Seismotectonic characteristics of the Taiwan collision-Manila subduction transition: The effect of pre-existing structures

Based on the distribution of an earthquake swarm determined from an ocean bottom seismometer (OBS) network deployed from 20 August to 5 September 2015 in the northernmost part of the South China Sea (SCS) and data from inland seismic stations in Taiwan, we resolved a M _L 4.1 earthquake occurring on 1 September 2015 as a NE-SW-trending left-lateral strike-slip event that ruptured along the pre-existing normal faults generated during the SCS opening phases. The direction of the T-axes derived from the M _L 4.1 earthquake and the background seismicity off SW Taiwan present high consistency, indicating a stable dominant NW-SE tensional stress for the subducted Eurasian Plate (EUP). The distinct stress variations on the two sides of these reactivated NE-SW trending features suggest that the presence of pre-existing normal faults and other related processes may lessen the lateral resistance between the Taiwan collision and Manila subduction system, and facilitate the slab-pull process for the subducting portion, which may explain the NW-SE tensional stress environment near the transition boundary in the northernmost part of the SCS.