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Journal of Tropical Oceanography >

2017 , Vol. 36 >Issue 6: 71 - 81

DOI: https://doi.org/10.11978/2017010

Progress and prospect of anisotropic study of hydrothermal field (49#cod#x000b0;39#cod#x02032;E) on the Southwest Indian Ridge

  • ZHANG Jiazheng , 1 ,
  • ZHOU Jianping 2 ,
  • ZHAO Minghui , 1 ,
  • QIU Xuelin 1
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  • 1. CAS Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Guangzhou 510301, China
  • 2. Second Institute of Oceanography, State Oceanic Administration, People#cod#x02019;s Republic of China, Hangzhou 310012, China;
Corresponding author: Zhao Minghui. E-mail:

Received date: 2017-01-19

  Request revised date: 2017-04-22

  Online published: 2018-01-18

Supported by

National Natural Science Foundation of China (41606064, 41730532, 91428204)

Foundation of China Ocean Mineral Resources Research and Development Association (Y4GQ051001)

Open Foundation of Key Laboratory of Marginal Sea Geology of Chinese Academy of Sciences, South China Sea Institute of Oceanology (MSGL15-03)

Open Foundation of Key Laboratory of Submarine Geosciences, State Oceanic Administration, People#cod#x02019;s Republic of China (KLSG1501).

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热带海洋学报编辑部
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Abstract

Dragon Flag Hydrothermal Field (DFHF, 49#cod#x000b0;39#cod#x02032;E), located on the ultraslow-spreading Southwest Indian Ridge (SWIR), was the first active hydrothermal vent discovered in this area; and it is an essential location for seafloor exploration of polymetallic sulfide resources and deep ocean studying for China. A three-dimensional tomographic study successfully revealed the deep structure characteristics of the DFHF, but it only provided the static velocity information. For comparison, seismic anisotropic study is an effective method for investigating the dynamic mechanism of the DFHF. In this paper, we briefly introduce the oceanic bottom seismometer (OBS) surveys of both active and passive sources. Our preliminary analysis shows a cosine relationship between travel-time residuals and azimuths based on the velocity structure obtained from previous three-dimensional tomography, indicating that there is an anisotropy on the velocity structure at the DFHF; but, the source of the anisotropy is unclear until now. Seismic anisotropic study at the DFHF will contribute greatly to hydrothermal circulation mechanism and dynamic evolution process of hydrothermal field on the SWIR. Hence, we intend to conduct seismic anisotropic study by combining azimuthal seismic anisotropy and shear-wave splitting techniques based on the OBS data generated by the active (airguns) and passive (earthquakes) sources. Combined with three-dimensional velocity model and regional geological background data, anisotropic parameters of fast-wave direction and travel-time difference between fast and slow waves are used to depict the distribution of crustal cracks, stress variation and mantle flow. In addition, hydrothermal circulation mechanism, lithospheric deformation and deep dynamics process are further revealed.

Key words： Southwest Indian Ridge (SWIR); Dragon Flag hydrothermal field; seismic anisotropy; mechanism of hydrothermal circulation

Cite this article

ZHANG Jiazheng , ZHOU Jianping , ZHAO Minghui , QIU Xuelin . Progress and prospect of anisotropic study of hydrothermal field (49#cod#x000b0;39#cod#x02032;E) on the Southwest Indian Ridge[J]. Journal of Tropical Oceanography, 2017 , 36(6) : 71 -81 . DOI: 10.11978/2017010

西南印度洋洋中脊(SWIR)是一类新的尚未充分认识的超慢速扩张板块边界(Dick et al, 2003), 具有明显分段性、显著变化的轴部水深地形、广泛分布的大型拆离断层、大量出露的蛇纹石化橄榄岩和斜向扩张等特征(Cannat et al, 1999, 2006; Zhou et al, 2013), 使之成为研究洋中脊-热点相互作用、岩浆活动、构造作用和热液循环等相互关系的最佳场所(Mendel et al, 2003; 张涛 等, 2011; 李三忠 等, 2015)。2007年在SWIR龙旂热液区(Dragon Flag Hydrothermal Field, DFHF, 49#cod#x000b0;39#cod#x02032;E)发现超慢速扩张洋中脊上的首个活动热液喷口(Tao et al, 2012), 以及国际大洋钻探(IODP)360航次选取SWIR的#cod#x0201c;亚特兰蒂斯浅滩#cod#x0201d;作为钻穿地壳/地幔边界的钻点, 更是将SWIR的研究提到了崭新高度。
龙旂热液区是我国走向深海、走向大洋的重点研究区域, 具有重要的科学意义和资源战略意义。国际海底管理局2011年第17次会议核准了我国10000km2的享有优先开采权的SWIR硫化物勘探合同区(SWIR 49#cod#x000b0;#cod#x02014;53#cod#x000b0;E), 同时要求我国在合同签署8年和10年内分别放弃50%和75%的勘探区面积(陶春辉 等, 2014)。因此, 尽快摸清龙旂热液区的成因模式, 可为合同区的取舍提供科学依据。截至目前, 该区开展过水深地形、浅表地质、地球化学取样、摄像、重力、磁法和地震等调查(李小虎 等, 2008; Zhu et al, 2010; 赵明辉 等, 2010; 张涛, 2010; 韩宗珠 等, 2012; 于淼 等, 2013; 梁裕扬 等, 2014; 陶春辉 等, 2014), 为研究热液系统成因机制积累了丰富的数据资料。
自1977年在东太平洋洋隆21#cod#x000b0;N处发现热液活动和硫化物堆积体以来, 至今在全球洋中脊和弧后扩张中心发现了300多处海底活动热液区和大量的非活动热液区。活动热液区的深入研究发现, 热液喷口分布与海底裂隙发育密切相关。洋中脊附近的海底常出现一系列线性形态的裂缝(Dauteuil et al, 1993; Wright et al, 1995), 它们不仅使岩体的渗透率提升几个数量级, 起到促进岩石圈-水圈物质交换和洋中脊岩浆系统持续冷却的重要作用(Dunn et al, 2001), 而且还会导致洋壳的各向异性渗透, 从而影响热液循环过程及热液喷口分布(Kleinrock et al, 1996)。因此, 摸清这些裂缝的穿透深度、连通性和定向排列程度, 对揭示洋中脊形成及其热液循环机制至关重要。
2010年我国首次在龙旂热液区成功开展了海底地震仪(OBS)深地震探测实验, 其三维层析成像研究成功揭示了该区的深部结构特征(Zhao et al, 2013), 为从地球内部视觉看清热源与热液通道提供了有力证据, 但该研究仅仅提供了静态信息, 其深部动力学机制尚不清楚。地震波各向异性是指地震波穿过各向异性介质后波速、偏振方向等参数所呈现的差异性现象。开展地震波各向异性研究, 可以获得地下矿物性质、介质内部结构、地球内部物质流动和运动方式等多元信息, 是深入解决龙旂热液区成因机制及其深部动力学问题的有效手段。
本文首先简要介绍了龙旂热液区开展的主、被动源实验, 并初步分析了前期三维层析成像的走时残差数据, 发现其走时残差与方位角之间存在显著的余弦三角函数关系, 说明该区存在各向异性, 但各向异性来源尚不明确, 值得深入研究。然后总结了目前常用的两种地震波各向异性研究方法, 并对龙旂热液区的地震波各向异性研究工作进行了展望, 拟结合方位地震波各向异性方法和横波分裂方法, 对主动源和被动源地震数据进行分析, 通过探索龙旂热液区的各向异性来源, 揭示其裂隙展布和深部动力学特征, 进而揭示龙旂热液区的热液循环机制, 为我国硫化物勘探区的取舍和认识超慢速扩张SWIR热液区形成的动力学演化过程提供科学依据。

1 OBS深地震探测研究

1.1 主动源OBS探测实验

为了揭示龙旂热液区的深部结构特征, 2010年1#cod#x02014;3月, 中国大洋21航次第6航段在该区成功实施三维主动源OBS深地震探测实验(图1a), 实验总共投放40台OBS, 最终成功回收38台, 回收率高达95%, 放炮数量达10832炮。通过后期速度结构模拟获得了龙旂热液区的深部结构(Zhao et al, 2013; Zhang et al, 2013), 以横穿裂谷的主剖面Y3Y4为例, 研究显示(图1b, c): 1)南北两翼表现为与拆离断层有关的、强烈的不对称性, 拆离断层至少持续活动了~2.0Myr, 扩展至洋脊南部16km, 这个长期活动的拆离断层为活动热液喷口提供了热液通道; 2)洋脊南翼大洋核杂岩(OCC)发育, 表现为高速特征, 首次定义为龙旂OCC, 主要成分推测为下地壳辉长岩或辉绿岩; 3)新火山洋脊(neo-volcanic ridge, NVR)下方存在低速异常区, 说明该区扩张脊处岩浆充足, 其丰富的岩浆也可能充当热液喷口长期活动的热源。该研究成功揭示了龙旂热液区的热源和热液通道, 为我们从内部视角认识龙旂热液区提供了重要依据, 也为深入研究超慢速扩张SWIR的形成演化机制奠定了基础。

1.2 被动源OBS探测实验

为了揭示龙旂热液区的断裂展布和构造活动特征, 2014年12月至2015年4月, 中国大洋34航次第1、4航段在该区成功实施被动源OBS深地震探测实验(图2a), 实验重点围绕活动热液喷口和OCC布放了4台国产长周期OBS和3台德国产长周期OBS, 最终成功回收5台, 回收率达到70%, 实验期间还通过气枪放炮对OBS位置进行了定位, 保证了OBS位置的精确性。这是继中国大洋30航次以来, 在超慢速扩张SWIR上开展的最为成功的一次被动源探测实验。此次实验采集的地震数据记录时间长达128天, 通过地震数据的初步处理分析, 发现OBS记录到了大量微震数据(图2b), 目前正在开展该区的微震活动研究, 已初步完成微震识别和定位工作, 为利用微震横波震相开展横波分裂研究打下了坚实的数据基础。

2 三维层析成像走时残差分析

Zhao等(2013)利用FAST软件对65634个初至波走时进行了反演, 获得了龙旂热液区的三维速度结构, 为分析龙旂热液区热液喷口的成因机制提供了深部结构证据。然而, FAST软件在进行速度结构模拟时, 并未考虑地下介质中各向异性对观测走时的影响, 而仅仅将观测走时的起伏归因于地下介质的不均一性, 因此, 前期三维层析成像结果获得的只是各向同性速度结构, 潜在的各向异性信息往往蕴藏于走时残差数据中(Hudson, 1981)。因此, 本文分析了65634个走时残差与其方位角的关系, 发现它们之间存在显著的cos(2#cod#x003b8;)余弦函数关系(图3), 说明龙旂热液区的地下介质不仅存在不均一性, 同时还具有各向异性。走时残差最小值所对应的方向(快波方向)近乎平行于扩张脊走向, 暗示其各向异性可能与洋中脊扩张形成的张性断裂有关, 但由于65634个走时残差数据来自不同深度和不同区域, 因此具体的各向异性来源尚不清楚, 有待进一步的深入分析。

3 地震波各向异性研究进展

海底裂隙的发育不仅可以使岩体渗透率提升几个数量级, 而且还能引起各向异性的渗透率, 从而控制着热液循环路径和热液喷口分布(Kleinrock et al, 1996; Dunn et al, 2001; Singh et al, 2006; Hayman et al, 2007)。摸清这些裂隙的深度、连通性和定向排列程度等, 对认识热液系统成因机制至关重要。龙旂热液区的前期研究认为长期活动的拆离断层为活动热液喷口提供了热液通道(Zhao et al, 2013; 张涛 等, 2013), 但是拆离断层的破裂程度有多大?穿透深度有多深?海底广泛分布的断裂构造之间如何相互关联?它们在热液循环中又起何种作用?此外, SWIR作为典型的超慢速扩张洋中脊, 具有特殊性和复杂性, 其水深变化、地壳和岩石圈厚度、构造和成岩发育历史等均明显有别于其他类型的洋中脊, 再加上前期大量热液浊度异常的发现, 特别是龙旂热液区发现了原本以为不可能存在于超慢速扩张洋中脊上的活动热液喷口, 更是让世界各国科学家对其神秘性愈加好奇。龙旂热液区位于SWIR的洋脊轴部, 是岩浆活动、构造活动和热液活动相互作用的复杂区域, 深部动力学演化过程是揭示其成因机制的根本。SWIR为何如此与众不同?其岩浆活动、构造活动和热液活动之间关系如何?深部动力学过程究竟有何特殊性?又通过何种方式影响浅部构造形态?
Fig. 1 Experiment of active source oceanic bottom seismometer (OBS) and velocity modeling at the Dragon Flag Hydrothermal Field (DFHF).

(a) Multi-beam bathymetry map and active-source seismic survey at the DFHF. Origin of local coordinate system is at (49#cod#x000b0;39#cod#x02032;E, 37#cod#x000b0;46#cod#x02032;S), with the X-axis oriented along 82#cod#x000b0;. Red star shows the location of active vents. Black circles with white numbers and thin black dotted lines show the locations of OBSs and shots, respectively. Red line of Y3Y4 shows the location of the profile in (b). Thick white lines with #28 (in red) and #29 (in black) indicate the neo-volcanic ridges (NVRs) for segments 28 and 29 (defined by Cannat et al, 1999). Thick dashed lines indicate the non-transform discontinuities (NTDs). Red star in the inset at the top-right corner shows the location of the seismic survey area. The inset at the bottom-right corner shows the corrugated surface of Dragon Flag oceanic core complex (OCC). (b) The final model of P-wave crustal structure along Profile Y3Y4. Red star is the horizontal projection of active vents. The inverted triangles indicate locations of OBS. Red lines on the Moho mean there are PmP rays being covered. (c) The interpreted geological model. BA: Breakaway; T: Termination; and ODF: Oceanic Detachment Fault (Zhang et al, 2013)

图1 龙旂热液区的主动源OBS探测实验及速度结构模拟

a. 龙旂热液区多波束水深及三维主动源OBS探测实验。笛卡尔坐标原点经纬度为49#cod#x000b0;39#cod#x02032;E, 37#cod#x000b0;46#cod#x02032;S, X轴方向为82#cod#x000b0;; 红色五角星代表活动热液喷口位置, 黑色圆圈和点线分别代表OBS台站和炮点, 红线Y3Y4指示图1(b)剖面位置; 标注#28和#29的白色粗线表示新火山洋脊(NVRs)位置, 白色虚线表示非转换不连续(non-transform discontinuities, NTDs); 右上角插图红色五角星指示龙旂热液区在SWIR上的位置, 右下角插图为龙旂OCC(大洋核杂岩)的波瓦状条痕构造。b. 主剖面Y3Y4的纵波速度模型。红色五角星为活动热液喷口横向投影, 红色倒三角为OBS, Moho面上红线表示有PmP射线覆盖。 c. 主剖面Y3Y4的地质解释模型。红线指示拆离断层(oceanic detachment fault, ODF)位置(Zhang et al, 2013) , BA表示breakaway, T表示Termination

Fig. 2 Experiment of passive source OBS and records of microseismicities at the DFHF.

(a) Red, pink and white triangles represent the locations of imported, domestic and lost OBS, respectively. Red star shows the location of active vents. (b) Filtered four-channel seismic records of OBS B65 and B71 for vertical channel (labeled V), two horizontal (H1 and H2) seismometer channels and hydrophone channel (H). The traces are scaled to equal maximum amplitude and band-pass filtered of 3~20 Hz. The start time of trace was 23:55:15.015 on Feb 18th, 2015. P waves, S waves and PwP phase are indicated by solid red line, dashed red line and dotted gray line, respectively

图2 龙旂热液区的被动源动源OBS探测实验和微震记录

a. 红色、粉红色和白色三角形分别代表进口OBS位置、国产OBS位置和丢失OBS位置, 红色五角星代表活动热液喷口位置。b. 台站B65和B71滤波后四通道地震记录; V、H1、H2和H分别代表垂直分量、两个水平分量和水听器分量, 数据经过归一化和3~20Hz带通滤波, 地震记录初始时间为2015年2月18日23:55:15.015, 纵波(P)、横波(S)和水面反射纵波(PwP)分别用红色实线、虚线和点线表示

Fig. 3 Travel-time residuals plotted against their source-receiver azimuths.

(a) A total of 65634 pair travel-time residuals plotted against their source-receiver azimuths. The 0#cod#x000b0; azimuth corresponds to the general trend of NVR (#28), and n is the number of travel-time residuals. (b) Travel-time residuals binned and plotted against their source-receiver azimuth. The black line shows best-fitting curve of cos2#cod#x003b8;. The circles show the mean residual in each 20#cod#x000b0; bin, and the bars indicate the ranges of a standard deviation. Fast direction is indicated by the triangle with standard error bar located below the upper horizontal axis, and n is the number of bins

图3 走时残差数据与其震源及接收仪器方位角的关系

a．65634对走时残差数据与方位角的关系。0#cod#x000b0;方位角对应于扩张脊走向, n为走时残差数量。 b. 分组后的走时残差数据与方位角的cos2#cod#x003b8;余弦函数关系。黑色曲线为最佳拟合曲线, 白色圆圈为20#cod#x000b0;方位角间隔的平均走时残差, 竖线为一个标准差#cod#x003c3;的误差条, 顶部黑色三角形对应快波方向, n为平均走时残差数量

地震波各向异性研究是解决上述问题的有效手段之一(Barclay et al, 2003; Tong et al, 2004)。通过地壳介质地震波各向异性研究可以刻画裂隙展布特征, 进而反映地壳应力场特征; 地幔地震波各向异性关系到板块运动、地震活动、地球动力学和深部结构等众多地球物理基本问题, 通过地幔介质的地震波各向异性研究可以推测地球内部物质运动模式, 进而探讨岩石圈等地球内部深部动力学问题(高原 等, 2005; 石玉涛, 2014)。目前, 运用比较广泛的地震波各向异性研究方法主要有两类, 一是纵波方位各向异性方法, 二是横波分裂方法。

3.1 纵波方位各向异性

纵波方位各向异性方法是指根据不同方位观测到的不同地震波属性(速度或偏振情况)来判断介质的各向异性, 进而判断地下介质的构造特征(图4), 该方法适用于全方位覆盖的观测系统。根据前人的理论研究(Crampin et al, 1977; Hudson, 1981), 各向异性介质的走时残差与方位角满足函数关系$T_{res}(\theta)=A_{0}+A_{2\theta} cos(2\theta+\alpha)+A_{4\theta}cos(4\theta+\beta)$, 其中, Tres为走时残差, #cod#x003b8;为炮点-接收器(OBS)的方位角, A0、A2#cod#x003b8;和A4#cod#x003b8;分别为常数项、$2\theta $项和$4\theta$项的振幅, #cod#x003b1;和#cod#x003b2;为相位偏移量。通过求取走时残差和方位角的最小二次拟合曲线, 就可获得上述公式的参数值。
Fig. 4 Theoretical diagram of anisotropy inverted from travel-time analysis.

(a) Top view for anisotropic medium. Black dashed lines represent dry cracks, and thin dashed gray lines represent saturated cracks. Black lines and arrows represent axis and spreading direction. (b) Side view for anisotropic medium. Thin dashed gray lines represent saturated cracks. According to different penetrating depths of shot rays, the anisotropic characteristics from different depths can be obtained. (c) The relationship between travel-time residuals and corresponding azimuths. Black, blue and red curves indicate the relationship of cos2#cod#x003b8;, cos4#cod#x003b8; and simultaneous cos2#cod#x003b8; and cos4#cod#x003b8;, respectively. Black line means larger percentage of dry cracks, and shows closer relationship with cos2#cod#x003b8;. Blue dashed line means larger percentage of saturated cracks, and shows closer relationship with cos4#cod#x003b8;. Red curve means equivalent effects of dry and saturated cracks

图4 各向异性的走时分析方法原理示意图

a. 各向异性介质俯视图。黑色粗虚线代表干燥裂隙, 灰色细虚线代表饱和裂隙, 黑色实线为脊轴, 黑色箭头代表扩张方向。b. 各向异性介质侧视图。根据射线穿透深度的不同, 可以分析不同深度介质的各向异性特征。c. 走时残差与方位角的关系图。黑色、蓝色和红色曲线分别表示满足cos2#cod#x003b8;、cos4#cod#x003b8;以及同时满足cos2#cod#x003b8;和cos4#cod#x003b8;关系, 黑色曲线代表干燥裂隙所占比例较大, 更多地表现为cos2#cod#x003b8;形式, 蓝色曲线代表饱和裂隙所占比例较大, 更多地表现为cos4#cod#x003b8;形式, 红色曲线代表干燥裂隙和饱和裂隙其同等作用

对于裂隙介质而言, 当其为干燥裂隙时, 拟合曲线主要表现为cos(2#cod#x003b8;)形式, 而当其为饱和裂隙时, 拟合曲线主要表现为cos(4#cod#x003b8;)形式(White et al, 1984; Barclay et al, 2003); A4#cod#x003b8;和A2#cod#x003b8;的比值还与裂隙的纵横比(aspect ratio)有关, 当影响2#cod#x003b8;项和4#cod#x003b8;项的裂隙方向一致时, 可利用A4#cod#x003b8;/A2#cod#x003b8;来判断裂隙的宽度(Tong et al, 2005); 拟合曲线的极小值所在方位角指示地震波传播的快波方向。虽然上述函数关系的提出是基于裂隙各向异性理论模型, 但它同样适用于地幔橄榄石优势定位等引起的各向异性(Chung, 1992; Dunn et al, 2005)。
纵波方位各向异性方法常见于海洋岩石圈。例如, Hess(1964)通过分析14条0~180#cod#x000b0;的地震剖面的上地幔纵波速度, 发现了上地幔各向异性的存在, 并根据橄榄岩在流动或形变作用下会产生优势定向排列的特性, 推断地幔发生过流动, 解答了海底扩张的动力来源问题; Chung(1992)通过分析日本海东南部Yamato盆地内两条相互垂直的地震剖面记录的Pn震相, 同样发现了上地幔各向异性的存在, 进一步印证了海底扩张的存在; Tong等 (2004)利用东太平洋洋隆(EPR)9#cod#x000b0;N重叠扩张中心西侧扩张脊上的三维层析成像走时残差数据, 分析不同位置由浅到深的各向异性, 从而揭示了该区裂隙展布特征, 并提出了平行扩张脊的海底热液循环模式(图5)。
Fig. 5 Result of azimuthally seismic anisotropy technique on the western limb of the 9#cod#x000b0;N overlapping spreading center on the East Pacific Rise (EPR) (Tong et al, 2004).

(a) Cosine function relationship between travel-time residuals and their azimuths; (b) estimated percentage anisotropy; (c) geometric configuration of along-axis hydrothermal circulation (arrow) consistent with crack structures inferred from anisotropic study

图5 EPR 9#cod#x000b0;N重叠扩张中心西侧扩张脊上的纵波方位各向异性研究成果(Tong et al, 2004)

a. 走时残差与方位角的余弦函数关系; b. 估算的各向异性大小; c. 基于各向异性研究提出的海底热液循环的地质解释模型

3.2 横波分裂

与光学中的双折射类似, 当横波穿过各向异性介质时, 会分裂成两个相互垂直且传播速度不同的快、慢波, 通过分析快波和慢波之间的到时差, 就可以间接获得各向异性大小和各向异性介质层的厚度(图6)。由于横波分裂现象受各向异性的影响非常敏感, 且不受方位角覆盖的限制, 使得横波分裂方法在地震波各向异性研究中运用最为广泛。例如, 在20世纪90年代时, 就有大量学者利用横波分裂测量方法开展对青藏高原上地幔各向异性的研究(McNamara et al, 1994; 郑斯华 等, 1994; Hirn et al, 1995; 吕庆田 等, 1997); 进入21世纪以来, 随着宽频带数字地震台站的增加, 上地幔各向异性的研究更是蓬勃发展(Huang et al, 2000; 姜枚 等, 2001; Flesch et al, 2005; 廖武林 等, 2007; 王椿镛 等, 2007; 吴晶 等, 2007; 常利军 等, 2008, 2015; 王琼等, 2013; 张洪双 等, 2013; 太龄雪 等, 2015; Singh et al, 2016)。
Fig. 6 Shear-wave splitting occurred when a shear wave traveled through an anisotropic medium (W#cod#x000fc;stefeld et al, 2008).

When incident shear wave arrived at an anisotropic medium, it split into two shear waves of perpendicular polarization along seismic fast and slow directions, respectively. Traveling through anisotropic medium, the two waves accumulated a delay time #cod#x003b4;t. Shear-wave splitting techniques inverted for #cod#x003b4;t and the fast polarization direction

图6 横波穿过各向异性介质时发生的横波分裂现象(W#cod#x000fc;stefeld et al, 2008)

当横波遇到各向异性介质时, 会分裂成一快一慢的两个相互垂直的极化波, 而且它们穿过各向异性介质后, 会产生一个时间差#cod#x003b4;t, 横波分裂方法就是求取快极化波和#cod#x003b4;t, 进而揭示各向异性介质的属性

虽然横波分裂研究多用于大陆岩石圈, 但在海洋岩石圈也取得了一定研究成果。Barclay等 (2003)通过在大西洋中脊(MAR)35#cod#x000b0;N轴部区域开展横波分裂研究, 发现快、慢波的到时差与射线路径长短、方位角和OBS位置均无关, 其各向异性主要来源于洋壳浅部(0~3km范围内)发育的一系列垂直、充水的平行裂谷方向定向排列的裂隙(图7)。该研究还表明, 纵波方位各向异性方法虽然具有很好的纵向分辨率, 但是太浅的地方约束不了, 对独立且充水的窄裂隙也不敏感; 而横波分裂方法虽然具有很好的横向分辨率, 对各种裂隙也都很敏感, 但却无法提供深度方面的信息; 因此, 只有联合使用纵波方位各向异性和横波分裂手段, 才能充分发挥两者的高纵向分辨率和高横向分辨率的优势, 才能够更全面地揭示热液循环机制。
Fig. 7 Result of shear-wave splitting at the axis of Mid-Atlantic Ridge (MAR) near 35#cod#x000b0;N (Barclay et al, 2003).

(a) Rose histograms of fast polarization directions for six OBSs. (b) Examples of shear-wave splitting in horizontal particle motions. The shear wave arrivals are shown for earthquakes at four different OBSs (52, 56, 60, and 63), with fast wave moving in a nearly NS direction and slow wave moving in a nearly EW direction. Numbers in parentheses are the earthquake epicenters in minutes of latitude and longitude, respectively. Each trace was 203ms long, with sample points (dots) every 7.8125ms. The open circle is the origin of particle motion

图7 MAR 35#cod#x000b0;N区域的横波分裂研究成果(Barclay et al, 2003)

a. 展示了6个OBS台站的快波方向玫瑰图。b. 以水平质点运动展示的横波分裂示例图。图中展示了4个OBS台站(52、56、60和63)记录到的横波分裂现象, 可见质点近乎先南北向运动, 后东西向运动, 表明快、慢波先后到达; 括号内的标注为震中坐标, 每个质点轨迹长203ms, 采样点(黑色圆点)间隔为7.8125ms, 空心圆代表质点运动起点

4 展望

基于龙旂热液区开展的主动源和被动源OBS探测实验, 下一步将围绕纵波各向异性和横波分裂开展地震波各向异性研究工作。首先利用纵波各向异性方法对前期三维层析成像的65634对走时残差数据进行深入分析, 根据OBS、炮点与射线拐点位置关系, 以及速度界面与射线拐点深度关系, 将走时残差数据进行不同方式的组合, 试图揭示不同区域不同深度上的各向异性特征。然后利用横波分裂方法获得不
同OBS台站下方的各向异性参数-快波方向和快、慢波到时差#cod#x003b4;t。最后将获得的纵波各向异性和横波分裂参数进行对比分析和有机融合, 结合三维速度模型和区域地质背景资料, 深入分析龙旂热液区的裂隙分布和应力场变化特征, 从而揭示该区的热液循环机制、岩石圈形变和深部动力学过程等科学问题, 该研究成果可为我国SWIR硫化物矿区的取舍和认识SWIR热液区形成的深部动力学机制提供科学依据。

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