Mechanisms of two P-phase observations in earthquakes within the upper mantle beneath Northeast Taiwan: Insights from 2D waveform simulations

Understanding the upper mantle structures beneath northeast Taiwan is crucial for interpreting the region's geodynamics. The deployment of the Formosa Array since 2017 has facilitated this investigation. In this study, we thoroughly examine the observations of two P phases from a representative deep earthquake, starting with a compilation of arrival time patterns and waveform characteristics, followed by two-dimensional waveform simulations. After successfully reproducing the key observations using models that include a high Vp anomaly in the mantle wedge above the Ryukyu subduction zone and a slightly faster Eurasian lithosphere to the west, we conclude that the second P phase originates from a head wave along the sub-vertical boundary of the Eurasian lithosphere. These two-dimensional model structures also replicate the observed arrival time patterns of nearby events with two P phases. The results of this study provide alternative constraint on the boundary between the Philippine Sea Plate and the Eurasian Plate at the surface. The findings support the active role of the Eurasian Plate in accommodating the northwesterly-indented Philippine Sea Plate slab beneath northeast Taiwan, rather than a passively torn model.