Abstract:

The duration of the aftershock sequence following large earthquakes is a crucial issue in seismicity research and is significant for seismic hazard estimation. Previous studies based on three large earthquakes that occurred since 1966 have provided important insights into aftershock activity and duration in North China. However, understanding the persistence of aftershock sequences following historical large earthquakes remains limited. In this study, we aim to comprehensively study the duration of long-lived aftershocks following large earthquake sequences in North China. Based on the modern earthquake catalog from 1970 to 2022, combined with historical earthquake records, we systematically investigate the aftershock characteristics of six large earthquakes with a magnitude of M≥7 in North China, including the 1668 Tancheng M8½, 1679 Sanhe-Pinggu M8, 1937 Heze M7.0, 1966 Xingtai M_S7.2, 1975 Haicheng M_S7.3, and 1976 Tangshan M_S7.8 earthquakes.

The determination of the aftershock zone is crucial in this study. We conducted a thorough review of existing research on earthquake rupture, including field investigations, fault slip inversions, and aftershocks distribution. By synthesizing these results with the seismic catalog data used in this study, we selected the aftershock zones. According to the analysis of the spatial distribution of earthquakes, temporal frequency variation, and data fitting based on the Omori-Utsu law, our study shows that the aftershocks of the six large earthquakes are still ongoing.

The aftershock zones of the Xingtai M_S7.2 and Haicheng M_S7.3 earthquakes are dominated by events with magnitudes around M_L≥3. In contrast, M_L≥4 earthquakes at a rate of approximately 1.5 events/a from 2013 to 2022 are observed in the Tangshan M_S7.8 aftershock zone. We fit the data of the Haicheng and Tangshan earthquake sequences according to the Omori-Utsu law. The p value is around 0.8, indicating a slow decaying rate. For historical earthquakes, the spatial-temporal distribution of events indicates that aftershocks are alive. In the aftershock zone of the 1668 Tancheng M8 earthquake, an average of 0.63 earthquakes/a with M_L≥3.0 occurred from 2013 to 2022. Additionally, the 1679 Sanhe-Pinggu aftershocks with magnitudes M_L≤2 are still active. Following the 1937 Heze M7 earthquake in Shandong Province, a M_S5.9 strong aftershock occurred in 1983. The aftershock zone records an average of 1.8 earthquakes/a with M_L≥2.0 during the last decade, which is higher than the rate in surrounding areas. Hence, the aftershock sequences of the Tancheng and Sanhe-Pinggu earthquakes have persisted for 354 and 343 years, respectively, and the Heze earthquake sequence has lasted for nearly 90 years.

According to the rate-and-state friction law, we deduce that the aftershocks can last for more than 300 years with a fault slip rate less than 1mm/a. This also supports the ongoing long-lived sequences of the 1668 Tancheng and the 1679 Sanhe-Pinggu earthquakes. The tectonic loading rate in North China is slow, causing a long recurrence interval of large earthquakes, as well as the slow decaying of aftershocks.

In all, our results indicate that aftershock sequences following M≥8 earthquakes in North China can persist for over 300 years, and the aftershock sequences of M>7 earthquakes such as the Xingtai, Haicheng, and Tangshan earthquakes, will continue for a long period. This study provides us with a further understanding of the aftershock decay in North China.

The aftershock sequences of the Tancheng and Sanhe-Pinggu earthquakes have persisted for 354 and 343 years, respectively; the Heze earthquake sequence has lasted for nearly 90 years; and the Xingtai, Haicheng, and Tangshan regions remain seismically active with frequent small earthquakes. These findings indicate that aftershock sequences of M≥8 earthquakes in North China can persist for over 300 years, and thoses of modern large earthquakes, such as Xingtai, Haicheng, and Tangshan, will continue for a long period.

Key words: North China, Large earthquakes, aftershock duration

摘要：

余震持续时间是地震活动研究中的关键问题, 对区域地震危险性分析和抗震设防具有重要意义。文中基于1970—2022年期间的现代地震目录与历史地震记载, 系统研究了6次7级以上大地震的余震持续性, 包括1668年郯城M8½、 1679年三河-平谷M8、 1937年菏泽M7.0、 1966年邢台M7.2、 1975年海城M7.3及1976年唐山M7.8地震。通过分析余震区小地震的空间分布和震级-频次随时间的变化, 发现上述地震的余震活动均未完全衰减。其中, 郯城地震和三河-平谷地震的余震序列已持续354a和343a; 菏泽地震余震持续近90a; 邢台、 海城及唐山地震震区仍有小地震频发。研究结果表明, 华北地区8级以上大地震的余震活动可持续超过300a, 而邢台、 海城及唐山等近代大地震的余震序列至完全衰减仍需较长时间。

关键词: 华北地区, 大地震, 余震持续时间