(a) The 18-hr vertical spectra at stations in the Taiwan region showing significant coupled modes ₀S₂₀-₀T₂₁ after the 2010 February 27 Chile earthquake. (b) Comparison between the 18-hr and 48-hr vertical spectra at six BATS stations. Coupled modes ₀S₂₀-₀T₂₁ are very strong in the 18-hr spectrum but not identified in the 48-hr spectrum, indicating a rapid attenuation of the coupling process. Hanning-tapered Fourier transform is applied to the records, which start at 1000 s after the origin time.