Evidence for a mega-tsunami from flank collapse of Mauna Loa

In the late 1980s and early 1990s the U.S. Geological Survey carried out extensive sonar surveys in the waters surrounding the Hawaiian islands.  A surprise outcome of this project was the discovery of vast hummocky debris deposits extending for many tens of kilometres from shore.  These deposits are interpreted to be the products of slow-moving slumps and fast-moving debris avalanches derived from the periodic collapse of parts of the Hawaii volcanoes.  The largest avalanches have volumes in the order of 5000 km3, making them the largest known slope failures on earth (Decker and Decker, 1998). 

It is widely accepted that avalanches of this type could have generated very large local tsunamis.  The possible scale of those tsunamis is graphically illustrated by the recent identification and interpretation of ancient tsunami deposits on the Kohala volcano (McMurtry et al., 2004).

The sedimentary beds in question, some of which are situated just above the existing shoreline, were initially interpreted to be beach deposits from a former marine highstand (Stearns and Macdonald, 1946).  Subsequent work has shown that Hawaii has been consistently subsiding at a rate of about 2.6 mm/y for at least the past 475 ka (475,000 y).  Subsidence at this rate is not consistent with marine highstand deposits.  McMurtry et al. have described geological evidence that the beds were formed through tsunami-type depositional processes.  They have also dated the deposits.

The deposits occur sporadically within a 7-km long section of the Kohala coast, at elevations ranging up to 60 m above sea level.  They comprise angular to subrounded basalt boulders and cobbles supported in a matrix of weathered cemented carbonate consisting of broken marine shells and rare coral clasts.  Reverse grading is present in some instances.  These deposits lack the organic framework structure typical of reefs, and the seaward dipping fabric of beach deposits.  They overly a paleosoil, indicating deposition on land, but the abundance of marine materials is inconsistent with fluvial or debris flow deposition.

The carbonate-bearing deposits have been dated using U-series methods.  the results give ages of between 102 and 110 ka (+ 10 ka), although these values are interpreted to represent a minimum age, since U-series ages can be decreased by weathering. 

Based on an age of 110 ka, and the observed subsidence rate of 2.6 mm/y, the Kohala tsunami deposits were deposited at a time when relative sea level on this part of Hawaii was approximately 290 m below current sea level.  This means that some of the observed deposits were at least 350 m above sea level at the time of their deposition.  The mega-tsunami event that produced these deposits is assumed by McMurtry et al. to be the massive Alika 2 debris avalanche (see picture below), which has been dated at 112 + 15 ka.

If an event of this type were to occur today the consequences for the present-day Hawaiian islands would be devastating.  It appears unlikely, however, that distant parts of the Pacific rim would be severely impacted.  As described by Pararas-Carayannis (2002), a tsunami generated by a landslide, even a massive slide like Alika 2, would have a significantly shorter wavelength and period than a tsunami generated by a large subduction-zone earthquake.  Such short-period waves attenuate much more significantly over great distances than long-period earthquake-generated waves, and it is unlikely that catastrophic wave run-ups would affect mainland areas on either side of the Pacific.

 


References

Decker,  R and Decker, B, 1998, Volcanoes, Freeman and Co., New York, 321 p.

McMurtry, G, Fryer, G, Tappin D, Wilkinson, I, Williams, M, Fietzke, J, Garbe-Shoenberg, D and Watts, P, 2004, Megatsunami deposits on Kohala volcano, Hawaii, from flank collapse of Mauna Loa, Geology, V. 32, p. 741-744. (Sept. 2004)

Pararas-Carayannis, G, 2002, Evaluation of the threat of mega tsunamis from postulated slope failures of island stratovolcanoes on La Palma, Canary Islands, and on the Island of Hawaii, Science of Tsunami Hazards, Vol 20, pages 251-277.

Stearns, H and Macdonald, G,  1946. Geology and Ground-Water Resources of the Island of Hawaii, Hawaii Division of Hydrography. Bull. 9,  363 p.


Steven Earle, 2004. Malaspina University-College, Geology Department, Return to Earth Science News