Volcano Watch — Volcano slides are often the cause of strong earthquakes

Moderate to large earthquakes located beneath the south flank of Kīlauea Volcano are commonly caused by seaward sliding of that part of the volcano. On Friday, May 29, at 2:03 p.m., a 3.3-magnitude earthquakeoccurred about five miles deep and 15 miles offshore from Kīlauea's shoreline. Like many previous earthquakes located at similar depths beneath Kīlauea, this one was caused by a small movement of the south flank of Kīlauea down and toward the sea.

Similar movement along parallel faults, repeated during long time periods, has produced the steep cliffs of the Hilina Pali. This seaward movement of the flank of the volcano is actually a large-scale landslide caused by the rapid growth of Kīlauea on top on an unstable foundation (sediment and volcanic debris that were shed down the slope), which results in instability of the volcano's flanks. If the flanks of active volcanoes are buttressed against previously formed volcanoes, their stability is increased. However, the flanks of some volcanoes, like Kīlauea's south flank, are unbuttressed (they have only the sea to lean against) and are, therefore, highly unstable.

Most of the largest earthquakes that have occurred in Hawai`i are related to seaward movement of these unstable, unbuttressed slopes. Most of the time, these earthquakes result in small-scale displacement of the coast downward and toward the sea, as occurred during the 1975 and 1989 earthquakes near Kalapana. These types of events, if repeated many times, result in large-scale slumping of the flank of the volcano. Very rarely, these slump events apparently continue to move and result in the catastrophic failure of the entire flank of the volcano as a debris avalanche.

The deposits from these debris avalanches were first proposed many years ago, but the evidence to support such an interpretation was unconvincing until sonar mapping of the sea floor around the Hawaiian Islands was completed a few years ago by the U.S. Geological Survey. These sonar images, which detect differences in the hardness and texture of the sea floor, clearly show that such enormous landslides have been common, if not abundant, occurrences in the history of the Hawaiian Islands.

The landslide deposits consist of an array of individual blocks ranging from the smallest size detectable by the sonar (about the size of a football field), up to a mammoth block nearly 19 miles long, three miles wide, and nearly one mile high. There are at least 17 major deposits extending away from the islands, and probably a comparable number that now underlie previous landslide deposits or have been buried by the growth of a younger volcano.

The figure shows the locations and extents of the landslide deposits mapped by the GLORIA sonar system. The map is oriented with north at about two o'clock. The heavy dashed line is the axis of the Hawaiian Trough or Moat. This trough is formed due to loading of the heavy volcanoes on the ocean crust and its downward bending as it flexes. For this discussion of the landslides, the important point about the trough is that landslides that move across the trough away from the islands have had to flow uphill. In order to do this, the landslide must have been moving rapidly (catastrophically) rather than in small, incremental steps.

Other evidence also supports the idea that these debris-avalanche type landslides occur catastrophically. In particular, there is a cobble bed of coral blocks at 1,000 feet elevation on Lana`i that has been interpreted as a tsunami deposit. The coral blocks have been dated using uranium-series at 105,000 years old. We suspect that this enormous tsunami was caused by a landslide that originated on the west coast of Mauna Loa Volcano and produced the landslide deposit we have called the `Alika-2. This landslide is the youngest of the major debris-avalanches around the island.

Clearly, these landslides would trigger enormous earthquakes. The largest earthquake in Hawai`i in historical times was that of 1868, which had a magnitude estimated at 8.0. This earthquake may have been triggered by seaward movement of the entire south flank of Hawai`i. Such movement would be one small, incremental slip of the south flank, rather than the type of catastrophic failure that produced the debris avalanche landslides in the more distant past.

Such events will surely occur again as they have been a continuing part of the growth and degradation of Hawaiian volcanoes over many millions of years. However, they occur very rarely, perhaps once every 150,000 years.

Volcano Activity Update

The geological history of the Hawaiian Islands continues to be written. This past week the eruptive vents on the west flank of the Pu`u `O`o cone were inactive until between 10 and 10:30 a.m. Monday when activity resumed and continued through Friday afternoon.