Three-dimensional relocated seismicity in eastern Taiwan reveals two north-to-northeast trending seismogenic zone, one located close to the Longitudinal Valley and the other near to the crest of the Luzon arc. Earthquake focal mechanisms obtained from P-wave first motion polarity data are presented in this study for 70 events in these two seismic zones. The focal mechanism solutions for both seismic zones show mainly thrust and strike-slip faulting in the area south of 23.2°N. The horizontal projection of the events' P-axis indicates a pattern consistent with the regional NW-SE compression. However, the orientation of the events' T-axes indicates that there are different patterns for the two seismic zones. By combining evidence from seismicity, P- and T-axes, as well as detailed bathymetry, we infer that the two seismic zones (fault systems) might mark the east and west boundaries of the Luzon forearc. According to the transpressional strain model proposed in this study, the Luzon forearc represents a deforming zone, between two relatively rigid blocks, that undergoes shearing from the transcurrent component of oblique convergence in eastern Taiwan.