Late Holocene great earthquakes in the eastern part of the Aleutian megathrust

The great earthquake, Mw 9.2, of AD 1964 may not be typical of other megathrust earthquakes in the region during the last 4000 years. We present new field data from three sites: Copper River Delta, the lower Katalla River valley and Puffy Slough, to enhance the temporal and spatial resolutions of the paleoseismic records of multiple great earthquakes. Differences in the spatial patterns of coseismic uplift and subsidence suggest different rupture combinations of the Kodiak, Prince William Sound and western Yakutat segments of the plate boundary. The longest and most comprehensive records all come from the Prince William Sound segment. Most sites here reveal net subsidence over multiple earthquake cycles except where probable upper plate faulting contributes locally to net uplift, with measurable differences between sites only a few kilometres apart. We identify the Katalla area as a source of local seismic hazard, similar to other locations in the western part of the Yakutat microplate, including the two Mw8+ ruptures in AD 1899. We use a Bayesian radiocarbon modelling approach to estimate the age and recurrence intervals of multiple great earthquakes for the Prince William Sound segment of the megathrust. The long interval, 883 +/- 34 (2s) years, between the penultimate earthquake and AD 1964 contrasts with the older earthquakes that have intervals ranging from ~420 to ~610 years, with a mean of ~535 years.