Abstract:

Minshan active block is located in Bayan Har block of Qinghai-Tibet Plateau. It is bounded by the Huya Fault and Minjiang Fault on the east and west sides of the block. In less than 100 years, there have been four earthquakes with M_S≥7.0 occurring along the eastern and western boundary faults, namely, the Diexi earthquake with M7.5 in 1933, two Songpan earthquakes with M_S7.2 in 1976, the Jiuzhaigou earthquake with M_S7.0 in 2017, and several earthquakes with M6.0~6.9. Such intensity and frequency of seismicity on either side of a relatively small intraplate active block is rare. Because the landforms along the active fault are mostly relatively gentle valleys with dense population and there is large terrain difference between the two sides of the valleys, each of the major earthquakes and the large-scale landslides it triggered were liable to cause serious casualties and property losses.
Therefore, how does the destructive seismic activity around the active block migrate in space, and is it closely related to the segmentation and coalescence of active faults?And what are the temporal development characteristics of major earthquake activities and earthquake sequences?The discussion of these questions will not only deepen our understanding of the location and time of future destructive earthquakes, but also promote the development of the hypothesis of active block theory. Compared with the Bayan Har block, the Minshan active block located in the eastern margin of the Qinghai-Tibet Plateau provides a unique experimental field for studying the temporal and spatial regularity of earthquake occurrence in the active block.
In this paper, 39 076 small earthquakes in Minshan active block and its adjacent areas from 2000 to 2019 were relocated using the double-difference location method, and 48, 110 seismic events in the study area were obtained by combining the earthquake catalogues recorded by instruments in the same area from 1972 to 1999. For the major earthquakes since the 1933 Diexi M7.5 earthquake, a thorough analysis was made on the spatial distribution characteristics of earthquake sequences in different periods, especially on the basis of formation of small earthquake bands, and the results show that: Since the Diexi M7.5 earthquake in 1933, the four M≥7.0 earthquake sequences are all distributed along the boundary zone of Minshan active block in space, indicating that the active block plays a controlling role in the process of large earthquake preparation. In terms of the determination of seismogenic structure, the strike of the seismogenic fault of the 1976 Songpan M_S7.2 earthquake is basically the same with that of the 2017 Jiuzhaigou M_S7.0 earthquake, but differs by 60°~70° with that of the 1976 Pingwu M_S7.2 earthquake. So, it is more reasonable that the seismogenic faults of these three major earthquakes belong to two earthquake rupture segments, among them, the seismogenic fault of Jiuzhaigou M_S7.0 earthquake in 2017 and Songpan M_S7.2 earthquake in 1976 is the NW-trending Shuzheng Fault, and that of the 1976 Pingwu M_S7.2 earthquake is the north segment of the Huya Fault. From the perspective of seismicity, the seismogenic fault of the 1933 Diexi earthquake should be the southern segment of Minjiang Fault. The 2017 Jiuzhaigou M_S7.0 earthquake occurred in the gap between the 1976 Songpan M_S7.2 earthquake and the Minjiang Fault. There are probably two seismic hazard areas around Minshan active block, which are located in the southern segment of Huya Fault and the middle segment of Minjiang Fault. The large earthquakes around Minshan block probably belong to foreshock-main shock-aftershock type. Therefore, from the perspective of earthquake prediction, it is suggested to strengthen monitoring of these two seismic gaps.

Key words: Minshan active block, relocation, earthquake sequence, seismogenic fault, earthquake hazard area

摘要：

深入理解活动地块如何控制区域强震活动是地震危险性评价的关键。文中采用双差定位法对岷山活动地块及其邻近地区2000—2019年39 076个小地震进行重新定位, 并结合1972—1999年同区域仪器记录的地震目录, 共计获得该区48 110个地震的位置。在此基础上, 对研究区内自1933年以来的4个M≥7.0大地震不同时段的地震序列空间分布特征进行了精细分析。结果表明: 这4次M≥7.0的地震序列在空间上均沿着岷山活动地块边界带分布, 显示活动地块对区域大地震的孕育和发生具有明显的控制作用。同时, 区域地震重定位结果和不同时段地震序列的空间分布较好地限定了4次大地震的发震断裂及其位置。综合分析结果认为, 1976年松潘 M_S7.2 地震与2017年九寨沟 M_S7.0 地震的发震断层走向基本相同, 但它们与1976年平武 M_S7.2 地震的发震断层在走向上存在约60°的差异。这3次大地震的发震断层可能分属于2条断裂。其中, 2017年九寨沟 M_S7.0 地震和1976年松潘M7.2地震的发震断层为NW向的树正断裂, 而1976年平武 M_S7.2 地震的发震断层为近SN向的虎牙断裂北段。从地震活动性角度来看, 1933年叠溪M7.5地震的发震断层应为岷江断裂南段。2017年九寨沟 M_S7.0 地震发生在1976年松潘 M_S7.2 地震与岷江断裂之间的空区。在岷山活动地块周缘很可能还存在2个地震危险区, 分别位于虎牙断裂南段和岷江断裂中北段。岷山地块周缘的大地震类型很可能属于前震-主震-余震型。因此, 从地震预报的角度出发, 建议加强对这2个地震空区的监测。

关键词: 岷山地块, 小地震精定位, 地震序列, 发震断层, 地震危险区