High-quality first-arrival data collected with a high-density temporary seismic array and regional seismic network were used to construct a P-wave velocity model and identify the precise location of earthquakes in the active collision zone of southeastern Taiwan. A crustal-scale weak zone, defined by high seismicity, is characterized by a steeply east-dipping lower-velocity anomaly and represents a boundary between an uplifted upper-mantle and the Luzon volcanic arc. The main features of the weak zone, with its associated low-velocity zone and seismicity pattern, vary significantly along the collisional boundary of southeastern Taiwan. Along the weak zone, unlithified sediments or highly fractured materials are compacted as the Philippine Sea plate moves toward the Eurasian plate. In areas of more advanced collision, soft material is more compacted and can therefore store significantly greater amounts of strain energy. This energy is released through episodic earthquakes.