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Tsunami
Generated by Magnitude 7.7 Earthquake on November 29, 1975

Demolished home in Punaluʻu, Hawaiʻi
House in Punaluʻu demolished by tsunami related to earthquake in November 1975
Photograph by David Shapiro, Honolulu Star-Bulletin

The largest tsunami generated locally in Hawaiʻi in the 20th century was triggered by sudden, violent ground motion associated with the magnitude 7.7 earthquake on November 29, 1975. Waves reached 6 to 14 m above sea level on the southeastern coast of the Big Island and generally less than 4 m elsewhere on the island. The tsunami killed two people and caused property damage of about $1.4 million (about $4.6 million in 1999 dollars).

The tsunami spread east and southwest from its source near Halapē, 30 km west of the earthquake epicenter. At Halapē, the sea began slowly rising within 10-30 seconds after ground shaking had diminished and then rapidly developed into a rushing wave. At least two more large waves followed (see story of campers at Halapē). No withdrawal of water was observed before the initial wave. The tsunami reached Hilo in only 20 minutes, Kailua-Kona in 27 minutes, and Honolulu in 49 minutes.

What is a tsunami?
A tsunami (tsoo-NAH-mee) is a series of waves of extremely long wave length (distance between wave crests can be >100 km) generated in the ocean by a disturbance that displaces the water. Tsunamis are primarily associated with earthquakes in oceanic and coastal regions. Landslides, volcanic eruptions, nuclear explosions, and even impacts of objects from outer space (such as meteorites, asteroids, and comets) can generate tsunamis. For more information about tsunamis see tsunami information from the University of Washington Geophysics Program and the physics of tsunamis from the West Coast and Alaska Tsunami Warning Center.

About 50 tsunamis have been reported in the Hawaiian Islands since the early 1800's. All but two (in 1975 and 1868) were triggered by earthquakes located several thousand kilometers from Hawaiʻi around the rim of the Pacific Ocean. The most damaging tsunamis to hit the Big Island in this century occurred in 1946 and 1960 and were caused by earthquakes located in Alaska and Chile, respectively. Tsunamis from these distant earthquakes arrive in the Hawaiian Islands 4 to 15 hours after the earthquake, plenty of time for people to evacuate low-lying coastal areas in the state.

 

1975 Tsunami Wave Heights

Strand line of tsunami wave at Keauhou Landing, 

Hawaiʻi Strand line of tsunami wave at Halapē, Hawaiʻi
Highest waves from the tsunami reached 13.1 m above the shoreline at Keauhou Landing (left) and 14.6 m at Halapē (right).

Maximum heights of tsunami on Island of Hawaiʻi generated by the 7.7 earthquake on November 29, 1975. Wave heights are given in meters relative to the postsubsidence shoreline. Epicenter of the earthquake shown by star.

 

Damage Caused by the Tsunami

Car at Punaluʻu, Hawaiʻi
Damaged car, Punaluʻu
Foundation remains of a warehouse 

    destroyed at Honuʻapo, Hawaiʻi
Remains of warehouse, Honuʻapo
Boat washed onto dock, Keauhou Bay,

    Kona, Hawaiʻi
Boat atop dock, Kona
Overturned boat, Hilo, Hawaiʻi
Overturned boat, Hilo

 

Warning Time for a Locally Generated Tsunami:
Seconds to Minutes

The time between a large earthquake on the Island of Hawaiʻi and arrival of first wave of a tsunami ranges from less than a minute to as long as 15 to 25 minutes—generally too short to activate the Hawaiʻi County Civil Defense warning system. The only practical warning for such local events is the earthquake itself. If ground shaking makes standing difficult, produces significant rockfalls, and damages structures, one should immediately move from low-lying coastal areas to higher ground, ideally at least 12-15 m above sea level. And because a tsunami consists of a series of waves instead of a single wave, people are advised to stay out of low-lying danger areas for several hours after a large earthquake.

Reference

Tilling, R.I., Koyanagi, R.Y., Lipman, P.W., Lockwood, J.P., Moore, J.G., and Swanson, D.A., 1976, Earthquake and related catastrophic events, Island of Hawaii, November 29, 1975: a preliminary report: U.S. Geological Survey Circular 740, 33 p.


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The URL of this page is: http://hvo.wr.usgs.gov/earthquakes/destruct/1975Nov29/tsunami.html
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Updated: 17 December 1998 (pnf)