Tsunami are produced by undersea earthquakes, landslides or volcanic eruptions. In the deep ocean tsunami pass almost unnoticed at only 1-2 metre swells but as they approach shallower water and land, they change dramatically and can reach over 30metre in height. Some tsunami come from the Hikurangi Trough and fault lines just offshore, such as the 10 metre high wave that washed a house & bridge at Te Mahanga in 1947, while most come from distance sources across the Pacific Ocean such as Chile and Alaska, such as the 3-4 metre wave that moved buildings and boats in Ahuriri harbour in 1960. Scientists are also finding signs of huge waves before human settlement.
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