Monday, October 20, 2014

Big tsunami: Just 500 years ago, a 9.25 magnitude Aleutian quake blasted Hawaiian shores

A stunningly large Aleutian-sourced tsunami hit the Hawaiian Islands 4-500 years ago, requiring a much more aggressive assessment of potential shoreline damage from future waves.

Much of the preliminary evidence for the big wave comes from the Makauwahi Sinkhole on south Kaua`i, but scientists expect to find more evidence once they start looking for it on other islands.

(Image: Researchers simulated earthquakes with magnitudes between 9.0 and 9, and found that the unique geometry of the eastern Aleutians would direct the largest post-earthquake tsunami energy directly toward the Hawaiian Islands. The red circles are centered on Kaua‘i and encircle the Big Island. Credit: Rhett Butler)

And what that means is that it could happen again.

A report on the wave was published this month in the journal Geophysical Research Letters. The article, “Paleotsunami evidence on Kaua‘i and numerical modeling of a great Aleutian tsunami,” was written by Rhett Butler of the Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, David Burney of the National Tropical Botanical Garden and David Walsh of the Pacific Tsunami Warning Center.

 Burney’s investigations of the sinkhole revealed a huge layered collection of marine debris that he determined could only have come from a tsunami, ripping up coral and rock from the ocean floor and depositing it over a limestone shelf into the sinkhole. 

The wave came 300 feet inland and rose more than 20 feet to dump debris into the sinkhole.

It was the massive 2011 Tohoku quake in Japan, with a magnitude of 9.0, that raised the awareness of researchers that such events were possible. They now assume at a quake that large may occur from the Aleutians every 1,000 years or so.

To account for the Makauwahi dune debris layer, they figure the quake would have needed to be even bigger than Tohoku.

“Using high-resolution bathymetry and topography we model tsunami inundation of the sinkhole caused by an earthquake with a moment magnitude of Mw ~9.25 located in the eastern Aleutians. 

“A preponderance of evidence indicates that a giant earthquake in the eastern Aleutian Islands circa 1425–1665 A.D… created the paleotsunami deposit in Kaua‘i. A tsunami deposit in the Aleutians dated circa 1530–1660 A.D. is consistent with this eastern Aleutian source region,” said the paper.

And why is this kind of study important? The authors write:

“The focus of tsunami energy from the Aleutians directed toward the State of Hawaii, and the short 4.5 (hour) tsunami propagation time, underscores the importance of tsunami readiness for Aleutian events. Hawaii State Civil Defense must make evacuation decisions 3 (hours) prior to tsunami arrival.”

© Jan TenBruggencate 2014

1 comment:

Reid Ozaki said...

There has been documentation of a magnitude 8.7 - 9.2 Cascadia subdution zone quake dated January 1700. It's the work of Brian Atwater and uses Japanese records as well as geological and tree ring data along the OR - WA coast.
Have been curious to see if there has been documentation of this event in the Hawaiian oral history.