Based on a dense seismic array deployed to monitor the seismic gap between the Wenchuan and Lushan earthquakes, a joint inversion of dispersion curves and receiver functions yielded a fine velocity structure in this region with the highest resolution of approximately 20 km. The results show that the upper crust of the Songpan‐Ganzi terrane overthrusted onto the basement of the Sichuan Basin, indicating that the crustal shortening model may be the major mechanism that accounts for the growth of the Longmenshan. The results also show a low‐velocity zone (Vs < 3.5 km/s) and thickened crust (>67 km) beneath the seismic gap, which could be associated with partial melting. The heat transferred from the partial melting at the base of the seismic gap may increase the temperature of the faults to as high as 300–400 °C and cause the segment of the Longmenshan Fault in the seismic gap to become “aseismic.” In contrast, the Shuangshi‐Dachuan Fault, Dayi Fault, and other blind faults to the east‐southeast of the seismic gap are located above a high‐velocity body. The seismogenic environment of these faults is similar to the segments ruptured during the Wenchuan and Lushan earthquakes, both of which occurred above the westward extension of the strong Sichuan Basin. Higher seismic activity was also observed along these faults than that above the low‐velocity zone. Therefore, we deduce that a major earthquake on these faults is possible and that the magnitude could be larger than that of the Dayi earthquake (M 6.2).