龙门山中南段地壳上地幔三维密度结构

引用本文

徐志萍, 王夫运, 姜磊, 赵延娜, 杨利普, 唐淋. 龙门山中南段地壳上地幔三维密度结构[J]. 地震地质, 2019,41(1): 84-98.
XU Zhi-ping, WANG Fu-yun, JIANG Lei, ZHAO Yan-na, YANG Li-pu, TANG Lin. THE THREE DIMENSIONAL DENSITY STRUCTURE OF CRUST AND UPPER MANTLE IN THE CENTRAL-SOUTHERN PART OF LONGMENSHAN[J]. SEISMOLOGY AND GEOLOGY, 2019,41(1): 84-98. 复制到剪切板

DOI:10.3969/j.issn.0253-4967.2019.01.006
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龙门山中南段地壳上地幔三维密度结构

徐志萍¹, 王夫运^1,^*, 姜磊¹, 赵延娜¹, 杨利普¹, 唐淋²

1中国地震局地球物理勘探中心, 郑州 450002

2四川省地震局, 成都 610041

*通讯作者: 王夫运, 男, 1962年生, 研究员, 固体地球物理学博士, 长期从事深地震测深综合研究工作, E-mail: fuyunwang@x263.net。

〔作者简介〕徐志萍, 女, 1987年生, 2012年于中国地质大学(武汉)获地球探测与信息技术专业硕士学位, 工程师, 主要研究方向为重磁电方法与深部结构构造, 电话: 0371-63757378, E-mail: xuzhipingcom@126.com。

收稿日期: 2018-05-23

基金项目: 国家自然科学基金(41774072)与川滇国家地震监测预报实验场项目(2016CESE0103)共同资助

摘要

基于高精度布格重力异常资料, 以川滇地区 P波速度三维层析成像结果为约束建立初始模型, 采用预优共轭梯度(Preconditional Conjugate Gradiem, PCG)反演方法得到了龙门山断裂带中南段的地壳上地幔(深度范围0~65km)三维密度结构(网格间距为10km(横向)×10km(纵向)×5km(深度))。密度成像结果表明: 龙门山断裂带中南段两侧地壳密度结构存在明显差异, 四川盆地有约10km厚的低密度沉积层, 松潘-甘孜块体因沉积层较薄, 且部分地区有基岩出露, 上地壳表现为高密度结构; 松潘-甘孜块体中、下地壳有大范围低密度层分布, 介质强度明显低于高密度的四川盆地, 青藏高原东移物质受到四川盆地阻挡后更易于在低密度的一侧发生挤压形变及隆升, 从而形成龙门山逆冲推覆构造带; 龙门山断裂带内部在地壳结构上具有明显的分段特征, 表现为沿着龙门山断裂带地壳密度变化不连续, 以汶川地震和芦山地震震中为界, 形成多个高、低密度异常区; 同时, 结合地震精定位结果分析, 汶川地震及其余震多分布于壳内中央断裂带西侧高密度体内, 芦山地震及其余震则集中在地壳密度变化梯级带附近并偏向高密度体一侧。四川盆地下地壳密度较高, 其前缘随深度增加向青藏高原方向扩展, 在上地幔顶部接近龙门山断裂带以西。松潘-甘孜块体中、下地壳虽然有一定规模的低密度体分布, 但其连通性差, 在平面上多形成局部低密度异常区, 是否存在下地壳流仍无法给出明确的证据。

关键词: 三维密度结构; 预优共轭梯度; 龙门山中南段; 汶川地震; 芦山地震

中图分类号:P315.3⁺1 文献标志码:A 文章编号:0253-4967(2019)01-0084-15

THE THREE DIMENSIONAL DENSITY STRUCTURE OF CRUST AND UPPER MANTLE IN THE CENTRAL-SOUTHERN PART OF LONGMENSHAN

XU Zhi-ping¹, WANG Fu-yun¹, JIANG Lei¹, ZHAO Yan-na¹, YANG Li-pu¹, TANG Lin²

1)Geophysical Exploration Center, China Earthquake Administration, Zhengzhou 450002, China

2)Sichuan Earthquake Agency, Chengdu 610041, China

Abstract

In recent years, strong earthquakes of M_S8.0 Wenchuan and M_S7.0 Lushan occurred in the central-southern part of Longmenshan fault zone. The distance between the two earthquakes is less than 80 kilometers. So if we can obtain the inner structure of the crust and upper mantle, it will benefit us to understand the mechanism of the two earthquakes.

Based on the high resolution dataset of Bouguer gravity anomaly data and the initial model constrained by three-dimensional tomography results of P-wave velocity in Sichuan-Yunnan region, with the help of the preconditioned conjugate gradient(PCG)inversion method, we established the three dimensional density structure model of the crust and upper mantle of the central-southern segment of Longmenshan, the spatial interval of which is 10 kilometers along the horizontal direction and 5 kilometers along the depth which is limited to 0~65km, respectively. This model also provides a new geophysical model for studying the crustal structure of western Sichuan plateau and Sichuan Basin. The results show obvious differences in the crustal density structure on both sides(Songpan-Ganzê block and Sichuan Basin)of Longmenshan fault zone which is a boundary fault and controls the inner crustal structure. In Sichuan Basin, the sedimentary layer is represented as low density structure which is about 10km thick. In contrast, the upper crust of Songpan-Ganzê block shows a thinner sedimentary layer and higher density structure where bedrock is exposed. Furthermore, there is a wide scale low density layer in the middle crust of the Songpan-Ganzê block. Based on this, we inferred that the medium intensity of the Songpan-Ganzê block is significantly lower than that of Sichuan Basin. As a result, the eastward movement of material of the Qinghai-Tibet plateau, blocked by the Sichuan Basin, is inevitably impacted, resulting in compressional deformation and uplift, forming the Longmenshan thrust-nappe tectonic belt at the same time. The result also presents that the crustal structure has a distinct segmental feature along the Longmenshan fault zone, which is characterized by obviously discontinuous changes in crustal density. Moreover, a lot of high- and low-density structures appear around the epicenters of Wenchuan and Lushan earthquakes. Combining with the projection of the precise locating earthquake results, it is found that Longmenshan fault zone in the upper crust shows obvious segmentation, both Wenchuan and Lushan earthquake occurred in the high density side of the density gradient zone. Wenchuan earthquake and its aftershocks are mainly distributed in the west of central Longmenshan fault zone. In the south of Maoxian-Beichuan, its aftershocks occurred in high density area and the majority of them are thrust earthquake. In the north of Maoxian-Beichuan, its aftershocks occurred in the low density area and the majority of them are strike-slip earthquake. The Lushan earthquake and its aftershocks are concentrated near the gradient zone of crustal density and tend to the side of the high density zone. The aftershocks of Lushan earthquake ended at the edge of low-density zone which is in EW direction in the north Baoxing. The leading edge of Sichuan Basin, which has high density in the lower crust, expands toward the Qinghai-Tibet Plateau with the increase of depth, and is close to the west of the Longmenshan fault zone at the top of upper mantle. Our results show that there are a lot of low density bodies in the middle and lower crust of Songpan-Ganzê Block. With the increase of the depth, the low density bodies are moving to the south and its direction changed. This phenomenon shows that the depth and surface structure of Songpan-Ganzê Block are not consistent, suggesting that the crust and upper mantle are decoupled. Although a certain scale of low-density bodies are distributed in the middle and lower crust of Songpan-Ganzê, their connectivity is poor. There are some low-density anomalies in the floor plan. It is hard to give clear evidence to prove whether the lower crust flow exists.

Keyword: 3-D density structure; preconditioned conjugate gradient (PCG) method; central and southern part of Longmenshan; Wenchuan earthquake; Lushan earthquake

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0 引言

受印度板块的挤压, 青藏高原地壳物质东移被稳定的四川盆地阻挡, 使得龙门山地区发生强烈隆升, 沿着青藏高原东缘形成NNE向、全长500km的龙门山造山带。除此之外, 最近深部结构的研究结果显示, 青藏高原东缘的隆升可能还与印度板块深俯冲至地幔转换带形成大地幔楔密切相关(Lei et al., 2009b, 2014, 2016; 雷建设等, 2018)。从构造分区上看, 龙门山造山带位于南北地震带中段, 西邻松潘-甘孜微块体, 东接稳定的扬子地块, 是南北地震带地壳形变和地震活动最强烈的地区之一(图1)。新生代以来, 该区构造运动强烈, 在多期岩浆活动和构造变形作用下, 形成了多个规模和性质不同的活动断裂(Lei et al., 2009a; 刘保金等, 2009; Zhang et al., 2010; 徐锡伟等, 2013)。

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	图 1 龙门山区域构造及地形图Fig. 1 The regional structure and topographic map of Longmenshan.

近年来, 龙门山断裂带中段和南段先后发生了2008年汶川M_S8.0和2013年芦山M_S7.0强烈地震, 2次地震相距80多km, 均为逆冲型地震(Zhang et al., 2009; 易桂喜等, 2016)。为研究它们的深部孕震环境及动力学演化过程, 不少学者在龙门山及其邻区开展了大量地球物理深部探测工作, 包括地壳上地幔三维速度结构(郭彪等, 2009; 雷建设等, 2009; 刘启元等, 2009; 吴建平等, 2009; 胥颐等, 2009)、二维电性结构(王绪本等, 2008; 赵国泽等, 2009)、高精度人工地震探测剖面(王椿镛等, 2003; 嘉世旭等, 2014; 王帅军等, 2015)、地壳上地幔密度结构(张季生等, 2009; 唐新功等, 2012; 张恩会等, 2015; 杨光亮等, 2015), 他们通过分析川西高原和四川盆地地壳上地幔速度、密度、电性结构的差异, 探讨了龙门山断裂带的深部特征以及深部孕震环境。这些成果为更深入认识汶川地震和芦山地震深部构造环境提供了依据。但由于地球物理探测数据本身的限制, 且天然地震台网间距较大, 电法、重力、人工地震探测剖面较少, 密度反演结果空间分辨率较低, 使得人们对汶川地震和芦山地震的深部孕震环境和动力学背景的认知程度仍然十分有限。

本文收集全国地质资料馆 1︰20万、 1︰50万、 1︰100万布格重力资料, 并对其进行重新采样, 得到研究区(28.5° ~32.5° N, 101° ~105° E)范围内点距为10km的布格重力异常数据, 以川滇地区 0.3° × 0.3° 网格P波速度三维层析成像结果(数据来自川滇实验场项目课题组内部交流结果)为初始约束, 经速度-密度转换及插值处理后得到三维网格间距为10km(横向)× 10km(纵向)× 5km(深度)的密度反演初始模型, 利用预优共轭梯度(Preconditional Conjugate Gradiem, PCG)反演方法得到了龙门山断裂带中南段及其周围地区地壳上地幔的三维密度结构, 然后结合汶川地震、芦山地震及其余震的精定位结果探讨了川滇地区强震构造背景、深部孕震环境等问题。本次研究的结果, 可以为川滇地震预报试验场的各类监测、实验和研究项目提供一个地壳结构的基础模型。

1 数据与方法

1.1 数据

为得到地壳上地幔的密度结构, 在进行反演之前, 需消除深部密度差异引起的重力异常。Borwin(1983)提出了地球深部场源深度与重力场球谐表达式位系数关系式:

$Z = \frac{R}{n + 1} (1)$

式中, Z为场源深度, R为地球半径, n为位系数阶数。根据式(1)计算出EGM2008卫星重力模型中低于97阶的重力场, 近似代替深度> 65km的场源引起的重力异常; 用布格重力异常减去该重力场值, 得到基本反映地壳上地幔密度分布信息的剩余重力场, 并对其进行三维密度反演(图2)。获取数据的范围为28.5° ~32.5° N, 101° ~105° E。

如图2所示, 研究区重力异常西低东高, 异常值变化范围-520× 10^-5~-40× 10^-5m/s², 青藏高原地区为低重力异常区, 四川盆地为高重力异常区, 在二者之间沿着龙门山形成1条NE-SW向的重力异常变化梯级带。

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	图 2 重力异常图红色圆点为汶川地震、芦山地震震中; 黑色实线为剖面位置Fig. 2 Gravity anomaly.

1.2 方法

本研究采用了张世晖(2003)提出的改进三维密度成像方法, 该方法将Occam’ s反演方法的光滑因子和包含深度加权信息的预优矩阵结合起来引入反演的目标函数之中。同时引入深度加权函数(Li et al., 1998), 用以克服反演结果趋于地表的现象, 即 “ 趋肤效应” , 改善解的非惟一性, 并提高了运算效率和反演精度。算法上, 该方法使用了基于规则网格的长方体快速正演算法和包含深度加权信息的预优共轭梯度反演算法。下面简要介绍该方法的原理。

假设a、 b、 c分别为长方体单元在x轴、 y轴和z轴方向上的半边长, $(ξ_{c}, η_{c}, ζ_{c})$ 为长方体单元中心点坐标, 在观测点 $P (x, y, z)$ 处的重力异常为(《重力勘探资料解释手册》编写组, 1983)

$Δ g (x, y, z) = σγ \int_{ξ_{1}}^{ξ_{2}} d ξ \int_{η_{1}}^{η_{2}} d η \int_{ζ_{1}}^{ζ_{2}} \frac{(ζ - z)}{r^{3}} d ζ (2)$

式中, $σ$ 为剩余密度, $γ$ 右为万有引力常数, $r = \sqrt[]{ξ^{2} + η^{2} + ζ^{2}},$ 右侧的积分项可写为

$\begin{array}{l} T (x, y, z) = [ξ \cdot \ln (η + r) + η \cdot \ln (ξ + r) + ζ \cdot \arctan (\frac{ζ \cdot r}{ξ \cdot η})] |\begin{array}{l} ξ_{2} \\ ξ_{1} \end{array} |\begin{array}{l} η_{2} \\ η_{1} \end{array} |\begin{array}{l} ζ_{2} \\ ζ_{1} \end{array} (3) \\ \begin{array}{l} ξ_{1} = ξ_{c} - a - x, η_{1} = η_{c} - b - y, ζ_{1} = ζ_{c} - c - z, \\ ξ_{2} = ξ_{c} + a - x, η_{2} = η_{c} + b - y, ζ_{2} = ζ_{c} + c - z, \end{array} \end{array}$

第k层长方体在每1个观测点产生的重力异常为

$Δ g^{(k)} (x_{1} + m \cdot δx, y_{1} + n \cdot δy) = γ \overset{M}{\sum_{i = 1}} \overset{N}{\sum_{j = 1}} σ_{ij} T_{t, p}^{(k)} (4)$

式中,

$t = \{\begin{array}{l} (m + 2) - i & 当 1 \leq i \leq m 时; \\ i - m & 当 m < i \leq M 时; \end{array} p = \{\begin{array}{l} (n + 2) - j & 当 1 \leq j \leq n 时; \\ j - n & 当 n < j \leq N 时; \end{array}$

令k=1, 2, …, L, 就可以计算出所有层长方体单元产生的重力异常值, 将每一个观测点上的重力异常值叠加, 就得到地下模型体在地面上形成的重力异常。

光滑密度成像的目标函数为

$Φ (m) = {(R_{m} Δ m)}^{2} + μ^{- 1} {(Δ d - G Δ m)}^{2} (5)$

令式(5)在满足 $(Δ d - G Δ m) < ε (ε$ 为拟合精度)的条件下, 粗糙度达到最小, 即 ${(R_{m} Δ m)}^{2}$ 为最小值。

深度加权函数可以补偿核函数随深度的自然衰减。Li等(1998)在3-D磁化率成像中采用了深度加权函数:

$w (z) = \frac{1}{(z + z_{0})^{β / 2}} (6)$

其中, $z_{0} 和 β$ 为常数, $z$ 为块体单元中心点埋深, $z_{0}$ 的大小由观测面的高度和网格单元的尺寸决定。我们在设定好观测平面的高度, 并对地下三维空间进行网格单元划分之后, 通过改变 $β 和 z_{0}$ 值, 该深度的加权函数就可以近似地表达核函数随深度的衰减效应。

为降低迭代次数, 提高收敛速度, Van Decar等(1994)提出通过预优矩阵来改善方程组的条件数, 利用预优共轭梯度算法来求解类似 $Am = b$ 的方程组, 将其修改为

$S A^{T} A Δ m = S A^{T} b (7)$

式中, $S 为预优矩阵, 近似等于 (A^{T} {A)}^{- 1}, 即 S (A^{T} A) \approx I (I 为单位矩阵) 。 S (A^{T} A)$ 的奇异值集中分布在对角线上, 非常接近单位矩阵。

2 结果

利用冯锐等(1986)提出的速度-密度转换公式, 将川滇地区0.3° × 0.3° 网格P波速度三维层析成像结果转换成三维密度模型, 作为反演初始模型。同时, 考虑到边界效应的影响, 将反演范围确定为29° ~32° N, 101.5° ~104.5° E, 将最大反演深度确定为65km。在进行三维密度反演之前, 对初始模型进行了插值和正则化处理, 得到立方体网格步长为10km× 10km× 5km的三维密度模型, 其中横向网格大小为10km× 10km, 网格深度为5km, 节点共计23i534个。反演时采用消除了深部场源影响的剩余重力异常(图2), 重力异常网格重采样间距为10km, 对三维空间网格模型进行全三维反演, 经过96次反演迭代后(图3), 最终得到了拟合误差≤ 2.0× 10^-5m/s²的三维密度结构。

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	图 3 反演收敛曲线Fig. 3 Inversion convergence curve.

2.1 三维密度反演结果

图 4给出了龙门山中南段地壳上地幔三维密度反演结果, 其中绿色色阶代表不同深度处地壳的平均密度。

图 4

地壳上地幔三维密度分布图
黑色实线为活动断裂(邓起东等, 2003); 红色圆点为汶川地震、芦山地震震中; 黑色圆点为余震震中; 余震参考文献(朱艾斓等, 2008; 易桂喜等, 2016)

Fig. 4

The 3-D density structure of the crust and upper mantle.

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	图 4 地壳上地幔三维密度分布图黑色实线为活动断裂(邓起东等, 2003); 红色圆点为汶川地震、芦山地震震中; 黑色圆点为余震震中; 余震参考文献(朱艾斓等, 2008; 易桂喜等, 2016)Fig. 4 The 3-D density structure of the crust and upper mantle.

深度为5km的密度分布图像中, 四川盆地大部分地区以低密度异常为主, 龙门山断裂带的茂汶断裂为高密度异常条带, 位于鲜水河断裂带东侧的康定附近存在一明显的高密度区, 结合已有的地质资料(许志琴等, 2007), 高密度异常与龙门山彭灌杂岩和康定杂岩体位置一致, 低密度异常与第四纪沉积层相一致。

深度为10km的地壳密度横向不均匀性明显, 在川西北高原的金川和理县以西表现为局部低密度异常区, 但其密度仍高于四川盆地; 四川盆地的低密度异常区向盆地内部移动, 表明在四川盆地相应区域存在厚约10km的沉积盖层, 在成都附近有一幅值和范围较小的局部高密度异常圈闭; 以汶川地震震中为界, 龙门山断裂带北段和南段密度结构出现明显的分段特征, 其北段的密度总体上高于南段, 与刘启元等(2009)得到的S波速度结果一致; 芦山地震震中附近有一近EW向低密度条状构造带将龙门山中段和南段分开, 沿着龙门山中、南段表现为南北高、中间低的密度分布特征。

15km深度处, 四川盆地低密度异常幅值和范围明显减小, 仅分布在成都南部部分地区; 龙门山断裂带自北向南, 以汶川地震震中和芦山地震震中为界分为3段, 北段、中段和南段均为高密度异常区, 但三者之间密度差异不大, 汶川地震位于龙门山北段和中段2个高密度异常区中间, 此深度处, 该区域的密度略高于地壳的平均密度, 芦山地震位于龙门山中段和南段低密度异常区的边缘, 密度接近该深度处地壳的平均密度。

20km深度处, 龙门山断裂带中段和北段以茂汶断裂为界, 西侧为松潘-甘孜低密度异常区, 且幅值低于上地壳15km处密度值, 表明在松潘-甘孜块体下方中地壳存在低密度层, 该结果与嘉世旭等(2014)的人工地震探测结果具有较好的一致性, 但自北向南, 松潘-甘孜块体内低速层并不连续, 在小金附近仍为高密度异常区; 在龙门山断裂带中、南段之间宝兴附近的低密度异常区范围近一步增加。

25km深度处, 沿着龙门山断裂带自北向南, 松潘-甘孜块体内低密度层连成一片, 幅值与20km处密度值接近, 而四川盆地则为大范围高密度异常区, 且幅值和范围随深度的增加而增加。

30km深度处, 松潘-甘孜块体下方为大范围低密度异常区, 地壳较为软弱, 而四川盆地为高密度异常区, 地壳强度高, 可阻挡松潘-甘孜块体中、下地壳低密度物质向E运移; 在龙门山断裂带南侧的雅安附近有一EW向分布的高密度异常体, 阻断了龙门山断裂带向S延伸。

45km深度处, 根据人工地震探测结果(嘉世旭等, 2014; 王帅军等, 2015), 松潘-甘孜块体在该深度处为下地壳, 呈现出大范围低密度异常, 地壳较为软弱, 仅在小金附近形成局部高密度异常圈闭; 四川盆地上地幔顶部为高密度异常区。

65km深度处, 位于青藏高原东缘的松潘-甘孜块体整体密度较四川盆地偏低, 二者之间的龙门山断裂带为密度变化梯级带, 同时也是莫霍面深度变化梯级带。

表1 汶川地震M≥ 3.0余震及投影深度 Table1 The aftershocks(M≥ 3.0)of Wenchuan earthquake and the projection depth

为进一步分析地壳密度结构与汶川地震、芦山地震之间的关系, 分别将朱艾斓等(2008)、易桂喜等(2016)得到的汶川地震、芦山地震余震定位结果投影到相应深度的地壳密度图像中(图4), 余震投影深度如表1、表2所示。汶川地震余震震源主要分布在深度≤ 20km的区域, 深度15km处地震分布最多, 余震主要发生在龙门山中央断裂带西侧, 以茂县— 北川为界, 南部余震震源主要位于高密度体内, 震源机制以逆冲推覆为主, 兼有少量右旋走滑分量, 北部余震震源位于低密度体内, 震源机制以右旋走滑为主, 兼有少量逆冲分量(朱艾斓等, 2008)。芦山地震余震震源主要分布在深度≤ 20km的区域, 且范围较小, 在地壳10km深处, 余震震源多位于高、低密度变化梯级带上高密度体一侧, 与龙门山断裂带走向一致, 呈条带状分布; 在地壳15km深处, 余震震源主要分布在宝兴低密度异常区东南缘, 震源附近密度接近该深度地壳平均密度; 在20km深处, 芦山地震西北侧垂直龙门山断裂带方向的密度变化等值线走向由NNE向转换为NWW向, 密度值由等于或高于地壳密度变为低密度, 基本没有余震分布, 表明芦山地震余震终止于地壳脆韧转换带上方; 在20~42km深度范围内, 汶川地震在龙门山断裂带北段仍有余震分布, 根据朱艾斓等(2008)的解释结果认为, 25~40km深度范围内的下地壳余震与壳内脆性变形的断裂活动有关, 20~25km深度范围内的缺震层可能对应于龙门山推覆构造带的深部滑脱面。

表2 芦山地震M≥ 3.0余震及投影深度 Table2 The aftershocks(M≥ 3.0)of Lushan earthquake and the projection depth

2.2 部分剖面结果

为进一步研究汶川、芦山地震下方地壳密度结构分布特征, 垂直构造走向过汶川、芦山地震震中切取了2条剖面AA'和BB', 沿构造走向切取了2条剖面CC'和DD'(图2), 结果如图 5所示。

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	图 5 部分剖面密度结构红色圆点为汶川地震、芦山地震震源; 黑色圆点为余震震源Fig. 5 The density structure along some profiles.

AA'剖面: 龙门山断裂带两侧的松潘-甘孜块体和四川盆地具有明显的深、浅构造背景差异, 在上地壳(深度≤ 15km), 松潘-甘孜块体密度较高, 四川盆地密度较低; 在中、下地壳(深度范围20~45km), 松潘-甘孜块体密度较低, 有低密度层分布, 与嘉世旭等(2014)通过人工地震得到的川西北高原(松潘-甘孜块体)下地壳介质速度大幅降低、岩性强烈弱化的结果相对应, 四川盆地中、下地壳密度逐渐增加, 结构稳定, 密度明显高于松潘-甘孜块体; 在龙门山断裂带下方, 莫霍面深度由松潘-甘孜块体内的50~60km迅速抬升至40~45km; 汶川地震发生在上地壳高、低密度变化带中高密度的一侧, 在汶川地震震源下方, 地壳内高、低密度层交互出现, 表明在青藏高原中、下地壳低密度物质东移过程中, 受坚硬的四川盆地阻挡, 向上逆冲并形成龙门山推覆构造带。汶川地震余震投影宽度为70km, 主要分布在上地壳20km深度以上的高密度体内。

BB'剖面穿过龙门山断裂带南段, 过芦山地震震中, 密度结构与AA'基本相同, 四川盆地上地壳为低密度沉积层, 中、下地壳为稳定的高密度结构, 松潘-甘孜块体上地壳为强度较大的高密度介质区, 中、下地壳为相对软弱的低密度介质, 且横向密度不均匀性明显, 芦山地震发生在上地壳内高、低密度变化区域中低密度的一侧。将易桂喜等(2016)给出的芦山地震余震定位结果投影到BB'剖面, 投影宽度为30km, 芦山地震余震发生在上地壳深度范围为9~17km的区域, 位于2个低密度体之间的高密度区域。

CC'剖面与龙门山构造带走向一致, 上地壳沉积层较薄, 甚至有基岩出露; 龙门山构造带自北向南分段特征明显, 以汶川地震震中为界, 将龙门山分为南、北两段, 汶川地震震中位于两侧高密度体中间; 在龙门山断裂带中、南段之间有一明显的低密度体, 结合图 4给出的三维密度分布结果, 芦山地震发生在该低密度体东南侧。沿着龙门山断裂带, 汶川地震余震投影宽度约57km, 且主要分布在上地壳高密度体内, 结合三维密度切片结果分析可知, 由于受到南侧低密度体的阻挡, 余震终止于上地壳的高密度体内。

从图 5中AA'和CC'剖面可以看出, 在20~25km深度范围内存在一缺震层, 其地壳密度接近整个剖面平均密度(绿色), 且为密度变化梯度带, 朱艾斓等(2008)推测该深度可能对应于龙门山构成推覆构造带的深部滑脱面。为进一步分析汶川地震和芦山地震深部构造差异, 沿着龙门山断裂带过2个地震的震中切取了DD'剖面, 可以明显地看出, 汶川地震发生在2个高密度体之间, 莫霍面上隆的部位, 芦山地震余震投影宽度为23km, 主要发生在低密度体内, 莫霍面下凹的部位。

3 结论及讨论

本文根据龙门山中南段及其邻区的布格重力资料, 以川滇地区 0.3° × 0.3° 网格P波速度三维层析成像结果为约束, 将其转换为三维密度模型, 利用预优共轭梯度(Preconditional Conjugate Gradiem, PCG)方法, 反演得到研究区(29° ~32° N, 101.5° ~104.5° E)地壳及上地幔65km深度范围内的三维密度结构, 为研究川西高原和四川盆地的地壳结构提供了新的地球物理学模型。反演结果表明:

(1)松潘-甘孜块体和四川盆地的地壳上地幔密度结构具有显著差异, 龙门山断裂带是它们的边界断裂带, 对两侧地壳结构具有明显的控制作用。

(2)四川盆地上地壳低密度特征表明相应区域存在厚度约10km的松散沉积盖层, 松潘-甘孜块体中、下地壳有低密度层分布, 地壳较为软弱, 青藏高原物质东移受坚硬的四川盆地阻挡发生强烈变形和隆升, 形成龙门山逆冲推覆构造带。

(3)龙门山断裂带在上地壳具有明显的分段特征, 汶川地震和芦山地震均发生在地壳密度变化梯级带附近高密度体一侧。汶川地震余震主要分布在龙门山中央断裂带西侧, 以茂县— 北川为界, 南部余震多位于高密度体内, 以逆冲为主, 北部余震位于低密度体内, 以走滑为主; 芦山地震余震在平面上沿着龙门山南段上地壳密度变化梯级带分布, 分布范围小, 向N终止于宝兴附近EW向展布的低密度体边缘。

(4)在深度30km以下, 四川盆地密度较高, 前缘随深度的增加向青藏高原方向扩展, 在上地幔顶部接近龙门山断裂带以西。

(5)本研究中, 松潘-甘孜中下地壳, 尤其是30~65km深度内有较大范围低密度异常分布(图4), 相比于相邻的四川盆地, 表现出明显的低密度特征, 随着深度的增加, 低密度范围逐渐向S移动, 走向亦发生改变, 表明该区深浅构造不一致, 暗示了地壳与地幔物质运动过程的解耦(熊熊等, 2002)。松潘-甘孜块体中、下地壳存在大范围的相对软弱的结构, 但由于本研究所得结果中低密度体并没有表现出较好的空间连通性, 在平面图上(图4)多以局部低密度异常区为主, 因此, 本文研究成果仅能证明在青藏高原东部存在大范围低密度异常体, 但其是否为下地壳流存在的依据, 仍有待进一步研究。

致谢上海市地震局朱艾斓博士提供了汶川地震余震精定位结果, 审稿专家提出了建设性意见, 在此一并表示感谢!

The authors have declared that no competing interests exist.

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2003

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1986

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2009

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郭彪, 刘启元, 陈九辉, 等. 2009. 川西龙门山及邻区地壳上地幔远震P波层析成像[J]. 地球物理学报, 52(2): 346—355.

GUO

Biao

, LIU

Qi-yuan

, CHEN

Jiu-hui

, et al. 2009. Teleseismic P-wave tomography of the crust and upper mantle in Longmenshan area, west Sichuan[J]. Chinese Journal of Geophysics, 52(2): 346—355(in Chinese).

The 3D P-wave velocity structure of the crust and upper mantle within the depth range of 400 km was obtained by using teleseismic traveltimes data recorded by West Sichuan Seismic Array distributed in Longmenshan region and non-linear tomography inversion technique. For adapting to the complicated structure, the fast marching traveltime calculation method and Tarantola′s inversion method was used. Our results show the tectonic differences of the crustal and upper-mantle structure among the blocks of Chuandian, Songpan-Garzê and Sichuan basin. Our results show that: 1) the crustal structure of the study area correlates with the surface geological features. The Sichuan basin is imaged as a high-velocity feature, while the Songpan-Garzê and Chuandian block as low-velocity feature. The lithosphere thickness of Sichuan basin has lateral variations from 250 km in south part to 100 km in north part. There exist upwelling features under Songpan-Garzê block. 2) Sichuan basin vertically contacts with Chuandian block, but the thickness of Sichuan basin front in Longmenshan area decreases from east to west with the feature of Songpan-Garzê block incursion into Sichuan basin at upper mantle. This feature shows the difference in dynamic mechanism between Sichuan basin with Chuandian block and Sichuan with Songpan-Garzê block. 3) The Longmen Shan faults belt is divided into two parts at Yingxiu: the south part and north part. The great Wenchuan earthquake and aftershocks distribute in the north part of Longmen Shan faults. The eastward extrusion of Tibetan plateau and upper mantle upwelling caused the uplifting of Songpan-Garzê block. Because Yingxiu is located in the south edge of north part of Longmenshan faults belt, it′s possible to accumulate high stress. This should be the deep dynamic background of the Wenchuan earthquake. Based on our results, the velocity structure does not support the subduction of the Sichuan basin downward and channel flow.

1 State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China 2 Hongshan Central Seismic Station, Hebei Earthquake Administration, Xingtai 054300, China

本文利用川西地震台阵记录到的远震P波走时数据和非线性层析成像算法，获得龙门山地区400 km深度范围内的三维P波速度结构.为了适应川西地区复杂的地质结构，本文的层析成像方法采用了快速行进三维走时计算算法和Tarantola非线性反演算法.我们的结果揭示了川滇地块、松潘—甘孜地块和四川盆地三个不同地块构造差异及该区深部动力学特征.本文的研究表明：1）研究区地壳上地幔P波速度结构具有较为明显的分区特征，松潘—甘孜地块和川滇地块岩石圈速度较低，四川盆地岩石圈速度较高，四川盆地的岩石圈厚度从南250 km向北逐渐减薄至100 km.松潘—甘孜地块上地幔存在地幔上涌的特征.2）川滇地块和四川盆地仅是垂直接触关系，而在龙门山地区四川盆地前缘存在减薄的现象，并伴随松潘—甘孜地块上地幔低速物质有侵入四川盆地岩石圈下方的特征，这显示了四川盆地与松潘—甘孜地块和川滇地块的动力学关系的差异.3）以映秀为界，龙门山断裂带被从松潘—甘孜侵入的低速异常分为南北两段：龙门山南段和龙门山北段，汶川大地震及其余震序列均分布在龙门山断裂带的北段.在青藏高原向东挤压和地幔上涌的双重作用下造成松潘—甘孜地块隆升，由于汶川处于龙门山北段的最南端，应力容易在此集中.这些因素可能是汶川 M S 8.0地震的基本动力学背景.本文的结果不支持四川盆地的俯冲及层间流动的动力学模型.

... 为研究它们的深部孕震环境及动力学演化过程, 不少学者在龙门山及其邻区开展了大量地球物理深部探测工作, 包括地壳上地幔三维速度结构(郭彪等, 2009 ...

2014

0.0

... 嘉世旭等, 2014 ...

... 45km深度处, 根据人工地震探测结果(嘉世旭等, 2014 ...

2009

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雷建设, 赵大鹏, 苏金蓉, 等. 2009. 龙门山断裂带地壳精细结构与汶川地震发震机理[J]. 地球物理学报, 52(2): 339—345.

LEI

Jian-she

, ZHAO

Da-peng

, SU

Jin-rong

, et al. 2009. Fine seismic structure under the Longmenshan fault zone and the mechanism of the large Wenchuan earthquake[J]. Chinese Journal of Geophysics, 52(2): 339—345(in Chinese).

In this study we used a large number of P-wave arrival times recorded by the Sichuan permanent and portable seismic stations during January 2001 and June 2008 to obtain a fine three-dimensional P-wave tomographic model of the crust around the Longmenshan fault zone. The results show that there exist obvious differences between south and north of the Wenchuan mainshock in the seismic structure. North of the Wenchuan mainshock there exist much more heterogeneities, where many aftershocks occurred. These results have significantly improved over the previous results suggesting that the Longmenshan fault zone is only a transition zone between different blocks. The source area of the Wenchuan mainshock is underlain by obvious low velocity anomalies, suggesting that there exist fluids within the Longmenshan fault zone. These fluids may affect the occurrence of the large Wenchuan earthquake. The present results provide significantly seismological evidence for the upward intrusion of the lower crustal flow along the Longmenshan fault zone.

1 Institute of Crustal Dynamics, China Earthquake Administration, Beijing 100085, China 2 Department of Geophysics, Tohoku University, Sendai 980-8578, Japan 3 Earthquake Administration of Sichuan Province, Chengdu 610041, China 4 Earthquake Administration of Hebei Province, Shijiazhuang 050021, China

利用2001年1月至2008年6月四川固定地震台网和临时地震台站记录到的大量P波到时资料，反演了龙门山断裂带及周边地区的地壳精细三维P波速度模型. 结果表明，汶川主震以北和以南地区的结构存在较大差异，以北地区的龙门山断裂带具有很强地壳不均匀性，这与该区发生了大量汶川地震的余震相一致. 这些结果有意义地改进了前人对龙门山断裂带仅为不同块体过渡带的认识. 汶川主震震源区下方存在有明显低波速异常体，表明流体可能存在于龙门山断裂带内. 这些流体可能直接影响汶川大震的形成. 本文的成像结果为下地壳流沿龙门山断裂带上浸提供了可靠的地震学证据.

... 雷建设等, 2009 ...

2018

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... 雷建设等, 2018) ...

2009

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刘保金, 张先康, 酆少英, 等. 2009. 龙门山山前彭州隐伏断裂高分辨率地震反射剖面[J]. 地球物理学报, 52(2): 538—546.

LIU

Bao-jin

, ZHANG

Xian-kang

, FENG

Shao-ying

, et al. 2009. High-resolution seismic reflection profile across Pengzhou buried fault in the frontal areas of Longmen Shan[J]. Chinese Journal of Geophysics, 52(2): 538—546(in Chinese).

Pengzhou fault is an important buried fault in the frontal areas of Longmen Shan. In order to investigate the location, characteristics and activities of Pengzhou fault, after the Wenchuan earthquake ( M S 8.0) of May 12,2008, a high-resolution shallow seismic reflection experiment using vibrator sources was carried out to obtain the imaging of Pengzhou buried fault. In this paper,based on the shallow seismic reflection data and in combination with the Petroleum seismic reflection section in same region, the geometry of Pengzhou buried fault and the thicknesses of Cenozoic strata in both sides of the fault was determined. The results show that Pengzhou fault is an active thrust fault running in NE and dipping in NW, and its dip angle is about 58°～62°. This fault extends upwards into Quaternary sediments, and it merges downwards into the west-dipping detachment at depth about 8～10 km. The results also provide seismological evidence for estimating the fault activities and rebuilding the homeland after the disaster.

Geophysical Exploration Center, China Earthquake Administration, Zhengzhou 450002, China

彭州断裂是龙门山山前一条重要的隐伏断裂.为了调查彭州断裂的位置、性质及其活动性，5·12汶川 M S 8.0地震发生后，作者采用可控震源和高精度的地震反射勘探方法，对彭州隐伏断裂进行了高分辨率地震反射成像.本文利用获得的浅层地震剖面资料并结合石油地震反射剖面，给出了彭州断裂的空间展布特征以及断裂两侧的新生代地层厚度.结果表明，彭州断裂为一条走向NE、倾向NW、倾角约为58°～62°的逆断层，该断层向上错断了第四纪沉积层，具有明显的第四纪活动，向下大约在深度8～10 km左右收敛到向西缓倾的滑脱面之上.研究结果为评价断裂的活动性和灾后重建提供了地震学证据.

... 刘保金等, 2009 ...

2009

0.0

刘启元, 李昱, 陈九辉, 等. 2009. 汶川 M_S8. 0地震: 地壳上地幔S波速度结构的初步研究[J]. 地球物理学报, 52(2): 309—319.

LIU

Qi-yuan

, LI

, CHEN

Jiu-hui

, et al. 2009. Wenchuan M_S8. 0 earthquake: Preliminary study of the S-wave velocity structure of the crust and upper mantle[J]. Chinese Journal of Geophysics, 52(2): 309—319(in Chinese).

On 12th of May, 2008, a devastating M S 8.0 event shocking the global world occurred in the Wenchuan region of Sichuan province. Historically, the same kind of earthquakes is very rare inside of the continent. The study on the tectonic environment of this event is crucial for understanding its genesis. From the teleseismic waveform data recorded by the dense large-scale movable seismic array in the western Sichuan by the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration (CEA), we investigate the S-wave velocity structure of the crust and upper mantle within the depth range of 120 km and averaged Poisson′s ratio over the crust beneath 19 stations distributed along the profile of 31°N by using the non-linear receiver function inversion technique. This profile is about 420 km long and crosses the main shock area. Our results show the differences of the crustal structure among the blocks of Chuandian, Songpan-Garze and Sichuan basin. Their main features can be summarized as follows: (1)the crust-mantle boundary in the foreland of the Sichuan basin declines westward and has been deformed obviously; The crustal thickness has lateral variations of 46～52 km; the velocity of middle and lower crust has lateral variations; the Poisson′s ratio averaged over the crust reaches up to 0.28～0.31; however, the crust is hard nearby the Longmen Shan faults, where the Poisson′s ratio averaged over the crust is only 0.2; (2)in the Songpan-Garze block, from the west to the east, the crustal thickness becomes thinner from 60 km to 52 km; in the depth range of 14～50 km exists a wedged low-velocity zone with the S-wave velocity of 2.75～3.15 km/s, whose thickness decreases from ～30 km in the western side to ～15 km in the eastern side; the Poisson′s ratio averaged over the crust containing the low-velocity zone reaches up to 0.29～0.31; (3)the crustal structure of the Chuandian block looks simple to the west of the Xianshuihe faults and the crustal thickness reaches to 58 km; but a high-velocity layer with thickness of ～10 km exists in the depth of 26 km; the Poisson′s ratio averaged over the crust is ～0.25; (4)within the crust beneath the Wenchuan earthquake region exists a high-velocity structure of ～4.0 km/s at the depth of 12～23 km, and below it exists a low-velocity structure, where the Poisson′s ratio averaged over the crust is 0.31～0.32; the aftershocks of the Wenchuan earthquake are mainly distributed in the region of the upper crust with high-velocity structure. Our results manifest that the Songpan-Garze block has a soft and weakened crust, and the westward subduction of the Sichuan basin does not exist. It could be inferred that under the long-term pressure eastward from the Tibetan plateau, the Songpan-Garze block obstructed by the Sichuan basin has been deformed greatly and a large energy of deformation has been accumulated inside before the Wenchuan earthquake and that the upper crust decoupled with the lower crust at the top boundary of the low-velocity zone leads to the listric thrust of the upper crust. The rigid upper crust with high-velocity underneath the Wenchuan earthquake and its adjacent region as well as the obstruction of the Sichuan basin make it possible to accumulate high stress with low strain rate. The vast strain energy accumulated in the long-term deformation within the Songpan-Garze block should be the dynamic source of the Wenchuan great earthquake.

1 State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China 2 Kangding Central Seismic Station, Sichuan Earthquake Administration, Kangding 626001,China

2008年5月12日我国四川省汶川地区发生了震惊世界的 M S 8.0地震.历史上，同类地震在大陆内部极为罕见.该地震深部构造背景的研究对理解其成因极为重要.本文利用中国地震局地质研究所地震动力学国家重点实验室在川西地区布设的大规模密集流动宽频带地震台阵记录的远震P波波形数据和接收函数非线性反演方法，得到了沿北纬31°线的19个台站下方120 km深度范围内的S波速度结构及台站下方地壳的平均泊松比.该观测剖面穿越了主震区，总长度约为420 km. 我们的结果揭示了川滇地块、松潘—甘孜地块和四川盆地三个不同地块构造差异.上述三个地块的地壳结构特征可以概括为：（1）四川盆地前陆壳幔界面向西侧倾斜并有较为明显的横向变形，地壳厚度存在46～52 km的横向变化，中下地壳S波速度存在横向变化，地壳平均泊松比值较高（0.28～0.31），但在龙门山断裂带附近，显示了坚硬地壳的特征,地壳平均泊松比仅为0.2；（2）松潘—甘孜地块地壳厚度由西侧靠近鲜水河断裂的60 km，向东减薄为52 km，在14～50 km深度范围内存在S波速度2.75～3.15 km/s的楔状低速区，其厚度由西侧的～30 km向东逐渐减薄为～15 km，相应区域的地壳平均泊松比高达0.29～0.31；（3）鲜水河断裂西侧,川滇地块地壳结构相对简单，地壳厚度为58 km，并在26 km深度存在约10 km厚度的高速层，地壳内平均泊松比约为0.25；（4）汶川大震区在12～23 km深度上具有近乎4.0 km/s的S波高速结构，而其下方的地壳为低速结构，地壳平均泊松比0.31～0.32，汶川大震的余震序列主要分布在高速介质区域内. 本文的结果表明松潘—甘孜地块的地壳相对软弱；而且并不存在四川盆地向西侧的俯冲.我们认为在青藏高原东向挤压的长期作用下，四川盆地强硬地壳的阻挡作用可导致松潘—甘孜地块内部蓄积很大的应变能量以及上、下地壳在壳内低速层顶部边界的解耦，在龙门山断裂带附近形成上地壳的铲形逆冲推覆.汶川大地震及其邻近区域所具有的坚硬上地壳和四川盆地的阻挡作用为低应变率下的高强度应力积累创造了必要条件，而松潘—甘孜地块长期变形积累的高应变能构成了孕育汶川大地震的动力来源.

... 刘启元等, 2009 ...

2012

0.0

唐新功, 尤双双, 胡文宝, 等. 2012. 龙门山断裂带地壳密度结构[J]. 地震地质, 34(1): 28—38. doi: DOI: 103969/j. issn. 0253-4967. 2012. 01. 004.

TANG

Xin-gong

, YOU

Shuang-shuang

, HU

Wen-bao

, et al. 2012. The crustal density structure underneath Longmenshan fault zone[J]. Seismology and Geology, 34(1): 28—38(in Chinese).

It is of importance to study the crustal density structure of Longmenshan Fault zone for understanding the seismicity of this area. Using commercial software of Geosoft, gravity inversion for 6 lines of Bouguer gravity anomaly data was carried out under the constraint of deep seismic sounding (DSS) data obtained in the last two decades. The depth and distribution of sediments, crustal thickness and Conrad discontinuity of Longmenshan tectonic zone and its adjacent area(100°～105°E, 28°～33°N)are obtained. The inverted results indicate that the crustal density structure of the Longmenshan Fault zone is obviously different between the two sides. The sedimentary layer in western plateau is thinner and base rocks outcrop almost in most parts; while the thickness of sedimentary layer in eastern Sichuan Basin is much larger, in most parts, it is larger than 6km. The Conrad and Moho discontinuities are relatively smooth in both sides of the fault and the depths of them increase gradually from the eastern 24km and 42km to 35km and 67km under Tibetan Plateau, respectively. The whole fault of Longmenshan displays as a SN-direction gravity gradient zone, with each of its interfaces in the crust under the faults being offset. The displacements of the Conrad and Moho discontinuities are 6～7km and 3～5km, respectively. The sharp change in crustal thickness is maybe one of the causes of strong earthquakes in this area.

Key Laboratory of Exploration Technologies for Oil and Gas Resources of MOE, Yangtze University, Jingzhou, Hubei 434023, China

研究龙门山及邻区地壳密度结构对于认识该地区地震活动性具有重要意义。根据龙门山及邻区(100°～105°E,28°～33°N)的布格重力异常资料,选取了跨越龙门山断裂带的6条重力测线,在深地震测深资料约束下,使用Geosoft软件分别反演出了龙门山地区地下的沉积层、康拉德界面和莫霍面的深度分布。研究结果表明:龙门山断裂带两侧的地壳结构明显不同,西面高原地区沉积层较薄,大部分为基岩出露;而东边盆地沉积层明显较厚,多在6km以上。莫霍面和康拉德面在两侧均相对平缓,康拉德面从东部的大约24km增加到青藏高原山区的35km左右;莫霍面深度从东部盆地的大约42km增加到西部青藏高原的67km左右。龙门山断裂带整体表现为一条近SN向的陡变重力梯度带,并在其地壳内各界面均发生错断,莫霍面和康拉德面错断距离分别达6～7km和3～5km。该区地壳的这种陡变和不均匀性是导致地震活动性强烈的主要原因之一。

... 唐新功等, 2012 ...

2003

0.0

... 赵国泽等, 2009)、高精度人工地震探测剖面(王椿镛等, 2003 ...

2015

0.0

... 王帅军等, 2015)、地壳上地幔密度结构(张季生等, 2009 ...

... 王帅军等, 2015), 松潘-甘孜块体在该深度处为下地壳, 呈现出大范围低密度异常, 地壳较为软弱, 仅在小金附近形成局部高密度异常圈闭 ...

2008

0.0

... 胥颐等, 2009)、二维电性结构(王绪本等, 2008 ...

2009

0.0

吴建平, 黄媛, 张天中, 等. 2009. 汶川 M_S8. 0地震余震分布及周边区域P 波三维速度结构研究[J]. 地球物理学报, 52(2): 320—328.

Jian-ping

, HUANG

Yuan

, ZHANG

Tian-zhong

, et al. 2009. Aftershock distribution of the M_S8. 0 Wenchuan earthquake and three-dimensional P wave velocity structure in and around source region[J]. Chinese Journal of Geophysics, 52(2): 320—328(in Chinese).

3-D P-wave velocity structure in crust and uppermost mantle in and around Wenchuan earthquake source region was studied using long-term accumulated seismic travel time data in Sichuan-Yunnan region and aftershock data of Wenchuan earthquake. The result shows that shallow P-wave velocity structure has good correlation with surface geology. Longmenshan fault zone is imaged as high P-wave velocity region in 0~20 km depth. Pengguan complex and Baoxing complex are imaged as two local high velocity anomaly bodies. The upper crustal high velocity anomaly bodies in Longmenshan fault zone control the distribution of aftershocks. At the southern part of the aftershock zone, aftershocks occurred only in the northeast of the high velocity body related to Baoxing complex. In the middle part，the distribution of aftershocks seems to be controlled by the high velocity anomaly body corresponding to Pengguan complex to some extent; In the northeast part, the high velocity body around Ningqiang-Mianxian may prevent the further extension of aftershocks to the northeast. The existence of upper crustal high P-wave velocity zone in Longmenshan implies that the upper crust has relatively high strength, which may play an important role in obstructing the extrusion of Tibetan Plateau material to east, and are prone to accumulate energy in deep depth. Yangzi Block is characterized by high velocity region below 30 km depth, and its front extends to Tibetan Plateau with depth increasing and reaches to the west of Longmenshan fault zone in the lower crust and upper mantle.

Institute of Geophysics, China Earthquake Administration, Beijing 100081，China

利用川滇地区长期积累的地震走时观测资料和汶川地震余震观测资料对汶川地震震源区及周边区域地壳和上地幔P波三维速度结构进行了研究.结果表明，浅部P波速度分布与地表地质之间具有很好的对应关系.龙门山断裂带在20 km以上深度表现为高速异常带，彭灌杂岩体和宝兴杂岩体为局部高速异常区.龙门山断裂带中上地壳的局部高速异常体对汶川地震的余震分布具有明显的控制作用.在余震带南端，余震全部发生在与宝兴杂岩体对应的高速异常体的东北侧；在余震带的中段，与彭灌杂岩体对应的高速异常体在一定程度上控制了余震的分布；在余震带的东北端，宁强—勉县一带的高速异常体可能阻止了余震进一步向东北扩展.龙门山断裂带中上地壳的P波高速异常表明介质具有相对较高的强度，在青藏高原物质向东挤出过程中起到了较强的阻挡作用，有利于深部能量积累.在30 km深度之下，扬子地块具有明显的高速特征，其前缘随深度增加向青藏高原方向扩展，在下地壳和上地幔顶部已达到龙门山断裂带以西.

... 吴建平等, 2009 ...

2002

0.0

熊熊, 滕吉文. 2002. 青藏高原东缘地壳运动与深部过程的研究[J]. 地球物理学报, 45(4): 507—515.

XIONG

Xiong

, TENG

Ji-wen

. 2002. Study on crustal movement and deep process in eastern Qinghai-Xizang plateau[J]. Chinese Journal of Geophysics, 45(4): 507—515(in Chinese).

The eastward movement of crustal material in Qinghai Xizang Plateau is recorded in the eastern part of the plateau,during over 50Ma tectonic history. This fact draws extensive attention in the world. Modern geodetic study,combined with geological survey,exhibits the crustal movement pattern of the region and provides important boundary constraint conditions for the numerical simulation on geodynamics. The obvious discrepancy between the mantle convection stress field in the plateau and adjacent areas derived from gravity anomalies and the surface crustal movement pattern implies that the crust and the mantle are decoupled. Under the assumption of a depth dependant crustal Geology,the numerical simulation exhibits a great difference of crustal thickening in the interior and eastern part of the plateau. The crustal thickening in the eastern plateau is mainly contributed by the lower crust. The discrepancy between the deformation of the mantle and the crust also shows the decoupling of them. It is suggested that the low strength of the lower crust may be the important factor resulting in the decoupling between the crust and the mantle. The knowledge concerning the physics and mechanics properties of the crust and the upper mantle is crucial for the study on the material eastward flow of the plateau.

1. Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077, China; 2. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100101, China

由于青藏高原东部地区记录了高原约50 Ma演化历史中物质东流的构造史,因此受到地学界的广泛重视. 现代大地测量与地质研究结果给出了该区现代地壳运动的图像,为地球动力学数值模拟提供了重要的边界约束条件. 利用重力异常计算的高原及邻区地幔对流应力场与地表地壳运动格局的明显差异表征了高原东部地壳与地幔物质的运动解耦. 基于随深度变化地壳蠕变率的动力学模拟结果显示,高原东部地壳增厚与高原内部存在很大差异,高原东部地壳增厚主要表现为下地壳的增厚,并且地幔形变过程与地表变化也不一致,同样显示出地壳、地幔运动的解耦. 研究表明,下地壳低强度分布可能是导致这种解耦的重要原因,而了解高原东部地壳及上地幔物理力学性质对我们认识高原物质东流至关重要.

... (5)本研究中, 松潘-甘孜中下地壳, 尤其是30~65km深度内有较大范围低密度异常分布(图4), 相比于相邻的四川盆地, 表现出明显的低密度特征, 随着深度的增加, 低密度范围逐渐向S移动, 走向亦发生改变, 表明该区深浅构造不一致, 暗示了地壳与地幔物质运动过程的解耦(熊熊等, 2002) ...

2013

0.0

... 徐锡伟等, 2013) ...

2009

0.0

胥颐, 黄润秋, 李志伟, 等. 2009. 龙门山构造带及汶川震源区的S 波速度结构[J]. 地球物理学报, 52(2): 329—338.

, HUANG

Run-qiu

, LI

Zhi-wei

, et al. 2009. S wave velocity structure of the Longmen Shan and Wenchuan earthquake area[J]. Chinese Journal of Geophysics, 52(2): 329—338(in Chinese).

Using earthquake data from the Sichuan seismic network, the S-wave velocity structure of the Longmen Shan area was reconstructed by a body wave seismic tomography. It is followed by an analysis of the crustal structure along the Longmen Shan fault zone and in the Wenchuan earthquake area. Seismic rupture is well correlated with the Longmen Shan fault zone and the variation of crustal structure on the east and west of the fault zone. On the north of Wenchuan, the upper crust of the Longmen Shan has relatively high strength and is uplifted apparently. The Pengguan massif between Guanxian and Jiangyou is the main area for stress accumulation on the west of the Longmen Shan. The Wenchuan earthquake is located on the southern edge of this area. The rigid basement of the crust of the Sichuan basin underthrusts beneath the Longmen Shan, and the Wenchuan earthquake can be attributed to the collision of its western bulge with the base of the Pengguan massif along the Longmen Shan fault zone near Wenchuan. On the contrary, the Longmen Shan south of Wenchuan has a weak upper crust with lower mechanical strength, where stress is not easy to accumulate, compared with the northern Longmen Shan. It partly explains a lack of aftershocks along the Longmen Shan fault zone south of Wenchuan. Crustal thickening is observed beneath the Longmen Shan and this is because the strong lithosphere of the Sichuan basin obstructs the eastward flow of the weak crust of the eastern Tibetan. A large part of shortening is accommodated in a manner of ductile deformation, which causes the thickening of the deep crust and the downward flex of the Moho, while the upper crust is uplifted and folded, forming high mountains in the Longmen Shan.

1 Institute of Geology and Geophysics, Kay Laboratory of Petroleum Geophysics, Chinese Academy of Sciences, Beijing 100029, China 2 Chengdu University of Technology, State Key Laboratory of Geohazard Prevention & Geoenvironment Protection, Chengdu 610059, China

利用四川地震台网的观测资料和体波地震层析成像方法反演了龙门山地区的S波速度结构，据此分析了龙门山断裂带的地壳结构和汶川震源区的深部构造特征.反演结果表明，地震破裂与龙门山断裂及其两侧的地壳结构差异存在明显的对应关系，汶川以北的龙门山上地壳具备较高的强度且明显抬升，灌县至江油是龙门山西侧应力积累的主要地区，汶川8.0级地震位于其南部边缘；四川盆地的刚性地壳向西俯冲于龙门山之下，其凸出部与造山带古老基底在汶川附近发生碰撞是汶川成为8.0级地震破裂起始点的主要原因.汶川以南的龙门山地区地壳上层具有较大的韧性，岩石强度相对减弱，与龙门山北部相比不易于应力积累和产生破裂，因而汶川以南的龙门山断裂缺少余震活动.龙门山地区地壳厚度明显增加，其原因与中下地壳具备较大的柔韧性有关.由于青藏东部向东挤出时受到四川盆地刚性岩石层的阻挡，龙门山中下地壳的塑性变形和垂向物质的增加导致地壳厚度加大和莫霍面下沉，以此方式吸收了龙门山地区的大部分地壳缩短量，地表则强烈褶皱抬升形成数千米的龙门山脉.

... 胥颐等, 2009)、二维电性结构(王绪本等, 2008 ...

2007

0.0

... 深度为5km的密度分布图像中, 四川盆地大部分地区以低密度异常为主, 龙门山断裂带的茂汶断裂为高密度异常条带, 位于鲜水河断裂带东侧的康定附近存在一明显的高密度区, 结合已有的地质资料(许志琴等, 2007), 高密度异常与龙门山彭灌杂岩和康定杂岩体位置一致, 低密度异常与第四纪沉积层相一致 ...

2015

0.0

... 杨光亮等, 2015), 他们通过分析川西高原和四川盆地地壳上地幔速度、密度、电性结构的差异, 探讨了龙门山断裂带的深部特征以及深部孕震环境 ...

2016

0.0

... 易桂喜等, 2016) ...

2008

0.0

朱艾斓, 徐锡伟, 刁桂苓, 等. 2008. 汶川 M_S8. 0 地震部分余震重新定位及地震构造初步分析[J]. 地震地质, 30(3): 759—767.

ZHU

Ai-lan

, XU

Xi-wei

, DIAO

Gui-ling

, et al. 2008. Relocation of the M_S8. 0 Wenchuan earthquake sequence in part: Preliminary seismotectonic analysis[J]. Seismology and Geology, 30(3): 759—767(in Chinese).

2741 aftershocks of the May 12,2008 M S 8.0 Wenchuan earthquake from May 12 to June 26 were relocated using the double difference algorithm to investigate tentatively the seismotectonic deformation and rupture process of the earthquake.Most relocated aftershocks occurred in the upper crust at depths of 0～20km,and fewer in the lower crust from depth of 25 to 40km,which may correspond to faulting in the lower crust.An aseismic layer appears beneath the Longmenshan thrust nappe zone between 20 and 25km depth after relocation,which may be correlated with the detachment for thrusting of the nappe zone.Spatial distribution of the relocated aftershocks and focal mechanism solutions show that the earthquake ruptured from south to north unilaterally with segmentation characteristics.The seismic belt formed by the aftershocks may be divided into two segments by the Gaochuan stepover in Anxian.The width of the seismic belt becomes narrower from south to north in map view,implying that the dip of the seismic fault may become steeper northward.The focal mechanism solutions suggest that rupturing in the south segment where the initial rupture point locates may be dominated by thrust slip with less component of strike slip,while it may have both thrust slip and dextral strike slip in the north segment.From Beichuan to the northeast end of the rupture zone,the seismic belt cannot be correlated with any fault in the north segment of the Longmenshan thrust nappe zone,even it cuts athwart the Qingchuan Fault.We can conclude that no controlling fault in the north segment of the Longmenshan thrust nappe zone was involved in earthquake generating and rupturing.The hypocenters in this part delineate a near vertical rupturing plane perpendicular to the strike of the seismic belt,and the focal mechanism solutions show great dextral strike slip component.

1. Earthquake Administration of Shanghai Municipality, Shanghai 200062, China; 2. National Center for Active Fault Studies, Institute of Geology, China Earthquake Administration, Beijing 100029, China; 3. Earthquake Administration of Hebei Province, Shijiazhuang 050021, China; 4. Earthquake Administration of Sichuan Province, Chengdu 610041, China; 5. China Seismic Network Center, Beijing 100045, China

利用双差地震定位法对5月12日汶川 M S 8.0地震至6月26日四川地震台网整理形成观测报告的2741个余震进行了重新定位。在此基础上,初步探讨了汶川地震的地震构造及其余震的破裂扩展。重新定位后震源深度主要分布在0～20km间的上地壳,25～40km的下地壳也有少量地震发生,与下地壳存在脆性变形的断裂活动相对应,在20～25km深度范围内的上下地壳之间存在一个明显的缺震层,推测其可能构成推覆构造的滑脱面。从震源分布与震源机制解在空间的变化上,地震破裂由南向北单侧破裂且存在明显的分段活动性,推测可能存在逆冲推覆与右旋走滑破裂相互转换的过程:逆冲推覆滑动主要发生在高川以南的段落上,震源机制解表现为以逆冲为主;地震破裂向北并未沿龙门山推覆构造带北段扩展,而是斜切青川断裂,震源分布刻画的结构面陡直,震源机制解表现为以右旋走滑错动为主。

... 北川为界, 南部余震震源主要位于高密度体内, 震源机制以逆冲推覆为主, 兼有少量右旋走滑分量, 北部余震震源位于低密度体内, 震源机制以右旋走滑为主, 兼有少量逆冲分量(朱艾斓等, 2008) ...

... 从图 5中AA'和CC'剖面可以看出, 在20~25km深度范围内存在一缺震层, 其地壳密度接近整个剖面平均密度(绿色), 且为密度变化梯度带, 朱艾斓等(2008)推测该深度可能对应于龙门山构成推覆构造带的深部滑脱面 ...

2015

0.0

张恩会, 石磊, 李永华, 等. 2015. 基于抛物线密度模型的频率域三维界面反演及其在川滇地区的应用[J]. 地球物理学报, 58(2): 556—565.

ZHANG

En-hui

, SHI

Lei

, LI

Yong-hua

, et al. 2015. 3-D interface inversion of gravity data in the frequency domain using a parabolic density-depth function and the application in Sichuan-Yunnan region[J]. Chinese Journal of Geophysics, 58(2): 556—565(in Chinese).

As an important way to reveal the internal structure of the earth, inversion of the density interface is a main subject of gravity research for a long time. Inversion of field data in the frequency domain provides a quick and efficient data analysis method. Combining the advantages of the frequency domain and parabolic density function, this paper presents a processing method for gravity anomaly data of a three-dimensional underground interface using parabolic density-depth function in the frequency domain. The method we used to make forward modeling of the gravity anomaly is based on the algorithm proposed by Parker and Oldenburg which uses constant density contrast. We substituted the constant density function in gravity forward formula for the parabolic density function. Then the Fast Fourier Transform (FFT) was applied to both sides of the equation. We derived the gravity anomaly formula of 3D density interface with parabolic density contrast after calculating the involved integral. To invert the density interface depth, an iterative algorithm was preformed to adjust the interface depth step by step until the difference between the theoretical anomalies and observations reduced to the threshold value. In addition, in order to avoid the divergence of the algorithm which is difficult to avoid for the calculation in the frequency domain, we also added a low-pass filter to the codes. We applied our method to a theoretical model which has 91×71 grid data with 10 km interval in x -axis and y -axis, and a complex 3D interface with parabolic density contrast variation in depth. In the synthetic test, we compared the method proposed in this paper with the constant, exponential and binomial functions. Besides the points with acute topography fluctuation, the results show that the inversion using parabolic function provides good and stable estimates of the interface depth of the theoretical model with RMS 0.02 km, and that the constant density function inversion gives the worst estimate of the theoretical model. According to the comparison, the choice of density parameters is a key factor in interface inversion. For the density of real situation, which varies more rapidly at smaller depth and less rapidly at larger depth, the method provides better approximations to density interface depth. In addition, we applied our method to invert the satellite regional gravity anomaly data with 10 km interval in x -axis and y -axis for the Sichuan-Yunnan region, which has complex geological structures with a large number of major faults, and negative gravity anomalies varying from -540~-90 mGal. We applied the method to invert the Moho depth below this region using reference level 35.5 km and density contrast -0.63 g·cm -3 at the earth surface with attenuation coefficient 0.0018. We compared the result with receiver function from Li et al. (2014) and other geological information. The differences between results of receiver function and our method at most seismic stations are less than 6 km. Therefore the Moho discontinuation derived from this method shows good correlation with the results of receiver function, other prior geophysical and geological information. Density contrast interface inversion is one of the primary subjects in gravity field research for understanding the earth's interior structure. This work combines the merits of the parabolic density model and the algorithm in the frequency domain, and applies the parabolic density function to the Parker-Oldenburg forward modeling and inversion algorithm. The tests on synthetic data and real data prove the new approach is rapid and effective. By inverting gravity anomaly data using this new approach, we obtained the Moho depth distribution in the Sichuan-Yunnan area, which shows that the Moho interface is shallow in the southeast but deep in the northwest. The Longmen Shan fault zone is a transition zone of the Moho depth ranging from 42 km to 58 km.

1. Institute of Geophysics, China Earthquake Administration, Beijing 100081, China; 2. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 3. CNPC Xibu Drilling Engineering Sulige Gas Field Development, Ordos Uxin Qi 017300, China

密度界面反演作为了解地球内部结构的一种重要方法,长期以来都是重力学研究的主要内容.本文结合抛物线密度模型及频率域算法的优点,将抛物线密度函数应用于Parker-Oldenburg算法,经过理论推导得到了抛物线密度模型的频率域公式,从而建立了基于抛物线密度模型的三维密度界面重力异常正反演的算法和流程.理论模型数据试验表明本方法快速、有效,适用于大多数浅部比深部增加更快的实际地壳密度.研究中还利用该方法对川滇地区重力异常进行了反演,获得了该区的莫霍面深度分布,并与接收函数研究结果进行对比分析,进一步验证了本文方法的正确性和有效性.

... 张恩会等, 2015 ...

2009

0.0

张季生, 高锐, 曾令森, 等. 2009. 龙门山及邻区重、磁异常特征及与地震关系的研究[J]. 地球物理学报, 52(2): 572—578.

ZHANG

Ji-sheng

, GAO

Rui

, ZENG

Ling-sen

, et al. 2009. Relationship between characteristics of gravity and magnetic anomalies and the earthquakes in Longmenshan range and adjacent area[J]. Chinese Journal of Geophysics, 52(2): 572—578(in Chinese).

The gravity gradient belt in the Longmenshan fault zone is part of the great gravity gradient belt surrounding the Tibetan plateau in the north and the east. The density boundary plane, corresponding to the gravity gradient belt, exists beneath the surface. Based on seismic data, the immense boundary plane of density distribution might be extended downward several ten kilometers across the Moho. According to analysis for the Wenchuan earthquake and aftershocks, most of the earthquakes occurred in the upper and middle crust. Therefore, we suggest that the immense boundary plane of density distribution might be divided into two parts in depth. The deeper part in the westernmost of the Longmenshan fault zone is the boundary of lithosphere blocks, which dips steeply toward northwest. In contrast the shallower part located in the upper and middle crust and dips gently northwest. The boundary of the shallower part at the surface coincides roughly with the Anxian-Guanxian fault, the easternmost one of the Longmenshan fault system. The distance is about 40~50 km between the boundary of lithosphere block and the Anxian-Guanxian fault. When brittle part of the continental block move along the density boundary surfaces from boundary of lithosphere block to the tectonic boundary on the surface and rupture, earthquake will occur.  The area around the Qingchuan-Wenchuan-Dujiangyan exhibits negative magnetic anomalies. 〖JP2〗Thrust faulting resulted in the overturn of the strata, 〖JP〗which in turn caused reversal magnetization.

Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

本文通过研究龙门山构造带及邻区的区域重、磁场特征，以及龙门山断裂带的产状等特征，探讨其与地震关系.研究结果表明，龙门山断裂带是环绕青藏高原的重力梯度带的一部分.其对应密度分界面向西北方向倾斜，向下延深数十公里，切穿莫霍面.推测密度分界面分为两段，深部较陡的为岩石圈块体的边界，浅部较缓.基底隆起与凹陷的界线大体与大地构造单元的界线一致.由西部的岩石圈块体的边界至东部在地表的大地构造单元界线之间的距离约为40~50 km.隶属于中上地壳脆性变形层的地质体由岩石圈块体界线沿缓倾的密度界面推覆至地表的大地构造单元的界线处，在此过程中伴随岩层破碎，从而发生地震.龙门山构造带主要部分位于负磁异常区，这种反磁化和退磁的现象，可能与逆冲推覆作用所引起的深部岩层倒转有关.

... 王帅军等, 2015)、地壳上地幔密度结构(张季生等, 2009 ...

2003

0.0

2009

0.0

赵国泽, 陈小斌, 肖骑彬, 等. 2009. 汶川 M_S8. 0 级地震成因三 “层次”分析: 基于深部电性结构[J]. 地球物理学报, 52(2): 553—563.

ZHAO

Guo-ze

, CHEN

Xiao-bin

, XIAO

Qi-bin

, et al. 2009. Generation mechanism of Wenchuan strong earthquake of M_S8. 0 inferred from EM measurement in three levels[J]. Chinese Journal of Geophysics, 52(2): 553—563(in Chinese).

Wenchuan earthquakes occurred on May 12, 2008 is not an independent tectonic event confined in a local regions. It is reasonable to study the deep structure of the crust, upper mantle and mantle transition zone combining between the local and regional observation data for understanding the preparation and development of Wenchuan earthquakes. The analysis on Wenchuan earthquake generation in this paper is on basis of electromagnetic measurement data obtained recent years with considering the other data e.g., seismic data, which are relative to three level of tectonic movements, Pacific plate subduction and penetration, collision of India plate with Tibet plateau and pressure of Songpan-Garzê block upon Sichuan block. The stagnant slabs formed by Pacific plate subduction exist generally in the mantle transition zones under eastern China continent of North-South seismic belts, which can not be ignored when the generation of Wenchuan earthquakes are investigated. Both northward and eastward movements of different blocks in the Tibet caused by collision of India plate appear. Their eastward movements act on the eastern margin of Tibet i.e., mid- and south- sections of North-South seismic belt and affect the shocks occurred there including Wenchuan earthquakes. The south-east-east direction along which Songpan-Garzê block moves is approximately normal to Longmenshan faults and formed a T-geometry structure by both of them. The Longmenshan faults are represented by a large steep electric boundary in the resistivity profile. This kind of pattern of deep structure is prone to accumulate the stress before Wenchuan earthquakes and is a direct factor for generation of the earthquakes.

1 Institute of Geology, China Earthquake Administration, Beijing 100029, China 2 Earthquake Administration of Shandong Province, Jinan 250014,China 3 Earthquake Research Institute, the University of Tokyo, Japan

2008年5月12日汶川 M S 8级地震的发生不是局部地区孤立的构造事件，研究汶川地震的孕震机制，应该把局部分析和区域分析相结合，关注地壳上地幔直至地幔过渡带的深部结构.基于近年来在东北、华北和汶川地震附近地区进行的深部结构电磁探测结果，结合地震学等其他资料，从太平洋板块的俯冲、印度板块的碰撞和松潘甘孜地块的推挤三个“层次”探讨分析汶川特大地震的成因.太平洋板块向亚洲大陆的俯冲作用，导致中国大陆东部地幔过渡带深度较普遍地存在着停滞的板片，它对汶川地震的影响不可忽视.印度板块与青藏高原的碰撞，使组成高原的各地块发生向北和向东的运动，各地块向东的运动作用于南北地震带中南段，影响到该区域的地震活动.松潘甘孜地块向四川地块的推挤，使松潘甘孜地块运动方向和龙门山断裂带形成“丁”字形结构，龙门山断裂带显示为较陡直的电性边界，加剧了汶川地震前的应力积累，可能是汶川地震发生的最直接的诱因.

... 赵国泽等, 2009)、高精度人工地震探测剖面(王椿镛等, 2003 ...

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1983

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2014

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... , 2009b, 2014, 2016 ...

2009

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2016

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2009

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1998

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... 同时引入深度加权函数(Li et al ...

1994

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2010

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... Zhang et al ...

2009

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... 0强烈地震, 2次地震相距80多km, 均为逆冲型地震(Zhang et al ...