Variations of apparent resistivity before the 2022 MS6.9 Menyuan earthquake in Qinghai Province revealed by the virtual fault dislocation model
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摘要:
选取2022年1月8日青海门源MS6.9地震震中400 km范围内四个地电阻率观测台站的观测精度高、具有稳态年变、震前无显著干扰的地电阻率$ {\rho }_{{\mathrm{s}}} $观测数据，结合ERA5同化数据集中的多层土壤温度和土壤水分含量，在利用多项式拟合获取各台站（或测道）地电阻率正常年动态的基础上，分析了门源地震前地电阻率的异常变化。结果显示：金银滩台EW测道、武威台NS测道和山丹台EW测道、N45°W测道震前存在超阈值的异常变化，并呈现各向异性特征。基于断层虚位错模式分析了地电阻率异常变化与孕震过程之间的联系，结果表明：金银滩台震前处于压缩区并受到NNE方向的挤压，与主压应变近似正交的EW测道于震前10个月出现负异常；同样位于压缩区的武威台，受到了ENE向的挤压，NS测道的地电阻率在孕震早期（震前13个月）以负异常为主，孕震中晚期（震前3个月）出现了正异常；山丹台，位于膨胀区，受到近似NS向的拉张，与主张应变平行的NS测道未发现异常，但EW测道震前一年地电阻率出现正异常，N45°W测道的地电阻率也在震前半年左右出现超阈值并呈正异常。此外，金银滩台、山丹台和武威台距离门源地震震中的距离分别为92 km，113 km和139 km，相应的地电阻率异常最大变化幅值分别为−3.0σ，2.2σ和−2.1σ。此外，门源地震前地电阻率异常变化的时空特征与岩石实验结果及理论模型一致，也符合震源区应力应变积累程度较高、向外围方向逐渐衰减的分布特征。由此推断，2022年门源MS6.9地震前地电阻率的时空变化可能与区域介质变形及应力变化有关。
Abstract:An earthquake with MS6.9 struck the Menyuan County, Qinghai Province, northwestern China on 8 January 2022. There were four apparent resistivity observation stations with anomalies in an area within 400 km from the epicenter. Using the observed data with high quality and stable annual variation as well as the soil temperature and soil water provided by ERA5 assimilation datasets, a polynomial fitting was performed so as to subtract the normal annual dynamics and to detect the anomalies of the apparent resistivity prior to the earthquake. The results indicate that the above-threshold changes and anisotropic characteristics appeared before the earthquake in the EW channel at the Jinyintan station, NS channel at the Wuwei station, and the EW and N45°W channels at the Shandan station. A virtual fault dislocation model was used to examine the relationship between apparent resistivity changes and seismogenic process of the Menyuan earthquake. In the areas with compression along NNE direction, the decrease changes in the EW channel, which is approximately orthogonal to the direction of the principal compressive strain, has been observed at the Jinyintan station since ten months before the earthquake, the negative anomaly in the NS channel in the early stages of the earthquake preparation （13 months before the main shock） appeared at the Wuwei station, while it turned to an increase change three months before the earthquake. At the Shandan station, which is located in the relative extensional area, no anomalies were detected in the NS channel parallel to the principle tensile strain. However, an increase change was observed in the EW channel one year prior to the earthquake, and an increase change was also recorded in the N45°W channel of Shandan station half-year before the earthquake. Furthermore, at the Jinyintan station, Shandan station, and Wuwei station with epicentral distance of 92 km, 113 km, and 139 km, the corresponding maximum variation of anomalies are −3.0σ, 2.2σ and −2.1σ respectively. In addition, Variations of apparent resistivity before the 2022 MS6.9 Menyuan earthquake were consistent with the results from rock experiment and theoretical models. Moreover, the spatio-temporal characteristics of the variation of apparent resistivity before the MS6.9 Menyuan earthquake are likely consistent with the stress accumulation in the source region, as well as with the characteristics of stress accumulation by a high degree in the epicenter and gradual attenuation towards the periphery. Therefore, it can be deduced that the spatio-temporal variation of apparent resistivity before the MS6.9 Menyuan earthquake may be related to regional medium deformation and stress change.
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图 2 山丹台（左）和金银滩台（右）降雨量与表层土壤水分含量（a）、气温与表层土壤温度（b）的相关性
Figure 2. Correlations between precipitation and surface soil water content （a） and air temperature andsurface soil temperature （b） for the stations Shandan （left panels） and Jinyintan （right panels）
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图 3 定西台NS测道（上）和EW测道（下）土壤水分含量和土壤温度拟合获取的地电阻率时间序列（a）及残差曲线（b）
Figure 3. Time series curves of apparent resistivity （a） and residual variance curves （b） at the NS channel （upper） and EW channel （lower） of Dingxi station obtained by polynomial fitting soil water content and soil temperature
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图 4 山丹台、金银滩台和武威台土壤水分含量和土壤温度拟合获取的地电阻率时间序列（左）及残差（右）
（a） 金银滩台EW测道；（b） 山丹台NS测道；（c） 山丹台EW测道；（d） 山丹台N45°W测道；（e） 武威台NS测道；（f） 武威台EW测道
Figure 4. Time series curves of apparent resistivity （left panels） and residual variance curves （right panels） at Shandan，Jinyintan and Wuwei stations obtained by a polynomial fitting of soil water content and soil temperature
（a） The EW channel of Jinyintan station；（b） The NS channel of Shandan station；（c） The EW channel of Shandan station；（d） The N45°W channel of Shandan station；（e） The NS channel of Wuwei station；（f） The EW channel of Wuwei station
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图 5 基于断层虚位错模式计算的门源MS6.9地震前震中区域的面应变及主应变
图中红色为面应变膨胀区，蓝色为面应变压缩区，白色箭头为主张应变，黑色箭头为主压应变
Figure 5. Distribution of surface strain and principal strain calculated using the virtual fault dislocation model in the epicenter before the Menyuan MS6.9 earthquake
The region in red color is the relative expansion area of surfacestrain，and the blue is the compression area of surface strain.The white arrows are the principal tensile strain，andthe black arrows are the principal compressive strain
表 1 2022年门源MS6.9地震周边400 km范围内地电阻率台站的基础信息
Table 1 Basic information of the apparent resistivity stations within 400 km to the epicenter of the 2022 Menyuan MS6.9 earthquake
台站 测道 震中距/km 数据质量 历史数据干扰因素 观测仪器 年变动态 备注 嘉峪关 N50°E 347 差 降雨、线路漏电、公路 ZD8M 无规律 2021年8—9月漏电干扰 N45°W 中 降雨、线路漏电、公路 夏高冬低 山丹 NS 113 优 线路漏电 ZD8B 夏高冬低 EW 优 线路漏电 夏低冬高 N45°W 优 线路漏电 夏低冬高 白水河 NS 345 优 大风 ZD8M 夏低冬高 2021年8月—2022年1月电极故障 EW 差 大风 夏高冬低 金银滩 NS 92 差 大风 ZD8M 无规律 EW 优 大风 夏低冬高 拦隆口 NS 114 差 大风、灌溉 ZD8M 夏低冬高 EW 差 大风、灌溉 夏高冬低 兰州 NS 296 差 地铁、塑料大棚 ZD8M 无规律 EW 差 地铁、塑料大棚 无规律 武威 NS 139 优 仪器故障 ZD8MI 夏低冬高 EW 优 仪器故障 夏高冬低 临夏 NS 310 中 漏电、塑料大棚、金属网 ZD8M 夏低冬高 2020—2021年测区存在施工建设 EW 差 漏电、塑料大棚、金属网 夏低冬高 定西 NS 388 优 漏电、灌溉 ZD8M 夏高冬低 EW 优 漏电、灌溉 夏高冬低 注：蓝色字为最终参与地电阻率异常分析的台站及测道。 
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表 2 地电阻率与土壤水分含量和土壤温度的相关系数
Table 2 Correlative coefficient between apparent resistivity and soil water content and soil temperature
台站 测道 不同深度土壤水分含量与地电阻率的相关系数 不同深度土壤温度与地电阻率的相关系数 0—7 cm 7—28 cm 28—100 cm 100—289 cm 0—7 cm 0—28 cm 28—100 cm 100—289 cm 金银滩 EW −0.32 −0.39 0.08 −0.16 −0.63 −0.68 −0.78 −0.75 定西 NS 0.47 0.42 0.09 −0.25 0.62 0.67 0.77 0.80 EW 0.51 0.43 −0.02 −0.45 0.61 0.65 0.76 0.82 武威 NS 0.12 0.54 −0.33 0.29 −0.35 −0.30 −0.16 0.19 EW −0.03 −0.36 0.75 −0.79 0.01 0.00 −0.05 −0.12 山丹 NS 0.43 0.10 −0.27 −0.30 0.82 0.82 0.75 0.37 EW −0.32 −0.41 −0.34 −0.82 −0.21 −0.24 −0.27 −0.24 N45°W −0.65 −0.70 0.04 −0.22 −0.56 −0.66 −0.80 −0.87 注：蓝色数字为与地电阻率相关系数较高并参与拟合的土壤水分含量和土壤温度。
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表 3 2022年门源MS6.9地震的震源参数（引自潘家伟等，2022）
Table 3 The source parameters of the Menyuan MS6.9 earthquake in 2022 （after Pan et al，2022）
断层中心位置 长度/km 宽度/km 中心深度/km 滑动量/cm 节面Ⅰ 东经/° 北纬/° 滑动角/° 走向/° 倾角/° 101.26 37.77 31 16 4 300 21 284 82
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