Volcano Watch — Lest we forget, April is tsunami awareness month

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April is "Tsunami Awareness Month" in Hawai`i. Tsunami is the deadliest natural hazard in Hawai`i. The month of April is chosen to remind people of this hazard because on April 1, 1946, a tsunami, generated in the Aleutians by a magnitude-7.8 earthquake, swept through the islands and killed 159 residents.

Recent newspaper and television stories have emphasized tsunami that originated far from Hawai`i. These events from around the Pacific rim take 4 to 13 hours to reach Hawai`i. What the news stories failed to publicize is the locally generated tsunami that can strike immediately after a very large Hawaiian earthquake.

The largest earthquake in the Hawaiian Islands in the past 200 years happened at 3:40 p.m. on April 2, 1868. It had an estimated magnitude of 7.9 and probably originated beneath the southeast flank of Mauna Loa.

Within minutes following the earthquake, coastal villages from Ka Lae to Kumukahi were inundated by a huge tsunami, at places over 15 m (50 feet) high. There were five large waves in succession, and the largest came first. Many villages were destroyed, and 75 people were swept out to sea and drowned.

The next locally generated tsunami resulted from a magnitude-6.9 earthquake from the Kealakekua fault in Kona at 00:57 a.m. on August 21, 1951. Sizeable waves struck Napo`opo`o and Miloli`i, with the largest 2 m (6-7 ft) high. The tide gage in Honolulu harbor recorded at least seven distinct oscillations starting 38 minutes after the earthquake. The oscillations had a period of 14 minutes and a maximum amplitude of 9 cm (3.6 inches).

A possible local tsunami occurred on March 17, 1952, at 6:00 p.m., when a wave inundated the yard of Kalapana School, 180 m (600 ft) inland from the ocean. A large earthquake had shaken the area about two minutes earlier. The large earthquake was one of 4,553 earthquakes recorded during a seven-week period from March 13 to April 30. The swarm of earthquakes originated offshore of the south flank of Kilauea Volcano.

Kalapana and the rest of the south flank of Kilauea Volcano were also hit by the most recent locally generated tsunami. A magnitude-7.2 earthquake that occurred at 4:48 a.m. on November 29, 1975, was located near Kalapana, and a tsunami was triggered by the ground deformation resulting from the earthquake.

The earthquake caused the shore near Halape in Hawai`i Volcanoes National Park to permanently shift 3.5 m (11.5 ft) down and 8 m (26 ft) seaward. This movement, together with possible unmeasured submarine movements associated with the earthquake, produced a tsunami that reached a height of 14.6 m (48 ft) at Halape.

The tsunami struck immediately after the shaking had stopped, and campers at Halape were engulfed by the waves. Two campers and several horses died.

The tsunami radiated away from the epicentral area and reached Punalu`u in 10 minutes and Hilo in 20 minutes. The largest wave at Punalu`u was 5.8 m (19 ft) high, and the maximum height of the wave in Hilo was 2.4 m (8 ft). The tsunami reached Kailua-Kona in 27 minutes and had a maximum height of 1.8 m (6 ft).

The tide gage in Kahului harbor on Maui recorded the arrival of the 1975 tsunami at 5:28 a.m., 40 minutes after the earthquake; the maximum wave height was 0.9 m (3 ft). The Honolulu tide gage first registered the tsunami at 5:37 a.m., and the largest displacement was 0.2 m (8 inches). The tide gage at Nawiliwili harbor on Kaua`i detected the tsunami at 5:44 a.m., and the maximum height was 0.3 m (1 ft).

The 1975 tsunami was recorded by tide gages in California. The travel time was slightly more than 5 hours, and the maximum wave amplitudes varied from 0.2 m (8 inches) in San Francisco to 0.8 m (2.6 ft) at Port San Luis just south of San Luis Obispo.

The tsunami warning system provides Hawai`i residents ample time to evacuate from waves generated by distant Pacific rim earthquakes, but do not expect a warning for a locally generated tsunami. Only your awareness of the urgency to evacuate will save your life during a locally generated tsunami. Remember, if you are near the ocean when a large earthquake occurs, head for high ground immediately.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated and effusively at the Pu`u `O`o vent during the past week. Starting on the night of April 5-6, lava was extruded from several vents in the crater of Pu`u `O`o as well as in the Episode 55 pit and Puka Nui, two small craters at the southwest base of the cone. A lava lake formed in Pu`u `O`o's crater that persisted until April 7 or 8. Since then, glow can be seen at night from one or more vents in the crater. Bright glow persists over the "rootless" shield area, where short flows emanate from overflows of the perched ponds and from leaks at the base of the shields. Surface flows from breakouts of the tube system on Pulama pali and also on the fan at the base of the pali are frequently observed. There are no ocean entries.

One earthquake was reported felt during the week ending on April 11. A resident of Pahala felt a magnitude-2.7 shock at 8:36 p.m. on April 8. The earthquake was located about 4 miles northeast of Pahala at a depth of about 7 miles.