Seismogenic zones in the convergent margin, eastern Taiwan and its implications in the Luzon forearc deformation

Three-dimensional relocated seismicity in eastern Taiwan reveals two north- to-northeast trending seismogenic zones, one located close to the Longitudi nal Valley and the ether near to the crest of the Luzon are. Earthquake foc al mechanisms obtained from P-wave first motion polarity data are presented in this study for 70 events in these two seismic zones. The focal mechanis m solutions for both seismic zones show mainly thrust and strike-slip fault ing in the area south of 23.2 degreesN. The horizontal projection of the ev ents' P-axis indicates a pattern consistent with the regional NW-SE compres sion. However, the orientation of the events' T-axes indicates that there a re different patterns for the two seismic zones. By combining evidence from seismicity, P- and T-axes, as well as detailed bathymetry, we infer that t he two seismic zones (fault systems) might mark the east and west boundarie s of the Luzon forearc, According to the transpressional strain model propo sed in this study, the Luzon forearc represents a deforming zone, between t wo relatively rigid blocks, that undergoes shearing from the transcurrent c omponent of oblique convergence in eastern Taiwan.