Evidence for frequent, large tsunamis spanning locked and creeping parts of the Aleutian megathrust

Abstract

Elevations at Driftwood Bay were mapped with a Magellan PM500 real-time kinematic Global Navigation Satellite System (GNSS) survey instrument with ±1.5 cm horizontal and ±3 cm vertical accuracy. Tidal benchmarks at Nikolski were surveyed to provide comparison to tidal datums calculated by NOAA for the Nikolski, Alaska tide gage (NOAA station ID: 9462450). We also obtained a tidal curve for Driftwood Bay from a pressure sensor deployed near the study site 31 July-8 August, 2013 that was coupled to a barometric pressure sensor to correct for atmospheric pressure changes. We tied the pressure sensor deployed at Driftwood Bay to the NOAA tide gage at Nikolski during the GNSS elevation survey. The tidal curve from Driftwood Bay was converted to a tidal datum referenced to the National Tidal Datum Epoch using the Online Tidal Datum Computation Tool developed by JOA Surveys (ref to https://www.tidaldatumtool.com). All elevations we report are referenced to mean tide level (MTL) as defined by the local tidal datum tool at Driftwood Bay, which incorporates a-0.137 m deviation from MTL at Nikolski due to variation in the geoid. 2 1.2 Dating The timing of sand sheet deposition was estimated using multiple methods, including 137 Cesium activity and radiocarbon dating. Gamma spectroscopy, using low-background, high-efficiency, high-purity Germanium detectors, yielded 137 Cs activities in 1-cm-thick sediment intervals sampled within 17 cm of the surface in core 201 (Table S1). We used peak 137 Cs activity to identify the chronostratigraphic interval of AD 1963 (Pennington et al., 1973) and its relationship to the depth of the youngest sand sheet. Samples were initially dried, homogenized by grinding, packed into standardized vessels, and sealed for at least 24 h before counting. Activities were corrected for self-absorption using a direct transmission method (Cutshall et al., 1983; Cable et al., 2001). Radiocarbon dates obtained through the National Ocean Sciences Accelerator Mass Spectrometry facility in Woods Hole, Massachusetts. We sampled plant macrofossils from above and below the sand sheets and tephras for dating, including delicate peat moss (Sphagnum spp) stems and leaves, black stems of sedges (Carex spp), and woody twigs. We computed calibrated radiocarbon ages from lab-reported ages using OxCal (version 4.2.4) (Bronk Ramsey, 2009a) with the IntCal13 data set of Reimer et al. (2013). Table 2 reports one-sigma, analytic ages in radiocarbon years before 1950 (14 C yr BP), and two-sigma, calibrated ages in solar years before 1950 (cal yr BP). Posterior ages that estimate the times of sand sheet and tephra deposition are listed in Table 3 based on a Bayesian age-depth model constructed with OxCal (Bronk Ramsey, 2008). A Bayesian age-depth model incorporated radiocarbon analyses (Table 2) of samples from multiple cores and the activity of 137 Cs in the upper 25 cm of sediment in core 201 (Table S1). We used OxCal (Bronk Ramsey, 2009a), to construct the model, which computed probability

Sedimentary evidence of prehistoric distant‐source tsunamis in the Hawaiian Islands

“…Probability density functions (bars underneath show 95% confidence intervals) for each modelled age are shown in red for sand A3 in Anahola Valley, sand K1 in Kahana Valley and sand P3 in Pololū Valley. Probability density functions for Aleutian tsunami deposits from Stardust Bay (sands S2 and S3) and Driftwood Bay (sands C1 and C2) are shown in shades of blue (Witter et al ., , ). The 95% confidence interval age range derived from precise Uh/Th coral dating of the Makauwahi Cave deposit is represented by the symmetrical distribution (95% confidence interval) (Butler et al ., ).…”

“…Bulk sediment, wood, grass and Tryonia shells above, within and below sand A3 were also dated, but the resulting ages were too wide ranging and inconsistent to be used in the OxCal model ( (Witter et al, 2016(Witter et al, , 2018. The 95% confidence interval age range derived from precise Uh/Th coral dating of the Makauwahi Cave deposit is represented by the symmetrical distribution (95% confidence interval) (Butler et al, 2017).…”

“…Thirteen coastal wetlands were cored, searching for geological evidence of large tsunamis generated by great (>M w 8Á5) subduction zone earthquakes around the Pacific Rim. Stratigraphic, sedimentary and 14 C age evidence from three sites on Kaua΄i, O΄ahu and Hawai΄i suggest that a large tsunami struck the Hawaiian Islands sometime between 1250 and 1450 CE, which overlaps with the ages of tsunami deposits in the eastern Aleutians (Witter et al, 2018). Historic tsunami deposits at two of the three sites, determined by 137 Cs dating to be from the 1946 and 1957 CE tsunamis, also share an eastern Aleutian source.…”

Organic geochemical investigation of far‐field tsunami deposits of the Kahana Valley, O‘ahu, Hawai‘i

“…Table S1. Modelled ages of tsunami deposits at sites in the Aleutians and Hawaii (after Richmond et al, 2019;Witter et al, 2016Witter et al, , 2018. Table S2.…”

“…gastropod shells in about half of the cores containing the sand layer, which were used to correlate the sand layers between some cores. Radiocarbon dates from bulrush seeds and bulk sediment just above and below the sand layer informed statistical age models used to bracket the timing of deposition to approximately 690 to 508 cal yr bp (1320 to 1442 ce ) at the 95·4% confidence interval which overlaps the ages of tsunami deposits found at multiple field sites on Kauaʻi, the Big Island of Hawaiʻi, and the Aleutians (Table ; Richmond et al ., ; Witter et al ., , ). Further dating and age modelling is needed to better correlate the carbonate sand sheet between the proposed palaeo‐tsunami deposits in Hawaiʻi and the Aleutians.…”