Paleotsunamis and Hawai’i

Depending on where you live, being prepared for a tsunami is not something you think about. In some places, the thought is much more likely to occur. Hawai’i is one such place. It has seen its share of tsunamis (including in 1946 and 1957), and evidence also exists for tsunamis occurring in Hawaii before recorded history (paleotsunamis).
Evidence of a paleotsunami in Hawai’i is recorded in its geological record. Let me here introduce today’s article: Paleotsunami evidence on Kauai and numerical modeling of a great Aleutian tsunami, written by Rhett Butler, David Burney, and David Walsh. Published in Geophysical Research Letters  in 2014, this article looks at evidence of past tsunamis in Hawai’i (one in particular, as we will see) and what kind of earthquake could have been its source.
The best evidence for Hawaiian paleotsunamis comes from the Makauwahi Sinkhole on the southeastern coast of Kaua’i Island. The sinkhole is about 100 meters (328 feet) from the shore, part of a limestone cave system whose roof collapsed several thousand years ago.  The sinkhole is about 30 meters by 35 meters (98 by 115 feet) and ranges from 6 to 25 meters (20 to 82 feet) high. At the north and south ends there are remaining caves; the northern one opens to a stream, and is the only way in or out of the sinkhole. Its lowest edge is 7.2 meters (24 feet) above sea level.
Google Maps screenshot of Hawai'i. The pinkish marker indicates the island in question, Kaua'i. Imagery copyright 2017 Data LDEO-Columbia, NSF, NOAA, Data SIO, NOAA, U.S. Navy, NGA, GEBCO, Map data copyright 2017 Google.
Google Maps screenshot of Hawai’i. The pinkish marker indicates the island in question, Kaua’i. Imagery copyright 2017 Data LDEO-Columbia, NSF, NOAA, Data SIO, NOAA, U.S. Navy, NGA, GEBCO, Map data copyright 2017 Google.
Geologically speaking, the sinkhole shows evidence of gradual sedimentation, nothing unusual–until a sudden deposit of boulders, rocks, gravel, sand, mollusk shells and pieces of coral. Some of the rocks are the same as those found on shore. At 80 centimeters (31 inches) thick and found throughout the entire sinkhole, this deposit represents a volume of around 600 cubic meters (784 cubic yards), which is quite large. This suggests a tsunami, dated to be somewhere in the time range of 1420 to 1665 AD.
This evidence gives two possibilities; either the tsunami crashed over the 7.2 meters (24 feet) of the seafacing eastern wall, or shot through the opening on the north end with great force. This force would have been enough to turn rocks into projectiles, but there is no sign of the damage this would have caused to the walls of the sinkhole. Most likely then, the debris was brought in, at least for the most part, by the tsunami going over the eastern wall.
This is not the only site suggesting a paleotsunami. In the Aleutian Islands, Sedanka Island has evidence of multiple tsunamis from the area. Located 1 kilometer (0.6 miles) inland and 18 meters (59 feet) above sea level, it includes a deposit dated to between 1530 and 1665 AD–within the range of that suggested by the Makauwahi Sinkhole. Additionally, several sites on the northwestern coast of North America, from British Columbia to Oregon, show evidence of a tsunami in that same timespan.
The authors then took to simulations of earthquakes and their generated tsunamis to see what could have caused the inundation at Makauwahi Sinkhole. These were magnitude 9 to 9.6 earthquakes spanning the Aleutian-Alaska arc. They found that an earthquake of magnitude 9.25 in the east Aleutians would be a likely culprit. They tried shifting the earthquake east and west; this caused the sinkhole to remain uninundated. The setup of the east Aleutian scenario essentially aims the tsunami at Hawai’i. Such an earthquake would not effect Japan (no one would notice anything different than the usual tides without special instruments), though the Pacific Northwest would certainly notice (paleotsunami evidence in these regions has already been discussed). It is possible that an earthquake not quite in the east Aleutians could have had the same effect, but the earthquake would have had to had more slip on its fault than ever recorded–over 35 meters (115 feet).
It is possible that this kind of earthquake and resultant tsunami could happen in modern times, though when cannot be predicted. As it takes only 4.5 hours for such a tsunami to reach Hawai’i, the more early warning the better. While there are buoys that can detect tsunamis in the appropriate ocean, these are often awaiting repair after the stormy season. Some high-tech tsunami sensors are suggested by the authors in order for better tsunami preparedness.
ButlerR., D. Burney, and D. Walsh (2014),  Paleotsunami evidence on Kauaand numerical modeling of a great Aleutian tsunami, Geophys. Res. Lett., 4167956802, doi:10.1002/2014GL061232
Today’s main article.

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