A moment magnitude (M_w) 5.0 earthquake hit Qiaojia, Yunnan, China on 18 May 2020. Its hypocentre 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 M_w 5.0 earthquake occurred during its water-retaining. Thus, it is critical to investigate the seismogenesis of the Qiaojia M_w 5.0 main shock and evaluate the risk of inducing earthquakes near the Baihetan Reservoir after impoundment. In this study, we built a complete and accurate earthquake catalogue to analyse seismicity in the reservoir area before and after the M_w 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 M_w 5.0 Qiaojia main shock. 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 hypocentre of the M_w 5.0 earthquake. Along with focal mechanism solutions, we suggested that the M_w 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.

Combination of velocity, Poisson’s ratio, and attenuation structure is a powerful tool to investigate the characteristics of media, and it is especially useful to infer the lithology, porosity, water content of rocks, fluid infiltration depth, and distribution in the reservoir area. Using data from 4967 earthquakes recorded by a high‐density seismic network, we carried out high‐resolution 3D $VP$, $VP/VS$, $QP$, and $QS$ tomography research in the Three Gorges Reservoir area of China, where the world’s largest high‐capacity hydroelectric power station is located. We found two regions with significantly low $VP$, high $VP/VS$, and low $QP$ and $QS$; one is the region between the Shuitianba fault and the western edge of the Huangling dome, and the other is the Badong region on the upstream Yangtze River, where more than 2000 earthquakes occurred in four months after the reservoir was first impounded. We infer that there is significant fluid infiltration in the two regions. The interior area of the Zigui basin is shown as a low‐$VP$ region at depths of 0–6 km, which is consistent with the depth to crystalline basement. The Three Gorges dam area is a region with moderately high $VP$, low $VP/VS$, and prominently high $QP$ and $QS$, demonstrating no obvious water infiltration around the dam area. Combining present results with previous studies, we draw a conclusion that the distance away from main rivers, lithologies, and the presence of faults are the main factors contributing to the fluid infiltration distribution beneath the reservoir region.