Why is it that very often, the first wave in a tsunami is not the largest? Since the period of tsunamigrnic events are usually shorter than the period of s tsunami wave, it would seem more logical that the first oscillation is the tallest.
My hypothesis is that the smaller waves are generated later but quickly overtake the larger waves due to having a longer wavelength.
Stand by the shore and watch powerboat waves arrive. Long wavelength waves travel faster than shorter wavelength waves. The motorboat is like a tsunami: it makes an impulsive wave as it passes. The wave spectrum peaks at a wavelength dependent on the size of the disturbance. As the impulsive wave spreads out into leading long waves and trailing short waves, the impulsive nature is lost, and the waves get, well, wavy. The long waves arrive first, but they are of low amplitude, not excited well by the smaller boat/fault. Then, the waves of maximum amplitude follow, and then come waves too short to be well excited.
One other reason is "edge waves" ie total internal reflection of waves near the coast. If the reflection is in phase with the next wave, they will amplify.
