Abstract
We report stratigraphic evidence of land-level changes along the eastern portion of the Alaska-Aleutian megathrust. Four marshes on Shuyak Island record variable amounts of coseismic deformation during four pre-20th century earthquakes. We combine these data with paleoseismic evidence from across the Kodiak, Kenai and Prince William Sound segments of the megathrust. These indicate that in the last 2000 years, AD 1964 was the only one to rupture all three segments simultaneously and generate a Mw 9.2 earthquake. The Kodiak segment ruptured independently on four further occasions with magnitudes > Mw 8.0; in AD 1788 and c.400 (440â320) BP, and independently but around the times of great earthquakes in the Prince William Sound segment c.850 and c.1500 BP.
Highlights
We report stratigraphic evidence of land-level changes along the eastern portion of the Alaska-Aleutian megathrust
The Katmai tephra is pervasive across the landscape and a clear region-wide stratigraphic marker on Shuyak Island and the rest of the Kodiak archipelago
We will judge the evidence from Shuyak against the well-accepted criteria, originally proposed with respect to tidal marsh sequences adjacent to the Cascadia subduction zone (Nelson et al, 1996), and successfully applied elsewhere with some additions, to distinguish sediment couplets that result from great earthquake subsidence or emergence from those produced by other processes (Shennan et al, 2016)
Summary
We report stratigraphic evidence of land-level changes along the eastern portion of the Alaska-Aleutian megathrust. Four marshes on Shuyak Island record variable amounts of coseismic deformation during four pre-20th century earthquakes We combine these data with paleoseismic evidence from across the Kodiak, Kenai and Prince William Sound segments of the megathrust. Numerous palaeoseismological investigations based on Late Holocene coastal sedimentary records in Japan, Alaska and Chile suggest different patterns of rupture during major, M7w e 7.9, to great, >M8 w, Late Holocene earthquakes compared to those recorded in the 20th and 21st centuries (Briggs et al, 2014; Cisternas et al, 2017; Dura et al, 2015, 2016; Garrett et al, 2015; Kelsey et al, 2015; Nelson et al, 2015; Sawai et al, 2004; Shennan et al, 2014a, 2016; Witter et al, 2014). California and Hawaii highlights the hazard that ruptures of single segments of the Alaska subduction zone pose to Pacific coasts, but note the lack of the geological evidence for the ages, recurrence and rupture dimensions of previous earthquakes (Butler, 2012; Kirby et al, 2013; Ryan et al, 2012; SAFFR, 2013)
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