Abstract: On August 8, 2017, Beijing time, an earthquake of M7.0 occurred in Jiuzhaigou County, Aba Prefecture, Sichuan Province, with the epicenter located at 33.20°N 103.82°E. The earthquake caused 25 people dead, 525 people injured, 6 people missing and 170000 people affected. Many houses were damaged to various degrees. Up to October 15, 2017, a total of 7679 aftershocks were recorded, including 2099 earthquakes of M ≥ 1.0.
The M7.0 Jiuzhaigou earthquake occurred in the northeastern boundary belt of the Bayan Har block on the Qinghai-Tibet Plateau, where many active faults are developed, including the Tazhong Fault(the eastern segment of the East Kunlun Fault), the Minjiang fault zone, the Xueshan fault zone, the Huya fault zone, the Wenxian fault zone, the Guanggaishan-Daishan Fault, the Bailongjiang Fault, the Longriuba Fault and the Longmenshan Fault. As one of the important passages for the eastward extrusion movement of the Qinghai-Tibet Plateau(Tapponnier et al., 2001), the East Kunlun fault zone has a crucial influence on the tectonic activities of the northeastern boundary belt of Bayan Kala. Meanwhile, the Coulomb stress, fault strain and other research results show that the eastern boundary of the Bayan Har block still has a high risk of strong earthquakes in the future. So the study of the M7.0 Jiuzhaigou earthquake' seismogenic faults and stress fields is of great significance for scientific understanding of the seismogenic environment and geodynamics of the eastern boundary of Bayan Har block.
In this paper, the epicenter of the main shock and its aftershocks were relocated by the double-difference relocation method and the spatial distribution of the aftershock sequence was obtained. Then we determined the focal mechanism solutions of 24 aftershocks(M ≥ 3.0)by using the CAP algorithm with the waveform records of China Digital Seismic Network. After that, we applied the sliding fitting algorithm to invert the stress field of the earthquake area based on the previous results of the mechanism solutions. Combining with the previous research results of seismogeology in this area, we discussed the seismogenic fault structure and dynamic characteristics of the M7.0 Jiuzhaigou earthquake. Our research results indicated that:1)The epicenters of the M7.0 Jiuzhaigou earthquake sequence distribute along NW-SE in a stripe pattern with a long axis of about 35km and a short axis of about 8km, and with high inclination and dipping to the southwest, the focal depths are mainly concentrated in the range of 2~25km, gradually deepening from northwest to southeast along the fault, but the dip angle does not change remarkably on the whole fault. 2)The focal mechanism solution of the M7.0 Jiuzhaigou earthquake is:strike 151°, dip 69° and rake 12° for nodal plane Ⅰ, and 245°, 78° and -158° for nodal plane Ⅱ, the main shock type is pure strike-slip and the centroid depth of the earthquake is about 5km. Most of the focal mechanism of the aftershock sequence is strike-slip type, which is consistent with the main shock's focal mechanism solution; 3)In the earthquake source area, the principal compressive stress and the principal tensile stress are both near horizontal, and the principal compressive stress is near east-west direction, while the principal tensile stress is near north-south direction. The Jiuzhaigou earthquake is a strike-slip event that occurs under the horizontal compressive stress.

Key words: Jiuzhaigou M7.0 earthquake sequence, earthquake relocation, focal mechanism, tectonic stress field

摘要： 2017年8月8日四川省九寨沟县发生M7.0地震，截至2017年10月15日共发生M1.0以上地震2099次。文中采用双差定位法对九寨沟地震序列进行重新定位，获得了1565个地震的精定位结果，利用CAP（Cut and Paste）方法获得了24个3级以上地震的震源机制解，利用滑动拟合方法反演了此次九寨沟M7.0地震的构造应力场，并结合前人对该区域地震地质的研究成果，探讨了九寨沟M7.0地震的发震断层结构和构造应力场特征。研究结果显示：1）九寨沟M7.0地震序列震中沿NW-SE向呈条带状分布，长轴约35km，短轴约8km，震源主要集中分布在深度2~25km范围内，且沿断层走向由NW向SE逐渐变深，震源断层倾角较高，倾向SW，沿整个断层倾角没有发生明显变化；2）九寨沟M7.0地震的震源机制解节面I的走向、倾角和滑动角为152°、71°和-11°，节面Ⅱ的走向、倾角和滑动角为246°、80°和-161°，主震为纯走滑型地震，矩心深度约5km，余震序列的震源机制解绝大多数为走滑型，与主震震源机制解性质基本一致；3）震源区主压应力、主张应力倾角接近水平，主压应力方向近EW向，主张应力近SN向，九寨沟地震是在水平挤压应力作用下发生的走滑运动。

关键词: 九寨沟M7.0地震序列, 地震精定位, 震源机制解, 构造应力场