PLATE SEGMENTATION OF THE EASTERN PART OF THE ALEUTIAN SUBDUCTION ZONE

At sites close to the epicentre of the 1964 great Alaska earthquake (Copper River Delta and Turnagain Arm) we find paleoseismological evidence of multiple earthquake deformation cycles over at least the past 4000 years, with earthquakes dated ~900, ~1500, ~2100, ~2600, ~3200 and ~3800 BP. Elsewhere within the area that underwent coseimic displacement in 1964 the Late Holocene records are more variable. For example, around Kenai our previous investigations show only one previous great earthquake in the past 6000 years. In contrast, beyond the eastern limit of the 1964 rupture we have evidence to suggest combined rupture of the Prince William Sound segment with the western Yakutat segment ~900 BP and ~1500 BP. Here we present new evidence from Kodiak Island and Katalla, at the western and eastern limits of the 1964 rupture respectively. Marshes in Kodiak record both coseismic submergence and tsunami deposition. Although complicated marsh stratigraphy currently limits our confidence in quantifying the age and magnitude of cosesimic displacement there is evidence of a subduction zone earthquake ~500 BP, not recorded at Copper River Delta and Turnagain Arm. A series of uplifted ridges along the Katalla valley contrast markedly with the palaeoseismic record in the adjacent Copper River Delta (multiple cycles with coseismic uplift and interseismic subsidence, leading to net subsidence). We show that the Katalla ridges do not correlate simply, on a one to one basis, to coseismic uplift. Sediments from the marshes between ridges in the lower Katalla Valley record slight net uplift in the last 3000 years, resulting from multiple earthquake cycles of coseismic uplift and interseismic subsidence, and correlate with megathrust earthquakes at ~900, ~1500, ~2100 and ~2600 BP. The penultimate episode of coseismic uplift at Katalla occurred within the last 500 years, with no equivalent in the eastern part of the 1964 rupture zone.

With due acknowledgment of the different lengths and clarity of records at our different sites, no Late Holocene megathrust earthquake presently appears to mirror the 1964 earthquake in terms of pattern and magnitude of surface deformation.