The Hilina Slump, on the flank of the Kilauea Volcano on the southeast side of the Big Island of Hawaii, is the most notable of several slumpz (a form of mass flow) that ring each of the Hawaiian Islands. Slumps and debris avalanches are geological processes by which material deposited at vents of volcanoes are transferred downward and seaward, eventually spilling onto the seabed to broaden the island.
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 Hawaiian Islands are volcanoes, the newest part of the Hawaiian-Emperor seamount chain, created by eruption of magma from the Hawaii hotspot. As the Pacific plate, moving to the northwest, carries the existing volcanoes away from the hotspot, new volcanoes form at the southeastern end. The newest and largest island is the Big Island of Hawaii, formed by the merger of seven volcanoes. The largest, at the trailing edge of the island, is Mauna Loa Volcano, and on its seaward flank is the younger Kilauea Volcano, with the still submerged Loihi Seamount just off-shore.
The Hawaiian volcanoes are shield volcanoes, distinguished from the more familiar stratovolcanoes by their greater breadth and lower gradient slopes. (E.g.: Kiluaea's average slope angle is only 3.3°.) When the volcano is over the hotspot a plentiful supply of magma allows it to build a broad shield; when it loses its supply of magma it dies and is eroded back to sea level.
Like the rest, Kilauea is composed of alternating subaerial and submarine lava flows fractured by cooling joints and interbedded with weaker rock, sediments, and tephra, resulting in what has been characterized as a fractured rock mass. These discontinuities form zones of weakness that lead to slope failure. On Kilauea's seaward flank (where it is not resting against Mauna Loa) these weaknesses are under tension (stretching) due to gravity, leading to extension downslope, and the formation of rift zones – such as the Southwest Rift Zone (SWRZ) and East Rift Zone (ERZ) – parallel to the shore.
The Hilina slump is sliding seaward on top of the southern flank of the Kilauea volcano, at an average speed of 10 cm/year (3.9 in/year). Kilauea is the southeastern portion, about 13.7%, of the Big Island of Hawaii. Compared to the 25,000 to 35,000 km3 volume of Kilauea, the submarine slide is between 10,000 and 12,000 km3, making up about 10% of the island. Model results based on present day slope and sea level suggest that earthquake accelerations stronger than about 0.4 to 0.6 g are enough to exceed the static friction coefficient resulting in a slip along a failure surface. However, recent undersea measurements show that an undersea "bench" has formed a buttress at the forefront of the Hilina Slump, and "this buttress may tend to reduce the likelihood of future catastrophic detachment."
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.
- Excerpt from Figure 2 of Denlinger & Morgan 2014.
- Thompson, Watters & Schiffman 2008, p. 165; Moore et al. 1989.
- Moore et al. 1989, p. 17,465.
- Moore et al. 1989, p. 17,482; Denlinger & Morgan 2014, p. 155.
- Morgan, Moore & Clague 2003.
- Facts on the stability of Kilauea's south flank, past and present (May 14, 2018) at the Wayback Machine (archived 16 May 2018)
- Excerpt from figure 16 in Swanson, Duffield & Fiske 1976, p. 26.
- Clague & Sherrod 2014, p. 97.
- Mahukona (submerged, off the northwest corner of the island), Kohala, Mauna Kea, Hualalai, Mauna Loa, Kilauea, and Loihi (Moore & Clague 1992, p. 1471).
- Moore & Mark 1992, p. 257.
- Clague & Sherrod 2014, p. 97.
- Okubo 2004, p. 44.
- Okubo (2004, §2.1) has a detailed explanation.
- Swanson, Duffield & Fiske 1976, pp. 1, 2, 24, 31, and throughout.
- Smith, Malahoff & Shor 1999.
- Okubo 2004.
- Morgan, Moore & Clague 2003; Hawaiian Landslides —Slope failure on Kilauea's submarine south flank (Subsection)
- Tsunami! by Walter C. Dudley (ISBN 0-8248-1125-9)
- National Geophysical Data Center. "Tsunami runups". Retrieved 1 December 2012.
- Nunn 2009, pp. 134–135.
- "A Slow Emergency and a Sudden Slump". Berkeley Seismology Lab. May 7, 2018. Retrieved May 27, 2018.
- Clague, Dennis A.; Sherrod, David R. (2014), "Chapter 3: Growth and Degradation of Hawaiian Volcanoes" (PDF), in Poland, Michael P.; Takahasi, Taeko Jane; Landowski, Claire M., Characteristics of Hawaiian volcanoes, Professional Paper 1801, U.S. Geological Survey, pp. 97–146, doi:10.3133/pp1801, ISSN 2330-7102.
- Denlinger, Roger P.; Morgan, Julia K. (2014), "Chapter 4: Instability of Hawaiian Volcanoes" (PDF), in Poland, Michael P.; Takahashi, Taeko Jane; Landowski, Claire M., Characteristics of Hawaiian volcanoes, Professional Paper 1801, U.S. Geological Survey, ISSN 2330-7102.
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