APPARENT RESISTIVITY ABNORMAL CHANGES OF BOREHOLE MONITORING BEFORE FOUR MS≥6.0 EARTHQUAKES IN SICHUAN-YUNNAN AREA FROM 2021 TO 2022

doi:10.3969/j.issn.0253-4967.2023.06.007

Abstract

Abstract:

There have been four M_S≥6.0 earthquakes, i.e. the Yangbi M_S6.4, Lushan M_S6.1, Maerkang M_S6.0 and Luding M_S6.8, occurred in Sichuan-Yunnan area from 2021 to 2022. Three borehole apparent resistivity monitoring stations of Hongge, Mianning and Garzê, normally operated within 400km away from these earthquakes. These stations recorded different types of middle-short abnormal changes before the four earthquakes. The EW and NS arrays at Hongge station began to display synchronous decline change since December 2020. According to the empirical relationship between the duration of the anomaly and the magnitude, it is predicted that there is a possibility of a magnitude 6.0 earthquake in the Sichuan-Yunnan border area. But there had been no short-term abnormal changes that can give a forecast opinion on a time scale. Short-term abnormal changes appeared at Hongge station in March 2021. The maximum decline magnitudes of the NS and EW arrays were 2.1% and 1.4%, respectively. We believed that the risk of a strong earthquake has further increased. The distance from the Yangbi earthquake to Hongge station is 232km. Data of NS array recovered to normal status after the Yangbi M_S6.4 earthquake in 2021, but the data of EW array continued to decline. In the fault virtual dislocation(FVD)model, the coseismic slips are loaded in a way of equal in magnitude but opposite in direction, which provides the relative deformation around the epicenter. In a compressive tectonic region, compressive areas from the FVD model can be formed as the areas with compression enhancement. The dilatant areas cannot be distinguished between absolutely dilatant areas and compressive areas. However, they can be regarded as relatively dilatant areas where the original extensive stress is enhanced, or the original compressive stress is released to some extent. It is found that Hongge station is located in the relative expansion region generated by the Yangbi earthquake. Abnormal changes at Hongge station are not only affected by the seismogenic process of the Yangbi earthquake, but also possibly indicating the existence of another strong earthquake. The anomaly duration of the EW array was about 21 months long and the decline magnitude was 2.8%up to September 5, 2023, when the Luding M_S6.8 earthquake occurred, which is about 340km far away from Hongge station. The N10°E array of Garzê station began to show abnormal decline change since April 2022 with the maximum decline magnitude of about 0.5%. Apparent resistivity curve changed to rise change since late June, and the anomaly recovered to the normal status in October. Three earthquakes of Lushan M_S6.1, Maerkang M_S6.0 and Luding M_S6.8 occurred during the period of anomaly evolution, with a distance of 306km, 183km and 293km from the Garzê station, respectively. The regional deformation before Lushan and Maerkang earthquakes from the fault virtual dislocation model show that the seismogenic process of these two earthquakes has little influence on the abnormal changes detected from Garzê station. The abnormal changes are probably related to the preparation of another big earthquake. At the earthquake situation consultation meeting on August 26, the Electromagnetism Laboratory predicted that a magnitude 6.0 earthquake would occur in the northwest of Sichuan in the short term. After the Luding M_S6.8 earthquake, the observation data of the N10°E array returned to the normal status. The N60°W array showed distortion in annual variation pattern from September to November 2022. Another M_S5.6 earthquake occurred in Luding on January 26, 2023. The EW, NS and N45°W arrays of Mianning station showed synchronous rise changes since late June 2022, and anomalies recovered after the Luding M_S6.8 earthquake. After the Luding earthquake, the regional deformation characteristics before the earthquake are also analyzed by the fault virtual dislocation model. The Garzê station is located in the area of compression enhancement, while Mianning station is located in the area of relative expansion. Results from petrophysical experiments have demonstrated the decline changes when water-bearing rock and soil samples are under compression, and the rise changes as samples are undergoing stress unloading. Resistivity does not recover to the initial status when the compression stress is completely released, suggesting the existence of irreversible change in the samples. Therefore, the patterns of anomalies recorded at Garzê and Mianning stations are consistent with the area deformation where the stations were located. In summary, according to the abnormal changes of three borehole apparent resistivity observation in Hongge, Mianning and Garzê station from 2020 to 2022, we have made short-term prediction for the Yangbi M_S6.4 in 2021, Luding M_S6.8 in 2022 and Luding M_S5.6 in 2023, at the regular weekly and monthly earthquake situation consultation meetings.

Key words: Apparent resistivity, borehole observation, Sichuan-Yunnan area, earthquake, anomaly

摘要：

2021—2022年川滇地区先后发生了云南漾濞 M_S6.4、四川芦山 M_S6.1、马尔康 M_S6.0 和泸定 M_S6.8 4次M_S≥6.0地震。距这些地震震中400km的范围内布设有红格、冕宁、甘孜井下地电阻率观测站, 其观测数据在这4次地震前出现了不同程度和形态的中短期异常变化。红格站EW和NS测道自2020年12月开始出现同步下降变化, 2021年漾濞 M_S6.4 地震后NS测道的异常恢复, 但EW测道仍然持续下降, 截至2022年9月5日泸定 M_S6.8 地震发生, EW测道的异常持续时间约为21个月。甘孜站N10°E测道的下降异常变化始于2022年4月, 6月底开始出现转折回升, 10月异常恢复, 其间先后发生芦山 M_S6.1、马尔康 M_S6.0、泸定 M_S6.8 地震。2022年9—11月, 甘孜站 N60°W测道出现年变畸变异常, 2023年1月26日泸定再次发生 M_S5.6 地震。冕宁站EW、 NS、 N45°W测道自2022年6月下旬开始出现同步上升变化, 于泸定 M_S6.8 地震后异常恢复。依据这些异常变化, 在周、月会商中对2021年漾濞 M_S6.4、 2022年泸定 M_S6.8、 2023年泸定 M_S5.6 地震做出了短期预测。

关键词: 地电阻率, 井下观测, 川滇地区, 地震, 异常变化

XIE Tao, HAN Ying, YU Chen. APPARENT RESISTIVITY ABNORMAL CHANGES OF BOREHOLE MONITORING BEFORE FOUR M_S≥6.0 EARTHQUAKES IN SICHUAN-YUNNAN AREA FROM 2021 TO 2022[J]. SEISMOLOGY AND GEOLOGY, 2023, 45(6): 1370-1384.

解滔, 韩盈, 于晨. 2021—2022年川滇地区4次M_S≥6.0地震前井下地电阻率观测的异常变化[J]. 地震地质, 2023, 45(6): 1370-1384.

Figures/Tables 7

Fig. 1 The locations of apparent resistivity stations and the four MS≥6.0 earthquakes in Sichuan-Yunnan area from 2021 to 2022.

Table1 Configuration parameters of the three borehole apparent resistivity stations in Sichuan province

序号	观测站	测道	启用年月	AB/m	MN/m	H/m
1	红格	NS	2014-07	422	152	90
		EW	2014-07	345	118	90
2	甘孜	N10°E	2018-01	400	120	60
		N60°W	2018-01	400	120	60
3	冕宁	NS	2020-01	600	200	120
		EW	2020-01	600	200	120
		N45°W	2020-01	600	200	120

Fig. 2 The electrical sounding curve of the three stations.

Fig. 4 Apparent resistivity of borehole monitoring data at Hongge station from 2016 to 2023.

Fig. 5 Apparent resistivity of borehole monitoring data at Garzê station from 2018 to 2023.

Fig. 6 Apparent resistivity of borehole monitoring data at Mianning station from 2020 to 2023.

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