Observed high amplitude tsunami 0.5–20 km away from the northern Sumatra coast during the 2004 Sumatra earthquake

Tsunami travels across the open sea very fast. It slows down as it approaches the coast, begins to increase in height and the waves becomes steeper and higher. Thus, the destructive tsunamis are believed to be generated in the harbors or near the shore. Moreover, most of the documented destructions related to tsunami are usually those near and along the shore areas. However, fishermen onboard fishing vessels located about 0.5–20 km from the west coast of northern Sumatra when the 2004 Sumatra–Andaman tsunami happened recounted some observed destructions. Almost all interviewed fishermen felt the earthquake shocks while others even had difficulty controlling their vessels during the shaking. Notably, shortly after the shaking, 10–20 m high waves struck their vessels. These waves were more or less isolated high waves and hit their vessels repeatedly. A numerical simulation using a dispersive nonlinear long wave equation suggested that solitary waves were generated on the steeply uprising waves of long-period tsunamis, which are similar to the waves that struck the vessels. Based from the interviews and simulation, it is ascertained that tsunami even several kilometers offshore can pose danger when the water depths are shallower than 40 m.