In 1997, scientists discovered evidence of an asteroid with a four kilometre (2.5 mile) diameter that landed offshore of Chile approximately two million years ago, producing a huge tsunami that swept over portions of South America and Antarctica. Scientists have concluded that the impact of a moderately large asteroid, five to six kilometres (three to four miles) in diameter, falling between the Hawaiian islands and the west coast of North America would produce a tsunami that would attack cities on the west coasts of Canada, the United States and Mexico and would cover most of the inhabited coastal areas of the Hawaiian islands. (“Tsunami Teacher”, Educational material prepared by UNESCO-IOC www.unesco.org)
It has not happened, but conceivably, a tsunami could also be generated by very large nuclear explosions.