地球物理学报  2019, Vol. 62 Issue (5): 1633-1649   PDF    
帕米尔-兴都库什地区构造应力场反演及拆离板片应力形因子特征研究
崔华伟1, 万永革2, 黄骥超2, 盛书中2, 靳志同2,3     
1. 山东省地震局, 济南 250014;
2. 防灾科技学院, 河北三河 065201;
3. 中国地震局地球物理研究所, 北京 100081
摘要:从Global CMT目录搜集了1976年1月至2016年6月之间的震源深度大于70 km的255个震源机制解,用阻尼应力反演方法,分70~160 km和170~310 km两个深度,计算了帕米尔-兴都库什地区的构造应力场;同时以10 km为间隔计算了兴都库什地区深度介于70~310 km之间的应力形因子.得到以下初步结论:兴都库什板片向下俯冲和帕米尔地区断裂带的横向拉张,可能是导致应力场不同的原因.兴都库什俯冲带与帕米尔俯冲带碰撞,导致交汇地区(37°N-37.5°N)的应力场参数突变.兴都库什俯冲板片受到深部温度、压力等因素,出现薄弱面进而形成拆离板片.其脱离了主俯冲板片的束缚后,重力的上下拉张作用导致空区附近张轴倾伏角接近90°,拆离板片俯冲至上地幔不连续面,导致板片部分熔融进而应力形因子随着深度变小.而拆离板片受到地幔挤压其内部发生破碎,其压应力轴由西部的NS到东部NW-SE方向偏转及纵向张应力轴倾伏角变小.
关键词: 帕米尔-兴都库什      震源机制解      构造应力场      俯冲带      应力形因子     
Inversion for the tectonic stress field and the characteristic of the stress shape factor of the detachment slab in the Pamir-Hindu Kush area
CUI HuaWei1, WAN YongGe2, HUANG JiChao2, SHENG ShuZhong2, JIN ZhiTong2,3     
1. Shandong Earthquake Agency, Jinan 250014, China;
2. Institute of Disaster Prevention, Hebei Sanhe 065201, China;
3. Institute of Geophysics, China Earthquake Administration, Beijing 100081, China
Abstract: This work collected focal mechanism solutions of 255 events with depths larger than 70 km in the Pamir-Hindu Kush from Global CMT catalogue January 1976 to June 2016. By using the method of regional damped stress field inversion, the tectonic stress fields in the depths of 70~160 km and 170~310 km are determined, respectively. The stress shape factor is calculated with an interval 10 km in depth range 70~310 km beneath the Hindu Kush area. Results show that subduction of the detachment slab in Hindu Kush and the horizontal extension of fault zones under Pamir may cause different stress regimes in Pamir and Hindu Kush. The stress field parameters are dramatically changed in the collision area of the Pamir and Hindu Kush nearby 37°N-37.5°N. The detachment slab in the subduction slab may be formed due to the weakness resulted from the high temperature and high pressure. After the detachment slab separates from the main body, the extensional axis shows nearly vertical near the seismic gap under the gravity pulling of the detaching slab that dives to the discontinuity in upper mantle. Heated by high temperature, the detached slab is in partial melting state, resulting in decrease of the stress shape factor with depth. The interior of the detached slab would be broken under compression in upper-mantle, which makes the compressive axis rotate from NS in west to NW-SE in east and the dip of the extensional axis become smaller.
Keywords: Pamir-Hindu Kush    Focal mechanism solution    Tectonic stress field    Subduction zone    Stress shape ratios    
0 引言

帕米尔—兴都库什地区(图 1)是印度板块与欧亚板块碰撞产生的陆—陆俯冲带,位于青藏高原的西北,塔里木盆地的西缘,因其特殊的地质构造而备受关注.目前,研究学者关于构造应力场(Lukk and Yunga, 1988宁杰远和臧绍先,1990Lukk et al,1995孙文斌等,2009唐兰兰等,2012)、GPS(Zubovich et al., 2010Ischuk et al., 2013Zhou et al., 2016)、层析成像(Roecker,1982Van Der Voo et al., 1999雷建设等,2002Koulakov and Sobolev, 2006Li et al., 2008Sippl et al., 2013)、地震活动性(Chatelain et al., 1980张家声等,2005张浪平等,2014Bai and Zhang, 2015)、各向异性(Schoenecker et al., 1997)等的研究已经给出该地区大致的构造背景及动力学特征.

图 1 帕米尔—兴都库什构造及震源机制解分布图 位于蓝色矩形内的蓝色震源机制解的深度介于70~160 km;位于绿色矩形内的绿色震源机制解的深度介于170~310 km,黄色矩形是70~310 km R值随深度变化研究区域.黑色粗实线为断层分布,GPS数据来源于Ischuk等(2013)Zubovich等(2010)Jouanne等(2014)以及Zhou等(2016),断层数据来源于Koulakov和Sobolev(2006). Fig. 1 Tectonics and focal mechanism solutions in Pamir-Hindu Kush The depth of blue focal mechanisms in the blue rectangle is between 70 km to 160 km. The depth of green focal mechanisms in the green rectangle is 170~310 km. The yellow box is the area where R value changes with depth from 70 km to 310 km. Thick black lines are faults. GPS data is from Ischuk et al. (2013), Zubovich et al. (2010), Jouanne et al. (2014) and Zhou et al. (2016). Fault data from Koulakov and Sobolev (2006).

帕米尔—兴都库什是约50 Ma年前(Schwab et al., 2004)印度板块与欧亚板块相互碰撞向北持续运动产生的隆起高原和向下俯冲的本尼奥夫带(Roecker et al., 1980宁杰远和臧绍先,1990Fan et al., 1994Pegler and Das, 1998张浪平等,2014).两个板块现在以大约29~35 mm·a-1的速率在帕米尔—兴都库什地区相互汇聚(Molnar and Stock, 2009DeMets et al., 2010Mohadjer et al., 2010Zhou et al., 2016),并形成一系列的呈区域性分布的逆冲型和走滑型为主的深大断裂(Mohadjer et al., 2010张浪平等,2014)及弧形构造(图 1).该地区的压应力轴方向(Lukk and Yunga, 1988宁杰远和臧绍先,1990Lukk et al., 1995楼小挺等,2007孙文斌等,2009)与弧形构造几乎垂直的结合在一起呈扇形分布,在西北部呈NW向挤压,并逐渐顺时针旋转到东北部的NE向挤压(Lukk and Yunga, 1988).欧亚板块向南运动与印度板块碰撞,产生倾向S-SE倾斜角度大约为40°~50°(Billington et al., 1977Negredo et al., 2007)的帕米尔俯冲带;印度板块向北运动与欧亚板块碰撞,产生向北倾斜且倾斜角度大约为70°~80°(Billington et al., 1977Negredo et al., 2007)的兴都库什俯冲带.两个俯冲带在37°N附近(Roecker et al., 1980楼小挺等,2007)双向碰撞(Billington et al., 1977Chatelain et al., 1980).而兴都库什地区延伸到地幔深部的俯冲板片形成拆离板片及高速异常体(Roecker, 1982Van Der Voo et al., 1999雷建设等,2002Koulakov and Sobolev, 2006Li et al., 2008Sippl et al., 2013).

另外,兴都库什地区的俯冲板片拆离现象的成因(Ram and Yadav, 1984Lister et al., 2008Bai and Zhang, 2015)也是研究的热点问题之一.研究学者基于其复杂的构造做了大量工作.例如,Bai和Zhang(2015)发现拆离板片西部震源机制解的P轴整体呈现N15°E,东部震源机制解的P轴整体上呈现N75°W;对地震重定位研究显示拆离板片的东、西存在两个相距大概20 km的震群,并结合震源机制解分布特征,参考Duretz等(2012)拆离板片的动态模型,给出了兴都库什中深部地区的拆离板片的变形模式.Sippl等(2013)的重定位结果显示拆离板片内部存在三个震群,通过地震的分布图以及剖面图,建立了兴都库什地区中深部拆离板片复杂的俯冲几何模型.虽然前人在拆离板片的地震分布研究结果上存在略有不同(Roecker et al., 1980Pegler and Das, 1998Sippl et al., 2013Bai and Zhang, 2015),但均得出在70.90±0.05°E附近(Sippl et al., 2013Bai and Zhang, 2015)存在大约为20 km的地震空区(Roecker et al., 1980Pegler and Das, 1998Sippl et al., 2013Bai and Zhang, 2015)及该区域存在板片拆离现象.

鉴于前人(Ritsema,1966Lukk and Yunga, 1988宁杰远和臧绍先,1990Lukk et al,1995孙文斌等,2009唐兰兰等,2012Kufner et al., 2016)在帕米尔—兴都库什地区的应力场工作还需进一步明确,本文旨在基于更为丰富的震源机制解资料反演两个俯冲带的应力场,对比分析两个俯冲带应力场的差异性及其碰撞区域的应力场特征,分析讨论拆离板片的应力场特征及其可能反映的动力学作用.

1 数据和方法 1.1 数据

本文数据来源于Global CMT Project(Dziewonski et al., 1981; Ekström et al., 2012)所给出的震源机制解目录.地震发生时间为1976年1月至2016年6月30日,震源机制解震级介于4.6≤MW≤7.6之间,震源深度介于70~310 km之间,经纬度范围分别为68°E—77°E、33°N—41°N,共得到255个震源机制解(图 1).深度介于70~160 km之间的震源机制解用蓝色绘制(见图 1a),深度介于170~310 km之间的震源机制解用绿色绘制(图 1b).

1.2 方法

本研究采用Hardebeck和Michael(2006)提出的,基于地震震源机制解反演构造应力场的方法.该方法是基于Michael(1984)的线性反演方法,使每个网格的构造应力场与震源机制解的残差最小(Michael,1987),同时在反演的过程中加上了合适的阻尼系数(图 2),该阻尼的选取是为了使相邻网格的应力张量的差异最小化,有效地减小人为划分网格而产生的应力场的异常偏转,更好地凸显整体应力偏转特征.

图 2 阻尼曲线 图中横坐标是数据拟合残差,纵坐标是模型长度;黑色十字是最优阻尼系数.(a) 70~160 km应力场反演得到的阻尼曲线;(b) 170~310 km应力场反演得到的阻尼曲线. Fig. 2 Trade-off curves The horizontal axis is fitting residual, vertical axis is the model length. The black cross is the best damping parameter. The trade-off curve in Fig.(a) Obtained from stress field inversion in the depth of 70~160 km; (b) Obtained from stress field inversion in the depth of 170~310 km.

阻尼反演中的模型长度表示为向量的二阶范数,包含了每对相邻格点的每个应力张量的差异,公式为

(1)

数据残差记为

(2)

阻尼最小二乘的解,需要同时最小化(1)和(2)两式,即:

(3)

式(1)中,D是由多个单位矩阵构成的阻尼矩阵,mall是需要求解的模型矢量,其包含所有网格的应力张量;式(2)中,Gall是由所有地震断层面法向量构成的矩阵,Gall的转置矩阵是GallTdall是由所有地震的滑移矢量的分量组成;式(3)中e为标量阻尼参数,控制着最小化过程中数据残差和模型长度的相对权重.阻尼参数偏大,模型长度太小(计算的应力场太过平滑,可能会忽略应力场的某些变化特征),数据残差急剧升高反演的误差相应的变大(图 2),从而使计算结果不稳定;阻尼参数偏小,数据残差变小相应的反演误差降低,然而模型长度逐渐变得复杂(计算的应力场结果太过紊乱,可能看不到应力场的整体变化规律),从而使阻尼约束失去意义.因此,在应力场反演的过程中选取合理的阻尼系数(e)是非常重要的,本文经过多次计算,得到最优的阻尼系数计算应力场.

本文使用的阻尼应力反演方法得到了国内外众多学者的应用(Wu et al., 2010郑建常等,2013Hardebeck,2015),还被Martínez-garzón等(2014)发展为MSATSI软件包,使其更为广泛的传播和使用(高熹微等,2015罗艳等,2015李祥等,2016Xu et al., 2016崔华伟等,2017Wu et al., 2017).本研究使用MSATSI软件包,对研究数据进行网格划分并反演构造应力场,拟得到最优状态下的应力场参数和应力形因子(R值)(Gephart and Forsyth, 1984),公式为

(4)

其中σ1σ2σ3分别表示最大、中间、最小主压应力.

R值的范围介于0~1之间.当R=0时,张应力轴保持相对稳定,压应力轴和中间应力轴在张应力轴垂直的平面内自由旋转且均呈挤压状态;当R=0.5时,压应力轴、中间应力轴和张应力轴相对稳定;当R=1时,压应力轴相对稳定,张应力轴和中间应力轴在压应力轴垂直的平面内自由旋转且均呈拉张状态(Guiraud et al., 1989万永革等,2011黄骥超等,2016Wan et al., 2016).

2 构造应力场反演结果分析与讨论

本文分析了地壳厚度研究资料.重力异常(Burov et al., 1990Shin et al., 2007, 2009)和面波层析成像(Brandon and Romaniwicz, 1986)研究证实帕米尔—兴都库什地区莫霍面深度为70 km.Koulakov和Sobolev(2006)结合欧亚板块的地壳模型(Beloussov et al., 1991Li and Mooney, 1998),并采用平滑的1°×1°地壳模型,得出帕米尔—兴都库什地区莫霍面深度最大为65 km(Bassin et al. 2000;Laske,personal communication,2004).其他研究(Beloussov et al., 1980Mechie et al., 2012Schneider et al., 2013)也得出帕米尔—兴都库什地区地壳厚度大于60 km的结论.为了减少地壳应力场对深部构造的影响,本文选择最大莫霍面深度(70 km)作为划分研究区域深度的最小深度,详细研究帕米尔—兴都库什地区的应力场.

兴都库什地区地震空区的深度是前人(Chatelain et al., 1980Roecker et al., 1980Pegler and Das, 1998楼小挺等,2007张浪平等,2014)研究的工作之一.张浪平等(2014)根据震源机制解分布特征,提出150~180 km之间存在地震空区.Chatelain等(1980)根据微震活动性,确定兴都库什中源深度160 km存在15~20 km宽的地震空区.Pegler和Das(1998)地震重定位结果表明兴都库什地区150~170 km深度上地震活动性低并证实该地震空区的存在.Roecker等(1980)的研究显示兴都库什地区160 km深度存在地震空区.根据学者的研究成果,本文选择了一个地震活动性相对较低的160~170 km作为应力场反演深度划分的地震空区.

本研究基于震源深度70~160 km的震源机制解,研究印度板块从南向北俯冲的应力形态和欧亚板块向南—东南(张浪平等,2014)俯冲的应力场,以及两个俯冲板片碰撞区域的应力场特征;在170~310 km的研究区域内,着重讨论兴都库什地区深部的应力场特征,以及分析导致板片拆离的动力学作用.

2.1 深度介于70~160 km的构造应力场分析与讨论

本部分统计了深度70~160 km之间的122个震源机制解,去除距离比较远的震源机制解数据,使用剩余的114个震源机制解(见图 1)参与计算,得到了帕米尔—兴都库什地区的中深部构造应力场.

该区域采用0.5°×0.5°的网格划分,得到较为详细的应力场结果(见图 3表 1).从整体上看,压应力轴整体上呈NNW-NW方向分布(图 3表 1)倾伏角变化范围介于1.5°~33°之间,西南部(兴都库什B、C和E区)倾伏角偏小(< 18°);东北区域(帕米尔D区)倾伏角偏大(17°~28°);中间区域(碰撞带A区)倾伏角最大(26°~33°).张应力轴的倾伏角从南向北由大变小(表 1),西南部(兴都库什B、C和E区)张应力轴的倾伏角偏大(48°~78°);中间区域(碰撞带A区)张应力轴的倾伏角较大(55°~62°),东北部的(帕米尔D区)大多数子区域张应力轴的倾伏角小于20°,呈现出沿着帕米尔俯冲带走向拉张的趋势.根据前人的(Roecker et al., 1980张浪平等,2014)工作成果,印度板块在兴都库什地区向北运动,欧亚板块在帕米尔地区向SE运动,两个俯冲带在37°N附近处相碰撞.本文以37°N—37.5°N之间为帕米尔与兴都库什俯冲带的碰撞区域,分别对碰撞带南(兴都库什)、北(帕米尔)两个区域进行探究,然后对37°N —37.5°N(碰撞带A区)的区域进行讨论分析.

图 3 70~160 km主应力轴及应力形因子(R值)分布图 图中应力轴的方向表示方位角,长度表示倾伏角,具体数据见表 1. Fig. 3 The distribution of the stress axis and stress shape factor (R value) between 70~160 km The azimuth and plunge are presented by direction and length of the stress axis, respectively. The detailed data is listed in Table 1.
表 1 帕米尔—兴都库什区域70~160 km反演得到的应力场参数 Table 1 Inverted stress field parameters between 70~160 km in Pamir-Hindu Kush

37.5°N以北区域(D区域),构造应力场的反演结果(图 3表 1)显示压应力轴方向整体上呈NW-SE向挤压(表 1),倾伏方向为NW,倾伏角介于17°~28°之间;张应力轴方向以NE-SW向拉张为主,除D1和D9子区域外其他子区域倾伏角偏小(< 20°),构造应力场呈走滑机制.R值(见图 3)普遍大于0.7,说明该区域的压应力轴相对稳定.帕米尔D1区域的压应力轴的方向为NW(-32.11°),普遍小于A区(A1、A2、A3)的压应力轴方位角(-28.19°、-24.46°、-29.63°);D1张应力轴倾伏角(25.68°)明显大于D3(5.16°)和D2(0.01°)的张应力轴的倾伏角.本文根据计算结果分析推断,D1区域距离两个俯冲带的碰撞区域(A区)较近,其周围邻近区域(D1区域)的应力场受两个板块碰撞影响而发生了改变,进而呈现出压应力轴方向及张应力轴的倾伏角变大.虽然D区域个别子区域的压/张应力轴倾伏角存在涨落,但是整体上压应力轴为SE-NW向挤压为主,张应力轴以NE-SW向拉张为主.通过分析该区域的压/张应力轴的方向、倾伏角和R值,该地区的压应力轴倾伏方向为NW,与前人在其他俯冲带(Wu et al., 2010Hardebeck,2012)研究吻合,其压应力来源于帕米尔高原与天山SE-NW向的推挤.将帕米尔部分俯冲板片视为走向NEE向、倾向SSE(张浪平等,2014)的“断裂”,本文的计算结果支持欧亚板块向S-SE运动俯冲进入地幔形成俯冲板片,同时受到南部帕米尔高原的阻挡,导致近EW延伸的帕米尔俯冲板片在SE向的挤压过程中存在右旋走滑(Robinson et al., 2004)运动的论述.

37°N以南区域,压应力轴的方向(见图 3)总体上呈NNW-SSE向挤压,倾伏角普遍小于18°;绝大多数张应力轴的倾伏角介于60°~78°之间,构造应力场呈逆冲机制.该区域的R值(见图 3)介于0.47~0.88之间,说明压应力轴都比较稳定.B区域及其南部(C2、C3)区域的压应力轴呈NNW-SSE向挤压(图 3表 1),倾伏方向为NW;E区域和C1区域压应力轴呈NNW-SSE向挤压(图 3表 1),倾伏方向为SE与B区域及C2、C3的倾伏方向不同(表 1),倾伏方向的变化可能是因为该区域(B、C1、C2)毗邻两个俯冲板片的碰撞带(A),俯冲板片的碰撞致使附近区域(B1、B2)的应力场参数发生变化,从而与周围区域的应力场存在差异性.通过分析本文的计算结果,印度板块向北运动与欧亚板块碰撞,俯冲板片被推挤进入岩石圈以下向板片两侧排开地幔,地幔给予的挤压环境导致了兴都库什俯冲板片NNW-SSE向挤压为主的逆冲应力机制.

37°N—37.5°N碰撞带附近的A(A1、A2、A3)区域(见图 3),压应力轴呈NNW-SSE向挤压,倾伏方向为NW,其倾伏角(26.35°,32.89°,26.36°)大于南部区域(B1、B2)的压应力轴倾伏角(10.13°,17.73°),同时大于北部区域(D1、D3)的压应力轴倾伏角.从应力机制来看,37.5°N以北的区域是走滑机制,37°N以南区域的应力场呈现逆冲机制.Roecker等(1980)推断37°N的地震空去可能是两个俯冲板片交汇位置,楼小挺等(2007)在36.8°N发现相同现象,并支持Roecker等(1980)的结论.但是根据本文应力场的计算结果,两个俯冲板片相互碰撞的位置是一个区域(A区域)比一个分界线更加合理,说明37°N—37.5°N可能是欧亚板块在帕米尔下面的俯冲带(37.5°N以北区域)与印度板块在兴都库什下面的俯冲带(37°N以南区域)相碰撞的区域.

通过计算结果,得出帕米尔(37.5°N以北)和兴都库什(37.5°N以南)两个俯冲带的应力场参数及应力机制类型存在一定的差异(表 2),帕米尔地区的压应力轴倾伏角普遍大于兴都库什地区(表 2),以及帕米尔张应力轴倾伏角要小于兴都库什地区(表 2),然而导致这种差异的原因可能是多方面的,可能是俯冲带的形成的年龄不同(Negredo et al., 2007Replumaz et al., 2010),也可能是两个俯冲带俯冲的速率不同(Negredo et al., 2007).通过研究结果分析得出,在帕米尔—兴都库什受到欧亚板块相互挤压作用的控制下,帕米尔的压应力主要来源于俯冲板片的推挤.而帕米尔地区局部低速层的存在(Li et al., 2018)卸载了部分向下拉张应力,使拉张应力沿着走向NEE、倾向SSE的帕米尔板片及多条弧形的深大断裂带(Fan et al., 1994)进行NW挤压NE向拉张,产生走滑为主的应力机制(表 2);而兴都库什俯冲板片得挤压应力很可能来源于地幔物质的NNW向近水平挤压及深部板片向下的拖曳作用控制,呈现出逆冲型的应力机制(表 2).可能是因为帕米尔地区低速层的存在(Li et al., 2018),使帕米尔俯冲板片向下拉张的作用转换为NE-SW向拉张运动,削弱了帕米尔板片向下拉张的动力,从而使帕米尔地区的俯冲运动和地震活动性(宁杰远和臧绍先,1990孙文斌等,2009)要弱于兴都库什地区.

表 2 各分区应力场反演参数 Table 2 Stress field parameters in different areas
2.2 震源深度大于170 km的构造应力场分析与讨论

本部分利用筛选之后震源深度170~310 km的129个震源机制解(图 1b图 4a),计算兴都库什俯冲带深部(>170 km)的构造应力场.该范围的震源机制解分布相对集中,重定位结果(Bai and Zhang, 2015)显示,以70.95°E为界,东西两部分震源机制解P轴分布不同.文中基于Bai和Zhang(2015)的研究结果,采用0.6°×0.6°的网格划分,得到俯冲带深部相对详细的应力场结果(见图 4b).经过分析发现,以70.85°E为界,东西两部分的压应力轴方向和应力形因子(R值)存在明显差异(图 4bc表 3).为了更加详细的探究该区域应力状态,绘制震源机制解的剖面图(见图 4a图 5),以下将从压应力轴方向、应力形因子(R值)及震源机制解分布特征三个方面具体分析讨论.

图 4 170~310 km震源机制解及主应力轴和应力空间分布图 图(a)是兴都库什地区深度介于170~310 km的震源机制解分布图.紫色、黄色、蓝色和红色实线依次表示的是剖面AA′、BB′、CC′和DD′的位置;图(b)中黑色箭头表示压应力轴,红色箭头表示张应力轴;图(c)中绘制的是主应力空间分布,黄色和蓝色表示压应力轴和张应力轴;图(d)中的颜色表示170~310 km的应力形因子. Fig. 4 The distribution of the focal mechanism solutions, stress axis and stress radiation pattern in 170~310 km (a) Focal mechanism solutions between depths 170 to 310 km in Hindu Kush. The positions of profiles AA′, BB′, CC′ and DD′ are plotted with purple, yellow, blue and red, respectively; (b) The compressive and extensional stress axes are presented by black and red arrows, respectively; (c) The distribution of principal stresses. The compressive and extensive stress axes are plotted with yellow and blue, respectively; (d) Stress shape factor (R value) between 170 to 310 km.
表 3 兴都库什170~310 km的应力场参数 Table 3 Stress field parameters at depths 170~310 km in Hindu Kush
图 5 深度170~310 km震源机制解剖面图 剖面AA′、BB′、CC′和DD′的位置见图 4a中的紫色、黄色、蓝色和红色. Fig. 5 Cross-sections of focal mechanism solutions at depths from 170 km to 310 km The positions of cross-sections AA′, BB′, CC′ and DD′ are shown in Fig. 4a.

(1) 压应力轴倾伏角近水平,其方向(图 4b)由西部的近NS方向(表 3)向东部的NW方向偏转.张应力轴倾伏角范围71°~82°之间(表 3),其倾伏角的大小与前人(Singh et al., 2005Negredo et al., 2007)得到的兴都库什俯冲板片角度相近.同时,这一区域的张应力轴倾伏角(71°~82°),明显大于深度70~160 km范围内的张应力轴的倾伏角(50°~78°).

(2) 应力形因子(R值)由西部(R1-R4)近似0.5逐渐减小为东部(R5、R6)的趋于0(图 4b),说明西部(R1-R4)的压应力轴和张应力轴较稳定,在东部(R5-R6)随着R趋于0,中间应力轴也表现为压应力的性质(黄骥超等,2016Wan et al., 2016).R接近于0的东部区域,压应力轴和中间应力轴在张应力轴垂直的平面内自由旋转,表明东部地区构造应力场以上下拉张的张应力为主导作用.东西部应力形因子存在差异性,可能反映了拆离板片内部结构的复杂性(Sippl et al., 2013).

(3) 震源机制解(见图 4a)分布呈现一定的规律性.西部地区的震源机制解P轴方向大多数为NS向且倾伏角较小,一致性较好;东部地区的震源机制解P轴呈现NS、NE和NW向且倾伏角较小,一致性较差.如震源机制解剖面图所示(图 5),大多数震源机制解的T轴方向接近竖直.剖面AA′(69.7°E,36°N;71.5°E,36°N)宽度为66.7 km,西部(图 5剖面AA′黑色虚线为70.85°E)的震源机制解P轴方向近NS向,东部震源机制解的P轴规律性较差,两种震源机制解数目接近.剖面BB′(69.7°E,36.6°N;71.5°E,36.6°N)宽度为66.7 km,震源机制解P轴近NS向的数目少于P轴近EW向的震源机制解.剖面CC′(70.6°E,35.7°N;70.6°E,36.9°N)宽度40 km,位于70.85°E以西,绝大多数震源机制解P轴方向近NS向.剖面DD′(71.2°E,35.7°N;71.2°E,36.9°N)宽度60 km,位于70.85°E以东,大部分震源机制解P轴方向近EW向,少部分震源机制解P轴方向为NE和NW向,极少部分震源机制解P轴方向近NS向.

通过对这一地区的应力轴方向、应力形因子(R值)和震源机制解分布特征的综合分析,可以看出张应力轴倾伏角接近垂直,压应力轴方向由西到东从NS向挤压转变为NW-SE向挤压(图 4b),相应的应力形因子(图 4d)也从0.5左右趋于0,应力空间分布(图 4c)也发生较大变化.本文将该区域的应力轴方向、应力形因子(R值)和震源机制解分布特征与地震重定位(Bai and Zhang, 2015Kufner et al., 2016, 2017)、层析成像(Li et al., 2006, 2008Li and Van der Hilst,2010Kufner et al., 2017)等的研究成果相结合,发现兴都库什地区由西向东地震带展布方向由E-W向逐渐变为NE-SW向,压应力轴的方向由N-S向偏转为NW-SE向,与地震带的走向接近垂直.震源机制解分布、主压应力轴偏转及R值变化均以板片走向发生变化的70.85°E为界,本文分析认为,观察到的三种异常与板片走向之间的关系,很有可能揭示拆离板片内部破碎的作用方式,因此,我们将在下文中通过兴都库什地区地区纵向应力形因子变化,详细讨论可能导致拆离板片发生破碎的内部作用.

3 兴都库什地区纵向应力形因子(R值)变化

本文之前的研究显示,兴都库什地区(170~310 km)应力形因子偏小(< 0.5),张应力轴近乎垂直的上下拉张,压应力轴方向随着地震带的走向发生了偏转.然而,仍然存在一些问题急待解决:(1)兴都库什地区深部和浅部俯冲板片倾角不同,并且压/张应力轴的倾伏角也存在差异;(2)兴都库什地区(70~310 km)应力场的形态随着深度是如何变化的.这可能揭示出该地区独特的构造环境及动力学意义及拆离板片内部破碎的原因.

本部分基于Global CMT的震源机制解数据,选取69.5°E—72°E,35.5°N—37°N(图 1黄色矩形)区域内,深度大于70 km的186个震源机制解数据以深度10 km为间隔划分网格,反演计算应力形因子(R值)及绘制应力辐射花样(万永革等,2011),拟得到它们在深度上的分布特征,探求兴都库什地区从浅到深部相对应力大小的变化及可能揭示的动力学意义.经过计算得到了应力形因子(图 6a)和应力辐射花样随深度的变化图(图 6b),应力形因子和张应力轴倾伏角随深度变化曲线(图 7).

图 6 深度70~310 km的R值变化及应力辐射花样 图(a)背景的颜色代表R值,空白表示没有数据,左边的坐标表示深度,右边的坐标表示深度相对应的R值;图(b)不同深度代表的应力辐射花样,从左至右及从上到下深度依次增加. Fig. 6 The variation of R value and stress radiation pattern at depths 70 km to 310 km (a) R value presented with background color, blank showing missing of data. Depth scale is shown on the left side and R value scale is shown on the right side; (b) Stress radiation patterns at varied depth. The depth increases from left to right, and from upper to lower.
图 7 应力形因子和张应力轴倾伏角随深度的变化曲线 Fig. 7 The variation of the R value and plunge of the tensional stress axis with depth

从整体上看,研究结果显示应力形因子(R值)随着深度的增加而减小(图 6a图 7),说明了兴都库什地区的构造应力环境,由NNW向的压应力为主的状态,逐渐转变为以上下拉张为主的张应力状态(图 6b).大概以地震空区的深度(170 km)为界,浅部区域(70~160 km)主俯冲板片的应力形因子比较大,反映出是NNW向比较稳定的压应力为主;深部区域(170~310 km)拆离板片的应力形因子偏小,反映出的是以近乎垂直向的张应力为主.在210~240 km应力形因子由0.38突降为0.17(图 7),反映了主压应力轴和中间应力轴更为接近,拆离板片已经俯冲至上地幔低速层(B2)的下边界莱曼不连续面(Lehmann,1961),一般认为,地幔低速层采用部分熔融和断层(板片破碎)卸载来部分应力(Karato,1992Thybo and Perchuc′,1997).本次反演首次用应力形因子可能表明上地幔低速层存在对构造应力场(Karato,1992)有影响,并且这里的低速层(B2)和均匀层(B3)分界面(Lehmann Discontinuity)受到俯冲板片的影响存在向下延伸的趋势(Revenaugh and Sipkin, 1994),另外,拆离板片与主俯冲板片的应力结构有明显差别,可能反映了拆离板片已经脱离主俯冲板片(黄骥超等,2016)或处于拆离作用的后期.

应力形因子R随深度的变化可能反映出兴都库什俯冲板片的拆离进程.拆离板片的浅部区域(小于160 km)的压应力环境可能来源于地幔深部物质的NNW向挤压.地震空区附近的区域(130~190 km)R值为0.6~0.8左右,压应力轴倾伏角普遍较小(< 11°)且张应力轴倾伏角(图 7表 4)很大(74°~87°),该深度内应力机制是逆冲机制,与Duretz等(2011a2012)的颈缩模式相一致,可能反映出该深度内来自于地幔物质NNW向推挤作用形成了近水平挤压的构造应力环境,同时重力作用(Pegler and Das, 1998Pavlis and Das, 2000)与向上的合力(浮力、阻力及黏滞力等)提供了近垂直的拉张作用,在水平挤压及垂直拉张环境下促使板片发生拆离现象.深部(大于170 km)区域拆离板片的应力形因子(R值)整体较小且随深度逐渐趋于零,张应力轴倾伏角(图 6b表 4)与前文中计算的结果(图 4c表 3)相近且均偏大,整体应力状态是上下的张应力为主,说明该深度压应力轴和中间应力轴相近并在水平面内自由旋转(万永革等,2011; 万永革,2015黄骥超等,2016),张应力轴较稳定呈上下拉张起主导作用.通过分析研究结果,本文推测俯冲板片受温度、压力作用产生薄弱面,在薄弱面受到重力下拉和地幔NNW向挤压致板片拆离,进而呈现出地震空区(Roecker et al., 1980Pegler and Das, 1998Sippl et al., 2013Bai and Zhang, 2015)及拆离板片,以及拆离板片向下沉降向南偏移的高速异常体(Koulakov and Sobolev, 2006Li et al., 2006Negredo et al., 2007Li et al., 2008Koulakov,2011).同时这种视觉上的“偏移”实际上也是印度板块向NNW运动的证据(Replumaz et al., 2004Negredo et al., 2007Ischuk et al., 2013).

表 4 兴都库什70~310 km的应力场参数 Table 4 The stress field parameters between 70~310 km in Hindu Kush

拆离板片与主俯冲板片倾角的差异也是我们研究的问题之一.通过结合应力形因子和压应力轴倾伏角的特征及层析成像(Koulakov and Sobolev, 2006Li et al., 2006Negredo et al., 2007Li et al., 2008Koulakov,2011)的结果,兴都库什地区的拆离板片在重力及黏滞力共同作用下远离主俯冲板片,由于失去了主俯冲板片束缚作用,拆离板片的倾角(80°~90°)不再与主俯冲板片(60°~75°)一致(Billington et al., 1977宁杰远和臧绍先,1990楼小挺等,2007Sippl et al., 2013).

张应力轴倾伏角的变化可能揭示出拆离板片内部的动力学作用.拆离板片的张应力轴倾伏角随着深度(200~310 km)的增加逐渐变小(图 6b图 7)是一个比较明显的现象,结合图 4b(R5,R6)压应力轴的偏转,是应力场不均匀的表征.图 6b所示的深度大于230 km的应力形因子均由东部区域(图 5ab)的震源机制解计算得来,其张应力轴倾伏角向西倾伏(图 6b)与170~310 km(R5-R6)计算结果一致(图 4b表 3),东西部应力场的差异性(图 4b)证实了拆离板片内部破碎(Sippl et al., 2013),揭示了拆离板片内部复杂构造.而地震分布呈现分叉(Roecker et al., 1980Pegler and Das, 1998Bai and Zhang, 2015)及震源机制解存在东西分布不均匀等现象(Bai and Zhang, 2015)很好的证实了这一观点,其原因可能是拆离板片俯冲至上地幔不连续面,随着地幔深部的压力、温度等(Negredo et al., 2007万永革,2016)作用发生变化,该作用降低了拆离板片内部的强度,使板片熔融、破碎而卸载板片内部的部分应力,使拆离板片内部以应力场的不均匀性、震源机制解类型丰富、张应力轴倾伏角的变小及多个子震群(Sippl et al., 2013Bai and Zhang, 2015)等形式显现出来.

综上所述,本文基于应力场特征结合前人地震重定位(Sippl et al., 2013Bai and Zhang, 2015Kufner et al., 2016)、层析成像(Van Der Voo et al., 1999Replumaz et al., 2004Negredo et al., 2007Koulakov,2011Kufner et al., 2016)等的研究成果,提出了一个可能的构造动力学模型.印度板块自南向北推挤欧亚板块运动至现在的帕米尔—兴都库什地区,形成隆起的高原和向下的俯冲板片.兴都库什俯冲板片受到印度板块的推挤向下俯冲至300 km,并向板片两侧垂直于俯冲板片延伸方向排开地幔物质,同时在地幔物质对俯冲板片的反挤压作用下,压应力的方向随着俯冲板片的展布方向由NS向挤压转变为NW-SE向挤压.在俯冲板片向下俯冲的过程中,随着深度增加温度升高(Negredo et al., 2007)、压力增大(万永革,2016)及地幔的挤压等作用致使俯冲板片产生薄弱面,在重力(Pegler and Das, 1998Pavlis and Das, 2000)或合力(浮力、阻力及黏滞力等)及近水平的地幔挤压作用下板片开始慢慢撕裂产生地震空区及拆离板片.拆离板片脱离了主俯冲板片的束缚,在重力(Pegler and Das, 1998Pavlis and Das, 2000)及合力作用下倾角增大(80°~90°),向下俯冲至上地幔不连续面(Revenaugh and Sipkin;1994),而其内部受到温度、压力(Negredo et al., 2007万永革,2016)等因素的变化发生部分熔融,伴随着地震呈现出拆离板片地震延伸方向变化(楼小挺等,2007Kufner et al., 2016)、丰富的震源机制解类型(Bai and Zhang, 2015)、应力场偏转等现象,也使得拆离板片内部复杂的几何构造以多个(Sippl et al., 2013Bai and Zhang, 2015)不同的地震群呈现出来.

4 结论

本文根据帕米尔—兴都库什地区的特殊构造,将其分为70~160 km和170~310 km两个研究区域反演该地区的精细构造应力场,并通过深度划分计算应力形因子和应力辐射花样随深度的变化,得到以下初步认识:

帕米尔与兴都库什俯冲带的应力场存在差异的原因是形成应力场的机制不同.兴都库什地区的深部地幔物质近水平NNW-SSE向挤压及拆离板片提供的向下拉拖曳作用形成了逆冲的应力机制,而帕米尔地区来源于NW-SE向板片挤压及断裂带的横向拉张形成走滑机制.帕米尔和兴都库什俯冲带交汇区域(A区域位于37°N—37.5°N附近)与相邻区域的应力场参数及应力机制存在较大的差异,可能是兴都库什俯冲带与帕米尔俯冲带碰撞的区域.兴都库什俯冲至地幔深处,板片受到深部温度、压力等因素,出现薄弱面进而受到上下拉张作用形成拆离板片.其脱离了主俯冲板片的束缚后,重力提供的拉张作用导致空区附近张轴倾伏角接近90°.拆离板片延伸至上地幔不连续面,部分熔融导致的主压应力值和中间应力值趋于一致,应力形因子因此随着深度逐渐变小.在地幔挤压作用下,拆离板片内部发生破碎,导致西部NS向挤压至东部NW-SE向挤压的偏转,其内部也呈现出地震带走向变化、震源机制解丰富、应力场不均匀性及多个地震群等现象.

致谢  三位审稿人和主编对本文提出了建设性意见,河北省地震局王晓山副研究员为本研究提供了有益建议,本文的大部分图件采用GMT(Wessel and Smith,1995),特此致谢.
References
Bai L, Zhang T Z. 2015. Complex deformation pattern of the Pamir-Hindu Kush region inferred from multi-scale double-difference earthquake relocations. Tectonophysics, 638: 177-184. DOI:10.1016/j.tecto.2014.11.006
Bassin C, Laske G, Masters G. 2000. The current limits of resolution for surface wave tomography in North America. EOS, Transactions American Geophysical Union, 81(48): F897.
Beloussov V V, Belyaevsky N A, Borisov A A, et al. 1980. Structure of the lithosphere along the deep seismic sounding profile: Tien Shan—Pamirs—Karakorum—Himalayas. Tectonophysics, 70(3-4): 193-221. DOI:10.1016/0040-1951(80)90279-6
Beloussov V V, Pavlenkova N I, Egorkin A V. 1991. Deep Structure of the Territory of the USSR. Nauka, Moscow.
Billington S, Isacks B L, Barazangi M. 1977. Spatial distribution and focal mechanisms of mantle earthquakes in the Hindu Kush Pamir region: A contorted Benioff zone. Geology, 5(11): 699-704. DOI:10.1130/0091-7613(1977)5<699:SDAFMO>2.0.CO;2
Brandon C, Romanowicz B. 1986. A "no-lid" zone in the central Chang-Thang platform of Tibet: Evidence from pure path phase velocity measurements of long period Rayleigh waves. Journal of Geophysical Research: Solid Earth, 91(B6): 6547-6564. DOI:10.1029/JB091iB06p06547
Burov E V, Kogan M G, Lyon-Caen H, et al. 1990. Gravity anomalies, the deep structure, and dynamic processes beneath the Tien Shan. Earth and Planetary Science Letters, 96(3-4): 367-383. DOI:10.1016/0012-821X(90)90013-N
Chatelain J L, Roecker S W, Hatzfeld D, et al. 1980. Microearthquake seismicity and fault plane solutions in the Hindu Kush region and their tectonic implications. Journal of Geophysical Research: Solid Earth, 85(B3): 1365-1387. DOI:10.1029/JB085iB03p01365
Cui H W, Wan Y G, Huang J C, et al. 2017. Tectonic stress field in the source of the New Britain MS7.4 earthquake of March 2015 and adjacent areas. Chinese Journal of Geophysics (in Chinese), 60(3): 985-998. DOI:10.6038/cjg20170313
DeMets C, Gordon R G, Argus D F. 2010. Geologically current plate motions. Geophysical Journal International, 181(1): 1-80. DOI:10.1111/gji.2010.181.issue-1
Duretz T, Gerya T V, May D A. 2011a. Numerical modelling of spontaneous slab breakoff and subsequent topographic response. Tectonophysics, 502(1-2): 244-256. DOI:10.1016/j.tecto.2010.05.024
Duretz T, Schmalholz S M, Gerya T V. 2012. Dynamics of slab detachment. Geochemistry, Geophysics, Geosystems, 13(3): Q03020. DOI:10.1029/2011GC004024
Dziewonski A M, Chou T A, Woodhouse J H. 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. Journal of Geophysical Research: Solid Earth, 86(B4): 2825-2852. DOI:10.1029/JB086iB04p02825
Ekström G, Nettles M, Dziewoński A M. 2012. The global CMT project 2004—2010: Centroid-moment tensors for 13, 017 earthquakes. Phys Earth Planet Inter, 200-201: 1-9. DOI:10.1016/j.pepi.2012.04.002
Fan G W, Ni J F, Wallace T C. 1994. Active tectonics of the Pamirs and Karakorum. Journal of Geophysical Research: Solid Earth, 99(B4): 7131-7160. DOI:10.1029/93JB02970
Gao X W, Wan Y G, Huang J C, et al. 2015. Tectonic stress field analysis and static coulomb stress changes of the MS5.8 Inner Mongolia′s Alxa left banner earthquake. North China earthquake Sciences (in Chinese), 33(2): 48-54.
Gephart J W, Forsyth D W. 1984. An improved method for determining the regional stress tensor using earthquake focal mechanism data: Application to the San Fernando earthquake sequence. Journal of Geophysical Research: Solid Earth, 89(B11): 9305-9320. DOI:10.1029/JB089iB11p09305
Guiraud M, Laborde O, Philip H. 1989. Characterization of various types of deformation and their corresponding deviatoric stress tensors using microfault analysis. Tectonophysics, 170(3-4): 289-316. DOI:10.1016/0040-1951(89)90277-1
Hardebeck J L. 2012. Coseismic and postseismic stress rotations due to great subduction zone earthquakes. Geophysical Research Letters, 39(21): L21313. DOI:10.1029/2012GL053438
Hardebeck J L. 2015. Stress orientations in subduction zones and the strength of subduction megathrust faults. Science, 349(6253): 1213-1216. DOI:10.1126/science.aac5625
Hardebeck J L, Michael A J. 2006. Damped regional-scale stress inversions: Methodology and examples for southern California and the Coalinga aftershock sequence. Journal of Geophysical Research: Solid Earth, 111(B11): B11310. DOI:10.1029/2005JB004144
Huang J C, Wan Y G, Sheng S Z, et al. 2016. Heterogeneity of present-da stress field in the Tonga-Kermadec subduction zone and its geodynamic significance. Chinese Journal of Geophysics (in Chinese), 59(2): 578-592. DOI:10.6038/cjg20160216
Ischuk A, Bendick R, Rybin A, et al. 2013. Kinematics of the Pamir and Hindu Kush regions from GPS geodesy. Journal of Geophysical Research: Solid Earth, 118(5): 2408-2416. DOI:10.1002/jgrb.50185
Jouanne F, Awan A, Pecher A, et al. 2014. Present-day deformation of northern Pakistan from Salt Ranges to Karakorum Ranges. Journal of Geophysical Research: Solid Earth, 119(3): 2487-2503. DOI:10.1002/2013JB010776
Karato S I. 1992. On the Lehmann discontinuity. Geophysical Research Letters, 19(22): 2255-2258. DOI:10.1029/92GL02603
Koulakov I. 2011. High-frequency P and S velocity anomalies in the upper mantle beneath Asia from inversion of worldwide traveltime data. Journal of Geophysical Research: Solid Earth, 116(B4): B04301. DOI:10.1029/2010JB007938
Koulakov I, Sobolev S V. 2006. A tomographic image of Indian lithosphere break-off beneath the Pamir-Hindukush region. Geophysical Journal International, 164(2): 425-440. DOI:10.1111/gji.2006.164.issue-2
Kufner S K, Schurr B, Haberland C, et al. 2017. Zooming into the Hindu Kush slab break-off: A rare glimpse on the terminal stage of subduction. Earth and Planetary Science Letters, 461: 127-140. DOI:10.1016/j.epsl.2016.12.043
Kufner S K, Schurr B, Sippl C, et al. 2016. Deep India meets deep Asia: Lithospheric indentation, delamination and break-off under Pamir and Hindu Kush (Central Asia). Earth and Planetary Science Letters, 435: 171-184. DOI:10.1016/j.epsl.2015.11.046
Lehmann I. 1960. Structure of the upper mantle as derived from the travel times of seismic P and S waves. Nature, 186(4729): 956.
Lei J S, Zhou H L, Zhao D P. 2002. 3-D velocity structure of P-wave in the crust and upper-mantle beneath Pamir and adjacent region. Chinese Journal of Geophysics (in Chinese), 45(6): 802-811.
Li C, Van Der Hilst R D. 2010. Structure of the upper mantle and transition zone beneath Southeast Asia from traveltime tomography. Journal of Geophysical Research: Solid Earth, 115(B7): B07308. DOI:10.1029/2009JB006882
Li C, Van Der Hilst R D, Meltzer A S, et al. 2008. Subduction of the Indian lithosphere beneath the Tibetan Plateau and Burma. Earth and Planetary Science Letters, 274(1-2): 157-168. DOI:10.1016/j.epsl.2008.07.016
Li C, Van Der Hilst R D, Toksöz M N. 2006. Constraining P-wave velocity variations in the upper mantle beneath Southeast Asia. Physics of the Earth and Planetary Interiors, 154(2): 180-195. DOI:10.1016/j.pepi.2005.09.008
Li S L, Mooney W D. 1998. Crustal structure of China from deep seismic sounding profiles. Tectonophysics, 288(1-4): 105-113. DOI:10.1016/S0040-1951(97)00287-4
Li W, Chen Y, Yuan X H, et al. 2018. Continental lithospheric subduction and intermediate-depth seismicity: Constraints from S-wave velocity structures in the Pamir and Hindu Kush. Earth and Planetary Science Letters, 482: 478-489. DOI:10.1016/j.epsl.2017.11.031
Li X, Wan Y G, Cui H W, et al. 2016. Tectonic stress field analysis on the source region of the 2015 MW8.3 Chile earthquake. Acta Seismologica Sinica (in Chinese), 38(6): 847-853.
Lister G, Kennett B, Richards S, et al. 2008. Boudinage of a stretching slablet implicated in earthquakes beneath the Hindu Kush. Nature Geoscience, 1(3): 196-201. DOI:10.1038/ngeo132
Lou X T, Diao G L, Ye G Y, et al. 2007. Spatial distribution of intermediate earthquakes and focal mechanism solutions in the Pamir-Hindu Kush region. Chinese Journal of Geophysics (in Chinese), 50(5): 1448-1455.
Lukk A A, Yunga S L. 1988. Geodynamics and Stress-Strain State of the Lithosphere in Central Asia. Duschanbe: Donish (in Russian).
Lukk A A, Yunga S L, Shevchenko V I, et al. 1995. Earthquake focal mechanisms, deformation state, and seismotectonics of the Pamir-Tien Shan region, central-Asia. Journal of Geophysical Research: Solid Earth, 100(B10): 20321-20343. DOI:10.1029/95JB02158
Luo Y, Zhao L, Zeng X F, et al. 2015. Focal mechanisms of the Lushan earthquake sequence and spatial variation of the stress field. Science China Earth Sciences, 58(7): 1148-1158. DOI:10.1007/s11430-014-5017-y
Martínez-Garzón P, Kwiatek G, Ickrath M, et al. 2014. MSATSI: A MATLAB package for stress inversion combining solid classic methodology, a new simplified user-handling, and a visualization tool. Seismological Research Letters, 85(4): 896-904. DOI:10.1785/0220130189
Mechie J, Yuan X, Schurr B, et al. 2012. Crustal and uppermost mantle velocity structure along a profile across the Pamir and southern Tien Shan as derived from project TIPAGE wide-angle seismic data. Geophysical Journal International, 188(2): 385-407. DOI:10.1111/gji.2012.188.issue-2
Michael A J. 1984. Determination of stress from slip data: Faults and folds. Journal of Geophysical Research: Solid Earth, 89(B13): 11517-11526. DOI:10.1029/JB089iB13p11517
Michael A J. 1987. Use of focal mechanisms to determine stress: A control study. Journal of Geophysical Research: Solid Earth, 92(B1): 357-368. DOI:10.1029/JB092iB01p00357
Mohadjer S, Bendick R, Ischuk A, et al. 2010. Partitioning of India-Eurasia convergence in the Pamir-Hindu Kush from GPS measurements. Geophysical Research Letters, 37(4): L04305. DOI:10.1029/2009GL041737
Molnar P, Stock J M. 2009. Slowing of India′s convergence with Eurasia since 20 Ma and its implications for Tibetan mantle dynamics. Tectonics, 28(3): TC3001. DOI:10.1029/2008TC002271
Negredo A M, Replumaz A, Villaseñor A, et al. 2007. Modeling the evolution of continental subduction processes in the Pamir-Hindu Kush region. Earth and Planetary Science Letters, 259(1-2): 212-225. DOI:10.1016/j.epsl.2007.04.043
Ning J Y, Zang S X. 1990. The distribution of earthquakes and stress state in Pamir-Hindukush region. Acta Geophysica Sinica (in Chinese), 33(6): 657-669.
Pavlis G L, Das S. 2000. The Pamir-Hindu Kush seismic zone as a strain marker for flow in the upper mantle. Tectonics, 19(1): 103-115. DOI:10.1029/1999TC900062
Pegler G, Das S. 1998. An enhanced image of the Pamir-Hindu Kush seismic zone from relocated earthquake hypocentres. Geophysical Journal International, 134(2): 573-595. DOI:10.1046/j.1365-246x.1998.00582.x
Ram A, Yadav L. 1984. Focal-mechanism solutions of earthquakes and tectonics of the Hindukush region. Tectonophysics, 104(1-2): 85-97. DOI:10.1016/0040-1951(84)90103-3
Replumaz A, Kárason H, Van Der Hilst R D, et al. 2004. 4-D evolution of SE Asia′s mantle from geological reconstructions and seismic tomography. Earth and Planetary Science Letters, 221(1-4): 103-115. DOI:10.1016/S0012-821X(04)00070-6
Replumaz A, Negredo A M, Guillot S, et al. 2010. Multiple episodes of continental subduction during India/Asia convergence: Insight from seismic tomography and tectonic reconstruction. Tectonophysics, 483(1-2): 125-134. DOI:10.1016/j.tecto.2009.10.007
Revenaugh J, Sipkin S A. 1994. Mantle discontinuity structure beneath China. Journal of Geophysical Research: Solid Earth, 99(B11): 21911-21927. DOI:10.1029/94JB01850
Ritsema A R. 1966. The fault-plane solutions of earthquakes of the Hindu Kush centre. Tectonophysics, 3(2): 147-163. DOI:10.1016/0040-1951(66)90017-5
Robinson A C, Yin A, Manning C E, et al. 2004. Tectonic evolution of the northeastern Pamir: Constraints from the northern portion of the Cenozoic Kongur Shan extensional system, western China. Geological Society of America Bulletin, 116(7-8): 953-973.
Roecker S W. 1982. Velocity structure of the Pamir-Hindu Kush Region: Possible evidence of subducted crust. Journal of Geophysical Research: Solid Earth, 87(B2): 945-959. DOI:10.1029/JB087iB02p00945
Roecker S W, Soboleva O V, Nersesov I L, et al. 1980. Seismicity and fault plane solutions of intermediate depth earthquakes in the Pamir-Hindu Kush region. Journal of Geophysical Research: Solid Earth, 85(B3): 1358-1364. DOI:10.1029/JB085iB03p01358
Schneider F M, Yuan X, Schurr B, et al. 2013. Seismic imaging of subducting continental lower crust beneath the Pamir. Earth and Planetary Science Letters, 375: 101-112. DOI:10.1016/j.epsl.2013.05.015
Schoenecker S C, Russo R M, Silver P G. 1997. Source-side splitting of s waves from Hindu Kush-Pamir earthquakes. Tectonophysics, 279(1-4): 149-159. DOI:10.1016/S0040-1951(97)00130-3
Schwab M, Ratschbacher L, Siebel W, et al. 2004. Assembly of the Pamirs: Age and origin of magmatic belts from the southern Tien Shan to the southern Pamirs and their relation to Tibet. Tectonics, 23(4): TC4002. DOI:10.1029/2003TC001583
Shin Y H, Shum C K, Braitenberg C, et al. 2009. Three-dimensional fold structure of the Tibetan Moho from GRACE gravity data. Geophysical Research Letters, 36(1): L01302. DOI:10.1029/2008GL036068
Shin Y H, Xu H Z, Braitenberg C, et al. 2007. Moho undulations beneath Tibet from GRACE-integrated gravity data. Geophysical Journal International, 170(3): 971-985. DOI:10.1111/gji.2007.170.issue-3
Singh V P, Duda J, Shanker D. 2005. A plausible model for the present day seismicity and tectonic activity in the Hindukush complex zone. Journal of Asian Earth Sciences, 25(1): 147-156. DOI:10.1016/j.jseaes.2004.02.002
Sippl C, Schurr B, Yuan X, et al. 2013. Geometry of the Pamir-Hindu Kush intermediate-depth earthquake zone from local seismic data. Journal of Geophysical Research: Solid Earth, 118(4): 1438-1457. DOI:10.1002/jgrb.50128
Sun W B, He Y S, Chang Z, et al. 2009. The plate subduction and stress state in the Pamir-Hindu Kush region. Seismology and Geology (in Chinese), 31(2): 207-217.
Tang L L, Zhao C P, Wang H T. 2012. Study on the source characteristics of the 2008 MS6.8 Wuqia, Xinjiang earthquake sequence and the stress field on the northeastern boundary of Pamir. Chinese Journal of Geophysics (in Chinese), 55(4): 1228-1239. DOI:10.6038/j.issn.0001-5733.2012.04.018
Thybo H, Perchuc′ E. 1997. The seismic 8 discontinuity and partial melting in continental mantle. Science, 275(5306): 1626-1629. DOI:10.1126/science.275.5306.1626
Van Der Voo R, Spakman W, Bijwaard H. 1999. Tethyan subducted slabs under India. Earth and Planetary Science Letters, 171(1): 7-20. DOI:10.1016/S0012-821X(99)00131-4
Wan Y G. 2015. A grid search method for determination of tectonic stress tensor using qualitative and quantitative data of active faults and its application to the Urumqi area. Chinese Journal of Geophysics (in Chinese), 58(9): 3144-3156. DOI:10.6038/cjg20150911
Wan Y G. 2016. Introduction to Seismology (in Chinese). Beijing: Science Press.
Wan Y G, Sheng S Z, Huang J C, et al. 2016. The grid search algorithm of tectonic stress tensor based on focal mechanism data and its application in the boundary zone of China, Vietnam and Laos. Journal of Earth Science, 27(5): 777-785. DOI:10.1007/s12583-015-0649-1
Wan Y G, Sheng S Z, Xu Y R, et al. 2011. Effect of stress ratio and friction coefficient on composite P wave radiation patterns. Chinese Journal of Geophysics (in Chinese), 54(4): 994-1001. DOI:10.3969/j.issn.0001-5733.2011.04.014
Wessel P, Smith W H F. 1995. New version of the generic mapping tools. EOS, Transactions American Geophysical Union, 76(33): 329-329.
Wu W N, Kao H, Hsu S K, et al. 2010. Spatial variation of the crustal stress field along the Ryukyu-Taiwan-Luzon convergent boundary. Journal of Geophysical Research: Solid Earth, 115(B11): B11401. DOI:10.1029/2009JB007080
Wu W N, Lo C L, Lin J Y. 2017. Spatial variations of the crustal stress field in the Philippine region from inversion of earthquake focal mechanisms and their tectonic implications. Journal of Asian Earth Sciences, 142: 109-118. DOI:10.1016/j.jseaes.2017.01.036
Xu Z G, Huang Z C, Wang L S, et al. 2016. Crustal stress field in Yunnan: Implication for crust-mantle coupling. Earthquake Science, 29(2): 105-115.
Zhang J S, Shan X J, Li J H, et al. 2005. Recent deep subducting of continetal crust in Pamier: An interpretation on seismotectonics and geodynamics. Acta Petrologica Sinica (in Chinese), 21(4): 1215-1227.
Zhang L P, Shao Z G, Li Z H. 2014. Dynamic action of mutual subduction between the Indian and the Eurasia plates in Hindu Kush-Pamir region. Chinese Journal of Geophysics (in Chinese), 57(2): 459-471. DOI:10.6038/cjg20140212
Zheng J C, Wang P, Li D M, et al. 2013. Tectonic stress field in Shandong region inferred from small earthquake focal mechanism solutions. Acta Seismologica Sinica (in Chinese), 35(6): 773-784.
Zhou Y, He J K, Oimahmadov I, et al. 2016. Present-day crustal motion around the Pamir Plateau from GPS measurements. Gondwana Research, 35: 144-154. DOI:10.1016/j.gr.2016.03.011
Zubovich A V, Wang X Q, Scherba Y G, et al. 2010. GPS velocity field for the Tien Shan and surrounding regions. Tectonics, 29(6): TC6014. DOI:10.1029/2010TC002772
崔华伟, 万永革, 黄骥超, 等. 2017. 2015年3月新不列颠MS7.4地震震源及邻区构造应力场特征. 地球物理学报, 60(3): 985-998. DOI:10.6038/cjg20170313
高熹微, 万永革, 黄骥超, 等. 2015. 内蒙古阿拉善左旗MS5.8地震的构造应力场和静态库伦应力变化分析. 华北地震科学, 33(2): 48-54. DOI:10.3969/j.issn.1003-1375.2015.02.009
黄骥超, 万永革, 盛书中, 等. 2016. 汤加—克马德克俯冲带现今非均匀应力场特征及其动力学意义. 地球物理学报, 59(2): 578-592. DOI:10.6038/cjg20160216
雷建设, 周蕙兰, 赵大鹏. 2002. 帕米尔及邻区地壳上地幔P波三维速度结构的研究. 地球物理学报, 45(6): 802-811. DOI:10.3321/j.issn:0001-5733.2002.06.007
李祥, 万永革, 崔华伟, 等. 2016. 2015年智利MW8.3地震震源区构造应力场分析. 地震学报, 38(6): 847-853.
楼小挺, 刁桂苓, 叶国扬, 等. 2007. 帕米尔—兴都库什地区中源地震的空间分布和震源机制解特征. 地球物理学报, 50(5): 1448-1455. DOI:10.3321/j.issn:0001-5733.2007.05.020
罗艳, 赵里, 曾祥方, 等. 2015. 芦山地震序列震源机制及其构造应力场空间变化. 中国科学:地球科学, 45(4): 538-550.
宁杰远, 臧绍先. 1990. 帕米尔—兴都库什地区地震空间分布特征及应力场特征. 地球物理学报, 33(6): 657-669. DOI:10.3321/j.issn:0001-5733.1990.06.005
孙文斌, 和跃时, 常征, 等. 2009. 帕米尔—兴都库什地区板块俯冲及其应力状态. 地震地质, 31(2): 207-217. DOI:10.3969/j.issn.0253-4967.2009.02.001
唐兰兰, 赵翠萍, 王海涛. 2012. 2008年新疆乌恰6.8级地震序列震源特征及帕米尔东北缘应力场研究. 地球物理学报, 55(4): 1228-1239. DOI:10.6038/j.issn.0001-5733.2012.04.018
万永革. 2015. 联合采用定性和定量断层资料的应力张量反演方法及在乌鲁木齐地区的应用. 地球物理学报, 58(9): 3144-3156. DOI:10.6038/cjg20150911
万永革. 2016. 地震学导论. 北京: 科学出版社.
万永革, 盛书中, 许雅儒, 等. 2011. 不同应力状态和摩擦系数对综合P波辐射花样影响的模拟研究. 地球物理学报, 54(4): 994-1001. DOI:10.3969/j.issn.0001-5733.2011.04.014
张家声, 单新建, 李建华, 等. 2005. 帕米尔地区现今大陆深俯冲—地震构造和动力学解释. 岩石学报, 21(4): 1215-1227.
张浪平, 邵志刚, 李志海. 2014. 印度板块与欧亚板块在兴都库什帕米尔地区相互俯冲的动力作用分析. 地球物理学报, 57(2): 459-471. DOI:10.6038/cjg20140212
郑建常, 王鹏, 李冬梅, 等. 2013. 使用小震震源机制解研究山东地区背景应力场. 地震学报, 35(6): 773-784. DOI:10.3969/j.issn.0253-3782.2013.06.001