The difference between S‐wave and S‐to‐P‐wave conversion (SP phase) arrival times is enhanced with rectilinear motion detector filtering to describe alluvial‐sediment thickness in the Kaohsiung–Pingtung (Kaoping) plains area. A more complete understanding of the underground structures of the Kaoping area is provided in this paper and explains why the surrounding regions in Taiwan experience more earthquakes than the Kaoping area. Data are based on seismic activity recorded by the portable array for numerical data acquisition (PANDA) for the period from 1995 to 1997. The difference between S‐wave and SP‐phase arrival times shows that the sedimentary layer is thicker along the west and southwest coasts. P‐wave travel‐time residuals, high‐frequency attenuation parameters Kappa, and quality factor QP, QS, and coda waves confirm this result. We also determined the orientation of the Chaochou fault using the first motion of P‐wave arrivals. To the east of the Chaochou fault, stress trends southeast–northwest, while to the west, it trends northeast–southwest. The change in stress trends east and west of Chaochou fault suggests the presence of a highly fluid accretionary wedge in the Kaoping area. The Chaochou fault forms a seismically active tectonic boundary with the uplift of the hanging wall leading to westward tilting of the basement of the Kaoping plains. We demonstrate that these features are the reason there are relatively few earthquakes in the Kaoping area. The presence of a highly fluid accretionary wedge is indicated by a thick alluvial layer in the west and southwest Kaoping coasts; the Peikung High acts as the indenter that may allow seismic energy to escape and reduce the number of earthquakes in the region.

Online Material: Figures illustrating calculations of Kappa, Qc, P‐ and S‐wave spectra, QP, and QS from ground‐motion data.

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