Present-day convergence within the Longmen Shan fold-and-thrust belt (LSFTB) was released by the 2008 Mw 7.9 Wenchuan earthquake, which ruptured multiple thrust ramps beneath the range front structures. However, it is still unclear whether fault slip propagated eastward into the foreland, closer to densely populated areas around Chengdu. In this study, we provide constraints on the 3-D subsurface structure, fault activity, and seismic hazards of the Longquan fault located in the central Sichuan basin, ~100 km east of the range front structures of the LSFTB. We develop a seismotectonic model and assess the activity of the Longquan fault by a combination of methods, including interpretation of seismic reflection profiles, analysis of satellite images, field mapping, trench logging, and radiocarbon dating. Our survey reveals that at least two surface-rupturing events occurred on the Longquan fault. Our 3-D structural model and evidence for Holocene faulting events on the Longquan fault reveals that upper crustal shortening in the Sichuan basin is accommodated by a frontal thrust system that is linked to the recently active range front blind structures by a shallow detachment. We suggest that dynamic weakening following fault activity at the Longmen Shan range front might unlock the updip portion of this shallow detachment, sending slip eastward into the foreland and to the surface along the Longquan thrust ramps. Our results have important implications for seismic hazards assessment of active frontal thrusts linked by upper crustal detachments in active fold-and-thrust belts around the world.