Increasingly frequent earthquakes in the Longmenshan (LMS) tectonic belt are linked to the ongoing eastward growth of the Tibetan Plateau during the Late Cenozoic. Following the devastating 2008 Ms 8.0 Wenchuan earthquake in the central LMS and the 2013 Ms 7.0 Lushan earthquake in the southern segment, an unexpected Ms 5.4 Qingchuan earthquake occurred on September 30, 2017 in the northern LMS. We synthesize focal mechanism solutions, relocated aftershocks, regional geology, and seismic reflection profiles in order to determine the pre-existing faults and active fault system in the northern LMS, thus to better assess potential seismic hazards. At a seismically resolvable scale, we identify a large structural wedge at the basement that is cross-cut by numerous multiphase and multi-behavior active faults. Our seismotectonic model indicates that the seismogenic fault responsible for the 2017 Ms 5.4 Qingchuan earthquake is a pre-existing thrust fault with a dip of ∼34° in the frontal thrust ramp. Reactivation of this fault then triggered the activity of the co-seismic strike-slip fault that was formed in the 2008 Wenchuan event. The seismic interpretation, focal mechanism solutions, and aftershock distribution indicate that two sub-segment faults with different geometries are distinguished along the northern LMS. Accordingly, a transformation of the coseismic fault with different geometry and kinematics is formed. With the constraints that the mainshock and aftershocks occurred on two different faults, we propose that both the frontal ramp thrust fault and the recently-formed strike-slip fault are tectonically active. This study highlights the importance of the influence of the pre-existing fault system on the evolution of the northern LMS during the Late Cenozoic. The rapid uplift of the eastern Tibetan Plateau and the complex fault system indicate the potential for seismic risks in the northern LMS.