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In the 1980s, the U.S. Geological Survey used sonar to map the sea floor surrounding the Hawaiian Islands. Geologists discovered about 70 major giant landslides that cover half of the flanks of the Hawaiian Ridge (Moore and others, 1989; 1994).

These landslides are among the largest on Earth, attaining lengths of 125 miles (200 km) and volumes of 1,200 cubic miles (5,000 cubic km). Moore and others (1989) identified two types of landslides: slumps and debris avalanches. Slumps moved on an overall slope >3?, caused little disruption of the structural coherence of the volcano's flanks, and extended back to the volcanic rift zones and down to the base of the volcanic pile. Slumps may move slowly or surge abruptly forward several meters, causing large earthquakes. The 1868 and 1975 earthquakes, the largest of Hawaii's historic earthquakes, resulted from movement of the Hilina slump in south Hawaii.

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Generalized locations of damaging earthquakes of magnitude 6 or greater since 1868 on the Island of Hawaii. The same earthquakes are listed in the accompanying table. The magnitudes marked by an (*) are based on eyewitness accounts of the earthquakes' effects and on reports of damage. Since about 1950, the accuracy of location, magnitude, and depth determinations has continually improved as the capability of seismic instruments has increased.

Year	Date	Region	Magnitude	Depth (Mi)
1868	Mar. 28	Mauna Loa south flank	6.5-7.0*	No data
1868	Apr. 2	Mauna Loa south flank	7.5-8.1*	No data
1929	Oct. 5	Hualalai	6.5*	No data
1941	Sept. 25	Kaoiki	6.0*	No data
1950	May 29	Mauna Loa southwest rift	6.2	No data
1951	Apr. 22	Kilauea	6.3	20
1951	Aug. 21	Kona	6.9	5
1952	May 23	Kona	6.0	5
1954	Mar. 30	Kilauea south flank	6.5	5
1962	June 27	Kaoiki	6.1	6
1973	Apr. 26	Honomu	6.2	25
1975	Nov. 29	Kilauea south flank	7.2	6
1983	Nov. 16	Kaoiki	6.6	7
1989	June 25	Kilauea south flank	6.1	9

In 1975, Kilauea's south flank was the site of the magnitude 7.2 Kalapana earthquake, the highest magnitude event in this century. The Kalapana coast subsided as much as 11 feet, generating a huge tsunami that claimed two lives in Hawaii Volcanoes National Park, destroyed houses in Punaluu, sank fishing boats in Keauhou Bay, and damaged boats and piers in Hilo. The most recent large earthquake on Kilauea's south flank occurred in June 1989. With a magnitude of 6.1, this quake caused much less damage than the 1975 event.