By Dan DuBois, JBPHH Emergency Manager
Hawaii has a long history with tsunamis because of its location in the Pacific. Tsunamis can be caused by undersea earthquakes, but they are also a result of volcanic eruptions and major landslides both above and below the sea.
Hawaii sits on the mid-Pacific hot spot that is responsible for the formation of the Hawaii island chain. Mauna Loa and Mauna Kea, volcanoes that are both located on the Big Island of Hawaii, are surrounded by the Pacific ring of fire that generates major earthquakes.
Underwater earthquakes are caused by tectonic plate movement that displaces the sea bed and by pushing one plate upward and displacing a column of water up and out.
Since the 1800s, the state has been struck by 50 tsunamis, seven which caused major damage and five that were generated from outside the state.
Oahu faces two major tsunami dangers, local and distant. Local tsunamis are deadly because of the very short amount of time, between 20-30 minutes from earthquake to tsunami arrival. The 1868 tsunami that hit the islands destroyed two villages and the 1975 tsunami killed two people.
Distant tsunamis originate somewhere in the ring of fire and arrive anywhere from four to 16 hours after the earthquake event that generated them. The 9.5 earthquake in Chile on May 23, 1960 resulted in waves up to 35 feet high that hit Hilo on the Big Island and caused 61 deaths and $75 million in damage.
Tsunamis are difficult to see in the open ocean, only changing the surface level by 12 or 36 inches, but they are moving as fast as a commercial jet at 600 mph. When these waves reach land, they slow down and move upward causing the huge waves. They are not a single wave but a series of five to seven waves that build in amplitude.
They can also cause what is known as a “wrap around effect,” so a wave impacting the north shore of an island wraps around the island and pushes large waves inland on the south facing shores. The waves are not always consistent when they come ashore. In the 2010 Chile quake, Oahu only had waves of 12 inches while homes were pulled off foundations and out to sea on the Big Island.
The 2011 Japan quake and tsunami has caused scientists at the University of Hawaii and Pacific Tsunami Warning Center to rethink how tsunamis may affect the Hawaiian Islands. This research resulted in the redrafting of the tsunami evacuation zones for an extreme tsunami event.
The important thing to remember about preparing for tsunamis is the same as for hurricanes and other disasters: Be aware and be prepared.