地球物理学报  2019, Vol. 62 Issue (2): 520-533   PDF    
西北太平洋俯冲带日本本州至中国东北段应力场反演
李天觉1,2,3, 陈棋福1,2,3     
1. 中国科学院地质与地球物理研究所, 中国科学院地球和行星物理重点实验室, 北京 100029;
2. 中国科学院地球科学研究院, 北京 100029;
3. 中国科学院大学, 北京 100049
摘要:本研究基于Global CMT提供的1196个1976年11月-2017年1月MW>4.6地震矩心矩张量解,对西北太平洋俯冲带日本本州至中国东北段的应力场进行反演计算,得到了从浅表到深部俯冲带应力状态的完整分布.结果显示:俯冲带浅表陆壳一侧应力场呈现水平挤压、垂向拉伸状态,洋壳一侧的应力状态则相反,即近水平拉张、近垂向压缩.沿着俯冲板片向下,应力主轴逐渐向俯冲板片轮廓靠拢,其中位于双地震层(120 km深度附近)之上的部分,主张应力轴沿俯冲板片轮廓展布而又比其更为陡倾;双地震层内的应力模式同典型Ⅰ型双层地震带内的应力模式一致,即上层沿俯冲板片轮廓压缩、下层沿俯冲板片轮廓拉伸;双地震层之下,应力模式逐步转变为主压应力轴平行于俯冲板片轮廓.通观所研究的整个俯冲系统,水平面内主压和主张应力轴基本保持了与西北太平洋板片俯冲方向上的一致性,同经典俯冲板片的应力导管模型所预言的俯冲带应力模式相符;而主张应力轴在俯冲板片表面之下的中源地震深度范围内转向海沟走向,或许同研究区域横跨日本海沟与千岛海沟结合带,改变的浅部海沟形态致使完整俯冲板片下部产生横向变形有关.
关键词: 西北太平洋俯冲带      震源机制      应力场反演     
Stress regime inversion in the northwest Pacific subduction zone, the segment from northern Honshu, Japan to northeast China
LI TianJue1,2,3, CHEN QiFu1,2,3     
1. Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. Institutions of Earth Science, Chinese Academy of Sciences, Beijing 100029, China;
3. University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: The northwestern Pacific region is one of the most typical and concentrated subduction zone on the Earth, and it has been taken as a very important region for subduction zone study. The Benioff zone from northeastern Honshu, Japan to northeast China with abundant deep earthquakes provides a great opportunity for earth science study. Stress regime inversion in the subduction zone using focal mechanisms of earthquakes is an important field in the study of such area. This paper will adopt the Spatial and Temporal Stress Inversion program downloaded from USGS, which is compiled based on damped regional-scale stress inversion technique, to invert the spatial stress regimes in the full subduction zone (segments from its surface part to the deep end) by dividing all the Global CMT solutions of earthquakes between November 1976 and January 2017 into 10 schemes from a near east-west crossing profile.For the spatial stress regimes in the shallow part of the subduction zone, results of our inversion show that horizontal compression dominates on the landward side, while for the seaward side with predominant horizontal tension. Along the downward subducting slab, the principal stress rotates to be slab-parallel gradually. For the upper portion above the double seismic zone (depth of~120 km), the axis of the principal tension stress is parallel with the subducting slab while accompanied by a steeper dip. At the depth of~120 km, the stress regime appears to be the typical Ⅰ of double seismic zones (down-dip compression along the slab in the upper plane, and down-dip tension in the lower plane). For the deep portion of the slab, the stress regime turns to be down-dip compression gradually. For our study subduction zone, the compressional stress axes keep parallel to the subducting direction very well, suggesting significant action of slab pull and weak mantle support below. The tensional stress axes beneath the slab interface with depth range of intermediate-focus earthquakes tend to be parallel with the trench strike. It may be the subducting slab contortion below caused by the Kurile-Japan arc-arc junction that occurred in our study region.
Keywords: Northwest Pacific subduction zone    Focal mechanism    Stress regime inversion    
0 引言

俯冲带作为大洋岩石圈向其相邻板片底部斜插作用区域(Stern,2002; 张克亮和魏东平,2008),其应力分布是构造应力场研究的重要约束条件(谢富仁等,2003).西北太平洋俯冲带是世界上最典型、最集中的俯冲带之一(臧绍先和宁杰远,1996Schellart et al., 2007),从日本本州(Honshu)北部延伸至中国东北的贝尼奥夫带(Benioff zone)连绵不断(宁杰远和臧绍先,1987Zhao et al., 2009),频繁的地震活动为开展西北太平洋俯冲带研究提供了丰富的资料来源.

俯冲带应力场的研究因其特殊的地理位置,常规的应力测量方法(谢富仁等,2003王强等,2014)在此难以开展,基于地震事件的震源机制解分析震源区的应力状态便成为俯冲带应力场研究的主要方法(Isacks and Molnar, 1971宁杰远和臧绍先,1987吴忠良和臧绍先,1989Christova, 2004, 2005, 2015Christova et al., 2004; Kita et al., 2010Hasegawa et al., 2011, 2012Chiba et al., 2012俞红玉等,2013黄骥超等,2016).前人有关西北太平洋俯冲带应力场的研究多集中在九州—琉球海沟(Ryukyu-Kyushu)、伊豆—小笠原海沟(Izu-Bonin)及千岛海沟(Kuril)等俯冲带(吴忠良和臧绍先,1989Christova, 2004, 2005, 2015)及发生2011年日本东北9.0级大地震的日本海沟俯冲带上部(Kita et al., 2010Hasegawa et al., 2011, 2012Chiba et al., 2012俞红玉等,2013),仅宁杰远和臧绍先(1987)孙文斌和和跃时(2004)利用震源机制解统计分析了日本本州北部至中国东北的应力场分布特征.而李圣强等(2013)对2011年日本东北9.0级大地震后2个月发生的中国东北MW5.7深震的震源机制研究表明,该深震可能与日本东北近海9.0级地震活动相关,属于西北太平洋俯冲带动力作用过程中的响应活动.

已有的俯冲带应力场研究成果,丰富了我们对俯冲过程中俯冲板片的形态及俯冲带内部应力状态的认识,但仍缺乏对西北太平洋俯冲带日本本州至中国东北段应力状态的系统分析.本研究基于Hardebeck和Michael(2006)提出的区域阻尼应力场反演算法程序(Spatial and Temporal Stress Inversion,SATSI)(http://earthquake.usgs.gov/research/software/#SATSI),采用Global CMT(Dziewonski et al., 1981Ekström et al., 2012)网站(http://www.globalcmt.org/CMTsearch.html)的地震矩心矩张量解,系统分析西北太平洋俯冲带日本本州至中国东北段的由浅表到深部的应力状态.

1 方法原理

基于地震事件的震源机制解获取一个地区内的应力场分布,一种方法是直接对区域里所有事件的震源机制做统计分析,进行归纳分类.每个地震震源机制解都给出了地震发生时与应力释放有关的PT轴方位信息,当地震发生在新鲜破裂面上时,应力释放的PT轴方位可作为震源区实际应力主轴方位的较好近似(McKenzie,1969Yamakawa,1971).但现实中的许多地震发生在既有的断层面上,并不符合地震发生在新鲜破裂面上的假设.尽管单个地震所显示的与应力释放有关的方位信息往往不能直接等同于发震地区的真实应力主轴方位(McKenzie,1969刘斌,2009),但数值模拟实验结果(许忠淮等,1983)表明,若地震发生在一般预存的薄弱面上,只要发生的地震数目足够多,且产生这些地震的断层面取向比较随机,那么由多个地震的震源机制解求得的平均PT轴方位便可代表震源区应力主轴的方位,故可通过PT轴方位展示来直观检验应力反演结果的可靠性.

此外还有一类方法,它不考虑震源机制解中PT轴方位与应力主轴方位间的对应关系,而是在两个设定的前提下直接建立起震源机制解(断层面几何及滑移信息)同区域应力场之间的解析表达关系.这两个得到普遍接受的设定前提是(Michael,1987):断层会沿着破裂面上剪切应力的方向滑动;研究区内的应力场能够保持“足够的”稳定,这里“足够的”是指研究区内的应力场可以用统一的应力张量来表征,并且在一定的时期内都不会发生改变.依据这两个设定前提,Angelier(1979)Gephart和Forsyth(1984)以及Michael(1984, 1987)等人先后发展出了各自的应力场反演方法.本研究使用Hardebeck和Michael(2006)提出的区域阻尼应力场反演方法进行应力场解算.应力反演结果的精度依靠自助重采样方法(Bootstrap method)进行评估(Efron and Tibshirani, 1986Michael,1987).

反演结果最终以应力张量解的特征值及其对应的特征向量表示,其中特征值σ1σ2σ3分别为最大主压应力、中间主压应力及最小主压应力,它们之间存在关系σ1σ2σ3(以拉张为正).此外,以应力形状因子R=(σ2-σ1)/(σ3-σ1)来表征主应力量值间的相对大小关系.当R值趋于中间值0.5时,表示三个应力主轴方位可完全分辨;而当R值趋于取值端点0或1时,表示压缩或拉张应力状态完全不能分辨,只能确定它在垂直于拉张或压缩应力主轴的平面内.一般认为当R值大于0.5时,获得的应力状态偏压缩性质,而当R值小于0.5时,应力状态偏拉张性质(Guiraud et al., 1989).在展示应力主轴方位及其量值相对大小关系时,采用如图 1所示的主应力空间图样(万永革等,2011黄骥超等,2016),以形象直观地表征区域内的统一应力场.

图 1 主应力空间图样说明 三个主应力方位给定,不同的R值显示出不同形态的主应力空间图样.图中暖色代表压缩应力状态,冷色代表拉张应力状态. Fig. 1 Illustration of the spatial distribution for a given principal stress regime Given the three principal stress directions, different spatial distribution features correspond to different R values. In the picture, warm color represents the compressional stress state, while cold color corresponds to the tensional stress state.

断层面解的准确选取对反演结果有所影响,Michael(1987)Vavryčuk(2014)的正演分析结果表明断层面选取正确与否有时对所得应力方位影响不大,但却会对应力形状因子R产生较为显著的影响.因此,本研究直接采用应力反演给出结果的应力主轴方位解,而对应力形状因子R进行统计分析,取其置信区间内的均值.需要指出的是,在自助重采样的过程中仍然设定等概率随机地选取两组节面解中的其中一组作为真实断层面使用,以期在弱条件下捕捉分区内最为显著的应力模式.

2 数据处理及结果

从Global CMT网站下载1976-11—2017-01时段内MW>4.6的地震目录,将其投影至图 2所示垂直于俯冲带走向的近EW向AB剖面上,然后在该剖面上依据slab1.0(Hayes et al., 2012)的俯冲板片轮廓线将域内所有目录事件按照其位置、深度、矩心空间密集度等标准划分为10个区域,其中对浅部俯冲系统的不同部位如海沟外的大洋板片、弧前大洋板片平缓俯冲段、陡倾俯冲段(不区分垂向上的陆洋板片)、陆洋壳夹角处的地幔楔、岛弧区及弧后等着重做了区分, 每个分区内目录事件数目均不少于10个,如此共选用1196个GCMT目录事件,使用区域阻尼应力反演方法对各个分区内的应力场进行解算.

图 2 研究区域及1976年11月至2017年1月MW>4.6 GCMT解分布(地形数据来自Etopo1,Amante and Eakins, 2009) 研究区域见黑色矩形框内,红色实线AB为深度剖面分区所用剖面,震源机制为下半球等面积投影,按颜色对应于不同的发震深度,虚线为西北太平洋俯冲带等深线(据slab1.0,Hayes et al., 2012),白色实线和白色箭头分别表示板块边界、板块间相对运动方向及速度大小(据NUVEL-1A,DeMets et al., 1994),左下角内插图为区域地质构造概况,其中PA表示太平洋板块、PH表示菲律宾板块、NA表示北美板块、EU表示欧亚板块. Fig. 2 Study area and events distribution (Topography data comes from Etopo1, Amante and Eakins, 2009) Study area is marked by the black rectangle. Red solid line AB shows the near east-west depth profile used for this study, and dash lines represent depth contours of the NW Pacific subducting slab from slab1.0 (Hayes et al., 2012). Focal mechanisms shown here are the lower hemisphere equal-area projections, and different color of compressional quadrant corresponds to different focal depth. White solid line depicts plate boundary and white arrow indicates relative motion between two neighboring plates from NUVEL-1A Model (DeMets et al., 1994). The left-bottom inset shows the regional tectonic background, and PA represents Pacific Plate, PH is Philippine Plate, NA is North America Plate, EU is Eurasia Plate.

确定的十个分区(见图 3)中,R1与R2位于海沟前的大陆地壳一侧,所属位置分别对应于弧后区和岛弧区(Uyeda,1992Hasegawa et al., 1994Yoshida et al., 2015), 分区内可使用的GCMT地震数均不少于20个;R3位于陆壳同洋壳之间所夹的地幔楔中(Hasegawa et al., 1994, 2005Miura et al., 2005),该分区内可用的GCMT事件最少(只有11个);R4~5位于浅部俯冲系统的陆洋板片接触带,R6~10共5个分区均位于大洋岩石圈内,从近地表一直延伸至地幔转换带的下界面.具体而言,R6处于海沟之外的大洋板片内,该区受前端大洋板片俯冲牵带影响,其内可用GCMT事件众多,达到128个.R5段大洋板片从海沟处进入俯冲(Goto et al., 1985Miura et al., 2005),且其俯冲倾角不超过10°(Umino et al., 1995Miura et al., 2005),分区内可用GCMT事件增至374个之多.大洋板片于深部的陡倾俯冲形态在R4内基本成型,确立了其俯冲的方向与倾角(近30°)(Umino et al., 1995Miura et al., 2005),这里拥有最多的可用GCMT事件(481个).R7位于俯冲板片的内弯折处(inner fold hinge, Warren et al., 2015),因其同俯冲板片上界面有一定距离,故推测其位于大洋岩石圈地幔中(Kita et al., 2010),可用的GCMT解相对于R4~6出现了明显的下降(减少至45个),震源分布的集中度也同样出现下降,分布显得发散.沿大洋板片俯冲方向向下的R8内可用的GCMT解与R7基本相当.继续沿俯冲方向向下的R9完全没入地幔中,该分区在空间上的延伸最长,GCMT解分布表现为较为显著的不连续三区段.R10处于地幔转换带内,选用紧靠俯冲板片轮廓线的GCMT事件25个进行分析.

图 3 深度剖面分区示意及反演阻尼系数挑选 将研究区域内所有GCMT事件及俯冲带等深线投影到AB剖面上得到本深度剖面图中的地震事件分布及俯冲板片轮廓线,并分别以圆圈与灰色实线表示;地形曲线做了垂向压缩,以便展示大陆、弧后、岛弧、海沟、大洋板片所在位置.在剖面图中按照地震分布的位置、深度、空间密集度等标准将域内所有地震事件划分在10个分区内,并以不同颜色的圆圈表示,未划入应力分区内的事件以黑色圆圈表示.右下角内插图为挑选反演所用阻尼系数的trade-off曲线,反映着剪切应力方向同断层滑移方向间的拟合误差大小(data misfit)与相邻应力分区内应力模型间差值大小(model length)之间的相对变化关系,最终进行反演时选取曲线拐角处的量值2.0. Fig. 3 Illustration of 10 divisions along the depth profile and selecting the damping parameter Projecting the events′ focus and depth contours of subducting slab onto the profile AB results in this picture including events′ focus and the subducting slab outline in form of circles and gray solid line. Ten schemes are generated according to events′ location, depth and spatial cluster of focus, and each corresponds to the specific color circle. Events which′re outside all ten schemes are plotted as black circles. The topography in the upper panel shows the relative location of continent, back-arc, island arc, trench and oceanic slab. The lower right inset shows the trade-off curve between model length and data misfit in the inversion process while choosing various damping parameters, and the damping parameter is selected as 2.0 here finally.

从trade-off曲线图(见图 3右下角的内插图)中选取反演使用的阻尼系数e=2.0,同时计算e=0非阻尼情形下的应力反演结果,以比对约束阻尼系数e选为2.0时可能带来的过阻尼情况,最终反演所得结果见表 1.表 1所示结果中同时展示了e=2.0与e=0反演所得结果,这里以R1为例说明.为反演R1内应力模式,使用了21个地震的震源机制解,取阻尼系数e=2.0反演,结果显示R1内主压应力轴(最大主压应力σ1轴)指向N96°E,倾伏角4°,应力形状因子R=0.54.而当取e=0时,主压应力轴指向W3°N,倾伏角5°,应力形状因子R=0.50.

表 1 西北太平洋俯冲带日本本州至中国东北段深度剖面应力反演所得最优参数 Table 1 The best fitting parameters of stress regime inverted for the depth-profile division in the study region

图 4中展示了10个分区在西北太平洋俯冲带内所处位置、选用的事件震源机制解、GCMT提供的PBT轴方位解结果、SATSI反演所得结果(应力主轴方位及应力形状因子)及其置信度为95%的置信区间.以图 1所示直观的主应力空间图样综合刻画图 4中反演得到的各分区应力模式见图 5图 6则显示了反演所得的主应力轴方位的平面展布情况.

图 4 西北太平洋俯冲带日本本州至中国东北段应力场反演结果 剖面图中展示了该应力分区内包含的目录事件,事件震源机制为沿AB剖面震源球的里半球等面积投影,其中的红点代表P轴,白点代表T轴,俯冲板片轮廓由俯冲带等深线投影到AB剖面所得,为展示分区内事件震源机制,剖面图绘制时进行了横向拉伸.剖面图右GCMT结果中红、绿及蓝点分别代表PBT轴,SATSI应力反演结果中,红、绿及蓝点分别代表σ1轴、σ2轴和σ3轴方位的置信度为95%的置信区间,×表示最优应力主轴方位.结果投影圆内,细实线为倾角30°的俯冲板片轮廓,点画线为倾角10°的俯冲板片轮廓.柱状统计图取95%置信区间内应力形状因子反演结果,置信区间通过2000次自助重采样获得. Fig. 4 Stress inversion results in the study area Events used in each region have been shown using hemisphere equal-area projections along the profile, and the subducting slab outline is generated by projecting the depth contours of the oceanic slab onto the profile. The profile here is amplified horizontally in order to show events′ focal mechanisms. Right beside each profile, the GCMT solutions are plotted with red, green and blue points for P, B, and T axes of each event respectively, and the SATSI inversion result is shown with red, green and blue points for the σ1, σ2 and σ3 axes, respectively. Cross represents the best fitting principal stress axis. The solid and dot dash lines in both GCMT and SATSI results represent the plane of the subducting slab with dipping 30° and 10° respectively. Histogram depicts the stress shape ratio distribution. All inversion results are given with 95% confidence level obtained by 2000 bootstrap resamplings.
图 5 西北太平洋俯冲带日本本州至中国东北段应力场反演所得主应力的空间图样 归入各应力分区内的地震事件以对应颜色的圆圈表示,俯冲板片轮廓线由俯冲的大洋板片等深线投影至近东西向的AB剖面而得.图中所示的主应力空间图样为沿俯冲带走向视角,并对比展示了反演所用阻尼系数为0.0及2.0时的计算结果.(b)图为(a)图中矩形虚线所圈部分的放大. Fig. 5 The spatial distribution of stress inversion results in the study area Events used in each region have been shown using the corresponding color circles, and the subducting slab outline is generated by projecting the depth contours of the oceanic slab onto the profile AB. The pattern of principal stress spatial distribution shown here is rotated to the strike of main subducting slab, which means the view of presentation is perpendicular to the AB profile. Here, inversion results with damping parameter 2.0 and 0.0 are both shown to discuss the possible over-damping situation. (b) shows the amplified part outlined by dash rectangle in (a).
图 6 西北太平洋俯冲带日本本州至中国东北段应力场反演所得应力主轴方位角结果 图中红色箭头表示σ1轴,蓝色箭头表示σ3轴,中心圆圈颜色对应于相应的应力分区,黑实线为西北太平洋俯冲带等深线(据slab1.0),红点线亦为该俯冲带等深线(据RUM,Gudmundsson and Sambridge, 1998).(a)、(b)为取阻尼系数e=2.0反演所得结果,(b)图为(a)图中虚线右侧部分的放大;(c)、(d)为取阻尼系数e=0.0反演所得结果,(d)图为(c)图中虚线右侧部分的放大. Fig. 6 Azimuth of the principal stress axis results in the study area Here, red arrow represents σ1 axis, while blue arrow indicates σ3 axis. Events used in each region and circles showing the locations for all principal stress axes have been shown using the corresponding color. Black solid line represents depth contours of NW Pacific subducting slab from slab1.0. Red dot line also depicts the subducting slab, while from RUM Model (Gudmundsson and Sambridge, 1998). Inversion results with damping parameter 2.0 and 0.0 are both shown to discuss the possible over-damped situation. (b) shows the amplified part right of the black dash line in (a), and same process is applied for (c) and (d).

反演结果显示各分区应力形状因子基本靠近中间值0.5,未有趋向端点值0与1的情况,表明所得的三个应力主轴方位分辨较好,反演结果中各分区内的应力主轴方位确定.从图 4来看,十个分区中除R2和R8两分区内应力反演结果中的应力主轴95%的置信区间稍大,GCMT提供的PBT轴方位解结果也较发散外,其他各分区内的应力状态较为明确统一,使用区域阻尼反演与不加阻尼反演结果差异较大处也集中在这两个分区里,因此若无特别说明,以下描述与讨论均使用区域阻尼反演的结果.

具体来看,陆壳一侧的弧后区R1内三个应力主轴方位均较集中,其中主压应力轴近EW向水平展布,主张应力轴(最小主压应力σ3轴)WN-ES向近垂直展布.到了岛弧区R2,三个应力主轴的95%置信区间较大,在GCMT解中也未显示出优势的应力主轴方向,这可能与多重作用力汇聚于此导致发生剪切形变的偏应力水平较低(Yoshida et al., 2015)有关;区域阻尼反演得到的结果显示该分区内应力主轴方位相比于R1发生了微小的顺时针旋转,而非阻尼反演结果与之差异较大(显示主压与主张应力轴为近水平展布),考虑到GCMT解结果显示多数T轴方位近垂向展布,这里认为区域阻尼反演的结果更加可信.地幔楔R3内三个主应力轴方位分布集中,其中主张应力轴方位与R2内的情况一致,主压应力轴则出现了水平面内的小角度顺时针旋转.进入到海沟外大洋板片之内的R6,三个应力主轴方位更加集中,相比于大陆侧的R1和R2,这里的主张应力轴转而近EW向水平展布,主压应力轴则近EW向垂直展布.R5内,大洋板片从海沟起进入低倾角俯冲,三个应力主轴方位依然十分集中,在海沟外R6应力模式的基础之上逆时针旋转,逐渐向大洋板片轮廓靠拢.板片陡倾俯冲形态确立的R4段,三应力主轴方位集中且继续逆时针转动,主张应力轴达到比俯冲板片轮廓更为陡倾的状态.在俯冲板片内弯折区域的大洋岩石圈地幔中的R7处应力主轴方位集中,从深度剖面看主压应力轴垂直于板片的俯冲方向,主张应力轴则渐转向海沟的延伸方向,非阻尼反演结果中的这种趋势更加明显.

R8内三个应力主轴95%的置信区间范围也较大,与GCMT给出的PBT轴方位解结果较发散的情形一致,考虑到该深度范围内存在有双层地震带(Hasegawa et al., 1979, 1994张克亮和魏东平, 2008, 2011Kita et al., 2010)的情况,故对该应力分区再行细分分析,即距离俯冲板片表面24 km范围内划为上分区,范围之外划为下分区,且新得到的两个分区内包含的地震事件数目均超过10个,反演(取阻尼系数e=1.0)结果见表 2图 7.从图 7可见,R8的上分区内主压应力轴沿俯冲板片轮廓延伸,下分区内主张应力轴近平行于俯冲板片轮廓展现出与R7的应力模式有些相似之处.

表 2 R8再分区应力场反演所得最优参数 Table 2 The best fitting parameters of stress regime inverted from depth profile for subdivision in region 8
图 7 R8再分区应力反演结果 (a)剖面图中黄色震源球展示了该应力分区内包含的地震事件,事件分区依据矩心位置同俯冲板片表面间的距离,以24 km为界分区,震源机制为沿AB剖面震源球的里半球等面积投影,剖面图右为该分区内SATSI应力反演结果及其与GCMT解的对比,图解意义同图 4.同样为展示分区内事件震源机制,剖面图绘制时进行了横向拉伸. (b)主应力空间图样. Fig. 7 Stress inversion results for subdivision in region 8 (a) Events used in each region have been shown with yellow color using hemisphere equal-area projections along the profile, and division depends on the distance between the subducting slab surface and events′ centroid location. Comparison between GCMT solutions and SATSI results are also shown. Elements′ illustration is same as Fig. 4. Profile here is also amplified horizontally in order to show events′ focal mechanisms. (b) The pattern of principal stress spatial distribution shown here is rotated to the strike of main subducting slab.

R9内三个应力主轴方位比较集中,主张、压应力轴处在近水平面上.R10进入到了地幔转换带内,这里应力主轴方位集中,应力场表现为显著的沿俯冲板片轮廓压缩(Down-dip Compression,DDC)状态.需要注意的是,阻尼反演结果(图 6)显示R10内拉张应力轴近平行于海沟走向,与非阻尼反演结果中的沿板片俯冲方向展布的情况差异较大,可能是阻尼反演过程导致了模型的过平滑,即区内应力场可能被邻区应力模式平滑改造.

3 分析与讨论

根据深度剖面分区反演的结果可以归纳出俯冲带内从浅到深不同部位的应力分布如下(见图 8):浅部陆壳一侧,远离海沟(~500 km)的弧后区存在有水平方向挤压、垂直方向拉张的应力分布.事实上,位于日本岛弧弧后区的日本海早已停止扩张(Sato,1994),且受东侧太平洋板块显著的西向俯冲推挤作用(Pollitz,1986),而处于明显的水平挤压应力环境下(Uyeda,1992Hasegawa et al., 1994Sato,1994).岛弧部位,除承受来自海沟方向而来的板片推挤作用外,随俯冲板片下潜到深部的含水矿物到达一定深度失稳脱水加入到次级(secondary or induced)对流中,岩石发生部分熔融,进而底辟上升形成的热物质上升推力(Uyeda,1992Hasegawa et al., 1994, 2005Stern,2002)也将作用于此,两者的共同作用或造成了该部位应力主轴发生顺时针旋转;此外毗邻本州地区的太平洋沿岸时常发生的大地震事件如2011年311东日本MW9.1大地震,亦会对岛弧区应力状态产生短期内的重新塑造,如由大震前的挤压状态转变为拉张状态(Hasegawa et al., 2012Chiba et al., 2012俞红玉等,2013),因而使用长时期该区GCMT事件反演得到的应力状态涵盖了区域大地震发生后地区应力模式的扰动,GCMT提供的PT轴方位解分散复杂、反演的应力模式置信区间偏大及最优应力模式相较于弧后区的旋转,均可能与此相关.而研究区东部位于海沟之外的浅部大洋板片内,应力环境则相反,这里因前端板片俯冲下行的拖曳作用(slab pull)而发生小幅度弯曲,总体处于近水平向拉伸环境,挤压作用近垂向分布.从海沟起大洋板片进入低倾角俯冲,在这里俯冲板片的弯曲作用(bending)显著,拉张应力指向板片俯冲方向,而其倾角相比于俯冲板片轮廓却更为陡倾,这与九州—琉球海沟俯冲带内100 km以上部位的应力状态类似(Christova,2004).Christova(2004)认为是俯冲板片间的耦合程度弱或者解耦,致使深部地幔物质的阻挡作用难以独自同俯冲板片的拖曳作用相平衡,而出现过剩的竖直向下海沟吸力(trench suction,Forsyth and Uyeda, 1975),导致拉张应力方向于深度剖面内逆时针旋转至更为陡倾的状态.但本研究区所在的日本海沟处,太平洋板块与上覆北美板块间耦合紧密(Uyeda,1992Uchida et al., 2009Christophersen et al., 2015),因此过剩的海沟吸力不能用于解释该处出现的更为陡倾的拉张应力状态.一种可能的解释是,虽然本地区上覆板片同俯冲板片间的强耦合能够抵消掉部分俯冲板片的负浮力作用,但浅部软流圈地幔物质仍然无法对俯冲板片提供足够的支撑,如此过剩的俯冲板片负浮力(或者说海沟吸力)引导了沿俯冲板片轮廓展布的拉张应力方向变得更为陡倾(Toksöz et al., 1973).综合考虑俯冲板片的俯冲速率、俯冲角度及其随深度的变化、俯冲板片负浮力、弯曲作用、抗弯作用(unbending)、上下板片间的耦合作用、周边地幔物质的流变属性及其运动状态等多种因素开展详细的数值模拟研究,将可为确切地判断俯冲带浅部(~ 100 km以上)存在的主张应力轴沿俯冲板片轮廓展布但却比之更为陡倾的现象提供最具说服力的解释.

图 8 西北太平洋俯冲带日本本州至中国东北段应力模式汇总卡通说明 图中箭头表示各区域内应力主轴的方位,粗直线棒示意双层地震带位置,粗曲线棒示意板片内弯折处;为展示俯冲带各部位应力模式,对其120 km以上的部分进行垂向放大,其120 km以下的部分进行横向缩小. Fig. 8 Schematics illustration of the stress regime in the study area Arrows represent the direction of principal stress. Solid bold line represents the location of double seismic zone, while bold curve indicates the inner fold hinge zone. The subduction zone is amplified vertically above 120 km and compressed horizontally below to show stress regimes.

在板片陡倾俯冲形态确立的部位,存在三处情形复杂的区域:陆洋壳夹角处的地幔楔、俯冲板片内弯折处的大洋岩石圈地幔及两者之间的板片陡倾俯冲段.虽然本研究划归到地幔楔内的地震事件偏少,但反演结果显示该处应力主轴方位集中,应力模式与岛弧部位的情形一致.俯冲板片内弯折区域,从平面上看主压应力轴平行于板片俯冲方向,拉张应力转向了海沟延伸方向,此处由于受到板片内弯折区段的挤压作用理应发育沿俯冲方向上的压缩应力环境(Warren et al., 2015),但考虑到深度剖面内垂直于俯冲方向上大洋岩石圈地幔物质变形受到外侧更具刚性的洋壳部分的阻挡,物质变形便有可能转到水平方向上发展(夏永旭,1984),因此俯冲板片内弯折处大洋岩石圈地幔物质可能出现的侧向水平挤出或导致了该方向上的拉张应力状态.陆洋壳交汇区与板片内弯折区两处相间的板片陡倾俯冲段,主张应力轴基本沿板片俯冲方向展布,但达到比俯冲板片轮廓更为陡倾的状态,与板片低倾角俯冲段内的应力场情景相似,成因或许一致.此三区域内的复杂应力状态的更精细分析,有待今后收集更多的中小地震事件的震源机制解,在精定位基础上进行精细的分区应力反演.

在俯冲板片存在有双地震层活动的部位(深度120 km附近区域),反演结果表明靠近俯冲板片表面的主压应力轴沿俯冲板片轮廓延伸,而其下方的主张应力轴则与俯冲板片轮廓近平行,这与典型的Ⅰ型双地震带应力模式(Hasegawa et al., 1979, 1994张克亮等和魏东平, 2008, 2011Kita et al., 2010):即上层DDC、下层沿俯冲板片轮廓拉张(down-dip tension, DDT)相符.考虑到板片内弯折区域内的应力状态与双地震带下层DDT应力模式的相似性及其所处的位置,俯冲板片内弯折区域发生的地震极有可能也位于双地震带内(Hasegawa et al., 1979, 1994Kita et al., 2010).

俯冲板片更深处的应力场表现为DDC为主的状态,与俯冲下行的板片受到深部地幔物质阻挡进而内部产生压缩(Isacks and Molnar, 1971Forsyth and Uyeda, 1975),并开始在660 km间断面处平躺(何建坤和刘福田,1998Zhao et al., 2009)的判断相符.

通观所研究的整个俯冲系统,水平面内主压、主张应力轴基本保持了与西北太平洋板片俯冲方向上的一致,同经典的俯冲板片应力导管(stress guide)模型(Isacks and Molnar, 1971Christova,2004)预言的俯冲带应力模式相符.其中,主压应力轴状态稳定、一致性非常好,均同板片俯冲方向一致,这一点在各应力分区内应力形状因子普遍大于0.5的结果中亦有体现.而主张应力轴在位于俯冲板片表面下方的下层地震带(俯冲板片内弯折处大洋岩石圈地幔及深度120 km附近的双地震带下层)及更深处,出现了转向海沟延伸方向的趋势,表明张应力状态于俯冲板片内部较为“敏感”,易遭受横向作用的改造.这种横向作用除了前文给出的板片内弯折段可能存在的物质侧向挤出作用外,研究区域横跨的日本海沟与千岛海沟结合带(Kumira,1986DeMets,1992)出现的浅部海沟形态改变亦可能造成完整俯冲板片下部的横向变形(Isacks and Molnar, 1971).前人(Minamino and Fuji, 1981Takeuchi et al., 2008)研究已发现海沟结合带附近的浅部应力场较其周边远离结合带的部位变化明显,即由远离结合带部位的平行于俯冲方向挤压转为靠近结合带部位的垂直于俯冲方向挤压,而Minamino和Fuji (1981)的数值模拟结果显示浅部拉张应力亦转向了海沟延伸方向.海沟结合带(或岛弧走向转变处)附近,深部拉张应力趋向海沟的延伸方向在阿努阿图海沟(Vanuatu,Christova et al., 2004)和阿留申海沟(Aleutian,Stauder,1968)均有发现,Stauder和Mualchin (1976)Christova(2015)在千岛海沟俯冲带西南缘靠近日本海沟的中源地震深度范围内,也观察到了沿海沟走向的拉张作用存在.图 4所示的岛弧区R2内的PBT轴略显发散的结果亦可能与研究区横跨日本和千岛海沟结合带有关.

4 结论

本研究使用USGS网站提供的SATSI应力反演程序,对从Global CMT网站下载的1976-11—2017-01时段内发生的MW>4.6地震事件矩心矩张量解数据,按照深度剖面分区后共计使用1196个事件震源机制结果,对西北太平洋俯冲带日本本州至中国东北段的应力场进行了反演计算,反演得到的应力场分布完整展现了俯冲带不同部位的应力状态.通观所研究的整个俯冲系统,水平面内主压和主张应力轴基本保持了与西北太平洋板片俯冲方向上的一致性,同经典俯冲板片的应力导管模型所预言的俯冲带应力模式相符.具体来说,陆壳一侧弧后区及岛弧区应力状态表现为水平压缩和垂向拉伸;洋壳一侧海沟之外应力状态呈现近水平拉伸和近垂向压缩;沿俯冲板片向下至明显发育双地震层之上的部位,俯冲板片表层区域内的应力状态基本为DDT,但拉张应力轴相较于俯冲板片轮廓更为陡倾,而其下部区域中垂直于俯冲板片轮廓的压缩应力模式更为显著,同时也出现了拉张应力转向海沟走向的趋势;而在明显存在双地震层的部位(120 km深度附近),应力模式同典型Ⅰ型双地震带应力模式一致:上层DDC,下层DDT;随着俯冲板片深度继续增加,应力模式便逐渐过渡到主压应力轴指向板片的俯冲方向,显示俯冲板片受到了周边地幔物质的阻挡而产生沿自身轮廓的压缩应力.

基于地震震源机制解来研究俯冲带应力场的分布是目前获取俯冲带应力环境的基础手段,随着地震震源机制解资料的积累及对俯冲板片结构的精细勾勒,将可更准确地系统把握俯冲带的应力分布特征.俯冲带应力场环境,是俯冲板片在俯冲过程中所处状态的一种物理表示,其成因涉及到俯冲板片的几何形态、俯冲速率、俯冲角度、物质组成及与地幔物质的相互作用等诸多方面,完整地揭示俯冲带应力场分布特征及其成因,需要多学科的共同协作研究.俯冲带应力场的数值模拟可较好地融合多学科取得的研究成果,模拟结果既可检验各学科的俯冲带研究结果,也可为俯冲带现今应力环境及其成因解释提供有益的参考.

致谢  感谢万永革老师提供的绘制主应力空间图样程序,感谢两位匿名审稿人仔细审阅了稿件并提出中肯的修改意见使本文得以完善.文中图件采用GMT软件绘制(Wessel and Smith, 1998).
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