from the best data-fitting inversion result. The right panel is similar, corresponding to the Gaussian filter 2.5.

圖6　單臺(tái)S波速度反演結(jié)果兩條虛線指示模型搜索空間，灰色區(qū)域?yàn)閿M合度高的1000個(gè)反演模型集，紅實(shí)線為最佳反演結(jié)果，而白實(shí)線為1000個(gè)模型集的平均值.Fig.6　The shear-wave velocity structure for each stationThe black dashed lines denote the parameter space, the gray lines denote the 1000 acceptable inversion results, the red line denotes the best data-fitting model, the white line denotes the average of the ensemble of 1000 models.

圖7　測(cè)線下方二維S波速度結(jié)構(gòu)白色圓圈為區(qū)域79°E—81°E，31°N—35°N范圍內(nèi)自1900年至2015年發(fā)生的4級(jí)以上地震(地震目錄來自USGS：http:∥earthquake.usgs.gov/earthquakes/search/)，殼內(nèi)速度間斷面和莫霍面結(jié)構(gòu)(黑線)是該剖面接收函數(shù)結(jié)果(Zhang et al., 2014).Fig.7　The shear-wave velocity map along the survey lineThe white circles denote the MS>4 earthquakes in the region 79°E—81°E，31°N—35°N, the crustal velocity discontinuity and the Moho structure plotted by black lines are derived from the previous receiver function study (Zhang et al., 2014).

圖8　研究區(qū)Moho結(jié)構(gòu)及其橫向變化, 剖面位置參看圖1aFig.8　The Moho structures based on the passive-source seismic survey lines shown in Fig.1a and the lateral variations in the study area

5結(jié)論

本文利用青藏高原西部TW-80試驗(yàn)寬頻帶流動(dòng)臺(tái)陣，在接收函數(shù)基礎(chǔ)上反演S波速度結(jié)構(gòu).綜合現(xiàn)有的青藏高原西部寬頻帶剖面探測(cè)結(jié)果認(rèn)為：

(1)喜馬拉雅塊體和拉薩塊體下方，S波速度結(jié)構(gòu)顯示下地殼存在10～20 km 厚的高速層，可能是印度下地殼向北俯沖過程中發(fā)生榴輝巖化所致；

(2)地震活動(dòng)集中在剖面兩端：喀拉昆侖斷裂帶和龍木錯(cuò)斷裂帶附近.南端喀拉昆侖斷裂帶下方震源深度達(dá)53 km，北端龍木錯(cuò)斷裂帶下方震源深度達(dá)60 km，表明兩者均為深斷裂，斷裂活動(dòng)造成的剪切破碎域致使剖面兩端中上地殼表現(xiàn)為低速異常；

(3)與青藏高原東北緣上地殼增厚模式(Tian and Zhang, 2013; Tian et al., 2014；Xu et al., 2014; Zhang et al., 2011)、東南緣下地殼流增厚模式(Royden et al., 1997; Clark et al., 2005)不同，喀拉昆侖斷裂帶和龍木錯(cuò)斷裂帶之間的中上地殼沒有觀測(cè)到連續(xù)的低速異常，說明可能缺乏解耦層，支持青藏高原西部地殼以整體縮短增厚模式為主，正在開展的人工源寬角地震資料和背景噪聲成像研究有望進(jìn)一步提供約束.

致謝謹(jǐn)以此文紀(jì)念中國科學(xué)院地質(zhì)與地球物理研究所張忠杰研究員(1964—2013).感謝中國科學(xué)院地質(zhì)與地球物理研究所地震實(shí)驗(yàn)室提供的寬頻帶流動(dòng)地震儀.對(duì)參加野外地震數(shù)據(jù)采集工作的中國科學(xué)院地質(zhì)與地球物理研究所梁曉峰副研究員、鄧陽凡、王敏玲、蘭海強(qiáng)博士等所有人員表示衷心的感謝.

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(本文編輯何燕)

基金項(xiàng)目中國科學(xué)院戰(zhàn)略性先導(dǎo)科技專項(xiàng)(XDB03010700)，中國地震局公益性行業(yè)科研專項(xiàng)(201408023)和國家自然科學(xué)基金(41174075，41274070，41374062，41474068)聯(lián)合資助.

作者簡(jiǎn)介武振波，女，1988年生，博士生，主要從事殼幔結(jié)構(gòu)地震成像研究. E-mail: wzb@mail.iggcas.ac.cn *通訊作者徐濤，男，1978年生，研究員，主要從事地震射線理論與殼幔結(jié)構(gòu)成像研究. E-mail: xutao@mail.iggcas.ac.cn

doi:10.6038/cjg20160211 中圖分類號(hào)P315

收稿日期2015-05-22，2016-01-05收修定稿

Crustal shear-wave velocity structure beneath the western Tibetan plateau revealed by receiver function inversions

WU Zhen-Bo1,2，XU Tao1,3*，WU Cheng-Long1,2，ZHANG Ming-Hui1,2，TIAN Xiao-Bo1,3，TENG Ji-Wen1

1StateKeyLaboratoryofLithosphericEvolution,InstituteofGeologyandGeophysics,ChineseAcademyofSciences,Beijing100029,China2UniversityofChineseAcademyofSciences,Beijing100049,China3CASCenterforExcellenceinTibetanPlateauEarthSciences,Beijing100101,China

AbstractThe collision of Indian and Eurasian plates is the most significant geological event on the Earth since Cenozoic era. How the subduction of the Indian plates occurs under the Tibetan plateau is one of the topics receiving much attention. Whether the boundaries between inner micro-blocks of the Tibetan plateau cut through the crust is another focused issue, which is of great significance for the deformation mechanism of the Tibetan plateau. To help address these issues, we performed a passive-source seismic survey profiling through the western Tibetan plateau.

KeywordsTibetan plateau; Receiver functions; Shear-wave velocity structure; Indian plate; Moho

武振波， 徐濤， 武澄瀧等. 2016. 利用接收函數(shù)反演青藏高原西部地殼S波速度結(jié)構(gòu).地球物理學(xué)報(bào)，59(2):516-527,doi:10.6038/cjg20160211.

Wu Z B, Xu T, Wu C L, et al. 2016. Crustal shear-wave velocity structure beneath the western Tibetan plateau revealed by receiver function inversions.ChineseJ.Geophys. (in Chinese),59(2):516-527,doi:10.6038/cjg20160211.