Volcano Watch — Mauna Loa will not push Kīlauea into the ocean

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"Volcano Watch" often responds to a question from an interested citizen. A resident of Makawao, Maui, asked about the stability of Kīlauea and whether its south flank could slide away during an eruption of Mauna Loa.

A concern about Kīlauea's stability has been raised many times in the past. The facts are often embellished, and the result is that the south side of Kīlauea is thought by some to be teetering on the brink of wholesale collapse.

The facts are these. Part of the south flank of Kīlauea is cut by faults, known collectively as the Hilina fault system, which more or less parallel the coastline. Hilina Pali, Poliokeawe Pali, and Holei Pali are three of the better known faults. During large earthquakes, such as the magnitude-7.9 in 1868 and the magnitude-7.2 in 1975, the faults drop down suddenly. The observed dropping has been as much as 3.5 m (11.5 feet) in an instant. Local tsunamis accompanied both of these earthquakes and presumably others before them. Such a tsunami is very dangerous, because it starts so close to shore.

The coastal part of the Hilina fault system is currently moving seaward at a maximum observed rate of 8-10 cm (3-4 inches) each year. It is also going up very slowly, or remaining at about the same elevation. This works against the long-term subsidence of the entire island, which is sinking because of its great weight, and the global rise in sea level. Faulting will eventually drop the coastline again.

The southwest rift zone of Kīlauea is quite stable, as is the south flank of Kīlauea, both east and west of the Hilina fault system. The only areas moving much are the Hilina fault system and the sector between it and the east rift zone.

Consequently, we cannot say that the entire south side of Kīlauea is currently unstable. That may be what some geophysical models say, but the data tell us otherwise.

Another fact is that the seafloor topography around each Hawaiian island reflects large-scale underwater landslides in the geologic past. The seafloor is complex off the Hilina fault system, showing in places evidence for landslides and in other places evidence for huge, almost horizontal faults. We do not know if any submarine fault is currently moving, though a long-term GPS experiment by Scripps Institution of Oceanography may resolve this issue in a year or two.

Kīlauea has had no massive landslide in the last 100,000 years, nor is there evidence for a huge landslide anywhere in Hawai`i in that time. The chances of the south side of Kīlauea suddenly peeling away into the sea are, in Dizzy Dean's famous words, "slim and none."

The questioner from Maui wanted to know if swelling of Mauna Loa before or during an eruption might exert enough pressure against Kīlauea to push the south flank catastrophically into the ocean. We think this unlikely. One of Mauna Loa's largest eruptions ever, in 1950, took place along the southwest rift zone but had no impact on Kīlauea. Mauna Loa is currently not swelling much, if at all, and the western part of Kīlauea stands almost stock still. It is hard to imagine a situation in which the slight swelling accompanying a future eruption of Mauna Loa would have any major impact on Kīlauea.

The Great Crack, part of Kīlauea's southwest rift zone, is a large eruptive fissure similar to several along the east rift zone, though longer and wider. It is not undergoing unusual deformation and is not a gash where the south flank of Kīlauea is starting to tear away from the rest of the island. Its name conjures up something mysterious, and so it has become part of the lore about impending doom awaiting Kīlauea.

As an earlier "Volcano Watch" put it, we need to be realistic and worry about things that will happen within a time period of several generations, such as the 1975 and 1868 earthquakes and tsunamis, and not the things that happen in a time period of 100,000 years or more.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lavamoves away from the vent toward the ocean in a network of tubes and descends Pulama pali in several separate tubes. Many breakouts from the tube system feed surface flows above, on, and below the pali in the coastal flats. During the night of September 28, the western flow entered the ocean at Kamoamoa and is now building a new bench. Lava also continues to enter the ocean in the area east of Kupapa`u and provides visitors to the County's lava viewing area a great show.

The public is reminded that the benches of the two ocean entries are very hazardous, with possible collapses of the new land. The steam clouds are extremely hot, highly acidic, and laced with glass particles. Swimming at the black sand beaches of the benches can be a blistering or even deadly venture.

One earthquake was reported felt during the week ending on October 4. Residents of Kapulena and Ahualoa felt an earthquake at 6:27 p.m. on September 28. The magnitude-3.0 earthquake was located 11 km (6.6 mi) east-northeast of Hawi at a depth of 30.7 km (18.4 mi).