A dense linear seismic array consisting of 18 short-period seismic stations is deployed in the Western Foothills to study deep crustal structures across central Taiwan, where significant mountain-building processes are considered as a result of the oblique convergence between the Eurasian and Philippine Sea plates since a few million years ago. The identification of mantle reflections (PmP, SmS and SmP) from later phases generated by one earthquake (ML = 4.3) in eastern Taiwan and recorded by the linear seismic array shows that crustal thickness gradually increases eastward from 40 km beneath the Western Central Range to 50 km beneath the Eastern Central Range. The significant horizontal offset (~30 km) between the highest mountains and the deepest crust suggests that crustal deformation has not yet reached isostatic equilibrium. It is concluded that continental subduction might be acceptable for explaining the dynamic processes in the Taiwan orogeny.