= Solution
The <sea ice> acts as a frequency-selective filter. Over a long path, <wave attenuation in sea ice> is generally much stronger for shorter <surface gravity waves>, through repeated scattering, internal ice losses and water-side dissipation. Their amplitude can fall below the tiltmeter's detection level even if the source initially generated them. Thus the long-period swell survives while the shorter-period tail does not.
\b[The disappearance near 14 s is an attenuation and detectability limit, not a forbidden-frequency interval of the ideal elastic plate.] The <flexural-gravity wave> <dispersion relation> admits a real positive <wavenumber> for every positive <angular frequency>. In an impulsive-source picture, shorter-period <deep-water gravity waves> also arrive later because their <group velocity> is smaller, but delayed arrival alone does not explain a persistent observed cutoff.
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