Abstract:

The “sub-instability” earthquake prediction theory, combined with field-to-source and source-to-field techniques, offers a method for determining the location and timing of future earthquakes in key geological structures and active fault zones. Following the 2021 Maduo M_S7.4 earthquake, seven more earthquakes with magnitudes ≥5 occurred along the eastern section of the Altyn Tagh fault zone and the Qilian Mountain seismic belt. According to active earthquake period statistics, the Qilian Mountains tectonic belt remains one of the most active seismic zones in mainland China. Among the moderate to strong earthquakes, the Menyuan M_S6.9 earthquake in Qinghai had the most significant impact.
The evolution from earthquake incubation to development is complex and multifaceted. The successful spatiotemporal prediction of the Menyuan M_S6.9 earthquake exemplifies the effectiveness of integrating long-, medium-, short-, and imminent-term earthquake forecasting by Chinese seismologists. In late 2015, the China Earthquake Administration's M7 Task Force identified high-risk zones for major earthquakes(M≥7) in mainland China for the period 2016-2025, based on geological structures, seismological data, and geophysical changes. The central section of the Qilian Mountain seismic belt(specifically the Lenglong Ridge Fault)was designated as one of these high-risk zones, representing a long-term forecast. In 2021, the China Earthquake Administration identified the region from Jiayuguan in Gansu Province to Menyuan in Qinghai Province as a seismic hazard zone for the following year(with a forecast of around magnitude 6), reflecting a medium-term prediction. Additionally, in December 2021, the Qinghai Earthquake Agency issued a short-term forecast(one month prior to the event), warning of a potential magnitude 6-7 earthquake in the Qinghai region.
An analysis of seismic activity tracking and anomalous spatiotemporal evolution leading up to the earthquake revealed several key seismological anomalies detected by the Gansu-Qinghai seismic network. Notably, the Qilian Mountain seismic belt experienced 1.95 years of quiescence for magnitude 5 earthquakes, which was subsequently interrupted by the M_S5.5 Aksai earthquake on August 26, 2021. Additionally, the M_L3.6 earthquake in Guyuan, Ningxia, on June 22, 2021, ended an 880-day period of quiescence in the central-eastern section of the Qilian Mountains. These disruptions increased concerns about the likelihood of intermediate-to strong-magnitude earthquakes in the region.
Major geophysical anomalies, including 23 instances of localized deformation, electromagnetic, and subsurface fluid anomalies, were detected along the Qilian Mountain structural belt over the past four years. Analyzing the sequence of these anomalies revealed that: 1)5 background anomalies(lasting over 1 year)were distributed 300km from the epicenter; 2)2/3 of the 8 medium-term anomalies(lasting 0.3 months to 1 year)were within 200km of the epicenter; and 3)4/5 of the 10 short-term and imminent anomalies(less than 3 months)were within 180km of the epicenter. Since July 2021, medium-to short-term anomalies have become concentrated in the central-eastern section of the Qilian Mountain seismic belt, an area with one of the strongest monitoring capabilities and densest station distributions within the Gansu-Qinghai seismic network. As these anomalies emerged in clustered, synchronous patterns, seismic authorities promptly organized field verification and tracking.
By integrating significant seismic events with geological structural features, authorities issued short-term forecasts prior to the earthquake, leading to effective disaster mitigation outcomes. This demonstrates that, under tectonic conditions in areas covered by seismic monitoring networks, the “sub-instability” theory can guide the research on anomalous changes in observational data, providing valuable support for earthquake prediction and reducing potential losses from future major earthquakes.

Key words: Menyuan M_S6.9 earthquake, seismic activity, “Sub-instability” theory, spatiotemporal evolution of geophysical field anomalies, earthquake prediction.

摘要：

“亚失稳”地震预报理论、 以场求源和以源观场技术是对重点地质构造区和活动断层判定未来地震发生地点和时间的一种方法。2021年玛多 M_S7.4 地震发生后, 阿尔金断裂东段至祁连山地震带先后发生了7次5级以上的地震。从历史地震活跃周期统计, 目前祁连山地震带仍是中国大陆地震活跃地区, 在这丛中强地震中青海门源 M_S6.9 地震影响最大。通过对该地震前的震情跟踪及地球物理场的时空演化总结发现, 祁连山地震带上在近4a内共出现23台项形变、 电磁、 地下流体等地球物理场异常。通过梳理异常变化的序列, 发现空间上5项背景异常(1a以上)分布在距震中300km以外, 8项中期异常中(0.3个月~1a)有2/3 的异常分布在震中距200km以内, 10项短临异常中(3个月以内)有4/5 的异常分布在震中距180km内。2021年7月以后, 中短期异常集中出现在祁连山地震带中东段, 该地区是甘青地震台网监测能力强、 台站分布最为密集的地区之一, 因此在异常变化成组、 成片同步出现时, 地震部门立即组织了现场核实和跟踪, 结合显著地震事件和地质构造特征, 在震前进行了短临预报, 取得了一定的减灾实效。文中研究带来的启示是: 在地震监测能力覆盖区域构造条件下, 利用“亚失稳”理论指导研究观测资料异常变化, 能为地震预报提供有力支撑, 可降低未来强震造成的灾害损失。

关键词: 门源M_S6.9地震, 地震活动性, “亚失稳”理论, 地球物理场异常时空演化, 地震预报