High-Resolution 3-D P Wave Velocity Structures Under NE Taiwan and Their Tectonic Implications

Abstract

First P wave arrival-time data from local earthquakes recorded by a dense geophone array deployed on the Ilan Plain and by existing permanent stations were combined to invert for high-resolution P wave velocity structures under northeast Taiwan. With relatively high resolution, we were able to examine the structures in more detail and to investigate their significance and tectonic implications. We introduce two distinct groups of proposals for mechanisms of subduction polarity flipping in Taiwan, referred to as the “tear model” and “breakoff model.” While the predicted Philippine Sea Plate boundaries differ between the two models, those of the breakoff model and the related subducting indenter model are geodynamically similar. The surface junction and the west edge of the imaged high Vp anomalous Philippine Sea Plate comply with those predicted by the subducting indenter model and thus favor the breakoff model over the tear model. While the observed high Vp anomalous region in the mantle wedge can be explained as eclogitization of previously subducted crust, eclogitization of the overriding continental crustal roots cannot be ruled out. Those beneath the Taipei Basin and the Tatun Volcano Group exhibit a pattern potentially connected to the low Vp anomalies in the mantle wedge, suggesting the involvement of the Philippine Sea slab, either by asthenospheric upwelling due to extensional collapse or by fluid migration due to slab dehydration. Those beneath the Ilan Plain exhibit a low Vp pattern extending to deeper origins in the eastern offshore region, suggesting a connection with the opening of the Okinawa Trough.