Abstract:

In 2022, the Menyuan M_S6.9 and Luding M_S6.8 earthquakes occurred successively in the north-south seismic belt of China, both of which caused severe property damage. Prior to these two earthquakes, the China Earthquake Administration conducted multiple ground gravity observations in the North-South seismic zone. It successfully made mid-term predictions based on mobile gravity observation data. The actual epicenters of the two earthquakes were not more than 56km away from the center of the predicted earthquake risk zone in 2022, especially in the precise determination of the epicenter position in 2021 and the distance between the actual epicenter and the predicted epicenter was not more than 10km. This article analyzes ground gravity fields' dynamic evolution characteristics and laws at different temporal and spatial scales before the 2022 Menyuan M_S6.9 and Luding M_S6.8 earthquakes. It reviews the prediction process and basis of these two earthquakes, which is helpful for the study of medium and short-term earthquake prediction.

Overall processing and analysis of multi-source gravity observation data in the north-south seismic zone, obtaining regional gravity field changes based on absolute gravity as the unified benchmark in the measurement area and analysis and study of the dynamic changes of the regional gravity field at different spatiotemporal scales before the Menyuan M_S6.9 and Luding M_S6.8 earthquakes. Further study of the spatiotemporal distribution characteristics and precursor mechanisms of regional gravity field changes will be done by combining geological tectonic dynamic environment and strong regional seismic activity events.

1)Before the Menyuan M_S6.9 earthquake, the gravity change first appeared as a gradient zone of gravity change that was consistent with the direction of the Qilian Mountain fault structure belt and then showed certain four-quadrant distribution characteristics around the epicenter area. The earthquake occurred at the center of the four quadrants of gravity change and near the zero value line of gravity change. 2)Before the Luding M_S6.8 earthquake, the gravity change first showed a gradient zone of gravity change that was consistent with the direction of the Xianshuihe fault structural belt and then showed significant four-quadrant distribution characteristics around the epicenter area. The earthquake occurred at the center of the four quadrants of gravity change and near the zero-value line of gravity change. 3)Gravity data reflects both earthquakes well, and obvious four quadrant distribution characteristics and high gradient zones of gravity changes were observed in the epicenter area and its vicinity before the earthquake, which may be gravity precursor information observed during the earthquake preparation process.

In the past, it was very difficult to accurately determine the location of the three elements of earthquakes(time, location, and magnitude). However, suppose potential strong earthquake epicenters and possible magnitudes are delineated. In that case, the determination of the three elements of earthquakes in the future will become the determination of one element, which will greatly improve the prediction ability of earthquakes The next step is how to carry out earthquake monitoring and tracking work in the identified potential epicenter area and its vicinity, which may help with short-term and imminent prediction. Mobile gravity measurement is the main observation method for exploring earthquake prediction. By establishing a high-density atomic gravimeter absolute gravity observation network or a relative gravity observation network with reasonable absolute gravity control and conducting intensive monitoring, it is possible to reveal the distribution characteristics of underground structures near the epicenter of high-risk areas and extract absolute gravity change signals accompanying source changes during the preparation process of strong/large earthquakes, Develop its application and research in short-term earthquake prediction.

Key words: Menyuan earthquake, Luding earthquake, gravity variation, structural activities, medium-term forecast

摘要：

2022年中国南北地震带先后发生青海门源 M_S6.9 和四川泸定 M_S6.8 地震, 2次地震均造成严重的财产损失, 后者还造成100多人伤亡与失联。这2次地震前, 中国地震局在南北地震带开展过多期地表重力观测, 观测到震中区域地表重力正、负变化交替出现, 呈现出的显著重力异常变化具有四象限分布特征。文中回顾了对这2次地震成功的中期预测和预测依据, 2次地震的实际震中与2022年度的预测地震危险区中心距离均≤56km, 尤其是2021年度对震中位置的精准判定, 实际震中与预测震中距离均≤10km。流动重力测量是探索地震预测的主要观测手段, 建立高密度的原子重力仪绝对重力观测网络或具有良好绝对重力控制的相对重力观测网络, 开展密集的强化监测, 有可能揭示高风险区震中附近的地下结构分布特征, 提取强震、大地震孕育过程中震源变化伴随的绝对重力变化信号, 发展其地震短临预测应用与研究。

关键词: 门源地震, 泸定地震, 重力变化, 构造活动, 中期预测