A team of researchers, led by Rhett Butler, geophysicist in the Hawai‘i Institute of Geophysics and Planetology (HIGP), re-examined historical evidence around the Pacific and discovered the origin of the tsunami that hit Sanriku, Japan in 1586—a mega-earthquake from the Aleutian Islands that broadly impacted the north Pacific. Until now, this was considered an orphan tsunami, a historical tsunami without an obvious local earthquake source, likely originating far away.
Butler and scientists from the National Tropical Botanical Garden, SOEST, and NOAA’s Pacific Tsunami Warning Center analyzed material deposited into Makauwahi Cave, Kauaʻi during a tsunami—specifically, coral fragments that were previously dated to approximately the sixteenth century using carbon-14. Using specific isotopes of naturally-occurring thorium and uranium in the coral fragments, they determined a very precise age of the tsunami event that washed the coral ashore. Prior carbon-14 dates had an uncertainty of ±120 years, whereas the uranium-thorium date is more precise, 1572±21 years. This increased precision allowed better comparison with dated, known tsunamis and earthquakes throughout the Pacific.
“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.”