THE SEARCH FOR GEOLOGIC EVIDENCE OF PALEOTSUNAMIS IMPACTING THE HAWAIIAN ISLANDS

The location of the Hawaiian Islands in the north-central Pacific Ocean and their active volcanic heritage are responsible for a number of tsunami hazards from both distant-source and local events. Historical records of tsunamis over the last 200 years in the state of Hawai’i demonstrate that earthquakes in subduction zones around the Pacific “ring of fire” (Aleutian, Kuril-Kamchatka, Chile, Japan, and Alaska) pose a significant tsunami threat to the Hawaiian Islands. Deposits found along the ring of fire subduction zones provide evidence for older tsunamis, and here we report on efforts to identify and document Hawaiian paleotsunami deposits. We collected sediment cores searching for tsunami deposits in coastal marshlands on Oʻahu, Maui, and Kauaʻi in order to determine the extent of the temporal and spatial record of tsunami inundation.. Sites were selected based on historical tsunami runup, numerical inundation modeling results, the presence of a coastal marsh in close proximity to a sandy shoreline, the extent of undisturbed wetlands, and the breadth of prior geological investigations. Several marine sand layers within coastal wetland peat and/or mud deposits were identified as possible tsunami deposits on the north and northeastern shores of Kauaʻi and Oʻahu. At some sites, deposits from historical 1946 and 1957 Aleutian tsunamis aided identification of deeper, older, candidate tsunami deposits. At a few sites, radiocarbon ages bracketing deeper (~ 120 cm) sand layers suggest deposition around 800-600 cal yr B.P., which may correlate with deposits in the eastern Aleutians 660-560 cal yr B.P. (Witter et al., 2016). Further dating and analyses of these deposits is needed to determine from sedimentological and micropaleontological characteristics whether a tsunami formed the deposits, and if deposition could be synchronous on O’ahu and Kaua’I, implying a large, distant-source tsunami inundated the Hawaiian Islands.