Cause for abnormal decline of DC apparent resistivity at Tanggu station in Tianjin
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摘要: 依据塘沽台所在区域的水文环境、地质构造和动力学等特征，讨论了地表储水能力、地下静水位及构造变形等因素对视电阻率的影响和塘沽台直流视电阻率下降异常的成因机制。结果表明，长期构造位移变化与地下静水位下降是视电阻率呈下降趋势的主要原因；短期降水量大且雨季长，加之有利的地表储水能力是造成塘沽台视电阻率破年变的主要原因。虽然对于塘沽视电阻率异常中是否掺有前兆信息尚缺少确凿证据，但本文采取的分析方法仍可为典型的异常形态提供有益的参考。Abstract: The decline of DC apparent resistivity is a common abnormal variation in electromagnetic forecast, the reason is the change of underground electrical structure caused by rock stress change. Therefore, it is very important for timely identification and scientific judgment of earthquake precursory anomalies to effectively analyze their genetic properties. The EW direction of the resistivity of Tanggu station showed a downward trend since 2013, and the low value in 2021 was significantly lower than previous years, which was a breaking anomaly in the downward trend. The low range was 0.104 Ω·m, accounting for about 1% of the background value, and the low value time was delayed compared with previous years. Based on the hydrologic environment, geological structure and dynamic characteristics of the area where Tanggu station is located, we discussed the influence and genetic mechanism of surface water storage capacity, underground static water level and tectonic deformation. The results show that the long-term tectonic displacement change and the decline of groundwater static water level are the main reasons for the decline of the resistivity trend, and the large short-term precipitation and long rainy season, together with the favorable surface water storage capacity, are the main reasons for the formation of the break year change pattern. Although there is no conclusive evidence that is precursory information in the apparent resistivity anomaly of Tanggu station, the analytical method adopted in this paper can still provide useful reference for the typical anomaly pattern.
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Keywords:
- apparent resistivity /
- Tanggu station /
- decline change /
- electrical structure /
- lag effect
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图 9 GNSS基线值在日本MW9.0地震前后的变化
Figure 9. Changes of GNSS baseline values before and after the MW9.0 earthquake in Japan
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图 1 区域构造背景与电性结构
（a） 区域主要断裂、台站位置及2000—2022年M＞1.0地震分布图；（b） 观测区视电阻率布极图；（c） NS向与EW向电测深曲线图
Figure 1. Regional tectonic background and electrical structure
（a） Distribution of major faults，station locations and M＞1.0 earthquakes during 2000−2022； （b） The electrode layout of apparent resistivity at the observation area ；（c） Electrical sounding curves in NS and EW directions
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图 2 塘沽台视电阻率多年日均值曲线图
A，B为往年数据低值对比辅助线，C，D，E，F，G为历年低值时间对比辅助线
Figure 2. Curves of multi-year daily mean value for apparent resistivity at Tanggu station
A and B are contrast auxiliary lines of low-values of previous years，and C，D，E，F， and G are contrast auxiliary lines of low-value time of previous years，respectively
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图 3 塘沽视电阻率夜静时段（1:00—4:00）平均值数据
虚线为理想的虚拟年变趋势线
Figure 3. Apparent resistivity data of Tanggu station at night （1:00—4:00 am）. Dashed line is an ideal virtual annual trend line
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图 4 塘沽台NS向和EW向测线附近的水体位置分布图
长方形与不规则形状表示池塘或水渠，点划线为河流，直线为布极方向线，虚线为金属管线
Figure 4. Water locations near the NS and EW survey lines at Tanggu station
Rectangles and irregular shapes represent ponds or canals，dot dash lines are rivers，straight lines are survey lines，dotted lines are metal pipelines
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图 5 2017—2021年塘沽台降雨量统计图
黑色矩形为最大降雨月份后降雨量持续影响视电阻率变化的部分；红色竖虚线代表每年最大降雨量月份对应的视电阻率位置；红色横虚线为往年视电阻率低值辅助线
Figure 5. Statistical map of rainfall at Tanggu station from 2017 to 2021
The black box is the part where the rainfall continues to affect the change of apparent resistivity after the maximum rainfall month； the vertical red dotted line represents the position of apparent resistivity corresponding to the month of maximum rainfall of each year；the horizontal red dotted line is the low value auxiliary line of apparent resistivity in previous years
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图 6 2017—2021年7—10月塘沽台降雨量统计图
线A表示2021年降雨量，线B表示2017—2020年降雨量均值
Figure 6. Statistical diagram of rainfall at Tanggu station from July to October during 2017 to 2021
Line A refers to the rainfall in 2021，line B refers to the mean rainfall during 2017—2020
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图 7 塘沽台EW向视电阻率与静水位对比图
Figure 7. Comparison of EW direction apparent resistivity and static water level at Tanggu station
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图 8 2002—2022年塘沽台视电阻率长趋势曲线形态
Figure 8. Morphology of long trend curves of apparent resistivity at Tanggu station during 2002−2022
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图 10 塘沽台2018年以来视电阻率EW向数据图像
黑色曲线为视电阻率EW向数据的多项式拟合曲线；虚线A为多年低点趋势性下降的低值辅助线；虚线B为2021年非拟合低值的实际位置，该点略低于A；虚线C为2019年非拟合低值的实际位置，该点略高于A
Figure 10. EW trend data images of apparent resistivity at Tanggu station since 2018
The black line is the polynomial fitting curve of the EW direction data of the apparent resistivity；the dotted line A is the moving average of multi-year low values trending downward；the dotted line B is the actual location of the unfitted low value in 2021，which is slightly below A；the dotted line C is the actual location of the unfitted low value in 2019，which is slightly above A
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图 11 无人机航拍照片与土方堆积的实地拍摄照片
航拍图像叠加在百度地图上
Figure 11. Aerial photos of UAV and field photos of earthwork accumulation
Aerial images are overlaid on Baidu Maps
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图 12 1976年唐山MS7.8地震前后塘沽台视电阻率变化
Figure 12. Apparent resistivity variation of Tanggu station before and after the Tangshan MS7.8 earthquake in 1976
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图 13 塘沽台视电阻率EW向与地震时间对应关系
地震b，c相距34天可视为一组，地震d，e相距21天可视为一组，图中字母对应表2中的地震事件
Figure 13. Correspondence between EW direction of apparent resistivity at Tanggu station and historical earthquake time
Earthquakes b and c can be considered as a group of 34 days apart，so as the earthquakes d and e of 21 days apart，letters mean the events listed in Table 2
表 1 塘沽台降雨量数据统计表
Table 1 Statistical table of rainfall data at Tanggu station
月份 降雨量/mm 2017年 2018年 2019年 2020年 2017—2020年均值 2021年 1月 0.5 0 0 0 0.1 0 2月 0 0 0 0 0 0 3月 4.4 0 0.7 8.1 3.3 10 4月 2.0 31.1 12.7 15.9 15.4 18.6 5月 15.7 21.9 14.8 57.3 27.4 8.5 6月 56.8 56.2 18.1 5.9 34.3 30.7 7月 60.7 130.1 155.7 98.9 111.4 298.9 8月 236.4 139.3 58.3 176.0 152.5 88.7 9月 0 13.9 18.9 147.6 45.1 128.3 10月 62.5 14.1 4.1 7.0 21.9 86.5 11月 0 3.3 1.3 30 8.7 7.2 12月 0 0 1.9 0 0.5 0 总和 439.0 409.9 286.5 546.7 420.5 677.4 
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表 2 2002年以来塘沽台120 km范围地震目录
Table 2 List of earthquakes around Tanggu station with 120 km range since 2002
地震编号 发震日期
年-月-日发震地点 M 震中距/km 走向/° 倾角/° 滑动角/° 震前阶段 震源机制解 A 2006-07-04 河北文安 5.1 112.8 215 87 −115 年变下降段 
B 2020-07-12 河北古冶 5.1 105.6 61 73 −180 年变下降段 
a 2002-05-19 河北丰南 4.2 45.7 318 79 −32 年变下降段 
b 2010-03-06 河北唐山 4.2 99.9 354 45 −33 高值转折段 
c 2010-04-09 河北丰南 4.1 59.7 324 90 −27 高值转折段 
d 2012-05-28 河北唐山 4.8 100.1 314 76 −20 年变下降段 
e 2012-06-18 天津宝坻 4.1 55.6 131 77 −25 年变下降段 
f 2016-09-10 河北唐山 4.2 92.7 152 80 8 低值转折段 
g 2018-02-12 河北永清 4.3 83.9 52 62 −140 年变上升段 
h 2019-12-05 河北丰南 4.5 46.0 325 54 −170 低值转折段 
注：2008年以后地震目录来源于地震编目系统，2002—2007年地震目录来自天津地震局历史地震目录总结
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