Hilina Slump


The Hilina Slump, on the south flank of the Kīlauea Volcano on the southeast coast of the Big Island of Hawaiʻi, is the most notable of several landslides that ring each of the Hawaiian Islands. These landslides are the means by which material deposited at a volcano's vents are transferred downward and seaward, eventually spilling onto the seabed to broaden the island.
Kīlauea's entire south flank, extending out to Cape Kumukahi, is currently sliding seaward, with some parts of the central portion moving as much as 10 centimeters per year, pushed by the forceful injection of magma and pulled by gravity.
Current movement of the Hilina slump and recent volcanic activity, coupled with evidence of massive submarine slides in the geological past, has led to sensationalistic claims of megatsunamis that might result if the south flank of Kīlauea should suddenly fail. Geologists are confident no such failure is likely, and other experts have stated that the supposed threats of megatsunamis are exaggerated.

Geology

The Hawaiian Islands are volcanoes, the newest part of the Hawaiian-Emperor seamount chain, created by eruption of magma from the Hawaiʻi 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 Hawaiʻi, 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 Kīlauea, with the still submerged Lōʻihi Seamount just off-shore.
The Hawaiian volcanoes are shield volcanoes, distinguished from the more familiar stratovolcanoes by their greater breadth and lower gradient slopes. 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, Kīlauea 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. The weight of the rock mass causes extension downhill, favoring the formation of vertical structures, such as dip-slip faults and rift zones, parallel to the slope. These disconnect the rock mass from the upper flank, putting more stress on any non-vertical planes of weakness, which may fail and form a slip zone.
On Kīlauea's seaward flank these tendencies are evident where magma oozing out of the caldera turns east and west to form the Southwest Rift Zone and East Rift Zone, both parallel to the shore, and also in the cliffs of the Hilina Pali – coincident with dip-slip faults of the Hilina fault system – which form the head-scarp where a large block of rock has slumped down and outward.
The rift zones enable transport of lava tens of kilometers away from the caldera. They also serve as wedges, forcing the south flank of Kīlauea downslope across a décollement – a nearly horizontal fault where the volcanic deposits rest on the oceanic crust – about 8 to 10 km deep. The combination of rifting and gravitationally driven slumping results in seaward movement of the entire south flank, especially around the Hilina Pali, with seaward motions of up to 10 centimeters per year.

Hilina Slump

On the central portion of the south flank of Kīlauea the thousand-foot high cliffs of the Hilina Pali and similar scarps were recognized as early as 1930 as headscarps resulting from slumping of the coast. The Hilina Pali is the headscarp of the Hilina Slump, a type of landslide where a large and relatively intact block slips along a concave surface, dropping vertically at the head, with the toe often extending upward as well as outward The Hilina Slump extends seaward from both ends of the Hilina Pali out to a depth of 5000 meters. Whether this slump is shallow, or reaches down to the décollement that underlies the entire Kīlauea south flank, is still under debate.
With the discovery in the late 1980s that the entire south flank of Kīlauea is involved with submarine landslides the term "Hilina slump" has been applied by some scientists to the broader area.
The Hilina slump is sliding seaward on top of the southern flank of the Kīlauea volcano, at an average speed of 10 cm/year. Kīlauea is the southeastern portion, about 13.7%, of the Big Island of Hawaii. Compared to the 25,000 to 35,000 km3 volume of Kīlauea, 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."

Earthquakes

Earthquakes in Hawaiʻi result from either movement of magma, or sliding of the volcanic edifices which comprise the islands. Some of the seaward slippage of the flank occurs aseismically, without noticeable earthquakes. At other times there is a lurch, resulting in an earthquake of magnitude 6 or greater.

1868

An earthquake on April 2, 1868, rocked the southeast coast of Hawaiʻi with a magnitude estimated between 7.25 and 7.75. It triggered a landslide on the slopes of the Mauna Loa volcano, five miles north of Pāhala, killing 31 people. A tsunami claimed 46 additional lives. The villages of Punaluʻu, Nīnole, Kāwāʻa, Honuʻapo, and Keauhou Landing were severely damaged. According to one account, the tsunami "rolled in over the tops of the coconut 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."

1975

A similar earthquake occurred November 29, 1975, with a magnitude of 7.2. A 40-mile wide section of the Hilina Slump slid 11 feet into the ocean, widening the crack by 26 ft. This movement also caused a tsunami that reached a maximum height of 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.

2018

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 Kīlauea, 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.

Landslides

The breadth and gentle slopes of young shield volcanoes such as Kīlauea are in contrast to the steep, picturesque cliffs, deeply incised canyons, and narrow ridges typical of the older islands, and for a long while it was a bit of mystery how the latter got that way. In 1930 it was suggested that this might be the result of large landslides. In 1964 it was noted that certain areas of sea floor with very blocky topography lay downslope from what appear to be giant amphitheaters, suggesting the possibility of giant landslides. However, that such mass wasting was a ubiquitous feature of Hawaiian geology was not recognized until systematic mapping of the sea floor in the late 1980s identified 17 areas on the flanks of the islands that appear to be the remnants of large landslides. Some aspects of these slides – such as the large volume, the transport of large blocks 50 km or more, and evidence of rapid transport – suggests that some of these slides were, indeed, gigantic, and could have generated giant tsunamis. One of these areas is the south flank of Kīlauea, including the Hilina slump, which is currently in motion. This has raised concern that failure of the south flank of Kīlauea could generate a tsunami that "may threaten cities in the Pacific region", and that even relatively minor displacement of the Hilina slump "would be truly disastrous to life and property on Hawaii island, the rest of the archipelago, and possibly the Pacific Rim."
While the Hawaiian archipelago faces a significant threat from even relatively minor local events, the threat to other trans-Pacific regions has been deemed "exaggerated". Particularly, the larger, more powerful, and more dramatic landslides seen around the Hawaiian Islands are a type of landslide called debris avalanches, where the material in the slide has broken up into pieces small enough to flow as a relatively narrow, fast-moving stream that can run-out as far as. Debris avalanches, or flows, "commonly represent a single episode of rapid failure", where the potential energy of the slide is released suddenly, and could cause giant tsunamis. On the other hand, slumps are largely intact and undeformed blocks that are wider and thicker than debris avalanches, and generally slow moving. Slumps commonly move on a concave surface, with the head dropping down almost vertically, and the toe thrusting upward.
Seaward movement of Kīlauea's flank has been largely halted on the western end by the Lōʻihi and Papaʻu seamounts, and slowed east of the Hilina Slump by the Hohonu Seamount. Additional buttressing of Hilina Slump is provided by a "Midslope Bench" ; catastrophic detachment of this landslide is deemed unlikely, and "dire predictions of the future breakaway of the Hilina slump... may be overstated".

Megatsunamis

Would the collapse of a flank of a Hawaiian volcano result in "towering waves crash with the force of countless atomic bombs onto the coastlines of North America and eastern Asia"?
Public awareness of, and anxiety about, so-called "megatsunamis" was heightened in 2001 when a respected scientific journal published an article by Steven Ward and Simon Day claiming that 1) the Cumbre Vieja volcano in the Canary Islands "may experience a catastrophic failure of its west flank" that would drop a large volume the resulting tsunami would hit the African coast with waves as high as 100 meters, while Florida could expect to see 25 meter high waves. Although no time frame was given, certain "ominous" developments were described that implied "any time now". Coupled with the knowledge that the Hawaiian islands are ringed with debris fans where large portions of the various volcanoes have slid into the sea – the volume of the Hilina slump has been estimated at 10,000–12,000 cubic kilometers – it seems reasonable to consider the risk of volcanic and/or seismic acivity in Hawaiʻi wreaking havoc around the Pacific Rim.
In the word of a report prepared by the U.S. Geological Survey for the U.S. Nuclear Regulatory Commission the threat of megatsunamis is: "exaggerated." Lost in much of the popular reporting was that 25 meter waves hitting Florida was only in the worst-case scenario, and that with "more modest assumptions" the calculated height was only 3 to 8 meters. At almost the same time a physical modeling study at the Swiss Federal Institute of Technology, though not mentioning Ward and Day, examined the same worst-case scenario, and predicted that "the maximum wave amplitude off the U.S. east coast would be less than one meter."
An analysis by found methodological errors in Ward and Day's modeling, including unrealistic assumptions regarding source dimensions, slope instability, speed of failure, and the means by which a slope failure generates a tsunami. That study also pointed out that: Pararas-Carayannis concluded that neither geology nor historic events indicate that Kīlauea's southern flank is "unusually unstable or that a massive collapse is possible in the foreseeable future", but even if such a collapse happened as postulated the far-field effects of the resultant tsunami "have been greatly overstated".