High-frequency waves guided by the subducted plates underneath Taiwan and their association with seismic intensity anomalies

Energy from seismic events traveling up a subduction zone is frequently associated with significant large-amplitude, high-frequency signals with sustained long coda. Such seismic waves guided by the subducted plate with high wave velocity and high Q can cause surprisingly large seismic intensity in the fore-arc area. In this study, we characterize the guiding behavior of the subducted Philippine Sea plate (PSP) underneath Taiwan and investigate their relationship with anomalous peak ground acceleration (PGA) patterns. Oblique subduction of Philippine Sea Plate beneath northeast Taiwan complicates the guiding phenomena. Seismic waves from events deeper than 60 km offshore northern Taiwan reveal wave guide behavior: large, sustained high-frequency (3-10 Hz) signal in P and S wave trains. With increasing depth, a low-frequency (< 1Hz) first arrival becomes more significant especially for events deeper than 100 km. The time separation between the low-frequency onset and the later high-frequency arrival slightly increases with depth, while the value varies with station due to different travel distances in the shallow crust. The depth-dependent high-frequency content confirms the association with a waveguide effect in the subducting slab rather than localized site amplification effects. We attempt here to obtain a practicable quantification scheme to determine the duration of higher-frequency energy, which can be regarded as an indicator of the guiding effect of the Philippine Sea Plate.