Alaska Science Center
Six large tsunamis in the past~ 1700 years at Stardust Bay, Sedanka Island, Alaska
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Full Publication: http://adsabs.harvard.edu/abs/2013AGUFMNH44A..08W
Product Type: Pages In
Two great earthquakes in 1946 (Unimak Island, Mw 8.1) and 1957 (Andreanof Islands, Mw 8.6) ruptured parts of the central Alaska-Aleutian subduction zone, generating deadly pan-Pacific tsunamis that hit Hawaii. Here, we provide the first estimates of recurrence intervals of such destructive Aleutian-born tsunamis from evidence for tsunami inundation at Stardust Bay on the Pacific coast of Sedanka Island, ~25 km southeast of Dutch Harbor, Alaska. We used soil augers, outcrops and shallow pits to map 6 continuous sand deposits across four depositional environments in a ~500-m-wide, 35-hectare valley. Successive sandy deposits mantled the crests of beach ridges, buried peat formed in freshwater wetlands and upland muskeg, and accumulated to form unusual terrace remnants along the valley's inland margin. Dark, basaltic tephras interbedded in peat underlying two of the sands guided stratigraphic correlation across the study area. Thin, peaty horizons separate the six gray sand beds that can be distinguished from black tephra deposits by their compositions, which consist of subangular volcanic lithics similar to Stardust Bay beach sand. The youngest sand, often the thinnest (<1-13 cm) of the six deposits, underlies drift logs scattered across the landscape at elevations up to 18.5 m and as far as 800 m inland, which provide minimum limits on inundation for the most recent tsunami. The older sands vary in thickness from 6-50 cm and often have rounded gravel at the bases of multiple, normally-graded sand beds, some of which contain ripped-up mud or peat clasts. The sheet-like sand beds blanket topography, thinning over beach ridges and thickening in swales and bogs. Although marine foraminifera are absent in the sandy and peaty deposits in the valley, we infer a tsunami origin for the sand beds, based on their physical properties. The activity of 210Pb and 137Cs in organic-rich sediment above and below the youngest sand bed suggest it predates 1963, consistent with deposition by either the 1946 or 1957 tsunami. Radiocarbon (AMS) analyses of herbaceous seeds, herb stems, moss leaves and insect parts suggest the following ages for the older sand beds: 0.4 ka, sand 2; 0.6 ka, sand 3; 1.1 ka, sand 4; 1.4 ka, sand 5; and 1.6 ka, sand 6. Large tsunamis entered Stardust Bay on average every ~325 years. Remaining questions include: How has late Holocene relative sea-level change and beach-ridge formation influenced tsunami sediment transport and deposition? What were the characteristics of the earthquake sources that generated earlier tsunamis? Like the 1946 and 1957 events, the tsunamis evidenced by the older Stardust Bay deposits probably crossed the Pacific. For example, the age of the penultimate Stardust Bay deposit (290-420 cal yr BP) overlaps the time of an unusual marine flood (285-520 cal yr BP) at Maha'ulepu Caves on the south coast of Kaua'i, Hawaiian Islands.