Why is the first tsunami wave not always the most destructive one?

A tsunami is not a single wave but a train of progressive, oscillatory waves that can continue to impact a coastline for several days. The reason the first wave is not always the most destructive is that a tsunami consists of multiple major oscillations—typically between three and five—that generate the bulk of the damage.12

These waves are reflected and refracted by the topography of the seafloor and the configuration of the coastline, which causes their effects to vary significantly from place to place. Furthermore, the first arrival at a coast may be a wave trough rather than a crest, causing the water to recede and expose the seafloor before the subsequent, more powerful crest arrives. Because these waves have very long periods—ranging from five minutes to over an hour—the destructive power is often concentrated in these later, successive run-ups or the violent outflow of water that follows them.2