The Hilina Slump on Hawaii

The Hilina Slump, on the south flank of the Kilauea Volcano on the southeast coast of the Big Island of Hawaii, is the most notable of several landslides that ring each of the Hawaiian Islands.  These landslides are an on-going geological process by which material deposited at a volcano's vents are transferred downward and seaward, eventually spilling onto the seabed to broaden the island.

Kilauea's entire south flank, extending out to Cape Kumukahi, is currently sliding seaward, with some parts of the central portion (over looking the Hilina slump) moving as much as 10 centimeters (4 inches) per year, pushed by the forceful injection of magma and pulled by gravity.

Despite speculation in connection with the 2018 lower Puna eruption that the Hilina Slump could catastrophically collapse and generate a Pacific-wide tsunami, the U.S. Geological Survey reports that such an event is extremely unlikely because slumps are associated with a slower type of movement that is not associated with tsunamis.

The Hilina Pali (cliff) on Kīlauea Volcano's south flank is visible evidence of the steep Hilina Fault System. Beneath this system lies the flat-lying detachment fault that has no visible surface expression, but has produced several large earthquakes in the past 200 years.

An earthquake on April 2, 1868, rocked the southeast coast of Hawaii with a magnitude estimated between 7.25 and 7.75. It triggered a landslide on the slopes of the Mauna Loa volcano, five miles (8 km) north of Pahala, killing 31 people. A tsunami claimed 46 additional lives. The villages of Punaluʻu, Ninole, Kawaa, Honuapo, and Keauhou Landing were severely damaged. According to one account, the tsunami "rolled in over the tops of the coconut trees, probably 60 feet (20 m) 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."

A similar earthquake occurred November 29, 1975, with a magnitude of 7.2. A 40-mile (60 km) wide section of the Hilina Slump slid 11 feet (3 m) into the ocean, widening the crack by 26 ft (8 m). This movement also caused a tsunami that reached a maximum height of 47.0 feet (14.3 m) at Keauhou Landing.  Oceanfront properties were washed off their foundations in Punaluʻu. Two deaths were reported at Halape, and 19 other persons were injured. The shoreline at Keauhou Bay was dramatically altered.

A magnitude 6.9 earthquake on May 4, 2018 resulted in the slump moving about two feet. It appears to have been precipitated by vibrations caused by the movement of magma in the eastern rift zone of Kilauea, and in turn the earthquake preceded further volcanic activity.  This could fit into a model of correlation between earthquakes and eruption events described for the earthquakes in 1868 and 1975.