This is a very interesting find. From the io9 page, there is a link to the academic paper (
Paleotsunami evidence on Kaua[]i and numerical modeling of a great Aleutian tsunami - Butler - 2014 - Geophysical Research Letters - Wiley Online Library), and the abstract alone has so much more info. Reading the paper ("Article" tab on the same page) reveals even more. Here is what I found most interesting:
- This is the existing and well-known Makauwahi sinkhole near Poipu, on the SE shore of Kaua'i (the title of this thread makes it sound like the sinkhole was newly discovered). It has some cave passages and the large 100-foot across open air sinkhole. Much research has been ongoing in the layers of this "cave," including extinct endemic birds (the kind wiped out by the Hawaiians). You can easily see this sinkhole, there is a trail nearby (called the Maha'ulepu trail), and they have done some plant restoration work around there. You can also enter and visit the cave with one of the researchers, I think they have weekly tours.
- Also, the layer of tsunami debris was previously discovered and identified as such, as was a similar layer in a bog on Kaua'i's north shore. It was even already dated to about 400 years ago (1430–1665 A.D). What is new is the modeling and research to determine which earthquake in which location could've caused that tsunami.
- There is some doubt as to whether the debris could have entered the sinkhole through a connecting cave that opens at a lower elevation near a stream, as opposed to overtopping the rim of the sinkhole. Thus there is some doubt as to the height of the tsunami run-up. Though they did bring up this alternative theory, I thought it was too easily dismissed.
- Tsunami events in Hawaii are different from those in Alaska, Japan, Chile, and Indonesia. In those places, the quake is local, usually at a subduction zone, and the tsunami strikes quickly. Apart from the rare cliff-collapse tsunami, Hawaii receives the tsunami after they travel across the Pacific. As the models show, only certain locations generate waves that have the right orientation and propagation to strike Hawaii. By running their model on various locations and various earthquake strenghths, this paper found one area where an earthquake would generate the right waves aimed at the Hawaiian islands, and strong enough to wrap around to the SE side of Kaua'i.
- Then they researched that location, Sedanka Island, which is known to have produced earthquakes, and found evidence (in local tsunami deposits) for a big tsunami in the same timeframe. Based on their modeling, they also found that the tsunami should have hit California, and mention there is corroborating historical evidence in that timeframe (they didn't say what it was, so not very convincing).
- The authors admit that there are 6 historical tsunami deposits at Sedanka Island, but only one in Makauwahi cave (over the same time frame in the layers). They speculate that the type of earthquake and the extent of the fault each time may also significantly affect the tsunami and its strength when it hits Hawaii. Thus, it is not every quake from this location will create a huge tsunami--but the idea that at least one did happen within the past 500 years is the big revelation.
- Finally, the authors point out that the US does not have enough scientific ocean buoys between the Alutian and Hawaiian islands to reliably detect such a tsunami early, which is critical for evacuation.
I love science, it's so cool how it all comes together, even if there are some rough edges.
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