House in Punalu`u demolished by tsunami related to earthquake in November 1975
Photograph by David Shapiro, Honolulu Star-Bulletin

The largest tsunami generated locally in Hawai`i in the 20th century was triggered by sudden, violent ground motion associated with the magnitude 7.2 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 Halape, 30 km west of the earthquake epicenter. At Halape, 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 Halape). 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.

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 Hawai`i 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

Damage Caused by the Tsunami

Damaged car, Punalu`u	Remains of warehouse, Honu`apo
Boat atop dock, Kona	Overturned boat, Hilo

Warning Time for a Locally Generated Tsunami:
Seconds to Minutes

The time between a large earthquake on the Island of Hawai`i 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 Hawai`i 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