Mechanism analysis of spatial distribution of the geophysical observation anomalies before the 2013 Minxian-Zhangxian M_S6.6 earthquake and the 2017 Jiuzhaigou M_S7.0 earthquake

The 2013 Minxian-Zhangxian M_S6.6 earthquake and the 2017 Jiuzhaigou M_S7.0 earthquake are typical moderate-strong earthquakes developed in the southeast of Gansu Province in recent years. In order to further discuss the common characteristics of geophysical anomalies before earthquakes in this area, the two earthquakes are taken as the research objects in this paper. Based on the published data, it is concluded that there are obvious geophysical anomalies before the 2013 Minxian-Zhangxian M_S6.6 earthquake and the 2017 Jiuzhaigou M_S7.0 earthquake, including crustal deformation （fixed-station and cross-fault leveling）, electromagnetism and underground fluid. Among them, there are 49 precursory anomalies of the 2013 Minxian-Zhangxian M_S6.6 earthquake and 34 precursory anomalies of the 2017 Jiuzhaigou M_S7.0 earthquake. Meanwhile, the number of precursory anomalies in different disciplines are calculated and the regulations of precursory anomalies in different disciplines are summarized. Besides, the geophysical observation stations are divided according to active fault zones, and the number and percentage of precursory anomalies on relevant fault zones are counted to analyze the precursory anomalies controlled by active tectonics, which has been compared with GNSS velocity field. We also adopt Anderson fault stress model to discuss the different spatial distribution and intensity of precursory anomalies between these two earthquakes, as well as the focal mechanism effect on precursory anomalies in depth. According to the above, the following understandings are obtained:

1） Through the comparison of the two earthquakes, the precursory anomalies of cross-fault leveling and electro-magnetism of the 2013 Minxian-Zhangxian M_S6.6 earthquake are more obvious, with the respective anomaly percentages 35% and 28%, followed by fixed-station deformation and underground fluid anomalies, the anomaly percentages are 21% and 15%, respectively. While the electromagnetic anomalies of the 2017 Jiuzhaigou M_S7.0 earthquake are significant, with an anomaly percentage of 45%, followed by the cross-fault leveling and fixed-station deformation, with an anomaly percentage of 26%. While the underground fluid anomalies are the least, with an anomaly percentage of only 5%. Summarized from the above data, the geophysical anomalies before the 2013 Minxian-Zhangxian M_S6.6 earthquake are more significant than these of the 2017 Jiuzhaigou M_S7.0 earthquake. And precursory anomalies of the two earthquakes both reflect that the electromagnetic and cross-fault leveling anomalies are more obvious, while the underground fluid precursory anomalies are rare.

2） The distribution of geophysical anomalies before the earthquake is related to the active tectonics. Specifically, the precursory anomalies before the 2013 Minxian-Zhangxian M_S6.6 earthquake are mainly concentrated in the east Kunlun-west Qinling fault zone and the Liupanshan-Haiyuan fault zone, while the precursory anomalies before the 2017 Jiuzhaigou M_S7.0 earthquake are mainly concentrated in the Longmenshan fault zone and the east Kunlun-west Qinling fault zone. The east Kunlun-west Qinling fault zone, which is located in the boundary structure of the Bayan Har block, the Longxi block and the Qaidam basin, has significant precursory anomalies before these two earthquakes, indicating that as an active structure of the central orogenic belt, it plays an important role in the stress conduction of the present extension of the Qinghai-Xizang Plateau.

3） There is a good correspondence between the distribution of precursory anomalies before the two earthquakes and the GNSS velocity field. The GNSS velocity field is the direct manifestation of the current crustal movement and fault activity. The corresponding relationship between the GNSS velocity field and the precursory anomalies distribution further proves the difference in seismogenic tectonic setting between these two earthquakes, and helps to analyze the stress accumulation before the earthquakes, as well as the energy conduction in seismogenic process.

4） The stress mechanism of the 2013 Minxian-Zhangxian M_S6.6 earthquake and the 2017 Jiuzhaigou M_S7.0 earthquake are analyzed based on the Anderson fault stress model, which indicates that shear stress （corresponding to strike-slip fault） is more likely to form fault rupture than normal stress （corresponding to reverse fault）, and this viewpoint is supported by rock test laboratory results: an increase in shear stress at the fault plane along the direction of fault slip makes the fault more likely to rupture, while an increase in normal stress （pressure） at the vertical fault plane increases the friction strength of the fault and inhibits rupture. Therefore, although the magnitude of the 2013 Minxian-Zhangxian M_S6.6 earthquake （thrust type） is slightly smaller than the 2017 Jiuzhaigou M_S7.0 earthquake （strike-slip type）, it needs to accumulate more stress to rupture, showing a higher intensity of precursory anomalies.

This study summarizes the precursory anomalies regulation of two typical earthquakes in the southeast of Gansu Province, which attributes to accumulate earthquake cases data for the establishment of prediction practice in this area.