横向不均匀性对视电阻率各向异性变化的影响和地震前电阻率的变化深度

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解滔, 卢军. 横向不均匀性对视电阻率各向异性变化的影响和地震前电阻率的变化深度[J]. 地震地质, 2020,42(5): 1172-1187.
XIE Tao, LU Jun. THE EFFECTS OF LATERAL INHOMOGENEITY ON ANISOTROPIC CHANGES OF APPARENT RESISTIVITY AND THE DEPTH OF RESISTIVITY CHANGES BEFORE EARTHQUAKES[J]. SEISMOLOGY AND GEOLOGY, 2020,42(5): 1172-1187. 复制到剪切板

DOI:10.3969/j.issn.0253-4967.2020.05.009
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横向不均匀性对视电阻率各向异性变化的影响和地震前电阻率的变化深度

解滔, 卢军

中国地震台网中心, 北京 100045

〔作者简介〕解滔, 男, 1986年生, 2017年于中国石油勘探开发研究院获地球探测与信息技术专业博士学位, 副研究员, 主要从事地震电磁学方面的研究, E-mail: xtaolake@163.com。

收稿日期: 2019-12-12 修回日期: 2020-07-17

基金项目: 中国地震局地震科技星火计划项目(XH19054Y)和冬奥会保障晋冀蒙监测能力提升项目共同资助

摘要

采用对称四级装置的直流视电阻率地表观测, 其深度探测范围与供电极距 AB大致相当。测区地下介质可能存在电性横向不均匀性, 即同一深度范围内介质电阻率在水平方向存在变化。地层的横向不均匀性会引起不同方向的视电阻率观测值出现差异, 但是否会引起各向异性变化还没有定论; 大地震前构造应力对地下分层电阻率的影响能上升到距离地表的深度范围也未得到充分讨论。文中采用有限元数值分析方法, 分别计算横向均匀、横向不均匀模型中不同深度范围内介质电阻率变化时视电阻率的各向异性变化, 认为: 对于供电极距 AB=1 000m的观测, 距离地表约300m以下的地层其电阻率的各向异性变化所产生的视电阻率各向异性变化与实验和震例中的实际观测结果不符; 地层电阻率的变化深度需要上升至近地表的浅层范围, 视电阻率的变化幅度和各向异性变化特征才与实验和观测相吻合。对于横向不均匀地层, 同一深度以下的地层电阻率发生相同幅度的各向同性变化, 视电阻率并不出现各向异性变化, 仅当电阻率出现各向异性变化时视电阻率才出现各向异性变化, 但地层横向不均匀性对各向异性变化特征的影响较小。

关键词: 视电阻率; 横向不均匀; 各向异性变化; 变化深度; 地震

中图分类号:P315.72⁺2 文献标志码:A 文章编号:0253-4967(2020)05-1172-16

THE EFFECTS OF LATERAL INHOMOGENEITY ON ANISOTROPIC CHANGES OF APPARENT RESISTIVITY AND THE DEPTH OF RESISTIVITY CHANGES BEFORE EARTHQUAKES

XIE Tao, LU Jun

China Earthquake Networks Center, CEA, Beijing 100045, China

Abstract

Crustal medium has varying degrees of electrical conductivity. Electrical resistivity is an important physical property for geomaterials. Electrical resistivity is commonly used in geophysics to investigate the deformation of the crust. Resistivity of sedimentary rock and soil is a combination of resistivity of both solid matrix and crack/pore fluid, and also depends on the degree of fluid saturation, crack ratio or porosity, and crack shape. Since the electrical resistivity is related to mechanical properties, build-up of strain ought to be accompanied by resistivity changes, which might warn of an impending earthquake. Many studies on electrical resistivity changes of rock and soil have been carried out in an attempt to find a physical basis for earthquake prediction. Rock resistivity changes under deformation up to failure in uniaxial and triaxial experiments have been measured in laboratory. For water-bearing rocks, resistivity decreases or increases at low stress and decreases greatly at high stress, just before failure. A series of midpoint Schlumberger arrays were placed on rock samples. Apparent resistivity of the perpendicular array has the maximum decrease magnitude, while the parallel array has the minimum magnitude. The decrease magnitude of the oblique array falls within the upper and lower bounds of the other two arrays. In-situ experiments show the similar anisotropic changes in apparent resistivity. Apparent resistivity has been continuously monitored at fixed stations in China for more than 50 years, using Schlumberger arrays. Apparent resistivity has been monitored the same pattern of anisotropic changes before great earthquakes as the results from experiments. Surface DC apparent resistivity observation using a Schlumberger array has a depth detection range approximately equal to the electrode spacing of AB. There may also be lateral electrical inhomogeneity under the survey area, i.e. the resistivity of the medium varies in the horizontal direction within the same depth range. Lateral inhomogeneity within the detection range will cause anisotropy in apparent resistivity, but whether it will cause anisotropic changes is still uncertain. The depth range of stratum resistivity affected by tectonic stress before earthquakes has not been fully discussed. In this paper, we use finite element method to calculate the anisotropic changes in apparent resistivity caused by resistivity changes from different depth range. Lateral homogeneity and lateral inhomogeneity models are taken into account, respectively. The results show that for Schlumberger array with spacing of AB=1 000m, anisotropic changes in apparent resistivity caused by resistivity changes of stratum below 300m is inconsistent with the observed results in experiments and seismic examples. Under this situation, apparent resistivity of the perpendicular array has the minimum decrease magnitude, while the parallel array has the maximum magnitude. On the other hand, the apparent resistivity decreases in a small range, and the difference between the two monitoring directions is not significant. The depth of stratum where resistivity changes take place needs to rise to a range of tens of meters from the surface. Then the magnitude and feature of anisotropic changes are consistent with the experiments and field observations. For stratum with lateral inhomogeneity, apparent resistivity dose not display anisotropic changes when resistivity of different stratum units under the same depth has the same magnitude of isotropic variation. Anisotropic changes in apparent resistivity take place only when stratum resistivity shows anisotropic variations. However, lateral heterogeneity has little effect on anisotropic changes of apparent resistivity.

Keyword: apparent resistivity; lateral inhomogeneity; anisotropic changes; depth of resistivity changes; earthquake

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0 引言

含水介质的电阻率与孔隙度、饱和度、水溶液中离子浓度和微裂隙结构关系密切。岩石物理实验结果表明, 对于基质电阻率高于裂隙水的含水岩石, 在压应力加载过程中电阻率呈现下降变化, 卸载过程中呈上升恢复变化, 多数岩石的电阻率在临近破裂时加速下降, 岩石破裂后出现快速回返(Brace et al., 1965, 1966, 1968; Yamazaki, 1966; 张金铸等, 1983; Jouniaux et al., 2006)。当应力积累到一定程度之后, 新生裂隙开始不断产生, 无论初始裂隙如何分布, 最终形成的新裂隙系统将大致沿最大主应力方向展布(Scholz et al., 1973; 徐靖南等, 1993; Teisseyre, 1997; 李新平等, 2002), 裂隙沿最大主应力方向的微小变化可引起较大幅度的电阻率各向异性变化(Yamazaki, 1965, 1966; Morrow et al., 1981; 解滔等, 2020)。大地震发生前, 震中附近的视电阻率异常变化表现为: 偏离之前多年背景观测值范围, 持续数月至2a左右的下降或上升, 变化幅度约为1%~7%, 部分地震发生前约3个月内出现过加速变化; 不同观测方向的视电阻率呈现出与主压应力方位相关的各向异性变化, 且变化特征与实验室及野外原地实验结果相符(赵玉林等, 1995; 钱复业等, 1996; 杜学彬等, 2007); 视电阻率下降或上升的变化形态与震源机制解给出的台站所处位置的挤压或拉张应变状态有关; 在台站分布相对较密的近震中区域, 异常出现的起始时间由震中向外围方向逐渐延迟, 且变化幅度逐渐衰减(钱复业等, 1982; 赵玉林等, 2001); 邻近震中的台站能够完整地记录到震前中期变化— 短期加速— 准同震阶跃— 震后恢复的变化过程(Du, 2011; 解滔等, 2018)。这些现象说明, 视电阻率变化可能与地震孕育过程之间存在力学机制上的联系。

台站的实际观测条件与岩石物理实验及各向异性电阻率模型之间仍然存在一定的差异。实验中岩石样品的尺寸较小, 且通常为均匀介质, 电阻率模型同样为均匀各向异性介质, 应力作用下介质的电阻率可视为整体发生均匀各向异性变化。目前, 在对视电阻率各向异性变化进行分析时, 往往将测区地层简化为半无限空间的均匀方位各向异性介质(钱复业等, 1996; 杜学彬等, 2007; Du, 2011)。中国大陆浅源地震的震源深度为数km至数十km, 通常认为应力、应变主要由深部孕震区逐渐向外围和近地表方向扩展。中国视电阻率台站的观测极距AB的范围为500~2 400m, 探测深度与极距相当(赵和云等, 1982; 杜学彬等, 2008), 且地下介质存在一定的横向不均匀性。在构造应力的作用下, 测区地下介质是否可视为整体发生均匀变化, 以及应力、应变积累的影响可上升至距离地表的深度范围还未得到充分讨论。由于横向不均匀性的存在, 如果地下介质只发生各向同性变化, 视电阻率是否会出现或者出现怎样的各向异性变化等相关问题也尚未得到充分的讨论。因此, 对这2个问题进行分析将有助于进一步理解地震前视电阻率的变化机理。

本文拟依据地震前视电阻率的各向异性变化特征, 采用有限元数值分析方法分别计算横向均匀、不均匀模型中不同深度介质电阻率变化时视电阻率的各向异性变化, 分析介质横向不均匀性对各向异性变化的影响, 并讨论电阻率变化层位上升至浅表的深度范围。

1 视电阻率的各向异性变化

陈大元等(1983)选用质地均匀的饱和岩石样品, 布设平行、垂直和以45° 斜交于应力加载方向的对称四极视电阻率观测装置, 并通过观测值计算最小电阻率主轴的方向。在加载初期, 3个方向的视电阻率略微上升但差异较小; 当超过破裂应力的50%时, 3个方向开始非同步性地转变为下降变化; 随着应力的持续加载, 电阻率主轴的方位趋于稳定, 且最终的断裂面走向为其中一个电性主轴的方向; 垂直于应力加载方向的视电阻率变化幅度最大, 平行方向最小, 斜交方向介于二者之间。为进一步分析电阻率各向异性变化的主要影响因素, 在实验样品上布设多组观测装置。结果表明, 有裂隙通过的装置所观测到的各向异性变化明显, 电性主轴与裂隙展布方向基本吻合; 无裂隙经过的装置观测到的各向异性变化不明显。这说明裂隙的定向排列和扩展是电阻率各向异性变化的重要原因(陈峰等, 2003, 2013; Samouë lian et al., 2004)。据岩石样品的电阻率成像结果可知, 应力加载过程中裂隙主要在浅部产生和扩展, 而深部裂隙的产生和扩展速率相对较低(Zhu et al., 2012; 张斌等, 2017)。

实验室内岩石样品的尺度较小, 且样品含水量、节理、裂隙以及受力状态等条件与自然状态下的岩土介质存在差异。为研究自然状态下岩土介质的电阻率在应力作用下的变化, 赵玉林等(1983)在非饱和平整土层上, 通过单侧沟槽开挖的方式利用千斤顶进行压应力加载实验。沿力源方向布设了几组不同距离的观测装置, 每组包含3个方向的对称四级观测装置, 得到的视电阻率各向异性变化与岩石物理实验结果一致(图1a)。在土层四周开挖沟槽, 并采用双力偶模拟剪切应力加载过程(图1b), 所得结果显示: 平行于最大剪应力方向的视电阻率略微上升, 而在垂直方向则下降, 且变化幅度最大(钱复业等, 1998)。

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	图 1 野外原地应力加、卸载实验中的视电阻率各向异性变化 a 压应力实验(赵玉林等, 1983); b 双力偶模拟剪切应力实验(钱复业等, 1998)Fig. 1 Apparent resistivity variations under stress of in situ experiments.

中国视电阻率定点台站采用对称四级装置, 通常在地表布设2个正交方向的观测, 或再加一斜交方向共计3个方向的观测(图 2)。大地震发生前, 近震中区域台站的视电阻率会出现各向异性变化, 该现象已得到许多震例的证实, 且变化特征与实验结果相符(赵玉林等, 1995; 钱复业等, 1996; 杜学彬等, 2007)。例如: 对于2008年汶川M_S8.0地震, 已知成都台和江油台分别距主破裂区约35km和30km。根据主震的分段震源机制解(图3a)可知, 成都台附近区域的主压应力方位为 N51° W, 江油台附近为 N5° W(张勇等, 2009)。成都台N58° E的观测方向与主压应力轴的夹角为71° , 震前最大下降幅度约为7%; 观测方向N49° W与主压应力轴近平行, 震前未出现下降变化(图3b)。江油台 N70° W的观测方向与主压应力轴的夹角为65° , 震前观测值出现约1.5%的下降变化, 观测方向N10° E与主压应力轴大致平行, 震前未出现明显的下降异常变化(Lu et al., 2016; 解滔等, 2018)。对于部分大地震, 在临近发生前, 位于震中附近同一台站不同观测方向的视电阻率可能出现差异性的加速变化, 如: 1976年唐山M_S7.8地震前2个月内, 昌黎台(距主震72km, 距主震发生当天的M_S7.1余震震中40km)的SN向观测值出现加速下降, 幅度约为5%, 而EW向观测值在震前半月才开始加速下降, 幅度约为3.8%。这种快速的各向异性变化可能反映了断层预滑引起的主应力方向变化(赵玉林等, 1995; 杜学彬等, 2001)。

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	图 2 视电阻率定点台站对称四级观测装置的布极方式Fig. 2 The Schlumberger arrays used in the apparent resistivity observation stations in China.

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	图 3 2008年汶川M_S8.0地震前的视电阻率各向异性变化(解滔等, 2018) a 汶川地震主破裂的分段震源机制解(张勇等, 2009)与视电阻率的观测方位; b 成都台和江油台的视电阻率观测值Fig. 3 The anisotropic changes in apparent resistivity before Wenchuan M_S8.0 earthquake(XIE Tao et al., 2018).

2 各向异性变化的数值分析

2.1 稳恒电流场有限元方法

在固定台站进行视电阻率观测时, 在供电电极A和B输入直流电流, 在电极M和N之间测量电位差。该问题可视为稳恒电流场计算, 遵守电流连续性方程, 电位分布满足泊松方程:

$\nabla \cdot (σ \nabla V) = - Iδ (x, y, z)$ (1)

式中, V是介质中的电位分布; $σ$ 为电导率张量, 且 $σ = ρ^{- 1}$ , $ρ$ 为电阻率张量; $δ (x, y, z)$ 是Dirac delta函数。对于各向异性介质, 电阻率 $ρ$ 可表示为

$ρ = (\begin{array}{l} \begin{array}{l} ρ_{11} & ρ_{12} & ρ_{13} \end{array} \\ \begin{array}{l} ρ_{21} & ρ_{22} & ρ_{23} \end{array} \\ \begin{array}{l} ρ_{31} & ρ_{32} & ρ_{33} \end{array} \end{array})$ (2)

对于各向同性介质, 电阻率 $ρ$ 可表示为

$ρ = (\begin{array}{l} \begin{array}{l} ^{ρ} & 0 & 0 \end{array} \\ \begin{array}{l} 0 & ρ & 0 \end{array} \\ \begin{array}{l} 0 & 0 & ^{ρ} \end{array} \end{array}) = ρI$ (3)

式中, I为单位矩阵。直接求解电位分布时, 电流源附近的电位梯度较大, 将产生较大的计算误差。通常采用二次场的求解方式(Lowry et al., 1989; Zhao et al., 1996)将电位分解为均匀半空间介质(电阻率为ρ _P)引起的一次场电位V_P和非均匀介质(电阻率为ρ _S)引起的二次场电位V_S, 总的电位分布为V=V_P+V_S, 且ρ =ρ _P+ρ _S。

对于均匀半空间介质, 电流源I产生的一次场电位分布为(克拉耶夫, 1954)

$V_{P} = \frac{I}{2 π} \frac{\sqrt[]{{ρ^{p}}_{1} {ρ^{p}}_{2} {ρ^{p}}_{3}}}{\sqrt[]{{ρ^{p}}_{1} x^{2} + {ρ^{p}}_{2} y^{2} + {ρ^{p}}_{3} z^{2}}}$ (4)

将总的电位分布表达式带入式(1), 可得二次场电位V_S满足的微分方程:

$\nabla \cdot (ρ^{- 1} \nabla V_{S}) + \nabla \cdot (ρ_{S}^{- 1} \nabla V_{P}) = 0$ (5)

有限体积介质的全部边界为Γ , 一部分边界没有电流流出(如地表), 满足纽曼(Neumann)边界条件, 记为Γ _N, 其余边界记为Γ _D, 满足狄利克雷(Dirichlet)边界条件。因此, 方程(5)满足边界条件:

$Γ = Γ_{N} + Γ_{D}$ (6)

其中, $\frac{\partial V}{\partial n}|_{rN}=0$, n为边界指向区域外的法线方向, $V|_{rD}=p$, p为常数。

采用虚功原理, 可得到二次场电位V_S的Galerkin有限元弱解形式:

$\int_{Ω} ρ^{- 1} (\nabla V_{s} \cdot \nabla φ) dΩ + \int_{Ω} ρ_{S}^{- 1} (\nabla V_{p} \cdot \nabla φ) dΩ = \int_{Γ} ρ^{- 1} \frac{\partial V_{S}}{\partial n} φ d s + \int_{Γ} ρ_{S}^{- 1} \frac{\partial V_{P}}{\partial n} φ d s$ (7)

式中, Ω 为计算区域; $φ$ 为任意形式的虚位移函数。经单元离散化后, 虚位移函数与单元电位插值函数可采用相同的插值基函数。在满足狄利克雷边界条件的边界上, 虚位移函数 $φ = 0$ 。将边界条件(6)带入式(7)可得:

$\int_{Ω} (ρ^{- 1} (\nabla V_{S} \cdot \nabla φ) + ρ_{S}^{- 1} (\nabla V_{P} \cdot \nabla φ)) dΩ = 0$ (8)

令第i个单元的插值基函数为N_i, 则单元的虚位移函数和电位可表示为

$φ_{i} = N_{i} δ_{i}^{*}, {V^{S}}_{i} = N_{i} {δ^{S}}_{i}, {V^{P}}_{i} = N_{i} {δ^{P}}_{i}$ (9)

式中, ${δ^{P}}_{i}$ 为单元节点一次场的电位值, 记 $\nabla {V^{S}}_{i} = B_{i} {δ^{S}}_{i}, \nabla φ_{i} = B_{i} δ_{i}^{*}, \nabla {V^{P}}_{i} = B_{i} {δ^{P}}_{i},$ 则式(8)可写为

$\int_{Ω_{i}} δ^{*} {^{T}}_{i} (ρ_{i}^{- 1} {B^{T}}_{i} B_{i} {δ^{S}}_{i} + ρ_{S i}^{- 1} {B^{T}}_{i} B_{i} {δ^{P}}_{i}) d Ω_{i} = 0$ (10)

由于对于任意的虚位移函数式(10)均成立, 故需满足如下关系:

$ρ_{i}^{- 1} {B^{T}}_{i} B_{i} {δ^{S}}_{i} + ρ_{S i}^{- 1} {B^{T}}_{i} B_{i} {δ^{P}}_{i} = 0$ (11)

按式(11)对所有单元进行组装, 可求解二次场电位, 进而可得到总的电位分布。需要注意的是, 当全部采用纽曼边界条件时, 方程(1)需要满足如下关系, 才能在相差一个常数的情况下存在惟一解:

$\int_{Ω} I (x, y, z) dΩ = 0$ (12)

在计算地下介质电位分布时, 深度方向和水平方向可视为无穷远边界, 也可采用吸收边界条件(Li et al., 2005):

$\frac{\partial V}{\partial n} + \frac{\cos (r, n)}{r} V = 0$ (13)

无论采用何种边界条件, 三维模型在水平方向和深度方向都需要足够大的尺寸, 边界条件的影响才能降低至可忽略的水平。解滔等(2014)通过分析视电阻率计算值随模型尺寸的变化, 认为对于对称四级装置, 当模型水平方向的尺寸> 6AB、最底层介质的厚度> 2AB时, 可忽略边界条件对视电阻率计算值的影响。此外, 如分析时采用视电阻率的相对变化, 则边界条件的影响将更小。

图 4a为模型示意图, 其水平尺寸为8AB, 厚度为3AB, 可分为3层, 每层在横向存在不均匀性, ρ _i_L 和ρ _i_R 分别表示第i层介质左侧和右侧的电阻率。在模型表面布设8个夹角为22.5° 的共中心点对称四级装置, 供电极距AB均为1 000m, 测量极距MN均为300m(图4b), 中心点与横向不均匀地层分界线的距离为100m。图4c为模型有限元剖分网格, 图4d为观测装置布极区的剖分网格。通过调整各层介质的厚度和电阻率, 可在2层和3层模型、横向均匀和横向不均匀模型以及各向同性和各向异性介质模型之间进行转换。

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	图 4 视电阻率的有限元计算模型 a 模型结构示意图; b 模型表面8个方向的对称四级装置; c 模型网格剖分; d 模型表面布极区的网格剖分Fig. 4 Finite element model for apparent resistivity calculation.

地震是构造应力长期积累并最终导致断层失稳的结果, 视电阻率的异常变化通常出现在大地震发生前的1~2a内。在该时段, 测区的地下介质经过构造应力的持续作用, 已进入新裂隙系统优势展布阶段, 且裂隙的优势展布和扩展方向大致平行于最大主应力方向(Du, 2011)。对于含水的裂隙介质, 裂隙优势展布的方向为最小电性主轴方向; 在裂隙扩展过程中, 最小电性主轴方向的电阻率变化幅度最大, 其余2个主轴变化较小(解滔等, 2020)。以x方向为最小电性主轴, 在计算过程中, 当各向异性介质的电阻率发生变化时, 最小主轴方向的电阻率值下降10%, 其余主轴不变; 当各向同性介质发生变化时, 下降幅度也为10%。对于图4b所示的观测装置, 0° 或180° 为平行于最小电性主轴的方向, 90° 或270° 与之垂直, 其余方向则与之斜交。对于视电阻率变化, 下文中未明确指出为绝对变化时, 均指相对变化幅度。

2.2 地层横向均匀

采用2层横向均匀介质模型(图 5), 第1层介质的电阻率不变, 第2层发生变化。分别设定第1层介质的厚度为500m、 300m、 100m、 60m、 40m和20m, 以分析不同深度以下介质电阻率发生变化时, 地表观测的视电阻率的各向异性变化。

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	图 5 2层横向均匀介质模型 a 第1层介质各向同性, 第2层各向异性; b 第1层介质各向异性, 第2层各向同性; c 2层介质均为各向异性Fig. 5 Two-layer laterally homogeneous model.

第1层介质为各向同性、第2层为各向异性时(图5a)的计算结果示于图6a。随着各向异性变化层的深度逐渐向地表方向靠近, 视电阻率的变化幅度(最大值)逐渐增加, 且各向异性变化特征呈现出2种截然相反的形态。第1层的厚度为500m和300m时, 平行于最小主轴方向的视电阻率变化幅度大于垂直方向, 与大地震前实际观测和实验给出的各向异性变化特征相反。当第1层的厚度< 100m, 各向异性变化特征开始转变为平行于最小主轴方向的变化幅度最小、垂直方向最大。第1层的厚度越小, 则垂直方向的变化幅度越大, 且平行方向的变化幅度越小。尤其是当第1层的厚度< 40m时, 平行于最小主轴方向的视电阻率变化幅度已经十分微弱。第1层介质为各向异性、第2层介质为各向同性时(图5b)的计算结果示于图6b。随着第1层各向异性介质厚度的减小, 视电阻率的变化幅度逐渐增加。当第1层的厚度为500m和300m时, 平行于最小主轴方向的视电阻率变化幅度小于垂直方向, 但二者的差异并不太明显, 也与实际观测不符。当第1层的厚度< 100m时, 视电阻率的各向异性变化特征已经不明显。第1层和第2层介质均为各向异性时(图5c)的计算结果示于图6c。随着第1层厚度的减小, 视电阻率的各向异性变化特征与图6a一致。

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	图 6 横向均匀模型的视电阻率各向异性变化 a 第1层介质各向同性, 第2层各向异性; b 第1层介质各向异性, 第2层各向同性; c 2层均为各向异性Fig. 6 Anisotropic changes in apparent resistivity of the laterally homogeneous model.

2.3 地层横向不均匀

地层介质存在横向不均匀性的模型示于图 7。图7a— d中的介质分为2层, 第1层介质的电阻率不变, 第2层发生变化。同样地, 将第1层介质的厚度分别设定为500m、 300m、 100m、 60m、 40m和20m。

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图 7 地层介质横向不均匀模型
a 第1层为各向同性、横向不均匀, 第2层为各向同性; b 第1层为各向同性, 第2层为各向同性、横向不均匀; c 第1层为各向同性、横向不均匀, 第2层为各向异性; d 第1层为横向不均匀, 左侧各向同性、右侧各向异性, 第2层为各向异性; e 3层介质模型, 各层均存在横向不均匀, 第1层为各向同性, 第2层和第3层为各向异性Fig. 7 The laterally inhomogeneous model.

第1层和第2层均为各向同性介质、第1层存在横向不均匀时(图7a)的计算结果示于图8a。随着电阻率的变化深度逐渐向地表方向靠近, 视电阻率的变化幅度增加, 但均未出现各向异性变化的现象。第1层和第2层均为各向同性介质、第2层存在横向不均匀时(图7b)的计算结果示于图8b。随着第1层的厚度减小, 视电阻率的变化幅度增加, 但也未出现各向异性变化的现象。第1层介质为各向同性且存在横向不均匀、第2层为均匀各向异性时(图7c)的计算结果示于图8c。随着第1层的厚度减小, 视电阻率各向异性主要来自下伏地层电阻率的各向异性, 且各向异性变化特征与图6a一致。第1层介质存在横向不均匀, 左侧为各向同性、右侧为各向异性, 第2层为均匀各向异性时(图7d)的计算结果示于图8d, 视电阻率各向异性变化的特征与图6a一致。

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图 8 横向不均匀模型的视电阻率各向异性变化
a 第1层为各向同性、横向不均匀, 第2层为各向同性; b 第1层为各向同性, 第2层为各向同性、横向不均匀; c 第1层为各向同性、横向不均匀, 第2层为各向异性; d 第1层为横向不均匀, 左侧各向同性、右侧各向异性, 第2层为各向异性; e 3层介质模型, 各层均存在横向不均匀, 第1层为各向同性, 第2层和第3层为各向异性Fig. 8 Anisotropic changes in apparent resistivity of the laterally inhomogeneous model.

以上关于横向均匀和横向不均匀模型的计算均采用第1层厚度逐渐变小的方式表示地层电阻率变化的上界面逐渐上升至近地表的过程。图7e是3层介质模型, 第1层电阻率为各向同性, 第2和第3层均为各向异性, 各层均存在横向不均匀性。在计算过程中, 各层的厚度保持不变, 电阻率发生变化的上界面距离地表的距离H_c分别为500m、 300m、 100m、 60m、 40m和20m, 计算结果示于图8e, 视电阻率的各向异性变化特征依然与图6a类似。

3 讨论

中国固定台站在进行视电阻率观测时, 在深度方向的探测范围与观测极距AB尺度相当, 主要反映浅层介质电阻率的变化。地表浅层构造应力的2个主分量通常沿水平方向分布, 第3个主应力沿垂直方向分布(Crampin et al., 1984)。近震中区域附近台站测区的地下介质在构造应力长期积累的过程中, 微裂隙系统沿最大主应力方向优势展布和扩展(Nur, 1972; Scholz et al., 1973; Mjachkin et al., 1975; Teisseyre, 1997)。在这一过程中, 震中附近台站的视电阻率出现各向异性变化, 垂直于最大主应力方向的变化幅度最大, 平行方向最小或无明显变化, 斜交方向介于二者之间(杜学彬等, 2007; Du, 2011)。

在仅考虑岩土骨架、裂隙水和裂隙气体的固液气三相介质中, 介质电阻率张量的最小主轴与裂隙展布方向相同(解滔等, 2020)。将测区地层简化为均匀方位各向异性的介质, 则可依据地表3 个方向的视电阻率观测值计算出最小电性主轴方向, 从而对最大主应力方向做出估计(钱复业等, 1996)。地下介质广泛存在横向不均匀性, 台站固定装置的观测无法区分视电阻率的各向异性是来自电阻率各向异性还是横向不均匀性。在电阻率同时存在横向不均匀和各向异性时, 视电阻率的各向异性与二者均有关系。采用3个方向的视电阻率计算得到的最小主轴方位 $α$ 与真实的裂隙电性主轴方向 $α'$ 存在差异。但是, 从文中的计算可以看出, 横向不均匀性对视电阻率各向异性变化的影响较小。因此, 进行定点连续的视电阻率观测, 通过各向异性变化可以分析裂隙的优势扩展方向, 进而可对水平最大主应力的方位进行估计。

浅源地震的震源在地表数km以下, 震源区处于闭锁状态, 在孕震过程中该区域将积累足够高的应力应变。对于距离震中数十至上百km之外的视电阻率台站, 其深部介质的电阻率受孕震应力影响的变化幅度还没有直接的探测数据。不同深度的介质变化对视电阻率观测值变化的影响随深度递减。与浅部介质相比, 深部介质的电阻率需要更大的变化幅度才能引起相同幅度的视电阻率变化, 仅依据单一方向的视电阻率变化幅度难以有效地估计电阻率变化层位的深度和变化幅度。但是, 地震前近震中区域视电阻率的各向异性变化为分析这一问题提供了一定的窗口。多数视电阻率台站的供电极距约为1 000m, 这些台站在许多中强地震前记录的视电阻率各向异性异常变化是客观存在的(杜学彬等, 2007)。从本文的计算结果来看, 对于极距AB=1 000m的地表观测, 约300m深度之下的介质电阻率的各向异性变化所引起的视电阻率各向异性变化特征与地表的实际观测事实不符。介质电阻率变化层位的上界面需要上升至近地表范围内, 所引起的视电阻率各向异性变化才能与实际观测相吻合。

目前, 视电阻率观测面临台网稀疏和观测易受干扰这2个困难。地表大极距观测方式所需的测区面积较大, 测区内潜在干扰因素(尤其是金属管网)较多, 难以进行高质量的持续性观测。地表不定期出现的干扰源破坏了原有的稳定背景形态, 为异常分析带来了极大困难(解滔等, 2016)。随着城镇化建设的推进和农村经济的发展, 土地价格日益增加, 想要新建大极距观测台站以增加台网密度已十分困难, 而在以山地和丘陵为主要地貌特征且地震多发的南北地震带, 大极距观测方式的组网成场布局更是难以实现。潜在可行的方案是缩短极距以减小占地面积, 并将观测装置埋入地下一定深度, 增加装置与地表的距离, 以减小来自地表的影响。考虑到建设成本, 目前实验观测中的台站, 装置埋深约为100m。观测极距减小之后, 探测深度也将减小, 是否能够记录到地震前的异常变化是这一观测方式的核心问题。基于以上关于电阻率变化深度的分析, 可将地表以下数十m至300m深度范围内的介质作为视电阻率的主体目标探测区域。如此, 采用减小极距和增加埋深的观测方式, 场地勘选和获取将更为容易, 有助于建设足够密集的观测台网。当区域内多个台站出现异常变化时, 可相互印证。进一步通过分析不同台站异常出现的时间、幅度、形态、空间范围和各向异性变化, 将有助于更好地对未来地震的地点、震级和发震时间做出预判。

4 结论

本文采用有限元数值分析方法建立了电性结构横向均匀和不均匀的三维模型, 基于大地震前震中附近台站观测的视电阻率的各向异性变化现象, 讨论了地层横向不均匀性对各向异性变化的影响和地震前电阻率变化的深度范围。对于横向不均匀地层, 在一定深度之下的介质电阻率发生相同幅度的变化时, 横向不均匀性并不会改变视电阻率的各向异性变化特征。因此, 采用各向异性变化来分析裂隙的优势扩展方向, 进而对水平最大主应力方位进行估计是可行的。对于地表观测的视电阻率, 深部介质的电阻率变化产生的影响较小, 且各向异性变化与实验和震例中的实际观测不相符。要合理地解释大地震前视电阻率的各向异性变化这一观测事实, 介质电阻率发生各向异性变化层位的上界面需要上升至近地表的浅层范围。

致谢四川省地震局何畅提供了成都台和江油台的视电阻率观测数据; 审稿专家为本文提出了中肯的修改建议。在此一并表示感谢!

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1983

0.0

... 1 视电阻率的各向异性变化陈大元等(1983)选用质地均匀的饱和岩石样品, 布设平行、垂直和以45#cod#x000b0 ...

2003

0.0

陈峰, 安金珍, 廖椿庭. 2003. 原始电阻率各向异性岩石电阻率变化的方向性[J]. 地球物理学报, 46(2): 271—280.

CHEN

Feng

, AN

Jin-zhen

, LIAO

Chun-ting

. 2003. Directional characteristic of resistivity changes in rock of original resistivity anisotropy[J]. Chinese Journal of Geophysics, 46(2): 271—280(in Chinese).

Multiple electrodes are arranged on the surfaces of samples, which in clude three kinds of resistivity of anisotropic samples not formed by ingredient of rock (The original resistivity anisotropy of the samples is due to the integ rity of samples be damaged in the process of sampling and processing) and a kind of samples formed by ingredient of rock. The samples are full y saturated with water. The multiple electrodes are combined form arrays of resi s tivity changing anisotropy in different direction and spacing. The experiments o f dynamic rock resistivity change are done with uniaxially compression, low confi ning pressure and shear. The variations of resistivity during the whole loading pr ocess are observed. The experimental results are the following: The samples of o riginal resistivity anisotropy are essentiall y accordant with the samples of original resistivity isotropy in resistivity cha nging anisotropy. Namely, for the measuringpoints located at cracks or rupture b ands, the results of resistivity changing anisotropy are good, the four directio ns deduced from four kinds of combinatorial equation sets of resistivity are ess entially identical with each other and approximately coincide with the orientati on of cracks or main rupture bands; for the measuringpoints without cracks or r upture bands passing through, the four directions calculated by different combin atorial arrays of resistivity are inconsistent with each other, or the direction can not be determined, especially in the cases when the crack plane is parallel to the measuringsurface.

1 Department of Geophysics, Peking University, Beijing 100871, China2 Geomechanics Institutes of Territories Resources Ministry, Beijing 100081, China

在3种非含岩成分造成的岩石原始电阻率各向异性标本(在采集和加工过程中，标本完整性遭到破坏的标本)，和一种由含岩成分形成的岩石原始电阻率各向异性标本上，布设多个电极，并将标本用水饱和. 然后用多电极组合法，将己布电极组合成不同方向、不同极距的电阻率各向异性测线. 采用单轴压缩、低围压三轴压缩和剪切3种加载方式，对标本进行动态电阻率变化实验. 观测标本电阻率随承载力的变化. 实验结果表明∶对于岩石原始电阻率各向异性标本，在电阻率变化各向异性方面，与岩石原始电阻率各向同性标本相似，即∶裂隙和破碎带通过区域的测点，视电阻率变化各向异性结果好，４种组合求得的４个各向异性主轴方向趋向一致，且与破碎带方向基本吻合；裂隙和破碎带不经过区域的测点，４个视电阻率变化各向异性主轴方向不一致，或者根本求不出各向异性解. 这后一种情况，在裂隙面平行测量面时，表现最为明显.

... 这说明裂隙的定向排列和扩展是电阻率各向异性变化的重要原因(陈峰等, 2003, 2013 ...

2013

0.0

... 这说明裂隙的定向排列和扩展是电阻率各向异性变化的重要原因(陈峰等, 2003, 2013 ...

2007

0.0

杜学彬, 李宁, 叶青, 等. 2007. 强地震附近视电阻率各向异性变化的原因[J]. 地球物理学报, 50(6): 1802—1810.

Xue-bin

, LI

Ning

, YE

Qing

, et al. 2007. A possible reason for the anisotropic changes in apparent resistivity near the focal region of strong earthquake[J]. Chinese Journal of Geophysics, 50(6): 1802—1810(in Chinese).

In this paper, we discussed the physical processes of the medium in/near the focal region in the late preparation period of strong earthquake, theoretically proved the orthogonal relation between the maximum loading direction and the direction of most prominent change in apparent resistivity, and produced the constitutive relations of anisotropic changes in true resistivity and apparent resistivity to crack ratio ν , crack-water resistivity ρ f and framework resistivity　 ρ o , and then proposed a possible reason for the anisotropic changes in apparent resistivity. We consider that in/near the focal region in the late preparation period of strong earthquake, the micro-cracks in the crust medium develop fast in number and size, which strike predominantly along the maximum compressional stress orientation of focal mechanism solution, and then the low resistance crack-watercomes fast in and conductive aisles connect each other. The physical processes generate the anisotropic changes in true resistivity, in which the most prominent change is in the direction in or close to the maximum compressional stress orientation, and the true anisotropic changes cause the anisotropic changes in apparent resistivity, in which the most prominent change is in the direction perpendicular or nearly perpendicular to the orientation. The underground water with low resistance plays a very important role in the above physical processes.

1 Lanzhou Institute of Seismology, China Earthquake Administration, Lanzhou 730000, China;2 China Earthquake Network Center, Beijing 100036, China;3 Beijing Earthquake Administration, Beijing 100080, China

讨论了强地震、孕震晚期阶段、震源区及附近介质的物理变化过程，证明了视电阻率变化最显著的方向与最大加载方向之间的正交关系，建立了真、视电阻率各向异性变化与裂隙率 ν 、骨架电阻率 ρ o 、饱水裂隙电阻率 ρ f 的本构关系，解释了视电阻率各向异性变化的原因.研究认为：在强地震孕震晚期阶段，在震源区及附近地壳近地表的较深部，介质微裂隙发育，其走向沿最大主压应力方位优势取向，低阻水充填微裂隙，导电通道连通，引起最大主压应力方向真电阻率变化最显著的真各向异性变化，从而产生垂直主压应力方向视电阻率变化最显著的视各向异性变化，低阻水在其物理过程中起了显著作用.

... 杜学彬等, 2007) ...

... 杜学彬等, 2007 ...

... 杜学彬等, 2007) ...

... 在这一过程中, 震中附近台站的视电阻率出现各向异性变化, 垂直于最大主应力方向的变化幅度最大, 平行方向最小或无明显变化, 斜交方向介于二者之间(杜学彬等, 2007 ...

... 多数视电阻率台站的供电极距约为1 000m, 这些台站在许多中强地震前记录的视电阻率各向异性异常变化是客观存在的(杜学彬等, 2007) ...

2001

0.0

... 杜学彬等, 2001) ...

2008

0.0

杜学彬, 叶青, 马占虎, 等. 2008. 强地震附近电阻率对称四极观测的探测深度[J]. 地球物理学报, 51(6): 1943—1949.

Xue-bin

, YE

Qing

, MA

Zhan-hu

, et al. 2008. The detection depth of symmetric four-electrode resistivity observation in/near the epicentral region of strong earthquakes[J]. Chinese Journal of Geophysics, 51(6): 1943—1949(in Chinese).

In consideration of China′s observation of apparent resistivity at fixed sites with fixed electrode arrays for earthquake monitoring and prediction, we produced the theoretical detection depth for the symmetric four-electrode resistivity observation in the half-space of sloping and anisotropic medium, and researched the depth to detect the apparent resistivity precursory related to earthquake, and thus, obtained the following results: (1) In the late preparation period of strong earthquakes, the changes in apparent resistivity in the underground deep place could be recorded in/near the epicentral region, which is far deeper than that in homogeneous medium; (2) The detection depth is different for different magnitudes of earthquake, different epicentral distances and in different measuring directions or in different stages of earthquake preparation, the difference could be very large and varies dynamically. Besides, we discussed the problems of the complexity of precursory changes in apparent resistivity and the ground electrical condition of the station site in observation of apparent resistivity based on the theoretical detection depth.

1 Lanzhou Institute of Seismology, China Earthquake Administration, Lanzhou 730000, China 2 Lanzhou Base of Earthquake Science Institute, China Earthquake Administration, Lanzhou 730000, China 3 China Earthquake Network Centre, Beijing 100045, China

针对我国地震监测预报中固定地点和固定观测装置及参数的视电阻率观测，给出了半空间倾斜各向异性介质中电阻率对称四极观测的理论探测深度，研究了地震前兆信息的检测深度问题，得到：(1)在强地震、孕震晚期阶段，在震中区及附近可检测到地壳近地表较深部介质的电阻率变化，其深度大于或远大于均匀介质之；(2)震级、震中距、观测方向不同或在不同的孕震阶段，探测深度存在差异，甚至存在大的差异，且是动态变化的.并依据理论探测深度讨论了地震视电阻率前兆变化的复杂性、地电台址电性条件等问题.

... 杜学彬等, 2008), 且地下介质存在一定的横向不均匀性 ...

1954

0.0

... 对于均匀半空间介质, 电流源I产生的一次场电位分布为(克拉耶夫, 1954) ...

2002

0.0

... 李新平等, 2002), 裂隙沿最大主应力方向的微小变化可引起较大幅度的电阻率各向异性变化(Yamazaki, 1965, 1966 ...

1998

0.0

... 在土层四周开挖沟槽, 并采用双力偶模拟剪切应力加载过程(图1b), 所得结果显示: 平行于最大剪应力方向的视电阻率略微上升, 而在垂直方向则下降, 且变化幅度最大(钱复业等, 1998) ...

1996

0.0

... 钱复业等, 1996 ...

... 目前, 在对视电阻率各向异性变化进行分析时, 往往将测区地层简化为半无限空间的均匀方位各向异性介质(钱复业等, 1996 ...

... 钱复业等, 1996 ...

... 将测区地层简化为均匀方位各向异性的介质, 则可依据地表3 个方向的视电阻率观测值计算出最小电性主轴方向, 从而对最大主应力方向做出估计(钱复业等, 1996) ...

1982

0.0

... 在台站分布相对较密的近震中区域, 异常出现的起始时间由震中向外围方向逐渐延迟, 且变化幅度逐渐衰减(钱复业等, 1982 ...

2018

0.0

... 解滔等, 2018) ...

2016

0.0

... 地表不定期出现的干扰源破坏了原有的稳定背景形态, 为异常分析带来了极大困难(解滔等, 2016) ...

2020

0.0

... 解滔等, 2020) ...

... 在裂隙扩展过程中, 最小电性主轴方向的电阻率变化幅度最大, 其余2个主轴变化较小(解滔等, 2020) ...

... 在仅考虑岩土骨架、裂隙水和裂隙气体的固液气三相介质中, 介质电阻率张量的最小主轴与裂隙展布方向相同(解滔等, 2020) ...

2014

0.0

解滔, 王洪岐, 刘立波, 等. 2014. 四平台地电阻率相反年变有限元数值分析[J]. 地球物理学进展, 29(2): 588—594.

XIE

Tao

, WANG

Hong-qi

, LIU

Li-bo

, et al. 2014. Inverse annual variations of apparent resistivity at Siping earthquake station by using finite element method[J]. Progress in Geophysics, 29(2): 588—594(in Chinese).

Geological section and electric property data of Siping earthquake station, Jilin province, China, were collected in our analysis. According to these information the electric structure of medium beneath Siping station could be simplified as a two dimensional electric section along with NW direction, which maintains homogeneity in N40°E direction and the electric properties only changes in NW direction and with depth. Based on this assumption and combined with electrical sounding data of N40°E and N50°W directions observed at Siping station, a three dimensional finite element model was constructed to analyze apparent resistivity annual changes in the station. In the mathematical analysis, the actual resistivity variations of the shallow medium were substituted with resistivity changes of the first and second layers in the model. Analyzed results of the model suggest that apparent resistivity of N50°W direction increases when resistivity of two shallow layers increases, and decreases with deduction of resistivity of shallow layers. However, apparent resistivity of N40°E direction decreases when resistivity of two shallow layers increases, and increases with deduction of resistivity of the two layers. Amplitude of annual variation of N50°W direction is also larger than that of N40°EW direction, and annual variations of two directions play an inverse way. The mathematical results from the model are accord with the observed data, so we conclude that the inhomogeneous distribution of electric medium beneath Siping station causes the inverse annual variations of the two direction observation.

1. China Earthquake Networks Center, China Earthquake Administration, Beijing 100045, China; 2. Earthquake Administration of Jilin Province, Changchun 130022, China

本文依据吉林省四平台地质剖面、地下介质电性资料，将四平台地下介质电性分布简化为沿NW走向的二维电性结构，并结合台站电测深资料建立三维有限元模型，以模型浅层介质电阻率变化模拟测区浅层介质电阻率随季节的变化，计算了四平台N50°W和N40°E两测道地电阻率的年变化形态.计算结果表明，N50°W测道地电阻率在浅层介质电阻率升高时上升，在浅层介质电阻率降低时下降；N40°E测道地电阻率在浅层介质电阻率升高时下降，在浅层介质电阻率降低时升高.两测道地电阻率年变化表现出相反的形态，且N50°W测道年变幅度要大于N40°E测道，计算结果在变化形态上符合四平台实际的观测资料，因此认为四平台测区地下介质的非均匀分布是产生两测道地电阻率年变形态相反现象的原因.

... 解滔等(2014)通过分析视电阻率计算值随模型尺寸的变化, 认为对于对称四级装置, 当模型水平方向的尺寸#cod#x0003E ...

1993

0.0

... 徐靖南等, 1993 ...

2017

0.0

... 张斌等, 2017) ...

1983

0.0

... 张金铸等, 1983 ...

2009

0.0

张勇, 许力生, 陈运泰. 2009. 2008年汶川大地震震源机制解的时空变化[J]. 地球物理学报, 52(2): 379—389.

ZHANG

Yong

, XU

Li-sheng

, CHEN

Yun-tai

. 2009. Spatio-temporal variation of the source mechanism of the 2008 great Wenchuan earthquake[J]. Chinese Journal of Geophysics, 52(2): 379—389(in Chinese).

A linear inversion technique based on assumption of constant rupture velocity and fixed source time functions of sub-events is proposed in this paper for inverting teleseismic waveform data to image spatio-temporal variation of source mechanism of large earthquakes, and applied to the 2008 Wenchuan M S 8.0 earthquake, obtaining its image of the spatio-temporal variation of source mechanism by inverting 48 worldwide stations of long period waveform data. The image shows that the mainly thrust mechanism appeared in the southwest segment of the fault, and the strike-slip component was getting more and more with the fault extending northeastwards, with the turning point at which the strike-slip component became larger than the thrust-slip component appearing around 190 km northeast to the epicenter. In order to inspect the validity of the used method and reliability of the inverted results a specially designed numerical test was performed. The inspection indicated that the inversion method proposed in this paper was valid and the inverted results regarding the great Wenchuan earthquake were trustable (except those on one segment where the long-period component is weak). A comparison shows that the inverted results were coincident with the field-investigation results.

1 Institute of Geophysics, China Earthquake Administration, Beijing 100081, China 2 School of Earth and Space Science, Peking University, Beijing 100871, China

本文提出了一种基于恒定破裂速度和固定子事件震源时间函数的假定、利用远场地震波形资料获取大地震震源机制的时空变化图像的线性反演方法，并利用这种方法及全球范围内48个台站的长周期波形资料反演建立了2008年汶川 M S 8.0地震的震源机制随时间和空间变化的图像.根据这个图像可知，汶川大地震断层的西南端震源机制接近于逆冲，随着破裂向东北方向延伸，震源机制的走滑分量逐渐增大，走滑分量超过逆冲分量的转折点在震中东北大约190 km的位置.为了检验反演方法的有效性和反演结果的可靠性，我们特别设计了一个数值试验对反演结果进行了检验.检验结果表明，我们在本文中提出的反演方法是有效的，关于汶川大地震的反演结果也是可靠的（除长周期信号较弱的一段外).通过比较发现，反演结果与震后野外考察的结果也相当吻合.

... W(张勇等, 2009) ...

1982

0.0

... 中国视电阻率台站的观测极距AB的范围为500~2 400m, 探测深度与极距相当(赵和云等, 1982 ...

1995

0.0

... 不同观测方向的视电阻率呈现出与主压应力方位相关的各向异性变化, 且变化特征与实验室及野外原地实验结果相符(赵玉林等, 1995 ...

... 大地震发生前, 近震中区域台站的视电阻率会出现各向异性变化, 该现象已得到许多震例的证实, 且变化特征与实验结果相符(赵玉林等, 1995 ...

... 这种快速的各向异性变化可能反映了断层预滑引起的主应力方向变化(赵玉林等, 1995 ...

2001

0.0

赵玉林, 卢军, 张洪魁, 等. 2001. 电测量在中国地震预报中的应用[J]. 地震地质, 23(2): 277—285.

ZHAO

Yu-lin

, LU

Jun

, ZHANG

Hong-kui

, et al. 2001. The application on electrical measurements to earthquake prediction in China[J]. Seismology and Geology, 23(2): 277—285(in Chinese).

Resistivity measurement with an active source has been practised for thirty years at an enormous number of station in China. Resistivity decreases of several percent, which began 2～3 years prior to the Tangshan earthquake and other events and gradual increases afterward were observed. These anomalous variations were larger than their background fluctuations and hence statistically significant. Resistivity decreases were generally seen first and larger at stations close to epicenters and later and smaller at more distant stations. This implies that in the preseismic intermediate term stage, the fault was closed and the accumulated strain started first in the epicentral area and then migrated outwards from the epicentre. Anomalies in self-potential of the specific pattern of rapid onset and slow decay recorded at the station XZ are almost simultaneous with oil blowouts from the borehole W11. These short term or impending earthquake precursors migrated towards the epicentre, i.e., strain energy releasing occurred first at stations more distant to the epicentre, later at closer stations and at last the main shock took place. For short periods the industrial interference magnetotellurics (IIMT) determined that the Tangshan earthquake took place in the resistive crust. TM mode (H polarization) data at long periods of the MT measurements determined that electrical properties of resistive zone and the existence of a deep fault zone (fluid leakage paths) was found in resistive crust of the Tangshan earthquake area. Electrical measurements are the geophysical measurements most closely related to fluid volumes, connectivities and pressures, and applicable to earthquake prediction.

1. Institute of Geophysics, China Seismological Bureau, Beijing 100081; 2. Beijing Seismological Bureau, Beijing 100080; 3. Petros EikonInc. Milton, ON L9T 5E3 Canada

唐山等大地震地电阻率中期下降系压缩应变积累所致,为基线缩短、井水位下降所支持。地电阻率、自然电位对起潮力的异常响应及自然电位快急始、慢衰减特殊图象与废油井喷油同步等短临前兆,与中期前兆相反显示出向震中迁移的规律,表明能量积累从震中向外围传播,而能量释放从外围向震中传播。海城地震时喷砂冒水可引起自然电位垂直分量跃变及地光。IIMT法和过剩海洋电流H极化结果表明,唐山地震与地壳高阻层及其中存在深大断裂有关。

... 赵玉林等, 2001) ...

1983

0.0

... 为研究自然状态下岩土介质的电阻率在应力作用下的变化, 赵玉林等(1983)在非饱和平整土层上, 通过单侧沟槽开挖的方式利用千斤顶进行压应力加载实验 ...

1968

0.0

... , 1965, 1966, 1968 ...

1965

0.0

... 岩石物理实验结果表明, 对于基质电阻率高于裂隙水的含水岩石, 在压应力加载过程中电阻率呈现下降变化, 卸载过程中呈上升恢复变化, 多数岩石的电阻率在临近破裂时加速下降, 岩石破裂后出现快速回返(Brace et al ...

1966

0.0

... , 1965, 1966, 1968 ...

1984

0.0

... 地表浅层构造应力的2个主分量通常沿水平方向分布, 第3个主应力沿垂直方向分布(Crampin et al ...

2011

0.0

... 震后恢复的变化过程(Du, 2011 ...

... Du, 2011) ...

... 在该时段, 测区的地下介质经过构造应力的持续作用, 已进入新裂隙系统优势展布阶段, 且裂隙的优势展布和扩展方向大致平行于最大主应力方向(Du, 2011) ...

... Du, 2011) ...

2006

0.0

... Jouniaux et al ...

2005

0.0

... 在计算地下介质电位分布时, 深度方向和水平方向可视为无穷远边界, 也可采用吸收边界条件(Li et al ...

1989

0.0

... 通常采用二次场的求解方式(Lowry et al ...

2016

0.0

... E与主压应力轴大致平行, 震前未出现明显的下降异常变化(Lu et al ...

1975

0.0

... Mjachkin et al ...

1981

0.0

... Morrow et al ...

1972

0.0

... 近震中区域附近台站测区的地下介质在构造应力长期积累的过程中, 微裂隙系统沿最大主应力方向优势展布和扩展(Nur, 1972 ...

2004

0.0

... Samou#cod#x000eb ...

1973

0.0

... 当应力积累到一定程度之后, 新生裂隙开始不断产生, 无论初始裂隙如何分布, 最终形成的新裂隙系统将大致沿最大主应力方向展布(Scholz et al ...

... Scholz et al ...

1997

0.0

... Teisseyre, 1997 ...

... Teisseyre, 1997) ...

1965

0.0

... 李新平等, 2002), 裂隙沿最大主应力方向的微小变化可引起较大幅度的电阻率各向异性变化(Yamazaki, 1965, 1966 ...

1966

0.0

... Yamazaki, 1966 ...

... 李新平等, 2002), 裂隙沿最大主应力方向的微小变化可引起较大幅度的电阻率各向异性变化(Yamazaki, 1965, 1966 ...

1996

0.0

... Zhao et al ...

2012

0.0

... 据岩石样品的电阻率成像结果可知, 应力加载过程中裂隙主要在浅部产生和扩展, 而深部裂隙的产生和扩展速率相对较低(Zhu et al ...