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April 16, 1998

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Lest we forget

Govenor Ben Cayetano's proclamation of April as "Tsunami Awareness Month" in Hawai`i is a timely one, for a generation of residents has grown up oblivious of the destructive force of these waves. During the last tsunami alert, Honolulu Civil Defense authorities reported that over 400 surfers were in the water waiting for the waves to hit. Luckily for the surfers, the tsunami was small and did not kill them. What the surfers failed to realize is that a tsunami is unlike a typical storm wave.

Tsunami are a set of waves that are hundreds of miles apart, and as they approach shore, they appear as a rapid movement of the tide. This characteristic leads to the common misnomer, "tidal" wave. Tsunami are generated by large displacement of ocean water. The large displacement is generally caused by one of three sources: earthquakes, landslides, or volcanoes.

Great earthquakes, especially from the subduction trenches along the rim of the Pacific basin, generate tsunami by moving the ocean bottom either up or down. The subsidence or uplift displaces the water over it, and this disturbance radiates away from the source at speeds up to 800 km/hr (500 mph). The two deadly tsunami that struck Hilo on April 1, 1946, and on May 23, 1960, originated from sea-floor movements caused by subduction zone earthquakes.

Large landslides, either submarine or from coastal shores slipping into the ocean, are also responsible for causing tsunami. The highest recorded tsunami at 530 meters (1740 ft) occurred on July 9, 1958, in Lituya Bay, Alaska, from a landslide near the mouth of the bay. Miraculously, two boaters were able to ride out the waves. In 1792, part of Unzen Volcano slid into the ocean and produced a tsunami that killed 4,300 people on Kyushu, Japan.

Two Indonesian volcanoes are responsible for the deadliest tsunami. The explosions of Tambora Volcano in 1815 and of Krakatau Volcano in 1883 resulted in tsunami that killed 4,600 and 33,000 Indonesians, respectively. Large pyroclastic flows into the sea from island volcanoes are also responsible for creating tsunami. Flows from Augustine Volcano have caused tsunami in Cook Inlet, Alaska.

If a tsunami is generated from a distant source, Hawai`i residents have four or more hours to evacuate from low coastal areas. However, if a tsunami is locally generated, the warning time is reduced to minutes.

The last locally generated tsunami occurred on November 29, 1975, and killed two campers at Halape in Hawai`i Volcanoes National Park, where it reached a height of 14.6 m (48 ft). A magnitude-7.2 earthquake located near Kalapana caused the tsunami. The wave took 10 minutes to reach Punalu`u and another 10 to reach Hilo, where it attained a height of 2.4 m (8 ft).

The U.S. Geological Survey's Hawaiian Volcano Observatory is working with the Pacific Tsunami Warning Center to improve their capability in providing a warning when a tsunami is locally generated. Hawaii residents can help themselves by developing an automatic response of quickly leaving the sea coast when they feel a strong earthquake. The Hawaii Island telephone directory has maps of the tsunami inundation zones for the island on pages 75 through 81.

Eruption and Earthquake Update

The activity of the east rift zone eruption of Kilauea Volcano from the vent at Pu`u `O`o returned to normal. Lava continued to flow through a network of tubes down to the seacoast where it entered the ocean at two locations - Waha`ula and Kamokuna. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam cloud is highly acidic and laced with glass particles. There were two earthquakes reported felt in Honaunau during the past week. At 4:09 a.m. on April 13, a magnitude 3.3 earthquake originated 18 km (11 mi) southeast of Honaunau from a depth of 11 km (6.6 mi). The second earthquake was located 11.5 km (7 mi) southeast of Honaunau at a depth of 14.3 km (8.6 mi). It occurred on April 16 at 5:34 a.m. and had a magnitude of 3.1.

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Updated: 20 April 1998