|This map shows how many hours it would take for a tsunami generated anywhere on the rim of the Pacific basin to reach Hawaii. The dots indicate the locations of earthquakes that have generated tsunamis which have affected Hawaii.|
Tsunamis are large, rapidly moving ocean waves triggered by a major disturbance of the ocean floor, which is usually caused by an earthquake but sometimes can be produced by a submarine landslide or a volcanic eruption. Tsunamis are also referred to as "tidal waves," but they have no relation to tides.
Ships at sea cannot detect a passing tsunami, nor can the waves be seen from aircraft. While passing through deep oceans, a tsunami consists of a series of waves that are only a few feet high and a hundred miles or more apart. These waves typically travel at speeds of about 600 mph. As they reach shallow water, the waves slow down but greatly increase in height, and the distance between them shrinks. When the tsunami finally strikes the coast, the waves may crest to heights of 100 feet and travel inland at speeds of 30 mph. A series of waves may reach the coast at intervals of 5 to 40 minutes; the first wave is frequently not the largest.
Tsunamis potentially destructive to the Big Island may originate anywhere around the rim of the Pacific Ocean, or they may be locally generated by earthquakes on or near this island. A tsunami produced by an earthquake on the coast of Chile will reach the Hawaiian Islands in about 15 hours, while one that originates in the Aleutian Islands will arrive in 4.5 to 5.5 hours. A locally generated tsunami gives much less warning; the waves may strike almost immediately after the earthquake occurs.
About 50 tsunamis have been reported in the Hawaiian Islands since the early 1800's. Seven caused major damage, and two of these were locally generated. The most devastating tsunamis to hit the Island of Hawaii in this century occurred in 1946 and 1960. In both cases, the worst damage was inflicted on the northeastern coast of the island. The tsunami of 1946 originated in the Aleutian Islands, struck Hawaii without warning, and killed over 170 people, mainly at Laupahoehoe and Hilo where the wave heights averaged 30 feet. The maximum wave height was 55 feet at Pololu Valley on the northern tip of the island.
The 1960 tsunami originated in Chile and advanced upon the island from the southeast; again, its effects were greatest at Hilo. The arrival time of this tsunami was correctly predicted, but many people failed to heed the warnings, and authorities evacuated an insufficient area of Hilo. As a result, 61 lives were lost as waves up to 35 feet high crashed through homes. Whole city blocks were swept clean of all buildings, and 580 acres were flooded.
The tsunamis of 1868 and 1975 were locally generated by earthquakes beneath the southern coast of the island. The 1868 waves destroyed several coastal villages in the Ka'u and Puna districts, most of which were never rebuilt. The 1975 tsunami claimed two lives and caused widespread damage along the Kalapana coast.
Following the disastrous tsunami of 1946, a tsunami warning system for the Pacific basin was developed by the U.S. Coast and Geodetic Survey. Presently, the Pacific Tsunami Warning System, which has its headquarters in Honolulu, is administered by the National Weather Service under the National Oceanic and Atmospheric Administration (NOAA). The Tsunami Warning Center receives a constant flow of information from seismometers (instruments that detect earthquakes) and tide-gauge stations all around the Pacific Ocean. When an earthquake with the potential to generate a tsunami occurs, the Warning Center puts out a "tsunami watch" within 30-40 minutes, alerting Civil Defense and other authorities of a possible tsunami. The first positive evidence of a tsunami comes from tide stations near the origin of the earthquake. Once the presence of a wave is confirmed, the Honolulu center issues a "tsunami warning," giving an estimated time of arrival for the first wave. When a warning is issued, local authorities evacuate low-lying coastal areas.
Because the speed of a tsunami depends entirely on the depth of the water, the arrival time of a wave from any point on the Pacific rim can be predicted. The size of the wave is difficult to predict, however, because the Hawaiian Islands may be the first place to be hit by the wave. The effects of each new tsunami, and the effects on different coasts of the same tsunami, vary greatly. The size and destructiveness of the waves at any particular site are largely determined by the local topography, both onshore and offshore, and the direction from which the wave approaches. For these reasons, people should not become complacent just because one tsunami warning ends with only a few inches' rise in water level. The next time, a warning could precede a 55-foot wave.
generated tsunamis, such as those of 1868 and 1975, are potentially the most
hazardous type, because the time between their origin and the arrival of the wave
at the shoreline may be too brief to warn and evacuate people. In 1975, the first
wave reached Punalu'u immediately after the earthquake; it arrived at Hilo in 20
minutes. Any earthquake strong enough to cause difficulty in standing or walking
should be regarded as a tsunami warning by people in coastal areas, who should
immediately head for higher ground.
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