Taiwan is one of the most typical arc-continental collision systems in the world. The Taiwan orogeny is often explained by the thin-skin model, where a shallow decollement, with depths of less than 10 km, is considered as the crustal deformation boundary for generating significant mountains on the island of Taiwan. In this study, the seismic data generated by crustal earthquakes, recorded by seismic stations in the Chia-Yi area of western Taiwan was examined to find the crustal structures beneath the Taiwan orogeny. A significant velocity boundary below the decollement was found from unambiguous multiple secondary waves and phases of primary waves converted to secondary waves. Careful comparison between the observations and calculated results of the travel-time and ray-path indicates that this boundary is dipping to the west at ~10 degrees, which, surprisingly, is dipping to the opposite direction of the proposed decollement. It is also interesting to note that this boundary is not only a velocity boundary but also consistent with a seismicity boundary. This west-dipping boundary is the major seismogenic boundary in the upper crust, while the east-dipping decollement proposed earlier is a geological boundary. As a result, the westward dipping boundary observed in this study is more suitable, than the eastward dipping decollement, to be the seismogenic boundary in the Chia-Yi area.