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The Xinfengjiang Water Reservoir (XWR) in Guangdong, China, is one of the reservoirs that have triggered earthquakes of magnitudes greater than 6. Numerous earthquakes have occurred since the impoundment of the reservoir, making it one of the most active seismic zones in Guangdong. However, due to the lack of seismic stations, the detailed seismic structures and earthquake mechanisms within XWR have not been resolved, and the significance of XWR as a typical protracted earthquake location is not well judged. In this study, by collecting waveform data from both permanent and temporary stations from 2012 to 2015, we relocated 1,528 earthquakes and inverted both Vpand Vsstructures from traveltimes of these earthquakes. Using waveform data, we also investigated focal mechanisms of earthquakes with magnitude greater than 1.5 in this region. Our results reveal fine crustal structure that has never been shown before and show complicated crust structure with several low‐velocity zones extending to 5–10 km depth under the major faults. Earthquake focal mechanisms show more dip‐slip faults than strike‐slip faults, and the two types of earthquakes are roughly divided by the reservoir boundary. The direction of principle stress of the earthquakes is northwest‐southeast, consistent with the direction of tectonic principal stress. Combining the above results, and investigation of historical earthquakes and water level change, we suggest that water loading cycle and diffusion play important role in XWR seismicity. They increase the pore pressure, make the earthquakes migrate to deeper depth, and change the type of earthquakes. Fine‐scale upper crustal structures of Vp, Vs, and Vp/Vs, together with earthquake locations near the Xinfengjiang reservoir, are obtainedFocal mechanisms of 136 M1.5 or larger earthquakes are invertedWater may play important role in fault extension and earthquake mechanism change

By systemic processing, comprehensive analysis, and interpretation of gravity data, we confirmed the existence of the west segment of the coastal fault zone (west of Yangjiang to Beibu Bay) in the coastal region of South China. This showed an apparent high gravity gradient in the NEE direction, and worse linearity and less compactness than that in the Pearl River month. This also revealed a relatively large curvature and a complicated gravity structure. In the finding images processed by the gravity data system, each fault was well reflected and primarily characterized by isolines or thick black stripes with a cutting depth greater than 30 km. Though mutually cut by NW-trending and NE-trending faults, the apparent NEE stripe-shaped structure of the west segment of the coastal fault zone remained unchanged, with good continuity and an activity strength higher than that of NW and NE-trending faults. Moreover, we determined that the west segment of the coastal fault zone is the major seismogenic structure responsible for strong earthquakes in the coastal region in the border area of Guangdong, Guangxi, and Hainan.