ABSTRACT

We systematically study the detailed bathymetry, seismicity, and source parameters of large- and moderate-sized earthquakes that occurred in the northern Luzon arc-Taiwan region between 1964 and 1996. The purpose is to characterize the transition from oblique subduction to regional collision in terms of the distribution of morphological features, the characteristics of seismogenic structures, and the corresponding state of strain. To the south of 21.5N, the existence of a subduction zone is clearly shown by both bathymetry and seismicity. The subducted Eurasia slab reaches a maximum depth of ~200 km, and the depth decreases sharply as it approaches Taiwan. Focal mechanisms indicate that the subducted slab is in downdip extension for depths <150 km but switches to downdip compression at greater depths. To the north of 23N, collision is clearly the predominant process with broad deformation on both sides of the suture. In between, the transition is accommodated by a distributed thrust deformation zone in the frontal portion of the accretionary prism, closure of the forearc basin, and the right-lateral NE-SW striking Taitung canyon fault zone (TCFZ). These structures resolve the differential movements between the subduction and collision zones and will repeatedly change their positions as the collision propagates southwestward. The present-day plate boundary follows the axis of the Manila trench in the south, broadens significantly in the zone of transition, and finally becomes a zone of several tens of kilometers in width across eastern Taiwan. The deformation front, on the other hands, is located between the Western Foothills and Coastal plain in Taiwan, then connects to the Manila trench to the south. Seismic strain tensors indicate the region to the north of TCFZ has a compression rate ~15 times larger than that to the south. The northern Luzon arc shows characteristics of oblique subduction, but the effects of collision are not as significant within the subducted Eurasian lithosphere, as comparing to the subducted Philippine Sea plate beneath the southernmost Ryukyu arc.