Accurate prediction of the epicenters of the 2022 MS 6.9 Menyuan and MS 6.8 Luding earthquakes

Dynamic gravity field changes before the Minoan earthquake

From October 2018 to October 2020 (Figure 1a), a negative gravity change occurred on the north side of the Tulishan fault, with a corresponding positive gravity change on the south side of the fault. A westward-trending gradient zone was found between gravity changes near the Tuolaishan fault, and it rotated near the epicenter of the 2022 Minyuan earthquake. The differential gravity changes on both sides of the Tuolaishan and Lenglongling fault near the epicenter were larger than 60 × 10−8 ms.

From October 2020 to July 2021 (Figure 1b), positive gravity changes occurred in the Gulang area east of Minyuan, while negative gravity changes occurred south of Minyuan. Thus, a quadrilateral distribution of gravity changes was formed which was concentrated in the Minoan. Gravity changes at the epicenter of the Minyuan earthquake were weak and close to zero. The gravity differences between the southeastern and northwestern sides of Wuwei, Gansu-Minyuan, and Qinghai were larger than 90 × 10−8 ms, and the Minyuan earthquake in 2022 occurred near the zero line of gravity variation at the center of the Quaternary distribution. .

Figure 1

Ms 6.9 Minyuan earthquake (a) from October 2018 to October 2020 and (b) from October 2020 to July 2021. F1: Tulayshan fault; F2: Linglongling error; F3: Jinqianghe error; F4: Maomaoshan error; F5: Laohushan fault; F6: Ynaokao fault; F7: Changma-Ebu fault; F8: Qilianshan North Marginal Fault; F9: Tiangouqiao-Huangyangchuan fault; F10: Zhuanglanghe error; The graph was drawn using Generic Mapping Tools (GMT) software (https://www.generic-mapping-tools.org, version 6.5.0).

The dynamic gravity field changes before the Luding earthquake

From September 2019 to September 2020 (Figure 2a), gravity changes throughout the study area ranged from −40 to +70 × 10−8 m s, with an overall negative and positive gravity change trend W–E. The gravity change trend was consistent with The direction of the major faults in the area, indicating strong tectonic activity. The study area can be divided into eastern and western regions by a boundary located along Daofu-Kangding-Julong. The western region showed small changes in negative gravity on the Western Sichuan Plateau, while the eastern region showed large changes in positive gravity in the Xiaojin, Luoding, Shimian, and Mianying regions. The line of gravity change was curved and intersected near Kangding and Luoding in the Xianshuihe fault zone. The 2022 MS 6.8 Luding earthquake occurred near the inflection portion of a high gravity shift gradient zone.

From September 2019 to September 2021 (Fig. 2b), the gravity changes throughout the study area ranged from −60 to +70 × 10−8 ms, and the trend of WE gravity change was negative to positive. Two positive gravity variation anomalies (maximum 70 × 10 ms) were found in the Jiulong and Mianying areas (south of the epicenter), as well as in the Xiaojin area (north of the epicenter). The high gravity variation gradient zone is located along the Longmenshan fault and in the Xianshuihe fault zone in the Wenchuan, Ya'an, Luding and Kangding areas. The gravity changes showed a quadrilateral distribution centered at Moxi and Shimian. The epicenter of the 2022 MS 6.8 Luding earthquake is located near the turning part of the high gravity change gradient zone and the center of the quadrilateral gravity distribution.

Figure 2

Dynamic changes in the regional gravity field before the 2022 Ms 6.8 Luding earthquake (a) from September 2019 to September 2020 and (b) from September 2019 to September 2021. F1: Longmenshan fault; F2: Qianshuihe fault; F3: Yulongshi fault; F4: Xiaojinhe fault; F5: Anninghi fault; F6: Daliangshan fault; F7: Zhaogui-Putou fault; F8: Zimmohi error; F9: Xiaojiang Fault; The graph was drawn using Generic Mapping Tools (GMT) software (https://www.generic-mapping-tools.org, version 6.5.0).

Prediction of the epicenter of the Minyuan earthquake

In the 2021 and 2022 Earthquake Trend Study Report, the Second Crustal Monitoring and Application Center of the China Earthquake Administration provided a medium-range forecast of the MS 6.916 Minyuan earthquake. The forecast details are summarized in Table 1. The 2021 forecast indicated that the earthquake would occur in the Gansu Jinchang-Qinghai Qilian area. Forecasts for 2022 indicated that the earthquake would occur in the central or eastern Kilian Mountains.

On January 8, 2022, the MS 6.9 Minyuan earthquake (37.77°N, 101.26°E) occurred in the forecast area. The distances between the epicenters of the two earthquakes predicted in 2021 and 2022 and the actual epicenter measured by the China Earthquake Networks Center were 6 and 56 kilometers, respectively.

Regional gravity changes from October 2018 to October 2020 (Figure 1a) indicate that the southwestern Qilian and Minoan regions in the Qilian Mountains underwent positive gravity changes, while the Shandan and Wuwei regions in the Hexi Corridor underwent negative gravity changes (difference of 60 × 10− 8 ms-2). A high-gradient zone of gravity change was found along the Tuolaishan Fault and Lenglongling Fault, which changed their direction near Menyuan. Based on these changes and the relationship between gravity changes and seismic activity studied by our predecessors8,9, we proposed in 2020 that a strong magnitude 6 earthquake could occur in 2021 near the Minoan region, where a high-gradient gravity change zone curves southward. The regional gravity changes from October 2020 to July 2021 also indicate that the distribution of quadrant gravity change was centered in the Minyuan, and the gravity difference between the southeast and northwest sides of the Minyuan-Wui area in Qinghai was 90 × 10−8 m-2. In 2021, we move the center of the 2022 earthquake hazard area southeastward to the intersection of the zero line and the Lenglongling fault, which is located close to the center of the quadrilateral gravity distribution (Figure 1b). The expected volume has not changed.

Before the 2022 MS 6.9 Minyuan earthquake, the gravity change first showed a gradient zone of gravity change that was mainly consistent with the collision of the Tulayshan fault and the Linglongling fault (Figure 1a), and then showed a quadrilateral distribution around the epicenter (Figure 1b), which presented Basis for location determination and mid-term prediction of this earthquake. The Menyuan MS 6.9 earthquake occurred near the epicenter of the fourth quadrant and had zero gravity variation.

Prediction of the epicenter of the Luding earthquake

The 2021 and 2022 reports of the Second Crustal Monitoring and Application Center of the China Earthquake Administration also provided accurate medium-range forecasts of the 2022 Luoding earthquake. The forecast details are summarized in Table 1. The 2021 forecast indicated that the earthquake would occur in the Sichuan Daofu-Yunnan Zhaotong region, While the 2022 prediction indicated that the earthquake would occur along the eastern border between Sichuan and Yunnan (i.e. Daofu and Xiaojin, Kangding, Luoding, Shimian, Jiulong, Mianning, Xichang areas).

On September 5, 2022, the MS 6.8 Luding earthquake (29.59°N, 102.08°E) occurred in the forecast area. The distances between the expected epicenters in 2021 and 2022 and the actual epicenter were 8 and 44 km, respectively.

Gravity changes along the eastern Sichuan-Yunnan border from September 2019 to September 2020 (Fig. 2a) indicate that the high gradient zone of gravity change was located along the Xianshuihe and Yulongshi fault, with a bend near Luoding. The gravity changes east of the Daofu-Kangding-Jiulong gradient zone were positive, while those west of the gradient zone were negative, with a maximum difference of 100 × 10−8 ms. Based on the gravity anomaly changes, we suggested that the bend in the gravity gradient zone near Luoding is expected to be the likely epicenter of the earthquake in 2021, with a strong risk of a 7-magnitude earthquake in the area. However, data from September 2019 to September 2021 indicate that the region near Shimian was the center of the quadrilateral gravity change distribution, which was bounded by the Xianshuihe and Daliangshan fault (Fig. 2b) and had a maximum gravity difference of more than 100 × 10−8 m s−. 2. Therefore, we have updated our forecasts in 2021 to indicate the possibility of a magnitude 7 earthquake in this region in 2022.

Before the 2022 MS 6.8 Luoding earthquake, gravity changes first appeared in the progradation zone that was consistent with the collision of the Xianshuihe tectonic belt (Fig. 2a) and then showed a quadrilateral distribution around the central zone (Fig. 2b). The 2022 Luding earthquake occurred near the center of the Quaternary distribution and the zero gravity variation line. Thus, the mobile gravity data accurately predicted the location of the epicenter of the Luding earthquake in 2022. The change of the quadrilateral gravity center and the rotating part of the high-gradient zone which mainly corresponds to the Xianshuihe fault is the main basis for determining the location of the Luding earthquake.

In brief, both the Menyuan MS 6.9 and Luding MS 6.8 earthquakes occurred in Quaternary center of gravity shifts. Gravity data accurately identified the epicenters of the Menyuan M6.9 and Luding M6.8 earthquakes at 202213,16.

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