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The Baihetan Reservoir, the second largest in the world, is located at the intersection of multiple large active fault zones on the eastern boundary of the Sichuan-Yunnan rhombic block. After impoundment on 6 April 2021, many earthquakes occurred around the reservoir area submerged by water. The largest M L 4.7 earthquake in the reservoir area occurred after the water level reached its highest point. But the seismogenic structures and mechanisms of earthquakes in the reservoir area are still unclear. Based on dense array data from the reservoir area, this paper uses the experimental site submodel of USTC-Pickers, transfer learned with 'DiTing' data set of China to obtain a high-precision earthquake catalogue that is twice as large as that the manual catalogue. This study show that earthquakes in the reservoir region primarily occur on secondary faults of pre-existing ones, characterized by a prominent feature of high dip angles trending northwest to southeast. Combined with the spatiotemperal migration characteristics of earthquakes and the relationship between earthquakes and water levels, we infer that most earthquakes are rapid response type and may be induced by rapid increase in elastic stress. Only the spatiotemporal distribution image of the M L 3.2 earthquakes sequence in the dam site-Toudaogou section conforms to the law of pore pressure diffusion, and belongs to the fast response type, which may be induced by the poroelasiticity coupling mechanism. The M L 3.0 earthquake swarm with deep depths in the Heishui River section belongs to the delayed response type and may be induced by the poroelasiticity coupling mechanism. [ABSTRACT FROM AUTHOR]

After impounding the Baihetan Water Reservoir in a historically seismically active area, numerous earthquakes occurred, but it is uncertain if they are connected to the impoundment. Based on the dense seismic network, this study used local earthquake tomography to construct high‐resolution VP (P‐wave velocity) and VP/VS models (P‐ to S‐wave velocity ratio) and update earthquake locations after the impoundment of Baihetan reservoir. Our study revealed that the reservoir water spreads from the dam site to the Qiaojia Basin and the Heishuihe branch, and pore pressure diffusion along faults induces many earthquakes. Reservoir water migrates through hidden faults and fractures beneath Hulukou, spreading to both sides and saturating some rocks below 7 km beneath the dam site‐Lianhuatang section, causing multiple magnitude 3.5+ earthquakes. This study reveals that reservoir water migration drives earthquakes in the reservoir area, offering new insights into seismogenesis following the Baihetan reservoir's impoundment, potentially applicable to understanding reservoir‐induced earthquakes in other reservoirs. Plain Language Summary: Numerous earthquakes occurred after the Baihetan reservoir impoundment. It is worth considering whether the significant seismicity was caused by fluid injection. We used the local earthquake tomography to construct high‐precision earthquake locations and high‐resolution 3D VP and VP/VS models of Baihetan reservoir after the impoundment. The 3D VP and VP/VS models reveal the penetration and diffusion range of reservoir water. The relationship between seismic locations and velocity models reveals that microearthquakes are induced by pore pressure diffusion caused by fluid migration, and some of the water‐saturated areas induced bigger earthquakes. Key Points: The highest resolution 3D VP and VP/VS models of the Baihetan reservoir are obtained by using a dense seismic arrayThe relationship between the change of medium structure and seismicity caused by the impoundment of Baihetan reservoir is revealedA new understanding of the seismogenesis of a large number of earthquakes occurred after the Baihetan reservoir impoundment are obtained [ABSTRACT FROM AUTHOR]

The article discusses the physical process and seismogenic mechanism of the Yangbi Ms 6.4 earthquake that occurred in Dali, Yunnan Province in 2021. The earthquake did not occur on a pre-existing active fault but on a buried fault in the eastern boundary of Baoshan sub-block. The study uses various seismological methods to analyze the earthquake sequence, seismogenic environment, and focal parameter characteristics, providing insights into the earthquake's source characteristics and trigger mechanism. [Extracted from the article]

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. [ABSTRACT FROM AUTHOR]

• Obtained the highest resolution 3D velocity model for the XLD Reservoir so far. • Inverse velocity model uses both machine-learning data and phases of dense stations. • The seismicity in the head area and around the Manao Fault is influenced by fluids. • Two M4 events occurred on the east of the Lianfeng Fault are tectonic activity. The Xiluodu Reservoir (XLD), the third hydropower station developed downstream of the Jinsha River, is currently the fourth largest hydropower station worldwide. The reservoir has been impounded for 10 years, and numerous earthquakes have occurred, thus indicating an increased seismic risk. In existing studies, the deep tectonics and fine-scale structure in the reservoir area have not been sufficiently explained. Thus, the mechanism underlying reservoir-induced seismicity in this area remains unclear. In this study, we utilized a microseismic catalogue compiled based on the LOC-FLOW workflow (Li et al., 2023), alongside long-term observation data from a dense seismic array in the Jinsha River downstream, to obtain high-precision 3D V P , V S and V P /V S structures, as well as refined seismic relocations in the XLD Reservoir area. Additionally, we incorporated petrological, geomagnetic, gravity, crustal density, geodetic observation and numerical simulation data, which demonstrate that seismicity in the reservoir head area and near the Manao Fault is influenced by fluids. Moreover, the two M4 earthquakes that occurred east of the Lianfeng Fault are indicative of tectonic activity. Of these earthquakes, the Tianba earthquake cluster in the reservoir head area is located in a high-V P /V S zone, while a low-V S anomaly channel is observed here, which suggests that large amounts of fluid infiltrated along the fault, increasing the pore pressure and reducing the effective stress on the fault. The Wuji earthquake cluster in the head area is also located in a low-V P /V S zone, which indicates a low porosity and possibly undrained conditions. Thus, the earthquakes in this area are the result of coupled elastic stress-pore water pressure interactions. [ABSTRACT FROM AUTHOR]