Summary A moment magnitude (Mw) 5.0 earthquake hit Qiaojia, Yunnan, China on May 18, 2020. Its hypocenter is only approximately 20 km away from the Baihetan reservoir, the second largest hydropower station in China. The Baihetan Reservoir is located at the junction of multiple fault zones on the eastern boundary of the Sichuan-Yunnan rhombic block, an area with high background seismic activity. The Baihetan Reservoir was planned to be impounded in April 2021 and the MW 5.0 earthquake occurred during its water-retaining. Thus, it is critical to investigate the seismogenesis of the Qiaojia MW 5.0 mainshock and evaluate the risk of inducing earthquakes near the Baihetan Reservoir after impoundment. In this study, we built a complete and accurate earthquake catalog to analyze seismicity in the reservoir area before and after the MW 5.0 Qiaojia earthquake. We adopted a machine learning-based seismic phase picker, PhaseNet, to automatically detect seismic picks from continuous raw seismic data. Seismic phase picks were associated and located using sequential earthquake association and location methods, including REAL, VELEST and hypoDD. We eventually obtained high-precision locations of 1640 earthquakes by the hypoDD. The distribution of earthquake locations indicates that a concealed fault nearly vertical to the surface accommodated the MW 5.0 Qiaojia mainshock. The majority of its aftershocks is located within a narrow depth range of 8–13 km, indicating that the stresses in the hypocentral area were concentrated near the hypocenter of the MW5.0 earthquake. Along with focal mechanism solutions, we suggested that the Mw 5.0 Qiaojia earthquake is more likely a tectonic earthquake. However, we cannot exclude the possibility that earthquakes could be induced after the impoundment of Baihetan Reservoir, because the identified concealed fault is located in the middle of many large fault zones and only 20 km away from the Baihetan Reservoir.

The Jinsha River flows through the tectonically active Sichuan and Yunnan regions, which have frequent strong earthquake activities. Four large-scale hydropower dams have been built in the lower reaches of the river and had begun impounding water successively since October 2012, with the Baihetan (BHT) and Xiluodu (XLD) being the second and fourth largest hydropower stations in the world. Using the seismic data recorded by dense seismic stations in the lower reaches of the Jinsha River from January 2010 to December 2019, we obtain the 3D crustal VP, VS, and VP/VS models and seismic relocations by applying the VP/VS model consistency-constrained double-difference tomography method (tomoDDMC). After Xiangjiaba (XJB) and XLD started to store water, earthquakes were mainly clustered in two areas where the seismicity was weak before the impoundment, that is, (1) the western segment of the Mabian-Yanjin fault (MBYJF) zone on the west side of the river and the tail section of XJB, and (2) the southwest part of Leibo-Yongshan basin near the middle section of XLD reservoir. We find that the areas where earthquakes are concentrated have characteristics of high-velocity and low-VP/VS. These particular features may provide important information for predicting the potential earthquake risk areas. In addition, we found a well-resolved high-velocity and low-VP/VS zone sandwiched between two layers of low velocity and high VP/VS at depths ranging from 3 to 10 km beneath the Qiaojia area along the Xiaojiang fault (XJF), which suggests that the seismic activity in this area may increase with the storage of the BHT dam in April 2021.

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We image the Zhaotong-Ludian fault (ZLF) in the southeastern margin of the Tibetan plateau (SE Tibetan plateau) using waveforms from local and teleseismic earthquakes recorded by 14 seismic stations. We identify two types of fault zone head wave (FZHW) from two clusters of earthquakes by applying an automatic picking algorithm and a horizontal particle motion analysis. The first type of FZHWs shows a linear time–distance moveout and is only observed at stations on the southeast side of the fault in the northeastern section of the ZLF. The moveout slope suggests an average cross-fault velocity contrast of ∼2.5%. The second type of FZHWs exhibits a constant moveout and is recorded by stations on both sides of the ZLF in the southwestern section from a cluster of earthquakes located in a low-velocity zone. The difference in cross-fault velocity contrast between the northeast and southwest segments of the ZLF is also confirmed by teleseismic P-wave travel-time data. We attribute the prominent velocity contrast in the northeast section to a lithological difference between the South China block in the southeast and the Daliangshan subblock on the northwest side of the fault. The striking difference between the northeast and southwest sections also implies that earthquakes nucleating in one segment would hardly rupture through the entire fault, which can significantly affect our estimates of the maximum magnitude of future earthquakes occurring on the fault.