先望吓咩係gravity wave先
然後望一次wave equation 既solution
gravity wave 既dispersion relation 係
w = (g k tanh(d/k))^(1/2)
w 係 angular frequency, k 係 wave number, d = depth.
所以 phase velocity 就係
v = (g tanh(d/k))^(1/2)
佢個計法係, 因為tsunami wave既 wavelength ≈ 500km
所以 d/k 好細, 用taylor expansion 攞 linear term (tanh x ≈ x)
佢覺得計錯係因為個wave 係seismic wave, not gravity wave in origin
但其實當個wave 出左海之後就係dominated by gravity
所以佢冇計錯
唯一要留意係 d is a function of t, 所以phase velocity唔係constant
如果海床高度轉得比較steep, 咁個error應該唔會太大