地电阻率日变化成因分析

引用本文

解滔, 卢军, 闫伟. 地电阻率日变化成因分析[J]. 地震地质, 2019,41(6): 1464-1480.
XIE Tao, LU Jun, YAN Wei. THE MECHANISM OF DIURNAL VARIATION IN CONSECUTIVE APPARENT RESISTIVITY OBSERVATION[J]. SEISMOLOGY AND GEOLOGY, 2019,41(6): 1464-1480. 复制到剪切板

DOI:10.3969/j.issn.0253-4967.2019.06.010
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地电阻率日变化成因分析

解滔, 卢军, 闫伟

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: 2018-09-21

基金项目: 中国地震局地震科技星火计划青年项目(XH19054Y)、冬奥会保障晋冀蒙监测能力提升项目和国家自然科学基金(41374080)共同资助

摘要

中国部分长期连续观测的地电阻率台站的观测数据呈现出清晰的日变化形态, 依据单一测道的变化特征, 日变化可分为2类相反的形态: 一类观测数据白天下降夜间上升; 另一类则白天上升夜间下降。文中结合含水岩土介质电阻率的影响因素、台站电性结构和地电阻率影响系数理论, 分析认为固体潮引起的介质电阻率变化幅度太小, 以目前地电阻率观测仪器的分辨率还不足以记录到固体潮的影响, 地电阻率的日变化主要由昼夜温差所引起。昼夜温差将使浅层介质电阻率发生大幅度的昼降夜升变化, 对于浅层介质影响系数为正的测道, 其日变化呈现昼降夜升的形态, 而浅层介质影响系数为负的测道, 其日变化则呈现出昼升夜降的形态。

关键词: 地电阻率; 日变化; 成因机理; 温度; 固体潮

中图分类号:P315.72⁺2 文献标志码:A 文章编号:0253-4967(2019)06-1464-17

THE MECHANISM OF DIURNAL VARIATION IN CONSECUTIVE APPARENT RESISTIVITY OBSERVATION

XIE Tao, LU Jun, YAN Wei

China Earthquake Networks Center, CEA, Beijing 100045, China

Abstract

Parts of the consecutive apparent resistivity monitoring stations of China have recorded clear diurnal variations. The relative amplitudes of diurnal variations at these stations range from 1.3‰ to 5.8‰. The daily accuracies of apparent resistivity observation are better than 1‰, because the background electromagnetic noise is rather low at these stations. Therefore, the diurnal variations of apparent resistivity recorded at these stations are real phenomena. The diurnal variation shapes can be divided into two opposite types according to their characteristics. One type is that the apparent resistivity data decreases during the daytime but increases during the nighttime(Type 1). The other type is the apparent resistivity data increases during the daytime but decreases during the nighttime(Type 2). There is a correspondence between the diurnal and annual variation patterns of apparent resistivity. For the monitoring direction with diurnal variation of Type 1, the apparent resistivity decreases in summer and increases in winter. However, for the monitoring direction with diurnal variation of Type 2, the apparent resistivity increases in summer and decreases in winter.
We take an analysis on the mechanism of apparent resistivity diurnal variation, combining the influence factors of water-bearing medium's resistivity, the electric structure of stations, and the apparent resistivity sensitivity coefficient(SC)theory. Intuitively, diurnal variation of apparent resistivity is caused by diurnal variation of medium resistivity in the measured area. The diurnal variation of medium resistivity will inevitably be caused by the factors with diurnal variation. Among the possible factors, there is diural variation in earth tide and temperature.
Our analysis displays that apparent resistivity diurnal variation is not caused by the usually-believed earth tide, but by the ground temperature difference between daytime and nighttime. The earth tide strain is too small to cause remarkable effects on the apparent resistivity data. On the other hand, the daily tide strain has two peak-valley variations, and its phase and amplitude has a period of approximate 28 days. However, the apparent resistivity data do not show these corresponding features to earth tide. Furthermore, the detection range of current apparent resistivity stations is within a depth of several hundred meters. Within this depth range, the medium deformation caused by solid tide can be regarded as uniform change. Therefore, all monitoring directions and all stations will have the same pattern of diurnal variation.
In general, the temperature increases in the daytime but decreases in the nighttime. For most water-bearing rock and soil medium, its resistivity decreases as temperature increases and increases as temperature decreases. Diurnal temperature difference affects about 0.4m of soil depth. Therefore, resistivity of this surface thin soil layer decreases in the daytime while increases in the nighttime. Under layered medium model, SC of each layer represents its contribution to the apparent resistivity. For the stations with positive SC of surface layer, apparent resistivity decreases in the daytime but increases in the nighttime. While for the stations with negative SC of surface layer, apparent resistivity diurnal variations display the opposite shape.

Keyword: apparent resistivity; diurnal variation; mechanism; temperature; earth tide

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0 引言

中国自1966年河北邢台M_S7.2地震后开始了规范化和规模化的地电阻率(也称视电阻率)定点连续观测, 目前有80多个台站在中国主要地震活动断裂带和人口密集的大中城市附近的地震活动区担负着常规地震监测任务。地电阻率观测采用对称四极装置, 观测极距AB通常在500~2 400m不等, 同一台站在地表布设2~3个不同方向的测道, 以1h观测1次的方式进行长期连续观测。在超过50a的观测实践过程中, 多次记录到了发生在台网内和附近中强地震前出现的突出的年尺度中短期异常(钱家栋等, 1985; 桂燮泰等, 1989; 汪志亮等, 2002; 杜学彬等, 2015; 解滔等, 2018)。地电阻率是地下探测范围内介质电阻率的综合反映, 与孕震过程有关的异常变化主要反映构造应力作用下测区介质变形诱发的微裂隙活动引起的电阻率变化(Mjachkin et al., 1975)。地震异常变化多表现为年尺度的下降或上升, 并伴有年变畸变, 近震中区异常形态以负异常为主(Du, 2011), 走滑型大地震近震区的异常形态呈现出与震源机制解一致的四象限分布(钱复业等, 1982)。异常时空演化特征表现出与孕震过程应变扩散相一致的变化, 离未来地震破裂区越近的台站越早出现异常, 异常幅度也越大, 随着孕震应变向外扩散, 远离未来破裂区的台站相继出现异常(赵玉林等, 2001)。在短期阶段, 由于断层预滑, 近邻破裂区的介质应变加速积累, 地电阻率在原有异常的背景基础上通常会出现加速变化, 而近震区由于出现断层预滑导致应变被部分释放, 地电阻率通常表现为转折变化(汪志亮等, 2002; Du, 2011); 水平应力作用下不同电性主轴的电阻率并非均匀变化, 地电阻率在短期内也通常表现出各向异性的变化特征(钱复业等, 1996, 1998; 杜学彬等, 2007)。

地电阻率观测数据中, 除地震异常变化之外还包括各种非地震异常变化, 这些非地震异常变化的成因机理分析是进行地震异常分析的基础。非地震异常变化大致包括: 持续多年的趋势上升或下降、趋势转折、年变化、阶跃变化、不稳定的扰动变化和日变化。目前初步认为趋势变化主要反映区域应力的长期积累, 趋势转折则反映区域应力场的调整(杜学彬等, 2000; 沈红会等, 2017)。平常时期的阶跃变化与测区内出现的环境干扰因素有关(张磊等, 2010), 最为突出的是在测区埋设金属管线, 改变了测区地表的电性结构, 导致观测数据出现同步的大幅阶跃变化(解滔等, 2013; 石富强等, 2014)。准同震阶段的阶跃变化反映地震发生前后近震区应力的快速释放(Morrison et al., 1977; 钱复业等, 1982; Lu et al., 2016), 之后随着应力的进一步释放, 观测数据进入异常恢复阶段。不稳定扰动变化主要与观测系统、测区工农业漏电以及远距离大功率漏电有关, 由于漏电特征的不同, 观测数据通常伴有大幅的不稳定阶跃变化(刘昌谋等, 1985; 张国苓等, 2017)。几乎所有台站都会记录到不同幅度的年变化, 从单一测道变化特征上可分为夏低冬高和夏高冬低2种类型(金安忠, 1981)。多极距实验观测和理论分析表明, 年变化是由浅层介质电阻率随降水和温度的季节性变化引起的(赵和云等, 1987; 薛顺章等, 1994), 不同测道的年变形态受浅层介质影响系数(即灵敏度系数)控制(Lu et al., 2004; 解滔等, 2014)。

地电阻率的日变化幅度很小, 只有在仪器观测的分辨率足够高, 并且台站测区背景噪声足够低时才能被观测到。目前ZD8系列地电阻率观测仪的分辨率为万分之几, 背景噪声较低的台站观测精度优于1‰ , 中国有14个台站的整点观测数据出现清晰的日变化。清晰地认识地电阻率日变化的成因机理, 将有助于促进日变化信息在异常分析以及地震预测中的应用。此外, 从欧姆定律的微分形式可知, 地电场与介质电导率密切相关, 对地电阻率日变化的成因分析还可为地电场非正常日变化的机理分析提供参考。已有学者从数据形态和频谱对比的角度分析认为地电阻率的日变化与固体潮有关(赵国泽等, 2003; 戴勇等, 2013; 张国苓等, 2016), 但还需要在频谱和形态对比的基础上分析各影响因素对地电阻率的影响幅度。本文将结合含水岩土介质电阻率的影响因素、台站的电性结构和地电阻率的影响系数理论对地电阻率日变化的成因机理做进一步的分析。

1 地电阻率的日变化

整点观测数据具有清晰日变化形态的14个台站的空间分布如图 1 所示, 这些台站均为地表观测方式。地电阻率观测台网中有13个台站为井下观测方式(将电极埋入地下几十甚至百m以下的深处), 很大程度上减小或屏蔽了来自地表浅层的影响, 且井下观测台站未出现可识别的日变化形态。其余的地表观测台站由于受到城铁、高压输电线路和工农业漏电等因素不同程度的影响(从原始观测曲线和观测均方差可以看出), 日变化形态被淹没在背景噪声中。日变化表现为具有日周期的连续变化, 其中昌黎、阳原、锡林浩特、宝昌、阜新、小庙、玉门和拦隆口台的SN测道观测数据呈现白天下降、夜间上升的昼降夜升形态, 代县、临汾、乌加河、甘孜、白水河、柯坪和拦隆口台的EW测道则呈现出与之相反的白天上升、夜间下降的昼升夜降形态(图 2)。表1为这14个台站的相关信息, 其中温度数据为天气后报网站^①(① http://www.tianqihoubao.com/。)提供的台站所在地区的数据, 可以大致反映台站测区的温度变化。据前兆观测台网统计, 目前台站地电阻率观测的连续率> 97%, 完整率> 96%。日观测精度为整点单次观测相对均方差的日均值, 可以反映场地的噪声水平, 日观测精度越高(值越低)则场地噪声水平越低。从表1中可看出日变化幅度大于图 2 中相应时段的日观测精度, 说明日变化形态是真实可信的。各测道观测数据的日变化形态与年变形态具有一致的对应关系, 即: 具有昼降夜升型日变化形态的测道, 其年变化形态为夏低冬高型; 而具有昼升夜降型日变化形态的测道, 年变形态则为夏高冬低型。尤其是拦隆口台, EW和SN 2个测道具有相反类型的年变化, 日变化形态也表现出与之对应的相反类型。除四川小庙台之外, 其余台站的年均温度均较低, 多数台站低于10℃。

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	图 1 地电阻率观测台网及具有清晰日变化的台站分布台站名: 1 昌黎; 2 阳原; 3 代县; 4 临汾; 5 乌加河; 6 锡林浩特; 7 宝昌; 8 阜新; 9 小庙; 10 甘孜; 11 玉门; 12 拦隆口; 13 白水河; 14 柯坪Fig. 1 The distribution of apparent resistivity stations with clear diurnal variation.

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	图 2 地电阻率观测数据日变化曲线乌加河、宝昌、阜新和白水河台为去倾斜后的曲线Fig. 2 Diurnal variations of apparent resistivity data at the 14 stations.

表1 地电阻率具有清晰日变化特征的台站信息 Table1 Related information of apparent resistivity stations with clear diurnal variation

此外, 在观测中还存在另外一种与每日重复出现的环境干扰因素有关的日变化形态, 这类干扰源的出现和消失时间较为规律, 地电阻率观测数据在干扰源持续存在时和消失之后表现出截然不同的变化形态。图 3 为延庆、平谷、通州、宝坻、塘沽、青光、成都和长兴台受地铁干扰的观测曲线, 观测数据约在地铁停止运行时段(凌晨— 5点)较为平稳, 而在地铁运行期间出现了大幅扰动变化。地铁通过铁轨供电, 铁轨与大地之间存在许多连通之处, 在运行期间将产生强度较大且不稳定的漏电电流。由于电流强度较大, 对远距离的台站仍然能产生较为显著的干扰, 例如延庆台距地铁约40km, 仍受到影响。

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	图 3 受地铁干扰的地电阻率台站观测数据的日变化Fig. 3 The apparent resistivity diurnal variations monitored by stations affected by subway operation.

地电阻率是测区地下介质电阻率的综合反映, 观测时通过稳恒电流源由供电电极A、 B向测区注入已知强度的恒定电流, 测量2个测量电极M、 N之间的电位差, 然后依据固定不变的装置系数K、测量电位差和恒定电流强度计算地电阻率的观测值。依据观测原理, 地电阻率观测值的变化主要体现在测量电位差的变化, 而测量电位差的变化包括2种情况: 1)在供电电流激励下, 因测区介质电阻率变化产生电位差变化; 2)干扰漏电电流在测量电极之间产生附加电位差。图 3 所示台站的日变化成因属于第二种情况, 本文主要分析表1所示的由测区介质电阻率变化引起的日变化的成因机理。

2 岩土介质电阻率的影响因素

地壳岩土介质是多种矿物的混合体, 其中金属矿物颗粒具有良好的导电性, 而有些结晶矿物则基本不导电, 因而不同类型岩石的平均电阻率差异较大。除少部分以金属矿物为主的岩石属于电子导电型外, 大多数岩石为离子导电型。地表浅层覆盖层和沉积岩层含有一定的孔隙, 岩土介质的结构可大致分为2部分: 1)由矿物组成的岩石骨架; 2)岩石中含水(油或气)的孔隙或裂隙。通常情况下, 岩石骨架的导电性较差, 而裂隙含水时因溶解有导电离子而具有低阻性质, 含油或气时导电性较差, 因而岩石的导电性主要取决于裂隙中所含流体的导电性和裂隙结构(钱家栋等, 2010)。对于含水岩石, 实验室结果显示其电阻率与岩石孔隙度、含水饱和度、水溶液离子浓度和裂隙结构关系密切。含水饱和岩石的电阻率与孔隙度的关系如图4a所示,

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图 4 岩土介质电阻率与各影响因素之间关系的实验结果
a 含水饱和岩石的电阻率与孔隙度的关系(Т а р х о в а , 1963); b 亲水性岩石的电阻率与含水饱和度的关系(胡庆东等, 1993); c 含水岩石的电阻率与温度的关系(П а р х о м е н к о , 1965); d 含水岩石受压时电阻率与应力应变的关系 (张金铸等, 1983; 赵玉林等, 1983)Fig. 4 The relation between influence factors and resistivity of rock-soil medium.

孔隙度越高, 则岩石中的导电介质越多, 故其电阻率随孔隙度的增加而减小(Т а р х о в а , 1963); 在孔隙度一定的情况下, 含水饱和度越高, 岩石中的导电介质也越多, 岩石电阻率随饱和度的增加而减小(胡庆东等, 1993)(图4b)。温度主要影响裂隙水中导电离子的浓度和迁移速率(П а р х о м е н к о , 1965), 离子浓度和迁移速率随温度的升高而增加, 岩石电阻率随温度升高而降低, 尤其是温度低于裂隙水冰点时, 岩石电阻率随温度的降低而快速升高(图4c)。实验室内含水岩石标本和野外原地应力加卸载实验结果显示, 主压应力加载过程中电阻率呈现下降变化, 多数岩石临近破裂时电阻率加速下降, 岩石破裂后电阻率回升(图4d)。应力作用使岩土介质裂隙的大小、形状、数量和裂隙之间的连通性发生改变, 从而引起介质的电阻率发生变化(张金铸等, 1983; 赵玉林等, 1983)。

3 地电阻率的影响系数

地电阻率是测区地下各区域介质电阻率的综合反映。如果将测区划分为任意大小的N块区域, 将每个区域视为均匀介质, 电阻率为 $ρ_{i} (i = 1, 2, \dots, N)$ , 在观测装置和布极位置确定时, 地电阻率ρ _a是各区域介质电阻率的函数 (钱家栋等, 1985; Park et al., 1991; Lu et al., 2004):

$d (\ln ρ_{a}) = \overset{N}{\sum_{i = 1}} \frac{\partial \ln ρ_{a}}{\partial \ln ρ_{i}} d (\ln ρ_{i})$ (1)

通常情况下, 各区域介质在一定时间内的变化量很小, 即 $Δ ρ_{i} / ρ_{i} ≪ 1,$ 对式(1)进行泰勒级数展开, 忽略二阶及高阶项, 地电阻率的相对变化可以表示为各区域介质电阻率相对变化的加权和:

$\frac{Δ ρ_{a}}{ρ_{a}} = \overset{N}{\sum_{i = 1}} B_{i} \frac{Δ ρ_{i}}{ρ_{i}}$ (2)

式中, $B_{i}$ 被称为影响系数:

$B_{i} = \frac{\partial \ln ρ_{a}}{\partial \ln ρ_{i}} = \frac{ρ_{i}}{ρ_{a}} \frac{\partial ρ_{a}}{\partial ρ_{i}}$ (3)

影响系数 $B_{i}$ 满足(Roy et al., 1981; Wait, 1981)

$\overset{N}{\sum_{i = 1}} B_{i} = 1$ (4)

在水平层状介质模型下, 将测区划分为N层, 则每一层介质的影响系数表示该层介质的电阻率变化对地电阻率观测值变化的影响程度。

4 地电阻率日变化的成因分析

地电阻率日变化是由测区介质电阻率的日变化所引起的, 而介质电阻率出现日周期变化必然是由具有与之相对应的日周期变化的影响因素所引起。在众多的影响因素中, 固体潮和温度具有日周期变化。直观地看, 固体潮使测区介质产生周期性的张压体应变, 进而引起介质电阻率产生相应周期的上升和下降变化; 昼夜温差主要影响浅层介质的温度, 进而引起浅层介质电阻率白天下降、夜间上升的周期性变化。四川甘孜台的地电阻率日变化为昼升夜降形态, 内蒙古宝昌台为昼降夜升形态, 这2个台站代表了日变化形态的2种类型。下面将以甘孜台和宝昌台为例, 分别分析固体潮和温度变化对地电阻率日变化的影响。

4.1 固体潮对地电阻率的影响

岩石物理实验结果显示, 岩石电阻率的相对变化量与体应变之间存在一比例系数, 称为放大系数K, 小应变时的放大系数大于大应变时的放大系数(Yamazaki, 1966)。对于含流体岩石, 其电阻率ρ 与岩石孔隙度φ 、孔隙内流体的电阻率ρ _w 和饱和度S之间的关系可用扩展Achie公式表示(钱家栋等, 2010):

$ρ = α ρ_{w} φ^{- m} S^{- n}$ (5)

式中, $α$ 是与岩石类型和成分有关的常数, $m$ 是与裂隙导电结构有关的结构指数, $n$ 是与岩石含流体导电效应有关的常数, 通常 $m \geq 1, n \geq 2,$ 且 $m < n$ 对式(5)的两边取微分, 仅考虑体积变化, 可得到如下关系(钱家栋等, 2010):

$\frac{Δ ρ}{ρ} = [(n - m) \frac{1}{φ} + m] \frac{Δ V}{V}$ (6)

在使式(5)成立的孔隙度范围内, 岩石孔隙度越小则放大系数越大。通常情况下放大系数K为n× 10²~n× 10³(Brace et al., 1968)。

甘孜台地电阻率观测的供电极距AB=500m, 宝昌台的供电极距AB=560m。图 5 是甘孜台和宝昌台在深度分别为0、 50m、 100m、 200m和500m时的理论固体潮体应变曲线, 不同深度的体应变幅度和形态几乎一致。因此, 可以认为在固体潮作用下, 甘孜台和宝昌台探测深度范围内的介质具有均匀分布的体应变。固体潮体应变的量级为10^-8, 对于岩土介质覆盖层, 即使各层介质的放大系数K取上限值10⁴, 固体潮引起的电阻率相对变化量级为10^-4, 结合式(2)和式(4)可知, 地电阻率的相对变化量级也为10^-4。目前, 地电阻率观测仪的分辨率为万分之几, 因此, 从影响幅度的角度分析, 目前的地电阻率观测还不能记录到固体潮引起的电阻率变化。

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	图 5 甘孜台(a)和宝昌台(b)不同深度的理论固体潮体应变Fig. 5 The theoretical solid tidal strain at different depth at Garzê station(a)and Baochang station(b).

如果固体潮是目前地电阻率日变化的主要影响因素, 则其对观测的影响形态是怎样的?由于测区介质可视为均匀应变, 固体潮体应变压缩时介质电阻率下降, 拉张时电阻率上升。同样依据式(2)和式(4), 影响系数之和为1, 地电阻率相对变化将与介质电阻率同向变化, 即: 介质电阻率上升或下降时, 地电阻率观测值也上升或下降。中国国土覆盖4个时区, 同一天固体潮相位最大相差4h。因此, 所有存在日变化的台站将具有大致相同的形态, 而不会出现部分台站日变化为昼升夜降, 另一部分台站为昼降夜升的情况, 比如甘孜台和宝昌台的经度相差约15° , 但其日变化形态则相反。更不会出现像拦隆口台不同测道日变化形态相反的情况。

固体潮体应变每日具有2个波峰和波谷, 且幅度存在半月周期变化。地电阻率日变化波形只具有1个波峰和波谷, 日变化幅度也未呈现出与固体潮半月周期变化对应的关系(图6a, b, e, f)。固体潮相位存在月周期变化, 在相位相差为π 的2个日期, 体应变的拉张和挤压形态相反, 而地电阻率日变化的形态特征并未出现相反的情况。

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图 6 甘孜台和宝昌台地电阻率日变化与理论固体潮、温度和降雨之间的对应关系
a 甘孜台地电阻率日变化; b 甘孜台理论固体潮体应变; c 甘孜台温度数据; d 甘孜台降水量; e 宝昌台地电阻率日变化; f 宝昌台理论固体潮体应变; g 宝昌台温度数据; h 宝昌台降水量Fig. 6 The relation of apparent resistivity diurnal variation to theoretical tidal strain, temperature, and precipitation of Garzê station and Baochang station.

因此, 由于固体潮引起的介质电阻率变化幅度低于目前地电阻率观测仪器的分辨率, 故无法观测到固体潮的影响; 从形态分析来看, 固体潮也不是目前观测日变化的主控因素。在将来仪器分辨能力足够高时, 有望观测到在目前日变化波形上叠加的固体潮的影响。

4.2 温度对地电阻率的影响

地表浅层介质的电阻率存在季节性的年变化, 进入春季之后, 随着台站所在地区的温度逐渐升高和降雨量的增加, 电阻率逐渐降低; 进入秋季之后, 温度逐渐降低, 降雨量减少, 浅层介质的电阻率逐渐上升。成都台地电阻率多极距实验的观测结果显示, 短极距观测数据的年变幅度较大, 形态为夏低冬高型, 随着极距的增加, 年变幅度逐渐减小, 在观测极距超过一定距离之后, 年变化形态由夏低冬高型转换为与之相反的夏高冬低型。短极距观测主要反映地表浅层介质的电阻率, 随着极距的增加, 深层介质的电阻率对观测的贡献逐渐增加, 直至占主导地位。同时, 年变幅度随观测极距的增加而减小, 在观测极距增加至使浅层介质影响系数由正转为负时, 年变化呈现相反的状态, 说明年变化是由浅层介质电阻率的季节性变化引起的(赵和云等, 1987; 薛顺章等, 1994)。图7a是甘孜台电测深曲线和反演得到的层状电性结构, 图7b是甘孜台各层介质影响系数的分布, 图7c是宝昌台的电测深曲线和层状电性结构, 图7d是宝昌台影响系数的分布。甘孜台在观测极距AB=500m时浅层介质影响系数为负, 因而地电阻率的年变化为夏高冬低型; 宝昌台在观测极距AB=560m时浅层介质影响系数为正, 地电阻率的年变化为夏低冬高型(图 8)。

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图 7 甘孜台和宝昌台电性结构与影响系数分布
虚线表示影响系数为负值。 a 甘孜台的电测深曲线与层状电性结构; b 甘孜台各层介质影响系数的分布; c 宝昌台的电测深曲线与层状电性结构; d 宝昌台各层介质影响系数的分布Fig. 7 The layered electric structures and sensitivity coefficients distribution of Garzê station and Baochang station.

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图 8 甘孜台和宝昌台的地电阻率年变化、温度和降雨量
a 甘孜台NE 测道地电阻率; b 甘孜台NW测道地电阻率; c 甘孜台温度日均值; d 甘孜台日降雨量; e 宝昌台EW测道地电阻率; f 宝昌台SN测道地电阻率; g 宝昌台温度日均值; h 宝昌台日降雨量Fig. 8 The annual apparent resistivity variation, temperature and precipitation of Garzê and Baochang station.

地电阻率的日变化形态和年变化形态具有一致的对应关系, 具有昼降夜升型日变化的测道具有夏低冬高型年变化特征, 而具有昼升夜降型日变化的测道具有夏高冬低型年变化特征, 说明地电阻率的日变化也是由浅层介质电阻率的变化所引起的。在图7b和图7d给出的数据中, 甘孜台第一、二层介质的影响系数之和为-0.47, 第三、四层的影响系数之和为1.47; 宝昌台第一层的影响系数为0.06, 第二、三层的影响系数之和为0.94。这2个台站浅层以下介质的影响系数之和为正, 且绝对值大于浅层介质。如果日变化是来自浅层以下介质变化的影响, 则所有台站和测道将呈现相同的日变化形态, 而不会出现不同台站或同一台站不同测道日变化形态相反的情况。

地表浅层温度变化是由大气温度变化引起的, 温度的影响幅度随深度的增加而减小, 且满足如下规律(徐世浙, 1985):

$T = T_{0} e^{- \sqrt[]{\frac{ω}{2 κ} z}} \sin (ωt - \sqrt[]{\frac{ω}{2 κ} z})$ (7)

式中, $T_{0}$ 为温度变化幅度, $κ$ 为介质热扩散率, 对于地表土层, 取 $κ$ =0.004 9cm²/s, $ω$ 为温度变化角频率, 对于日变化而言, $ω = 2 π / d, z$ 为深度。取日均气温为15℃, $T_{0}$ =7℃(日温差为14℃), 分别计算ωt为0、$\frac{\pi}{2}$、π和$\frac{3}{2}\pi$时温度随深度 $z$ 的变化, 结果如图9a所示。从图中可以看出, 在地下约0.4m处温度的日变化幅度< 1℃, 因此可以认为温度日变化对地层温度的影响深度为0.4m。

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	图 9 温度日变化对地表介质电阻率的影响 a 温度日变化对地层的影响深度; b 温度日变化对不同深度电阻率的影响Fig. 9 Effects of diurnal temperature variation on resistivity of surface medium.

据实验室结果, 在室温变化范围内含水岩石的电阻率随温度的变化可由如下关系式表示(Т а р х о в а , 1963):

$ρ_{T} = \frac{ρ_{18 ℃}}{1 + α (T - 18)}$ (8)

式中, α 为温度系数, 一般取α =0.025/℃。 ρ _18℃为18℃时岩石的电阻率, 这里取ρ _18℃=50Ω · m, 则在日变化温度影响下浅层介质的电阻率随深度的变化如图9b所示。由此可见, 昼夜温差能引起地表浅层介质的等效电阻率出现大幅度变化, 在白天温度上升时浅层介质的电阻率下降, 而夜间温度下降时浅层介质的电阻率上升。对于浅层介质影响系数为负的测道, 地电阻率的日变化为白天上升、夜间下降的昼升夜降型(图6a, c); 对于浅层介质影响系数为正的测道, 地电阻率的日变化则为白天下降、夜间上升的昼降夜升型(图6e, g)。

5 结论

地电阻率观测中存在2类日变化形态, 一类为白天下降、夜间上升的昼降夜升型, 另一类为白天上升、夜间下降的昼升夜降型。有些台站的日变化为昼降夜升型, 也有一些台站为昼升夜降型, 还有一些台站不同测道的日变化呈现出相反的类型。通过对台站理论固体潮体应变和各层介质的影响系数进行分析可知, 固体潮对介质电阻率的影响幅度低于现行地电阻率仪器的分辨率。地电阻率的日变化形态与年变化形态存在一致对应关系: 具有昼降夜升型日变化的测道, 其年变化为夏低冬高型; 而具有昼升夜降型日变化的测道, 其年变化为夏高冬低型。地电阻率的日变化是由浅层介质电阻率随昼夜温度变化引起的, 日变化形态受影响系数控制: 浅层介质影响系数为正的测道日变化形态为昼降夜升型; 浅层介质影响系数为负的测道日变化形态为昼升夜降型。在今后观测仪器分辨能力进一步提高之后, 有望在现有日变化的波形中记录到固体潮产生的影响。

致谢台站所在区域的温度数据下载自中国天气后报网; 审稿专家提出了中肯的修改建议, 对文章的完善和提高给予了很大的帮助; 作者与甘肃省地震局杜学彬研究员进行了有益的讨论。在此一并表示感谢!

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[25]	张金铸, 陆阳泉. 1983. 不同三轴应力条件下岩石电阻率变化的试验研究[J]. 地震学报, 5(4): 440—445. ZHANG Jin-zhu, LU Yang-quan. 1983. An experimental study on the variation of rock resistivity under triaxially different stress[J]. Acta Seismologica Sinica, 5(4): 440—445(in Chinese). [本文引用:1]
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[40]	Yamazaki Y. 1966. Electrical conductivity of strained rocks, The second paper: Further experiments on sedimentary rocks[J]. Bulletin of Earthquake Research Institute, 44: 1553—1570. [本文引用:1]
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2013

0.0

... 戴勇等, 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) ...

2015

0.0

杜学彬, 刘君, 崔腾发, 等. 2015. 2次近距离大震前成都台视电阻率重现性相似性和各向异性变化[J]. 地球物理学报, 58(2): 576—588.

Xue-bin

, LIU

Jun

, CUI

Teng-fa

, et al. 2015. Repeatability, similarity and anisotropy changes in apparent resistivity recorded by station Chengdu at near distances before two great earthquakes[J]. Chinese Journal of Geophysics, 58(2): 576—588(in Chinese).

The precursor observations, such as electromagnetism, underground fluid, geodesy, etc., for earthquake monitoring and prediction have been internationally implemented for decades. However, nowadays it is highly debatable whether the earthquake-related precursory anomalies can be observed or not, owing to the lack of both the reproducibility and similarity of these anomalies. The anomalies of direct current apparent resistivity (AR, for short; denoted as ρ s ) recorded by station Chengdu before the M S 8.0 Wenchuan quake on May 12, 2008, and the M S 7.0 Lushan quake on Apr. 20, 2013 provide examples to address this issue. The reasons for this are as follows. First, this station is only 35 km from the epicenter in Wenchuan and 99 km from that in Lushan. Second, both the two events happened along the NE-strike Longmen Shan fault zone west of the station. Third, the two focal mechanisms and depths are almost the same. Whether did the reproducibility and similarity of the AR anomalies appeared before the two great shocks? The primary curve of AR observation data and the annual-variation-free curve from which the annual variation of AR is eliminated by using the moving Fourier method (MFM, for short) are traditionally adopted to analyze the anomalies in China. As for the traditional approaches, it is reliable for identifying the relative AR change with the amplitude of no less than 1% because the measurement accuracy is much better than 0.3% in the observation of AR. But, it is sometimes difficult to distinguish the normal and anomalous changes using the traditional approaches, which will result in the uncertainty of anomalies. To solve this problem, the normalized variation rate method (NVRM) is put forward and its principle is briefly described as follows. The long-term decreasing or decreasing variation in the AR time series { ρ s } (with the length N ), if any, is eliminated by the linear regression and the annual variation in it is also removed by using the MFM. The ξ continuous data in { ρ s } are selected to form the i th time sub-series and the variation rate is calculated from ρ si · '=K i ×R i (where K i is its slope coefficient and R i is linear correlation coefficient). Like this, the initial variation rate time series ρ s ·' is generated with the step length ξ fixed. Finally, the NVRM time series ρ s · (dimensionless) with its mean value m →0 and RMSE σ n -1 →1 is obtained from ρ si ·' after the processing such as the normalization, centralization, and noise reduction. The index of anomalies is consistently defined as ±2.4 on ρ s · for each station or channel. In this paper the traditional approaches and the NVRM are applied to process the AR data observed by station Chengdu. At station Chengdu, two measurement channels are respectively in N58°E and N49°W orientations at the same location and each channel employs the fixed Schlumberger array. Before the M S 8.0 Wenchuan and M S 7.0 Lushan events, the two orthogonal channels recorded pre-seismic anomalies with the repeatability, similarity and difference. (1) The two anomalies recorded by each channel showed the repeatability and similarity. The AR anomalies of channel NE, with the relative amplitude of ~-7.0% on the daily mean curve, before the Wenchuan event persistently decreased for 19 months, and then started a recovery (increasing) change about 3.5 months before the shock. Another decrease anomaly, -5.9%, before the M S 7.0 Lushan event continued for 8 months and started a recovery change immediately following the maximum drop. The earlier decreasing and then increasing changes had distinctly exhibited the reproducibility in their changing forms and processes, which tallied with the change process of the electrical resistivity within the focal area predicted by the dilatancy-diffusion model. Furthermore, they also exhibited the similarity in the large-amplitude decrease in the medium-term before the two events. The two anomalies recorded by channel NW in the medium-term clearly showed their reproducibility of the two positive anomalies on the NVRM curves. Their change forms were identical, their starting times were nearly the same, and only the amplitude was larger before the near-distance and greater event in Wenchuan than that before the slightly farther and smaller shock in Lushan. (2) Before each quake the two anomalies recorded by the two channels showed differences in their change forms, amplitudes and starting times. The channel NE recorded the two prominent decreasing anomalies in the medium-term while channel NW recorded two NVRM positive anomalies that indicated the increasing changes of AR, which manifested as the anisotropic changes in the reverse directions. The two anomalies on the monthly and daily mean curves of channel NE were larger in amplitude while those of channel NW appeared only on the NVRM curves that indicated the small-amplitude changes of AR, which showed the apparent anisotropic changes in amplitude. The two medium-term anomalies of channel NE started earlier while those on channel NW started later. Furthermore, the channel NE is obviously superior to channel NW in the responsiveness to the two quakes. (3) The possible reasons for the anomalies of AR is theoretically explained based on the anisotropic medium as follows. During the later preparation of the two great earthquakes, there occurred such the processes as the nonlinear growth and development of the micro cracks within the medium below the station, with the micro-crack strikes in the dominant arrangement roughly along the NW—SE direction and the penetration of low-resistance water within the medium. As a result, the processes brought about the anisotropic changes in electrical resistivity within the medium, with the change in the direction being larger in amplitude than that perpendicular to direction. It resulted in the apparent anisotropic changes, with the AR change of channel NE being larger than that of channel NW. Furthermore, the anisotropic changes in the electrical resistivity were rather strong due to the near distance and large magnitude, so the reverse changes of AR appeared; and before the Wenchuan quake the underground medium below this station had undergone the process mostly characterized by the NW—SE direction compression since August 2006 at latest, and before the Lushan quake the compression had been formed since August 2012. The M S 8.0 Wenchuan and M S 7.0 Lushan earthquakes all occurred on the Longmen Shan fault, very near to station Chengdu. Their focal characters and depths are almost the same, and the two measuring channels in the station record the AR anomalies with the following features: (1) Before the two events, the two anomalies recorded by each channel clearly exhibited the repeatability in the medium-term feature and changing form, and the similarity in the amplitude. Especially, the two anomalies recorded by channel NE show the repeatability in the "earlier decreasing and then recovery (increasing) change" process. (2) Before each quake, the two anomalies recorded by the two channels show the differences in the change form, amplitude and starting time. The repeatability and similarity prove the anomalies to be directly associated with the two great earthquakes, and the differences show that the anisotropic AR changes are directly related to the two source characters.

1. Lanzhou Institute of Seismology, China Earthquake Administration, Lanzhou 730000, China; 2. Lanzhou National Observatory of Geophysics, Lanzhou 730000, China; 3. Earthquake Administration of Beijing Municipality, Beijing 100080, China

本文评价了四川汶川 M S 8.0、芦山 M S 7.0地震前后成都台地电观测环境,研究了该台视电阻率变化.结果为:(1)两次大震发生在该台以西的龙门山断裂带、震源机制和震源深度接近,是近距离大震,相应地,在两次地震前该台两个正交测道中的每一测道视电阻率变化均显示了中期异常及其变化过程的重现性和异常幅度的相似性;(2)在每次地震前,两个测道表现了异常变化形态、幅度和起始时间的差异性.其重现性、相似性证明这些异常与两次大震晚期孕育有关;差异性主要展示了与震源机制有直接联系的视电阻率各向异性变化,揭示了震前该台地下介质经历了强烈的电性各向异性变化的过程.

... 杜学彬等, 2015 ...

2000

0.0

... 目前初步认为趋势变化主要反映区域应力的长期积累, 趋势转折则反映区域应力场的调整(杜学彬等, 2000 ...

1989

0.0

... 桂燮泰等, 1989 ...

1993

0.0

... 在孔隙度一定的情况下, 含水饱和度越高, 岩石中的导电介质也越多, 岩石电阻率随饱和度的增加而减小(胡庆东等, 1993)(图4b) ...

1981

0.0

金安忠. 1981. 地电阻率正常变化的初步研究[J]. 地球物理学报, 24(1): 92—106.

JIN

An-zhong

. 1981. Preliminary study of the normal changes of earth electrical resistivity[J]. Chinese Journal of Geophysics, 24(1): 92—106(in Chinese).

We used the method of ground resistivity measurement to show the variation with time of the earth's electrical resistivity near the surface as the precursor of an earthquake. It has been found that the variation of the resistivity is really very complicated even under conditions of no earthquake and assured accuracy of measurement, thus making the quantitative or even qualitative analysis of the resistivity variation rather difficult.In this paper, the data of variations of earth resistivity observed at a number of stations were analysized for empirical regularities, and on the basis of which a "model of profile section of varying resistivity" was proposed to describe the complex nature and the possible cause of the variation of resistivity in case no earthquake is to occur. From this model section, a formula is obtained for calculating the amplitude of relative apparent annual variation of resistivity at a station characterized by two resistivity layers. Through computations, it is found that, the use of stationary variations of resistivity data might be a possible way to resolve the above difficulty and a concrete method of its application is given.

Seismological Bureau of Shandong Province

观测地壳浅层电阻率随时间变化以寻找地震信息是地电阻率方法的目的。实际工作中,在保证观测精度的前提下,发现正常（无震）情况的地电阻率仍然存在着复杂的变化,给定量和定性分析地震异常带来相当大的困难。本文分析了一些台站正常地电阻率的变化规律,并在此基础上提出“变化电性剖面模式”,来阐明一些台站正常地电阻率变化的复杂过程及其产生的可能原因。从此观点出发,导出以二层电性剖面为特征的台站正常地电阻率相对似年变化幅度公式。通过典型计算,提出获取平稳的正常原始资料是解决上述困难问题的方向,并指出了获取平稳正常变化资料的具体途径。

... 几乎所有台站都会记录到不同幅度的年变化, 从单一测道变化特征上可分为夏低冬高和夏高冬低2种类型(金安忠, 1981) ...

1985

0.0

... 不稳定扰动变化主要与观测系统、测区工农业漏电以及远距离大功率漏电有关, 由于漏电特征的不同, 观测数据通常伴有大幅的不稳定阶跃变化(刘昌谋等, 1985 ...

1998

0.0

... 水平应力作用下不同电性主轴的电阻率并非均匀变化, 地电阻率在短期内也通常表现出各向异性的变化特征(钱复业等, 1996, 1998 ...

1996

0.0

... 水平应力作用下不同电性主轴的电阻率并非均匀变化, 地电阻率在短期内也通常表现出各向异性的变化特征(钱复业等, 1996, 1998 ...

1982

0.0

... 地震异常变化多表现为年尺度的下降或上升, 并伴有年变畸变, 近震中区异常形态以负异常为主(Du, 2011), 走滑型大地震近震区的异常形态呈现出与震源机制解一致的四象限分布(钱复业等, 1982) ...

... 钱复业等, 1982 ...

1985

0.0

... 在超过50a的观测实践过程中, 多次记录到了发生在台网内和附近中强地震前出现的突出的年尺度中短期异常(钱家栋等, 1985 ...

... _a是各区域介质电阻率的函数 (钱家栋等, 1985 ...

2010

0.0

... 通常情况下, 岩石骨架的导电性较差, 而裂隙含水时因溶解有导电离子而具有低阻性质, 含油或气时导电性较差, 因而岩石的导电性主要取决于裂隙中所含流体的导电性和裂隙结构(钱家栋等, 2010) ...

... _w 和饱和度S之间的关系可用扩展Achie公式表示(钱家栋等, 2010): ...

... 式中, α是与岩石类型和成分有关的常数, m是与裂隙导电结构有关的结构指数, n是与岩石含流体导电效应有关的常数, 通常 m≥1,n≥2,且 mn对式(5)的两边取微分, 仅考虑体积变化, 可得到如下关系(钱家栋等, 2010): ...

2017

0.0

... 沈红会等, 2017) ...

2014

0.0

... 石富强等, 2014) ...

2002

0.0

... 汪志亮等, 2002 ...

... 在短期阶段, 由于断层预滑, 近邻破裂区的介质应变加速积累, 地电阻率在原有异常的背景基础上通常会出现加速变化, 而近震区由于出现断层预滑导致应变被部分释放, 地电阻率通常表现为转折变化(汪志亮等, 2002 ...

2018

0.0

... 解滔等, 2018) ...

2013

0.0

... 平常时期的阶跃变化与测区内出现的环境干扰因素有关(张磊等, 2010), 最为突出的是在测区埋设金属管线, 改变了测区地表的电性结构, 导致观测数据出现同步的大幅阶跃变化(解滔等, 2013 ...

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) ...

1985

0.0

... 地表浅层温度变化是由大气温度变化引起的, 温度的影响幅度随深度的增加而减小, 且满足如下规律(徐世浙, 1985): ...

1985

0.0

1994

0.0

... 薛顺章等, 1994), 不同测道的年变形态受浅层介质影响系数(即灵敏度系数)控制(Lu et al ...

... 薛顺章等, 1994) ...

2016

0.0

... 张国苓等, 2016), 但还需要在频谱和形态对比的基础上分析各影响因素对地电阻率的影响幅度 ...

2017

0.0

... 张国苓等, 2017) ...

1983

0.0

... 应力作用使岩土介质裂隙的大小、形状、数量和裂隙之间的连通性发生改变, 从而引起介质的电阻率发生变化(张金铸等, 1983 ...

2010

0.0

2003

0.0

... 已有学者从数据形态和频谱对比的角度分析认为地电阻率的日变化与固体潮有关(赵国泽等, 2003 ...

1987

0.0

... 多极距实验观测和理论分析表明, 年变化是由浅层介质电阻率随降水和温度的季节性变化引起的(赵和云等, 1987 ...

... 同时, 年变幅度随观测极距的增加而减小, 在观测极距增加至使浅层介质影响系数由正转为负时, 年变化呈现相反的状态, 说明年变化是由浅层介质电阻率的季节性变化引起的(赵和云等, 1987 ...

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。

... 异常时空演化特征表现出与孕震过程应变扩散相一致的变化, 离未来地震破裂区越近的台站越早出现异常, 异常幅度也越大, 随着孕震应变向外扩散, 远离未来破裂区的台站相继出现异常(赵玉林等, 2001) ...

1983

0.0

... 赵玉林等, 1983) ...

1968

0.0

... 10³(Brace et al ...

2011

0.0

... Du, 2011) ...

2016

0.0

... Lu et al ...

2004

0.0

... 薛顺章等, 1994), 不同测道的年变形态受浅层介质影响系数(即灵敏度系数)控制(Lu et al ...

... Lu et al ...

1975

0.0

... 地电阻率是地下探测范围内介质电阻率的综合反映, 与孕震过程有关的异常变化主要反映构造应力作用下测区介质变形诱发的微裂隙活动引起的电阻率变化(Mjachkin et al ...

1977

0.0

... 准同震阶段的阶跃变化反映地震发生前后近震区应力的快速释放(Morrison et al ...

1991

0.0

... Park et al ...

1981

0.0

... 影响系数 Bi满足(Roy et al ...

1981

0.0

... Wait, 1981) ...

1966

0.0

... 1 固体潮对地电阻率的影响岩石物理实验结果显示, 岩石电阻率的相对变化量与体应变之间存在一比例系数, 称为放大系数K, 小应变时的放大系数大于大应变时的放大系数(Yamazaki, 1966) ...

1965

0.0

... 温度主要影响裂隙水中导电离子的浓度和迁移速率(#cod#x0041f ...

1963

0.0

... 孔隙度越高, 则岩石中的导电介质越多, 故其电阻率随孔隙度的增加而减小(#cod#x00422 ...

... 据实验室结果, 在室温变化范围内含水岩石的电阻率随温度的变化可由如下关系式表示(#cod#x00422 ...