Marine waves generated by sudden water displacements are referred to by the Japanese word tsunami, which has a literal translation ‘harbour wave’. This expression arises from local resonant amplification of tsunamis in some harbours. Tsunamis are examples of what are technically shallow water waves, that is, waves of wavelength much greater than the water depth. Unlike the familiar ocean waves, their propagation speed (Table 16.1) is independent of wavelength or frequency, but this is not always obvious. It depends on depth and, in water of variable depth, refraction/diffraction effects cause apparent dispersion, with lower frequencies (longer wavelengths) arriving first at great distances.