A tsunami is a series of water waves generated by any large displacement of the sea surface. Seafloor uplift from an earthquake is the most common cause of a tsunami, but volcanic eruptions, underwater landslides, or meteorite impacts can also generate tsunami waves. In the open ocean these waves may only be a few inches high, but can travel at speeds in excess of 500 mph (805 km/h)! In contrast, when tsunami waves reach shallow water, they slow down considerably and may reach heights up to tens of meters. Upon making contact with the shore, tsunamis can severely alter the coastal landscape through rapid erosion and deposition of sediment. High wave energy may transport coastal vegetation and marine debris and leave in place unstable barren dunes susceptible to further erosion. Tsunami events have impacted and led to the closure of coastal parks.
A tsunami generated by a magnitude 9.0 earthquake off the coast of Japan struck Pu‘uhonua O Hōnaunau and Kaloko-Honokōhau National Historical Parks on March 11, 2011. At Pu‘uhonua O Hōnaunau, repeated tsunami surges overtopped walls and travelled hundreds of feet inland. Several sites and features sustained damage, including collapsed, breached, and bulging walls, eroding cultural deposits, and washed out sections of trail. About 80% of the sand and fill material present in the Royal Grounds area of the park was either removed or displaced by the tsunami surges. Damaging events like this challenge the long term survival of historically significant coastal areas.
American Samoa 2009
In 2009 the 8.1 Samoa earthquake generated tsunami waves that reached up to 40 ft (12 m) in height and flooded areas more than a third of a mile (600 m) inland. The tsunami caused widespread damage and destroyed the National Park of American Samoa headquarters and visitor center. The buildings were located in Pago Plaza at the very head of Pago Pago Harbor. The narrowing of the harbor at this point funnels water and makes it particularly vulnerable to tsunami damage. A total of five waves swept the area causing damage to newly installed exhibits, park maintenance equipment, and cultural resources storage. Villages on every side of the island were affected and multiple homes were destroyed.