Massive Kaua‘i Sinkhole Reveals Source of 1586 Tsunami

Research of coral deposits at a massive sinkhole/cave on the island of Kauaʻi has revealed the origin of a tsunami that hit Sanriku, Japan in 1586.

The study determined that the Japan event was caused by a mega-earthquake measuring greater than a magnitude 9.5 from the Aleutian Islands that broadly impacted the north Pacific.

Makauwahi sinkhole. Credits: R. Butler (L), Gerard Fryer (R), GoogleMaps.

A team of researchers led by Dr. Rhett Butler, geophysicist at the University of Hawai‘i at Mānoa, re-examined historical evidence around the Pacific including coral fragments deposited into the Makauwahi Cave on Kauaʻi.

The Makauwahi geological feature is situated in a hardened sand dune about 100 meters from the ocean in the Māhāʻulepū area, and is the “only well-documented paleotsunami deposit in Hawaiʻi from the 16th century.”

An earlier study estimated the probability of a 9+ Magnitude earthquake in the Aleutian Islands, and its power to create a mega-tsunami in Hawai‘i.

Butler said the latest study identified a very precise age of the tsunami event that caused the coral deposits on Kauaʻi.

The coral deposits were previously dated to approximately the sixteenth century using carbon-14, which had an uncertainty of ±120 years. Using more specific isotopes of naturally occurring thorium and uranium in the coral fragments, researchers came up with a more precise, 1572±21 date.

This increased precision allowed for better comparison with dated, known tsunamis and earthquakes throughout the Pacific.

Coral fragments analyzed in this study (3–5 cm in longest dimension). Credit: Butler, et al.

Until now, researchers considered the event an orphan tsunami, “a historical tsunami without an obvious local earthquake source, likely originating far away.”

“Although we were aware of the 1586 Sanriku tsunami, the age of the Kauaʻi deposit was too uncertain to establish a link,” said Butler. “Also, the 1586 Sanriku event had been ascribed to an earthquake in Lima, Peru. After dating the corals, their more precise date matched with that of the Sanriku tsunami.”

“Even though there was no seismic instrumentation in the 16th century, we offer a preponderance of evidence for the occurrence of a magnitude 9 earthquake in the Aleutian Islands. Our knowledge of past events helps us to forecast tsunami effects and thereby enable us to assess this risk for Hawaiʻi.”

Further, re-analysis of the Peruvian evidence showed that the 1586 Peruvian earthquake was not large enough to create a measurable tsunami hitting Japan. They found additional corroborative evidence around the Pacific that strengthened the case. Earthquakes from Cascadia, the Alaskan Kodiak region and Kamchatka were incompatible with the Sanriku data in several ways. However, a mega-earthquake (magnitude greater than 9.25) in the Aleutians was consistent with evidence from Kauaʻi and the northeast coast of Japan.

Tsunami amplitudes for (a) Mw 9.25 earthquake in E Aleutians, (b) Mw 8.05 earthquake in Lima, Peru.

Butler and scientists from the National Tropical Botanical Garden, UHM School of Ocean and Earth Science and Technology and NOAA’s Pacific Tsunami Warning Center participated in the latest research.

“Hawaiʻi is surrounded by the ‘ring of fire’ where mega-earthquakes generate great tsunamis impacting our island shores—the 2011 Tohoku Japan is the most recent example,” said Butler.

Forecast models of a great Aleutian event inform the development of new maps of extreme tsunami inundation zones for the State of Hawai‘i. By linking evidence on Kauaʻi to other sites around the Pacific, researchers say they can better understand the Aleutian earthquake that generated the tsunami.

Butler and colleagues at UH Mānoa are now working to determine how frequently great earthquakes along the Cascadia margin of the Pacific Northwest might occur. These events have the potential to devastate the coasts of Oregon and Washington, and send a dangerous tsunami to Hawai‘i’s shores.

The coral dating was funded by Director’s funds from the UHM Hawai‘i Institute of Geophysics and Planetology. Tsunami forecast methods were provided by the Pacific Tsunami Warning Center of NOAA. Historical and scientific literature research used the resources of the University of Hawai‘i library.