Hilina Slump

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The Hilina Slump is outlined in red.

The Hilina Slump is a 20 cubic kilometre (4.760 mile³) chunk of the big island of Hawaii on the south slope of the Kilauea volcano which is breaking away from the island and slipping into the Pacific Ocean as an average of 4 inches (100 mm) per year. The slump has the potential of breaking away at a faster pace in the form of an underwater landslide. In Hawaii, landslides of this nature are called debris avalanches. If the entire Hilina Slump did slide into the ocean at once, it could cause an earthquake in excess of a 9 in magnitude and a tsunami 1,000 feet (300 m) tall, threatening the entire Pacific Rim.

On April 2, 1868, an earthquake in this area with a magnitude estimated between 7.25 and 7.75 rocked the southeast coast of Hawaii. It triggered a landslide on the slopes of the Mauna Loa volcano, five miles north of Pahala, killing 31 persons. A tsunami claimed 46 additional lives. The villages of Punaluu, Ninole, Kawaa, Honuapo, and Keauhou Landing were severely damaged. According to one account, the tsunami "rolled in over the tops of the cocoanut trees, probably 60 feet high .... inland a distance of a quarter of a mile in some places, taking out to sea when it returned, houses, men, women, and almost everything movable." This was reported in the 1988 edition of Walter C. Dudley's book, "Tsunami!" (ISBN 0-8248-1125-9).

On November 29, 1975, a 60 kilometre (37 mile) wide section of the Hilina Slump plunged 11 feet (3 m) into the ocean, widening the crack by 26 feet. This movement caused a 7.2 magnitude earthquake and a 15 metre (48 ft) high tsunami. Oceanfront properties were washed off their foundations in Punaluu. Two deaths were reported at Halape, and 19 other persons were injured.