Seismic Imaging of the Crust and Upper Mantle beneath Taiwan Using Full-Waveform Inversion of Teleseismic Body Waves

The tectonics of Taiwan are characterized by the complex convergence of the Philippine Sea Plate (PSP) and the Eurasian Plate (EP). While the general orogenic structure is known, resolving the detailed morphology of the subducting slabs, the forearc basement, and shallow volcanic reservoir systems remains challenging with classical travel-time tomography. Here, we present high-resolution 3-D tomographic models of density, V_P, V_S, and the V_P/V_S ratio of Taiwan, obtained by inverting complete teleseismic waveforms from 36 P and 18 SH events. We utilized data from 240 broadband stations, including those from permanent networks and temporary experiments. In our final FWI model, the Eurasian slab is imaged as a continuous eastward-dipping high-velocity anomaly in southern Taiwan but exhibits a distinct slab gap north of 23.6°N at depths greater than 130 km. This discontinuity likely facilitates toroidal mantle flow around the slab edge, consistent with geodynamic models. A high-velocity body in the upper crust along the eastern Central Range is interpreted as the underthrusted forearc basement. Our model also identifies distinct low-velocity, high V_P/V_S bodies beneath the Tatun Volcano Group (TVG) and Turtle Island, indicative of crustal magma reservoirs. The reservoir beneath the TVG appears dome-shaped with an apex at ~6 km depth, while the reservoir beneath Turtle Island is volumetrically larger and connects to a hydrated mantle wedge, suggesting a fluid-rich magmatic source. These findings provide new constraints on the tectonic evolution, slab dynamics, and volcanic structures in Taiwan, and highlight the potential of teleseismic FWI for crust and upper-mantle imaging.