A VIEW OF STRATIFIED RESPOSE BASED ON SHORT-TERM CHANGES OF APPARENT RESISTIVITY AFTER STRONG EARTHQUAKE

doi:10.3969/j.issn.0253-4967.20240136

Abstract

Abstract:

The relationship between pre-seismic anomalous trends in apparent resistivity(continuous decreases or increases) and the accumulation of seismogenic deformation has been confirmed by numerous earthquake cases as well as experimental and theoretical studies. After an earthquake releases the previously accumulated deformation, apparent resistivity is expected to change in the direction opposite to the pre-seismic anomaly. In practice, however, pronounced post-seismic step changes are uncommon in observational records because apparent resistivity is an indirect proxy for crustal deformation. Based on a literature survey and data analysis, we identify several stations that exhibited significant resistivity changes within days after the 1976 Tangshan M_S7.8 earthquake, the 1976 Songpan-Pingwu M_S7.2 earthquake, and the 2008 Wenchuan M_S8.0 earthquake. Although the recovery trends over months or longer are generally opposite to the pre-seismic anomalous patterns, the short-term responses during the first few days after the mainshock vary among stations. Some stations that showed a continuous pre-seismic decrease displayed an immediate post-seismic reversal. Others continued to decrease for several days after the earthquake before turning upward and entering a recovery phase opposite to the pre-seismic anomaly.
In this study, we compile electrical sounding data from five stations that recorded notable changes within days following the three earthquakes. Using apparent-resistivity sensitivity-coefficient theory, we evaluate the depth-dependent response of formation resistivity. The results suggest that, immediately after an earthquake, resistivity variations in shallow media are larger than those in the underlying strata. As stress and deformation continue to relax, changes in deeper strata increasingly control the longer-term recovery of the preceding anomaly. Thus, resistivity changes at different depths can diverge during the immediate post-seismic period, and the observed behavior cannot be primarily explained by water-level fluctuations.
We interpret these patterns in the context of post-seismic stress-strain relaxation. During rupture of a locked fault, accumulated deformation near the source is partially released. Because geological materials are not perfectly elastic, stress and deformation continue to be released after the mainshock, and aftershocks persisting for months further unload residual deformation. Surface rupture during the mainshock promotes more complete deformation release in shallow strata during the immediate post-seismic stage. In contrast, deformation release efficiency likely decreases with depth owing to higher confining pressure, producing larger resistivity increases in shallow layers than in deeper ones. As stress relaxation progresses in the source region, deformation release subsequently affects the observation-station area. Because shallow deformation has already been substantially released, resistivity variations in shallow strata become less pronounced during this phase, whereas continued deformation release at depth drives sustained resistivity increases. Consequently, resistivity changes in deeper strata dominate during the prolonged post-seismic recovery stage.
These findings help clarify the relationship between apparent-resistivity variations and regional deformation across three phases: pre-seismic deformation accumulation, short-term post-seismic adjustment, and long-term post-seismic recovery.

Key words: apparent resistivity, strong earthquake, short-term change, electrical structure, stratified response, sensitivity coefficient

摘要：

强震前视电阻率的持续性下降或上升异常变化与孕震变形积累之间的关系已经得到许多震例、实验及理论分析的证实。地震发生后, 随着原有变形积累的释放, 视电阻率应出现与地震前异常变化形态相反的变化。视电阻率作为间接反映地壳变形的测量方法, 地震发生后较为显著的阶跃变化并不常见。通过文献调研和数据分析发现, 1976年唐山 M_S7.8、松潘-平武 M_S7.2 和2008年汶川 M_S8.0 地震后数天内部分观测站出现了较大幅度的变化, 尽管地震发生后数月至更长时间尺度内的恢复形态与之前的异常形态相反, 但地震后数天内的变化形态却存在差异。在地震前出现下降异常的观测站中, 部分观测站在地震发生后立即出现转折回升; 而另一部分观测站则在地震发生后数天内先出现下降变化, 之后才开始转折回升进入异常恢复阶段。文中收集了这3次地震发生后数天内出现大幅变化的5个观测站的电测深曲线, 结合视电阻率影响系数理论分析了不同深度地层电阻率的响应能力, 认为地震发生后短时间内浅层介质电阻率的变化幅度大于下伏地层。在这之后, 下伏地层的电阻率变化才对异常的恢复阶段起主要控制作用。文中研究有助于认识在地震发生前、地震发生后短期内及震后更长时间的恢复阶段, 视电阻率变化与区域变形之间的关系。

关键词: 视电阻率, 强震, 短期变化, 电性结构, 分层响应, 影响系数

XIE Tao, HAN Ying, LI Xin-yan, YU Chen, LI Xiao-fan, ZHANG Guo-ling, ZHANG Li-qiong, LIAO Xiao-feng. A VIEW OF STRATIFIED RESPOSE BASED ON SHORT-TERM CHANGES OF APPARENT RESISTIVITY AFTER STRONG EARTHQUAKE[J]. SEISMOLOGY AND GEOLOGY, 2026, 48(2): 351-369.

解滔, 韩盈, 李新艳, 于晨, 李晓帆, 张国苓, 张丽琼, 廖晓峰. 从强震后视电阻率的短期变化看地层的分层响应[J]. 地震地质, 2026, 48(2): 351-369.

Figures/Tables 12

Fig. 1 Distribution of the 1976 Tangshan MS7.8, Songpan-Pingwu MS7.2, and 2008 Wenchuan MS8.0 earthquake.

Fig. 2 The short term apparent resistivity changes after the 1976 Tangshan MS7.8, Songpan-Pingwu MS7.2, and 2008 Wenchuan MS8.0 earthquake.

Table 1 The apparent resistivity changes before and after the three strong earthquakes

Table 2 Focal mechanism solutions and coseismic slip models of the three strong earthquakes

Fig. 3 Coseismic deformation caused by the 1976 Tangshan MS7.8 earthquake.

Fig. 4 Coseismic deformation caused by the 1976 Songpan-Pingwu MS7.2 earthquake.

Fig. 5 Coseismic deformation caused by the 2008 Wenchuan MS8.0 earthquake.

Fig. 6 The water level observation data at Changli station.

Fig. 7 The electrical sounding observation data from the five apparent resistivity stations.

Table 3 The monitoring arrays of apparent resistivity measurement

序号	观测站	测道	AB/m	MN/m	H/m
1	宝坻	NS	1000	200	2
		EW	1000	200	2
2	昌黎	NS	990	217	1.8
		EW	1000	200	1.8
3	武都	N72°W	924	308	2
4	成都	N58°E	736	226	2.5
		N49°W	846	270	2.5
5	江油	N10°E	710	90	2
		N70°W	660	90	2

Fig. 8 Sensitivity coefficient versus depth of the five apparent resistivity stations.

Fig. 9 The amplification coefficients of rock and soil resistivity on fracture change.

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